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Liquefaction hazard assessment and ground failure probability analysis in the Kathmandu Valley of Nepal

Liquefaction hazard assessment and ground failure probability analysis in the Kathmandu Valley of... During the 2015 Gorkha Earthquake (M 7.8), extensive soil liquefaction was observed across the Kathmandu Valley. As a densely populated urban settlement, the assessment of liquefaction potential of the valley is crucial especially for ensuring the safety of engineering structures. In this study, we use borehole data including SPT-N values of 410 locations in the valley to assess the susceptibility, hazard, and risk of liquefaction of the valley soil considering three likely-to-recur scenario earthquakes. Some of the existing and frequently used analysis and computation methods are employed for the assessments, and the obtained results are presented in the form of liquefaction hazard maps indi- cating factor of safety, liquefaction potential index, and probability of ground failure (P ). The assessment results reveal that most of the areas have medium to very high liquefaction susceptibility, and that the central and southern parts of the valley are more susceptible to liquefaction and are at greater risk of liquefaction damage than the northern parts. The assessment outcomes are validated with the field manifestations during the 2015 Gorkha Earthquake. The target SPT-N values (N ) at potentially liquefiable areas are determined using back analysis to ascertain no liquefaction improved during the aforesaid three scenario earthquakes. Keywords: Liquefaction, SPT-N, Hazard, Risk, Ground improvement, Kathmandu Valley Introduction Emilia-Romagna (2012, Italy) and Gorkha Earthquake Soil liquefaction is one of the common seismic conse- (2015, Nepal) (Ansal and Tönük 2007; Novikova et  al. quences that frequently lead to significant structural 2007; Sharma et  al. 2018; etc.). All these manifestations damage during earthquakes (Setiawan et  al. 2017). Dif- have led to an understanding that liquefaction occurs ferent parts of the world have observed liquefaction- mostly in fine loose and saturated silty sands, low-plastic induced ground as well as structural damages in the silty clays, and non-plastic silts as a result of substantial past in loose, saturated sands and other granular soils loss of material shear strength (Jalil et al. 2021). It is also (Setiawan et al. 2018). Surface manifestations of liquefac- witnessed that not only the larger magnitude earthquakes tion have been well recorded during various earthquake (i.e., M > 7) but moderate earthquakes (i.e., M = 5 to 6) w w events, such as Nepal-Bihar Earthquake (1934, Nepal), can also induce liquefaction (Boulanger and Idriss 2014). Alaska Earthquake (1964, USA), Niigata Earthquake With an approximate population of 5 million and a (1964, Japan), Loma Prieta (1989, USA), Kobe Earth- population density of 13,225/km , the Kathmandu Val- quake (1995, Japan), Chi-Chi Earthquake (1999, Taiwan), ley (KMC 2011), is one of the fastest urbanizing cities Bhuj Earthquake (2001, India), Chile Earthquake (2010), in South Asia. Many urban settlements within the val- ley have recently exhibited rapid developments on their outskirts (Chaulagain et al. 2016). As a seismically active *Correspondence: mandip.072phce104@ioe.edu.np area in the Nepal Himalaya, the Kathmandu Valley has a Department of Civil Engineering, Pulchowk Campus, IoE, Tribhuvan long history of large earthquakes. The valley experiences University, Lalitpur, Nepal © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Subedi and Acharya Geoenvironmental Disasters (2022) 9:1 Page 2 of 17 comparatively heavy damage during earthquakes because Earthquake. Additionally, the target SPT-N values (N im- of its ground features, which are composed of lacustrine ) at potentially liquefiable areas are determined proved sediments and have high earthquake wave amplification using backward analysis, ascertaining no liquefaction capacity (Chaulagain et  al. 2016). The lacustrine deposit during the aforementioned three seismic scenarios. also consists of near-surface fine to coarse granular mate - rial layers at different locations with considerably high Study area groundwater tables, which may potentially liquefy during Geology and seismicity earthquake shaking (Okamura et  al. 2015; Sharma et  al. The Kathmandu Valley deposit consists of soft sediment 2019). of mainly lacustrine and fluvial origin with a maximum To ensure the seismic safety of engineering structures depth of about 650  m at the center (Sakai 2001). Typi- in earthquake-prone regions, it is vital to determine cally, the sediment deposits consist of mixtures of gravel, the liquefaction potential of ground material (Naghiza- sand, silt, and clay (Mugnier et  al. 2011; Paudyal et  al. dehrokni et  al. 2018). So far in the Kathmandu Valley, 2013; Sharma et al. 2016; Subedi et al. 2018) with a shal- there are only a few case studies related to liquefaction low groundwater table (Pathak et al. 2009; Shrestha et al. hazard assessment including UNDP (1994), JICA (2002), 2016). Almost all the flood plains of the valley constitute and Piya (2004). UNDP (1994) and Piya (2004) adopted potential liquified sites (Gautam et al. 2017; Sharma et al. similar analysis techniques, but the former followed the 2019; Subedi et  al. 2018). The schematic geological map qualitative analysis method for determining the liquefac- of the valley is shown in Fig. 1. tion score, while the latter used borehole data to verify Nepal lies in one of the most seismically active regions the qualitative analysis results quantitatively. The quali - of the world, with a long history of earthquakes and tative assessment considered the surficial geology and experiences at least one major quake in about 100  years geotechnical characteristics such as SPT-N value, depth (Thapa 2018; KC et al. 2019). As a soft lacustrine deposit, of groundwater table, particle size distribution, Atterberg the Kathmandu Valley ground is prone to amplified shak - limit etc. The liquefaction hazard maps developed by ings during earthquakes, which is corroborated during UNDP (1994) and Piya (2004) contradict those prepared many earthquakes of the past, such as in 1803, 1833, by JICA (2002), which reported that most of the areas 1934, 1960, 1988 and 2015 (Dixit et  al. 2013; Gautam in the Kathmandu Valley are less susceptible to lique- et  al. 2017). Based on the shaking record of the valley as faction. All these three studies are argued to have been well as other parts of Nepal of recent earthquakes, the erroneous, incomplete, and to have underestimated the Department of Urban Development and Building Con- liquefaction susceptibility of the Kathmandu Valley (e.g., struction (DUDBC) has published a new seismic haz- Gautam et  al. 2017; KC et  al. 2020; Sharma et  al. 2016, ard map of Nepal (NBC 2020), which indicates that the 2018, 2019; Subedi et  al. 2021). Thus, it is imperative to peak ground acceleration during an earthquake may revise the liquefaction study in the valley through reliable reach 0.36  g to 0.4  g during probabilistic seismic hazard analysis and assessment methods in order to update the analysis (PSHA) with a 10% probability of exceedance existing liquefaction hazard maps and verify them with over a 50-year study period. This is more than twice the the field evidence from the 2015 Gorkha Earthquake. maximum acceleration recorded during the M 7.8, 2015 In this study, we use borehole data with SPT-N val- Gorkha Earthquake (i.e., 0.18  g as recorded in the Kath- ues of 410 locations in the Kathmandu Valley and mandu Valley). perform liquefaction analysis considering three likely- to-recur earthquakes viz. M 7.8 (0.18  g), M 8.0 (0.30  g) Geotechnical conditions w w and M 8.4 (0.36  g). The liquefaction analysis is done in The liquefaction potential of sediments can be predicted terms of factor of safety (FOS) against liquefaction, liq- using subsurface sediment properties and lithologi- uefaction potential index (LPI), and probability of ground cal units in a given area (Bansal and Nath 2011; Kramer failure (P ). For the factor of safety against liquefaction 1996). For this, total 410 borehole data with standard for each soil layer, we use stress-based technique pro- penetration tests (SPT) from different locations were posed by Idriss and Boulanger (2008) while for estimat- used for the study. Among them, primary data were ing the liquefaction potential index, we use Iwasaki et al. assessed and investigated with 10 borehole data; 5 bore- (1982) method. Likewise, for the quantitative evalua- holes at liquefaction sites of 2015 during the Gorkha tion, we refer to Li et al. (2006) and determine the lique- Earthquake and 5 boreholes at the core city area. Rest of faction-induced probability of ground failure (P ). The the data were obtained from geotechnical investigation validity of outcomes of the simplified procedure for liq - reports of 400 locations in the valley availed by local soil uefaction analysis in FOS, LPI, and P is carried out with laboratories and SAFER database prepared by the Uni- 25 observed liquefaction cases during the 2015 Gorkha versity of Bristol (Gilder et al. 2019). Subedi and A charya Geoenvironmental Disasters (2022) 9:1 Page 3 of 17 Fig. 1 Kathmandu Valley area and its geological formations (modified after Dhital 2015) together with the locations of the boreholes used in the study and locations of Manamaiju and Imadol where liquefaction was manifested during the 2015 Gorkha Earthquake (refer Fig. 2) The geotechnical investigation includes standard pen - boreholes ranging in depth from 15 to 20  m at an inter- etration tests and particle size distribution (sieve and val of 1.5  m. From this 3D model, it is understood that hydrometer analyses), Atterberg’s limits, dry unit weight, the subsoil of the study area comprises clay and silt layers natural moisture content, triaxial tests, and ring shear in a few meters’ surficial depth, and with the increase in tests. Two typical borehole logs of the liquefied sites depth, it has sand and silty sand layers. (Manamaiju and Imadol) during the 2015 Gorkha Earth- Kathmandu Valley subsoil possesses heterogeneous quake are shown in Fig. 2 along with pictures of observed lithology, soil variability and exposure to different over - liquefaction. At Manamaiju (Fig.  2a), the SPT-N values burden stresses. Standard distribution system is used to are less than 10 up to the depth of 6 m and less than 15 up provide a general characterization for the Kathmandu to the depth of 12 m while in case of Imadol (Fig. 2b), the soil typology. The number of data (n), minimum value, SPT-N values are less than 10 up to the depth of 15  m. maximum value, mean value (μ), standard deviation (σ), At both locations, the soil types are mostly low plastic and coefficient of variation (COV) were calculated for silt (ML) and silty sand (SM). Low SPT-N values, shallow analyzing variability and statistical analysis, as shown in groundwater table (1.5 m) and sand-silt type soil compo- Table  1. Likewise in Fig.  4, depth-wise variation of geo- sition preliminarily justify the possibility of soil liquefac- technical parameters of valley soil is presented. An anal- tion during earthquake shaking. ysis of the borehole log data indicates that about 85% Figure  3 is a 15 times vertically exaggerated (i.e., of the borehole locations have SPT-N values less than H:V = 1:15) 3D lithological model of the study area cre- 20 while more than 50% have SPT-N values below 10, ated in Rockworks 2016 with a solid modeling algo- especially at shallow depths in the core area of the val- rithm ‘litho blend’ using the lithological database of the ley. Moreover the average SPT-N value of Kathmandu Subedi and Acharya Geoenvironmental Disasters (2022) 9:1 Page 4 of 17 Fig. 2 Typical borehole logs and observed liquefaction during the 2015 Gorkha Earthquake at a Manamaiju (27.7453° N, 85.3007° E) and b Imadol (27.6668° N, 85.3383° E) (refer Fig. 1 for the site locations) soil at depths 1.5 m, 3 m, 6 m, 9 m and 15 m are 12, 15, fines content (FC) with depth in Kathmandu soils is 18, 20 and 23, respectively, with overall μ = 15, σ = 11, shown in Fig. 4b. The value of FC ranges from 0 to 100% and COV = 71.87% (Fig.  4a). Similarly, the variation of with a mean of 52.24, standard deviation of 36.69, and Subedi and A charya Geoenvironmental Disasters (2022) 9:1 Page 5 of 17 liquefaction. The Kathmandu Valley soil is predomi - nated by dark grey sandy silts followed by low to medium plasticity silts ranging into low to medium plasticity clays. Plotting the data acquired from bore- hole logs in the particle size distribution chart with liquefaction range given out by Tsuchida and Hayashi (1972) (Fig.  5a), it is illustrated that most of the soils from the Kathmandu Valley are liquifiable suggesting that the valley soil is highly susceptible to liquefaction. Figure 5b depicts the plasticity map of all cohesive soils used in the analysis, based on Casagrande (1947). The result ensures that fine-grained soils are mostly silt rather than clays. This finding is crucial for liquefac - tion researchers, as low plasticity silts are more vul- nerable to earthquake motion (Bray and Sancio 2006). Fig. 3. 15 times vertically exaggerated 3D lithological model of Seed et  al. (2003) proposed liquefaction susceptibility the Kathmandu Valley created in Rockworks 2016 ( The lithology requirements, represented in the same figure. descriptions are CH: high plastic clay, CL: low plastic clay, GM: silty gravel, GP: poorly graded gravel, GW: well-graded gravel, MI: intermediate plastic silt, ML: low plastic silt, SC: clayey sand, SM: silty Hydrological conditions sand, SP: poorly graded sand, and SW: well-graded gravel) Groundwater is a crucial factor for soil liquefaction and swelling of extremely fine sediments, making the depth to the groundwater table critical in assessing the extent of a coefficient of variation of 70.24%, which means most water concentration in-situ sediments (Yilmaz and Bagci parts of the valley have low FC in shallow depths. For 2006). The shallow groundwater level reduces the effec - most soils in the Kathmandu Valley, the plasticity index tive confining stress at any depth, making liquefaction (PI) values range from 10 to 20% (μ = 15.13, σ = 6.26, and more likely during an earthquake (Ayele et al. 2021; Nath COV = 41.34%) as shown in Fig.  4c. The average LL and et  al. 2014). The groundwater table study was conducted PI values are found to be 44.29% and 15.13% (Table  1), using data from two sources, i.e. borehole log informa- which clearly shows the abundance of medium plastic- tion and groundwater table map prepared by Shrestha ity soils in the Kathmandu Valley. Furthermore, the spe- et al. (2016). For this, groundwater map of monsoon sea- cific gravity (G ) of soil is between 1.89 and 2.89 (μ = 2.59, son (by Shrestha et al. 2016) and borehole locations were σ = 0.15, and COV = 5.79%) (Fig.  4d). The average value overlaid in ArcGIS. Moreover, groundwater depths for of G is 2.59 which indicates higher content of silty sand all borehole locations were extracted from the map and (which is vulnerable to liquefaction) in the valley. It is of compared with the observed groundwater depths during interest to note that the lower value of SPT-N, abundance borehole tests. Finally, the worst-case scenario for lique- of sandy and silty soils with low fines content and low faction study, i.e., shallow groundwater level, was chosen specific gravity support the soil liquefaction vulnerability for all boreholes from the above two data sources. The in the Kathmandu Valley. spatial map showing the maximum groundwater level Generally, cohesionless sands and coarse silts to of Kathmandu Valley is shown in Fig.  6. Groundwater in low plasticity fines are found to be susceptible to the valley is found at shallow depths ranging from 0.5 to Table 1 A statistical overview of the geotechnical properties of Kathmandu Valley soils Property Number of data Min Max Mean (μ) Standard deviation COV (%) (n) (σ) Specific gravity, G 302 1.89 2.89 2.59 0.15 5.79 Plastic limit, PL (%) 239 15.2 50.98 29.12 7.84 26.91 Liquid limit, LL (%) 239 22.8 69.12 44.29 11.29 25.49 Plasticity index, PI (%) 239 1.73 34 15.13 6.26 41.34 Standard penetration test, (SPT-N) 1167 0 50 15 11 71.87 Fines content, FC (%) 933 0 100 52.24 36.69 70.24 Subedi and Acharya Geoenvironmental Disasters (2022) 9:1 Page 6 of 17 Fig. 4 Depthwise distribution of a SPT-N, b fines content (FC), c plasticity index (PI), and d specific gravity (G ) showing mean (μ) and standard deviation (σ) of the data 5 m below the ground surface, which attributes the Kath- 2015 Gorkha Earthquake. After the 2015 Gorkha Earth- mandu soil to be highly susceptible to liquefaction during quake, NBC (2020) has recommended an earthquake seismic events. of M 8.4 with a PGA of 0.36  g for the Kathmandu Val- ley. All these recommended scenarios were used in the Methodology analysis of liquefaction susceptibility in terms of factor As also stated elsewhere above, the liquefaction analysis of safety (FOS), liquefaction potential index (LPI) and was conducted for three likely-to-recur scenario earth- probability of ground failure (P ). These parameters were quakes of magnitudes M 7.8, M 8.0, and M 8.4 consid- obtained using standard procedures as described in the w w w ering peak ground acceleration (PGA) of 0.18  g, 0.30  g below sections. The obtained result data were interpo - and 0.36  g respectively using existing standards and fre- lated in ArcGIS using inverse distance weighting (IDW) quently used analysis and computation methods. method and presented as spatial zonation maps in terms The PGAs and earthquake magnitudes were taken of the FOS, LPI and P . Moreover, graphical and statisti- from three standard seismic hazard assessments con- cal visualizations were prepared using OriginPro. Finally, ducted for the Kathmandu Valley. JICA (2002) reported the target SPT-N values (N ) at potentially liquefi - improved a PGA of 0.3 g for the scenario earthquake of M 8.0 with able areas were assessed using back analysis to ascertain a 10% probability of exceedance in 50  years (i.e., return no liquefaction during the aforementioned three scenario period of 475  years) in the Kathmandu Valley. A PGA earthquakes. of 0.18  g was observed in the valley during the M 7.8, w Subedi and A charya Geoenvironmental Disasters (2022) 9:1 Page 7 of 17 Determination of the factor of safety (FOS) against liquefaction Several methods based on cone penetration test (CPT), shear wave velocity and cyclic loading test (Bolton Seed et  al. 1985; Robertson and Wride 1998; Onder Cetin et  al. 2004; Moss et  al. 2006; Idriss and Boulanger 2008) are used to do accurate liquefaction potential assess- ment. However, as the CPT, shear wave velocity test, and cyclic loading test are not commonly practiced in Nepal, the liquefaction potential assessment in the valley largely relies on borehole data with SPT-N. In this regard, a method developed by Idriss and Boulanger (2008) is adopted in this study to perform an analysis of the factor of safety (FOS) against liquefaction. Idriss and Boulanger (2008) method is further modified and verified with the liquefaction cases during earthquakes. Additionally, the Iwasaki et al. (1982) method is adopted to compute lique- faction potential index (LPI) at the target locations. Idriss and Boulanger (2008) use the SPT-N data and geotechnical properties of the soil layers to predict the FOS against the liquefaction for each layer. The cyclic resistance ratio (CRR) of the soils is specified in the sys - tem, and the stress (loading) produced in the field as a result of a design earthquake that results in liquefaction is defined as the cyclic stress ratio (CSR). Equation  1 is used to determine the factor of safety against liquefaction: CRR 7.5 Fig. 5 a Particle size distribution graph of Kathmandu soils with a FOS = MSF • K (1) CSR demarcation of liquefiable soils suggested by Tsuchida and Hayashi (1972), b Casagrande’s plasticity chart with liquefaction criteria given where CRR is the cyclic resistance ratio calibrated for 7.5 by Seed et al. (2003) the earthquake of M 7.5; MSF is the magnitude scaling factor that accounts for the effects of shaking duration, and K is a factor for the presence of sustained static shear stresses, such as may exist beneath foundations or within slopes. MSF and K were calculated using Eqs. 2 and 3 4 (2) MSF = 6.9e − 0.058(≤ 1.8) K = 1 − C ln ≤ 1.1 (3) σ σ where C = √ ≤ 0.3 σ (4) 18.9 − 2.55 (N ) 60cs The SPT-N value derived from the field investigation was used to calculate the CRR, while Eq.  5 was used to correct the raw SPT-N value. (N ) = NC C C C C 1 N E B R S (5) Fig. 6 Spatial mapping of the maximum groundwater level obtained by analyzing borehole data and map by Shrestha et al. (2016) Subedi and Acharya Geoenvironmental Disasters (2022) 9:1 Page 8 of 17 Fig. 7 Box plots of the factor of safety (FOS) against liquefaction for three PGAs at depths of a 1.5 b 3 c 6 d 9 and e 15 m in the Kathmandu Valley soils where (N ) is the SPT-N normalised to an overburden the correction factor for rod length. C is the correction 1 60 S pressure of 101 kPa (i.e., atmospheric pressure) with a factor for samplers with and without liners. hammer efficiency of 60%. N is the measured SPT blow The CRR is calculated using Eq. 6. 7.5 count. C is the correction factor for overburden stress. C is the correction factor for borehole diameter. C is B E the correction factor for the hammer energy ratio. C is R Subedi and A charya Geoenvironmental Disasters (2022) 9:1 Page 9 of 17 Fig. 8 Cumulative distribution function (CDF) for FOS against liquefaction at PGA of a 0.18 g b 0.30 g, c 0.36 g against liquefaction, and d liquefaction potential index (LPI) for three seismic scenarios τ σ a 2 max vc max (N ) (N ) CSR = 0.65 = 0.65 r 1 60cs 1 60cs (9) ′ ′ CRR = exp + σ σ g 7.5 vc vc 14.1 126 (6) 3 4 where a is the peak horizontal ground acceleration max (N ) (N ) 1 1 60cs 60cs − + − 2.8 (PGA) at the ground surface, g is the gravitational accel- 23.6 25.4 eration, σ and σ are the total overburden stress and vc vc effective overburden stress, respectively, and r is the where (N ) is an equivalent clean-sand SPT blow 1 60cs stress reduction factor as given in Eq. 10. count. Equations 7 and 8 are used to calculate (N ) : 1 60cs r = exp −1.012 − 1.126sin + 5.133 11.73 (N ) = (N ) + �(N ) 1 60cs 1 60 1 60 (7) +M 0.106 + 0.118sin + 5.142 11.28 (10) 9.7 15.7 �(N ) = exp 1.63 + − where z is the depth of the soil layer in meters. FC + 0.01 FC + 0.01 (8) Estimation of the liquefaction potential index (LPI) where FC is the fines content in the soils obtained from The FOS in Eq.  1 above does not help to obtain precise sieve analysis of the borehole or split-spoon samples. information on the severity of the potential ground defor- The CSR is calculated by using Eq. 9. mation at a given depth while the liquefaction potential index (LPI) introduced by Iwasaki et al. (1982) considers Subedi and Acharya Geoenvironmental Disasters (2022) 9:1 Page 10 of 17 less than 1. The LPI is estimated by using Eq.  11 for the top 20 m or less soil profile (Iwasaki et al. 1982). LPI = F (z)W (z)dz (11) where z = depth of layer; F(z) = function of FOS against liquefaction and is defined as: F (z) = 1 for FOS ≤ 1 (12) F(z) = 0 for FOS > 1 (13) W(z) is a depth-weighting factor defined as: W (z) = 10 − 0.5z. (14) Based on the LPI value, the liquefaction sensitivity can be divided into four groups: very low, low, high, and very high. Estimation of the probability of ground failure (PG) For quantitative evaluation, the liquefaction-induced probability of ground failure (P ) was estimated using Eq. 15 (Li et al. 2006). P = (15) 4.71−0.71∗LPI 1 + e where LPI is the liquefaction potential index as in Eq. 11. Improved SPT‑N values The N is calculated to provide desired SPT-N after improved ground improvement ensuring no liquefaction condition. Improved SPT-N values were determined for all liquefi - able sites using calculations for desired FOS. Correlations in Eqs. 5, 6, 7, 8, and 9 are used to determine the amount of in-situ soil strength change needed to avoid liquefac- tion at previously defined liquefiable locations. In case any soil stratum is found to be liquefiable for a known improved value of CSR, the CRR is calculated using Eq.  16 7.5 as follows. improved CRR = CSR × FOS (16) 7.5 Then, (N ) is evaluated using Eq.  6, and Eqs.  7, 8, 1 60cs and 5 are subsequently used to obtain the final targeted Fig. 9 Spatial distribution of FOS against liquefaction in the SPT-N (i.e. N ) value. The obtained N values improved improved Kathmandu Valley for the seismic scenarios of a 0.18 g, M 7.8, b ensure no potential liquefaction with suitable methods of 0.30 g, M 8.0, c 0.36 g, M 8.4 based on Idriss and Boulanger (2008) w w ground improvement needed at liquefaction-prone sites. These values are supposed to prove highly evidential in planning and estimating the cost of ground improvement. the effect of the liquefiable soil layer’s width, depth, and FOS assuming that the severity of liquefaction is propor- tional to the thickness of the liquefied layer, its proximity to the ground surface, and the extent to which the FOS is Subedi and A charya Geoenvironmental Disasters (2022) 9:1 Page 11 of 17 exacerbated by the shallow water table of 0.5  m to 5  m in these regions. For M 8.4 (0.36 g) scenario earthquake, the FOS values were observed to be the lowest at these depths with an interquartile range of 0.3–0.6 and mini- mum value of 0.1 for both 6 m and 9 m depths. However, for the M 8.0 (0.30  g) earthquake, the FOS values were found to be the lowest for the depth of 9  m with inter- quartile range and minimum value of 0.4–0.7 and 0.2, respectively. As expected, for M 7.8 (0.18 g) scenario, the FOS values were the highest among the three scenarios with the minimum values of 0.4, 0.3, 0.3, 0.3 and 0.4 with the corresponding interquartile range of 1–1.6, 0.9–1.4, 0.7–1.1, 0.6–1.2 and 0.6–1.1 respectively for the depths of 1.5 m, 3 m, 6 m, 9 m and 15 m. Moreover, if the median value for a given depth at a particular location is less than one, soil liquefaction Fig. 10 Area in percentage of FOS against liquefaction in three is likely to occur at that location (Dixit et  al. 2012). In scenario earthquakes of M 7.8 (0.18 g), M 8.0 (0.30 g), and M 8.4 w w w Fig.  7, the median of the FOS values for a given depth (0.36 g) tends to be decreasing as the magnitude of the FOS val- ues increases. Similar results of increase in vulnerability of seismic phenomena with their intensification are also Results and discussions reported by Nath et al. (2018) and Dixit et al. (2012). To analyze the variation of factor of safety (FOS) values The FOS profiles were used to compute cumulative for each substratum encountered at all borehole sites, distribution functions (CDF) at different depths of 1.5, 3, the box plots of FOS against liquefaction are prepared 6, 9 and 15  m, which are shown in Fig.  8a–c. The prob - at depths ranging from 1.5 to 15 m for the three ground ability of liquefaction susceptibility at a depth for vari- motions of scenario earthquakes as shown in Fig.  7. Box ous earthquake magnitudes is depicted in this diagram. plots are a non-parametric representation of a data set’s Similarly, in Fig.  8d, CDF versus LPI is plotted for three statistical distribution which depict the minimum, maxi- ground motions. Using these distribution functions, the mum, mean, median, first quartile (25th percentile), and probability of occurrence of liquefaction (P(FOS < 1)) at third quartile (75th percentile) of FOS values. The red different depths can be computed. dots in the figure denote outliers in the FOS dataset. Figure  8a shows that, for the M 7.8 (0.18  g) scenario These figures show that the FOS values are estimated to earthquake, the probability of liquefaction (i.e., FOS < 1) be higher for the earthquake scenario of M 7.8 (0.18  g) is 0.25 at shallow depth and increases to 0.65 at 15  m followed by M 8.0 (0.30  g) and M 8.4 (0.36  g) for all w w depth. It is interesting to note that the probability of depths. In all three earthquake scenarios, the 6  m and liquefaction is greater than 0.5 at all depths (except for 9  m depths are assessed to be the most vulnerable. The the 1.5  m depth) up to 15  m below the ground surface liquefaction vulnerability of these depths may have been Table 2 Calculation of FOS-based liquefaction potential index (LPI) using Iwasaki et al. (1982) and probability of ground failure (P ) using Li et al. (2006) at Babarmahal (27° 41′ 39″ N, 85° 19′ 28″) Depth (m) FOS F(z) W(z) F(z) * W(z) LPI P 1.5 0.4 0.5879 9.25 5.4376 10 0.93 3 0.5 0.5257 8.50 4.4682 4.5 2.0 0.0000 7.75 0.0000 6 2.0 0.0000 7.00 0.0000 7.5 2.0 0.0000 6.25 0.0000 9 2.0 0.0000 5.50 0.0000 10.5 2.0 0.0000 4.75 0.0000 12 0.96 0.0417 4.00 0.1667 13.5 1.34 0.0000 3.25 0.0000 15 0.91 0.0889 2.50 0.2223 Subedi and Acharya Geoenvironmental Disasters (2022) 9:1 Page 12 of 17 Fig. 12 Area in percentage of liquefaction susceptibility based on liquefaction potential index (LPI) in three scenario earthquakes: M 7.8 (0.18 g), M 8.0 (0.30 g), and M 8.4 (0.36 g) w w (Gautam et al. 2017; Sharma and Deng 2019). Moreover, with the increase in magnitude and PGA of the earth- quake scenario from Fig.  8a–c, the probability of lique- faction at all depths has increased significantly, which is in agreement with the findings of Dixit et  al. (2012) and Raghu Kanth and Dash (2008). Figure 9 shows FOS against liquefaction maps for three different seismic scenarios for the Kathmandu Valley. All red-colored areas with FOS < 0.5 are highly susceptible to liquefaction. Orange-colored areas with 0.5 < FOS < 1 may also be considered susceptible to liquefaction. So, the liquefaction-susceptible areas under the three seismic scenarios are quantitatively summarized in Fig. 10, which indicates that 58.16%, 79.98%, and 96.73% of the study area may liquefy if the ground is shaken during earth- quakes with the PGA values of 0.18 g, 0.30 g, and 0.36 g respectively. Observing all the scenarios, almost 80% of the Kathmandu Valley is found to have FOS less than 1, which means a significant portion of the valley is poten - tially liquefiable. Gautam et  al. (2017), KC et  al. (2020), Piya (2004), Sharma et al. (2019), and Subedi et al. (2021) also bring it to the light that the valley has high suscepti- bility to liquefaction. Table  2 presents a typical calculation for LPI, and based on the LPI values for all the 410 boreholes, the LPI zonation maps were generated for the Kathmandu Val- ley considering the three scenario earthquake. The spa - Fig. 11 Liquefaction potential index (LPI) map of the Kathmandu tial variation of the LPI in the valley is shown in Fig.  11. Valley for the seismic scenarios of a 0.18 g, M 7.8, b 0.30 g, M 8.0, c w w 0.36 g, M 8.4 based on Iwasaki et al. (1982) Severe liquefaction is more likely to occur at locations with an LPI value greater than 15 and is improbable at locations with an LPI value less than 5 (Iwasaki et  al. 1982; Sonmez 2003). According to Sonmez (2003), liq- indicating that the soil is highly susceptible to liquefac- uefaction potential is very high for LPI > 15; high for tion under this earthquake scenario. This is consistent 5 < LPI ≤ 15; moderate for 2 < LPI ≤ 5; low for 0 < LPI ≤ 2 with the occurrence of widespread liquefaction in the and non-liquefied for LPI = 0. Kathmandu Valley during the 2015 Gorkha Earthquake Subedi and A charya Geoenvironmental Disasters (2022) 9:1 Page 13 of 17 Fig. 14 Area in percentage of the level of risks based on probability of ground failure (P ) in three scenario earthquakes of M 7.8 (0.18 g), G w M 8.0 (0.30 g), and M 8.4 (0.36 g) w w analysis indicates that 17.9%, 64.15%, and 80.4% of the sample region fall inside a zone with very high liquefac- tion potential for 0.18 g, 0.30 g, and 0.36 g PGA, respec- tively. Likewise, 55.69%, 32.02%, and 18.1% of the sample region are located within high liquefaction hazard areas for the aforementioned earthquake scenarios while 14.32%, 2.13%, and 0.8% of the region can be classified as having moderate liquefaction hazard. According to the LPI analysis, 70% of the study area in the Kathmandu Val- ley is liquefiable with LPI values greater than 5, which is in agreement with the results from FOS-based analysis in the preceding part of this study. The probability of ground failure (P ) for each target location, after calculation, interpolation, and mapping, is shown in Fig. 13. The red-colored area indicates a very high risk zone while dark green area indicates a very low risk zone. It was observed that the central and south- ern parts of the valley have higher risk of ground failure than the northern parts. Within the study area, 1.87%, 17.64%, and 41.76% of the area are respectively found to be at very high risk of liquefaction-induced ground fail- ure during the scenario earthquakes (i.e., 0.18  g, 0.30  g, and 0.36 g PGAs) while 13.06%, 0.61%, and 0% of the area were found to be at very low or no risk of ground failure Fig. 13 Probability of ground failure (P ) map of the Kathmandu (Fig. 14). Results from ground failure analysis are consist- Valley for the seismic scenarios of a 0.18 g, M 7.8, b 0.30 g, M 8.0, c w w 0.36 g, M 8.4 based on Li et al. (2006) ent with the preceding results based on FOS and LPI. Furthermore, the resulting maps of liquefaction analy- sis based on the FOS, LPI, and P were compared with the observed liquefaction cases during the 2015 Gorkha For further interpretation, the prepared maps are stud- Earthquake documented by the authors along with the ied, and the percentage of locations in the Kathmandu reports by Okamura et  al. (2015); Gautam et  al. (2017); Valley that fall under various liquefaction susceptibility and Sharma et  al. (2019) as shown in Fig.  15. The FOS, zones is comparatively shown in Fig. 12. The comparative Subedi and Acharya Geoenvironmental Disasters (2022) 9:1 Page 14 of 17 Fig. 16 Improved SPT-N (N ) distribution map of the improved Fig. 15 Liquefied sites during the 2015 Gorkha Earthquake Kathmandu Valley for FOS of 1.5 for the seismic scenarios of a 0.18 g, presented in a FOS, b LPI, and c P maps of Kathmandu Valley M 7.8, b 0.30 g, M 8.0, c 0.36 g, M 8.4 w w w prepared for earthquake scenario of M 7.8 (0.18 g) w Subedi and A charya Geoenvironmental Disasters (2022) 9:1 Page 15 of 17 Table 3 Tabular presentation of calculated FOS, LPI and P at liquefaction observed sites during the 2015 Gorkha Earthquake S.N Location Latitude Longitude FOS LPI P 1 Bagdol 27.6676° N 85.2980° E 0.5–1.0 High Medium 2 Bungamati 27.6222° N 85.2622° E 0.5–1.0 Very high High 3 Changunarayan, NEC 27.7090° N 85.4140° E 0.5–1.0 High High 4 Duwakot 27.7094° N 85.4139° E < 0.5 Very high Very high 5 Guheshwori 27.7093° N 85.3576° E < 0.5 High High 6 Gwarko 27.6670° N 85.3380° E < 0.5 Very high High 7 Harisiddhi 27.6549° N 85.3352° E < 0.5 High Medium 8 Hattiban 27.6668° N 85.3344° E < 0.5 High Medium 9 Imadol 27.6668° N 85.3383° E 0.5–1.0 High Very high 10 Itapakhe 27.6792° N 85.4289° E < 0.5 Very high High 11 Jharuwarashi 27.6151° N 85.3439° E < 0.5 Very high Medium 12 Kamalvinayak 27.6785° N 85.4370° E 0.5–1.0 Very high High 13 Khadka Gaon 27.6950° N 85.2714° E 0.5–1.0 very high High 14 Lokanthali 27.6748° N 85.3626° E < 0.5 High Very high 15 Malpokhari 27.6720° N 85.2958° E 0.5–1.0 Very high Medium 16 Manamaiju 27.7453° N 85.3007° E 0.5–1.0 Very high Very high 17 Mulpani 27.7025° N 85.7005° E 0.5–1.0 Very high High 18 Pakune Pati 27.6969° N 85.4401° E < 0.5 Very high Medium 19 Ramkot 27.7110° N 85.2622° E < 0.5 Very high Medium 20 Satdobato 27.6552° N 85.3264° E < 0.5 High High 21 Singhadurbar 27.6987° N 85.3200° E < 0.5 High High 22 Sitapaila 27.7200° N 85.2726° E 0.5–1.0 High Medium 23 Syuchatar 27.6972° N 85.2740° E 0.5–1.0 Very high High 24 Taudaha 1 27.6499° N 85.2829° E < 0.5 Very high High 25 Taudaha 2 27.6484° N 85.2811° E < 0.5 Very high Very high scenarios in the valley. As in the case of FOS, LPI, and Table 4 Improved SPT-N (N ) range for FOS of 1.5 to improved P mapping, the N values assigned to all liquefi - ascertain no liquefaction during three seismic scenarios i.e., G improved M 7.8 (0.18 g), M 8.0 (0.30 g), and M 8.4 (0.36 g) able borehole locations in the Kathmandu Valley were w w w mapped. Based on these maps, it is possible to observe S.N N range for FOS = 1.5 Seismic scenarios improved N values that ensure no liquefaction situation in improved 1 15–25 0.18 g, M 7.8 w potentially liquefiable zones for each of the three earth - 2 20–30 0.30 g, M 8.0 w quake scenarios. Moreover, Table 4 summarises the range 3 25–35 0.36 g, M 8.4 w of N values that could be obtained with each case improved with a FOS of 1.5. It is interesting to note that the range of N values obtained in this study is consistent with improved LPI, and P values were assessed for liquefied locations the results of Khan and Kumar (2020). Selective ground using the resulting maps of this study and are presented improvement methods may be used to reach the target in Table 3. It is observed that at all the 25 liquefied sites, value of N . The adoption of available approaches is improved the FOS values are less than 1, the liquefaction suscep- conditional on the site requirements and available funds. tibility varies from high to very high, and the ground failure risk level ranges from medium to very high. This Conclusion highlights that the findings of this study are coherent A larger part of the Kathmandu Valley is considered to with the field observations of liquefaction during the be highly susceptible to soil liquefaction during seis- 2015 Gorkha Earthquake. mic activities owing to its geological, geotechnical, and Figure  16 shows distribution of N values (or the improved hydrogeological conditions. In this study, we assessed improved SPT-N values), which were determined for all susceptibility, hazard, and risk of liquefaction phenom- liquefiable sites with FOS value of 1.5 to ensure no liq - enon in subsoil stratum of the valley using borehole uefaction condition, for the cases of three earthquake Subedi and Acharya Geoenvironmental Disasters (2022) 9:1 Page 16 of 17 Availability of data and materials data including SPT-N values and laboratory test param- The datasets used and/or analysed during this work are accessible upon eters. We used factor of safety (FOS) against liquefac- reasonable request from the corresponding author. tion, liquefaction potential index (LPI), and probability of ground failure (P ) as the main parameters consider- Declarations ing three likely-to-recur scenario earthquakes of M 7.8 Competing interests (0.18  g), M 8.0 (0.30  g) and M 8.4 (0.36  g). The w w The authors declare that they have no competing interests. obtained results are presented as liquefaction hazard maps showing FOS against liquefaction, assessing liq- Received: 19 August 2021 Accepted: 15 December 2021 uefaction manifestation in terms of LPI and P values. Based on these parameters, the calculated susceptibil- ity of soil liquefaction corroborates with the sites of soil References liquefactions observed during the 2015 Gorkha Earth- Ansal A, Tönük G (2007) Source and site factors in microzonation. In: Pitilakis quake. The resulting maps illustrate the quantitative KD (ed) Earthquake geotechnical engineering. Springer, pp 73–92 features of the liquefiable layers and the region where Ayele A, Woldearegay K, Meten M (2021) A review on the multi-criteria seismic hazard analysis of Ethiopia: with implications of infrastructural develop- ground failure due to liquefaction is likely. The majority ment. 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Liquefaction hazard assessment and ground failure probability analysis in the Kathmandu Valley of Nepal

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Abstract

During the 2015 Gorkha Earthquake (M 7.8), extensive soil liquefaction was observed across the Kathmandu Valley. As a densely populated urban settlement, the assessment of liquefaction potential of the valley is crucial especially for ensuring the safety of engineering structures. In this study, we use borehole data including SPT-N values of 410 locations in the valley to assess the susceptibility, hazard, and risk of liquefaction of the valley soil considering three likely-to-recur scenario earthquakes. Some of the existing and frequently used analysis and computation methods are employed for the assessments, and the obtained results are presented in the form of liquefaction hazard maps indi- cating factor of safety, liquefaction potential index, and probability of ground failure (P ). The assessment results reveal that most of the areas have medium to very high liquefaction susceptibility, and that the central and southern parts of the valley are more susceptible to liquefaction and are at greater risk of liquefaction damage than the northern parts. The assessment outcomes are validated with the field manifestations during the 2015 Gorkha Earthquake. The target SPT-N values (N ) at potentially liquefiable areas are determined using back analysis to ascertain no liquefaction improved during the aforesaid three scenario earthquakes. Keywords: Liquefaction, SPT-N, Hazard, Risk, Ground improvement, Kathmandu Valley Introduction Emilia-Romagna (2012, Italy) and Gorkha Earthquake Soil liquefaction is one of the common seismic conse- (2015, Nepal) (Ansal and Tönük 2007; Novikova et  al. quences that frequently lead to significant structural 2007; Sharma et  al. 2018; etc.). All these manifestations damage during earthquakes (Setiawan et  al. 2017). Dif- have led to an understanding that liquefaction occurs ferent parts of the world have observed liquefaction- mostly in fine loose and saturated silty sands, low-plastic induced ground as well as structural damages in the silty clays, and non-plastic silts as a result of substantial past in loose, saturated sands and other granular soils loss of material shear strength (Jalil et al. 2021). It is also (Setiawan et al. 2018). Surface manifestations of liquefac- witnessed that not only the larger magnitude earthquakes tion have been well recorded during various earthquake (i.e., M > 7) but moderate earthquakes (i.e., M = 5 to 6) w w events, such as Nepal-Bihar Earthquake (1934, Nepal), can also induce liquefaction (Boulanger and Idriss 2014). Alaska Earthquake (1964, USA), Niigata Earthquake With an approximate population of 5 million and a (1964, Japan), Loma Prieta (1989, USA), Kobe Earth- population density of 13,225/km , the Kathmandu Val- quake (1995, Japan), Chi-Chi Earthquake (1999, Taiwan), ley (KMC 2011), is one of the fastest urbanizing cities Bhuj Earthquake (2001, India), Chile Earthquake (2010), in South Asia. Many urban settlements within the val- ley have recently exhibited rapid developments on their outskirts (Chaulagain et al. 2016). As a seismically active *Correspondence: mandip.072phce104@ioe.edu.np area in the Nepal Himalaya, the Kathmandu Valley has a Department of Civil Engineering, Pulchowk Campus, IoE, Tribhuvan long history of large earthquakes. The valley experiences University, Lalitpur, Nepal © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Subedi and Acharya Geoenvironmental Disasters (2022) 9:1 Page 2 of 17 comparatively heavy damage during earthquakes because Earthquake. Additionally, the target SPT-N values (N im- of its ground features, which are composed of lacustrine ) at potentially liquefiable areas are determined proved sediments and have high earthquake wave amplification using backward analysis, ascertaining no liquefaction capacity (Chaulagain et  al. 2016). The lacustrine deposit during the aforementioned three seismic scenarios. also consists of near-surface fine to coarse granular mate - rial layers at different locations with considerably high Study area groundwater tables, which may potentially liquefy during Geology and seismicity earthquake shaking (Okamura et  al. 2015; Sharma et  al. The Kathmandu Valley deposit consists of soft sediment 2019). of mainly lacustrine and fluvial origin with a maximum To ensure the seismic safety of engineering structures depth of about 650  m at the center (Sakai 2001). Typi- in earthquake-prone regions, it is vital to determine cally, the sediment deposits consist of mixtures of gravel, the liquefaction potential of ground material (Naghiza- sand, silt, and clay (Mugnier et  al. 2011; Paudyal et  al. dehrokni et  al. 2018). So far in the Kathmandu Valley, 2013; Sharma et al. 2016; Subedi et al. 2018) with a shal- there are only a few case studies related to liquefaction low groundwater table (Pathak et al. 2009; Shrestha et al. hazard assessment including UNDP (1994), JICA (2002), 2016). Almost all the flood plains of the valley constitute and Piya (2004). UNDP (1994) and Piya (2004) adopted potential liquified sites (Gautam et al. 2017; Sharma et al. similar analysis techniques, but the former followed the 2019; Subedi et  al. 2018). The schematic geological map qualitative analysis method for determining the liquefac- of the valley is shown in Fig. 1. tion score, while the latter used borehole data to verify Nepal lies in one of the most seismically active regions the qualitative analysis results quantitatively. The quali - of the world, with a long history of earthquakes and tative assessment considered the surficial geology and experiences at least one major quake in about 100  years geotechnical characteristics such as SPT-N value, depth (Thapa 2018; KC et al. 2019). As a soft lacustrine deposit, of groundwater table, particle size distribution, Atterberg the Kathmandu Valley ground is prone to amplified shak - limit etc. The liquefaction hazard maps developed by ings during earthquakes, which is corroborated during UNDP (1994) and Piya (2004) contradict those prepared many earthquakes of the past, such as in 1803, 1833, by JICA (2002), which reported that most of the areas 1934, 1960, 1988 and 2015 (Dixit et  al. 2013; Gautam in the Kathmandu Valley are less susceptible to lique- et  al. 2017). Based on the shaking record of the valley as faction. All these three studies are argued to have been well as other parts of Nepal of recent earthquakes, the erroneous, incomplete, and to have underestimated the Department of Urban Development and Building Con- liquefaction susceptibility of the Kathmandu Valley (e.g., struction (DUDBC) has published a new seismic haz- Gautam et  al. 2017; KC et  al. 2020; Sharma et  al. 2016, ard map of Nepal (NBC 2020), which indicates that the 2018, 2019; Subedi et  al. 2021). Thus, it is imperative to peak ground acceleration during an earthquake may revise the liquefaction study in the valley through reliable reach 0.36  g to 0.4  g during probabilistic seismic hazard analysis and assessment methods in order to update the analysis (PSHA) with a 10% probability of exceedance existing liquefaction hazard maps and verify them with over a 50-year study period. This is more than twice the the field evidence from the 2015 Gorkha Earthquake. maximum acceleration recorded during the M 7.8, 2015 In this study, we use borehole data with SPT-N val- Gorkha Earthquake (i.e., 0.18  g as recorded in the Kath- ues of 410 locations in the Kathmandu Valley and mandu Valley). perform liquefaction analysis considering three likely- to-recur earthquakes viz. M 7.8 (0.18  g), M 8.0 (0.30  g) Geotechnical conditions w w and M 8.4 (0.36  g). The liquefaction analysis is done in The liquefaction potential of sediments can be predicted terms of factor of safety (FOS) against liquefaction, liq- using subsurface sediment properties and lithologi- uefaction potential index (LPI), and probability of ground cal units in a given area (Bansal and Nath 2011; Kramer failure (P ). For the factor of safety against liquefaction 1996). For this, total 410 borehole data with standard for each soil layer, we use stress-based technique pro- penetration tests (SPT) from different locations were posed by Idriss and Boulanger (2008) while for estimat- used for the study. Among them, primary data were ing the liquefaction potential index, we use Iwasaki et al. assessed and investigated with 10 borehole data; 5 bore- (1982) method. Likewise, for the quantitative evalua- holes at liquefaction sites of 2015 during the Gorkha tion, we refer to Li et al. (2006) and determine the lique- Earthquake and 5 boreholes at the core city area. Rest of faction-induced probability of ground failure (P ). The the data were obtained from geotechnical investigation validity of outcomes of the simplified procedure for liq - reports of 400 locations in the valley availed by local soil uefaction analysis in FOS, LPI, and P is carried out with laboratories and SAFER database prepared by the Uni- 25 observed liquefaction cases during the 2015 Gorkha versity of Bristol (Gilder et al. 2019). Subedi and A charya Geoenvironmental Disasters (2022) 9:1 Page 3 of 17 Fig. 1 Kathmandu Valley area and its geological formations (modified after Dhital 2015) together with the locations of the boreholes used in the study and locations of Manamaiju and Imadol where liquefaction was manifested during the 2015 Gorkha Earthquake (refer Fig. 2) The geotechnical investigation includes standard pen - boreholes ranging in depth from 15 to 20  m at an inter- etration tests and particle size distribution (sieve and val of 1.5  m. From this 3D model, it is understood that hydrometer analyses), Atterberg’s limits, dry unit weight, the subsoil of the study area comprises clay and silt layers natural moisture content, triaxial tests, and ring shear in a few meters’ surficial depth, and with the increase in tests. Two typical borehole logs of the liquefied sites depth, it has sand and silty sand layers. (Manamaiju and Imadol) during the 2015 Gorkha Earth- Kathmandu Valley subsoil possesses heterogeneous quake are shown in Fig. 2 along with pictures of observed lithology, soil variability and exposure to different over - liquefaction. At Manamaiju (Fig.  2a), the SPT-N values burden stresses. Standard distribution system is used to are less than 10 up to the depth of 6 m and less than 15 up provide a general characterization for the Kathmandu to the depth of 12 m while in case of Imadol (Fig. 2b), the soil typology. The number of data (n), minimum value, SPT-N values are less than 10 up to the depth of 15  m. maximum value, mean value (μ), standard deviation (σ), At both locations, the soil types are mostly low plastic and coefficient of variation (COV) were calculated for silt (ML) and silty sand (SM). Low SPT-N values, shallow analyzing variability and statistical analysis, as shown in groundwater table (1.5 m) and sand-silt type soil compo- Table  1. Likewise in Fig.  4, depth-wise variation of geo- sition preliminarily justify the possibility of soil liquefac- technical parameters of valley soil is presented. An anal- tion during earthquake shaking. ysis of the borehole log data indicates that about 85% Figure  3 is a 15 times vertically exaggerated (i.e., of the borehole locations have SPT-N values less than H:V = 1:15) 3D lithological model of the study area cre- 20 while more than 50% have SPT-N values below 10, ated in Rockworks 2016 with a solid modeling algo- especially at shallow depths in the core area of the val- rithm ‘litho blend’ using the lithological database of the ley. Moreover the average SPT-N value of Kathmandu Subedi and Acharya Geoenvironmental Disasters (2022) 9:1 Page 4 of 17 Fig. 2 Typical borehole logs and observed liquefaction during the 2015 Gorkha Earthquake at a Manamaiju (27.7453° N, 85.3007° E) and b Imadol (27.6668° N, 85.3383° E) (refer Fig. 1 for the site locations) soil at depths 1.5 m, 3 m, 6 m, 9 m and 15 m are 12, 15, fines content (FC) with depth in Kathmandu soils is 18, 20 and 23, respectively, with overall μ = 15, σ = 11, shown in Fig. 4b. The value of FC ranges from 0 to 100% and COV = 71.87% (Fig.  4a). Similarly, the variation of with a mean of 52.24, standard deviation of 36.69, and Subedi and A charya Geoenvironmental Disasters (2022) 9:1 Page 5 of 17 liquefaction. The Kathmandu Valley soil is predomi - nated by dark grey sandy silts followed by low to medium plasticity silts ranging into low to medium plasticity clays. Plotting the data acquired from bore- hole logs in the particle size distribution chart with liquefaction range given out by Tsuchida and Hayashi (1972) (Fig.  5a), it is illustrated that most of the soils from the Kathmandu Valley are liquifiable suggesting that the valley soil is highly susceptible to liquefaction. Figure 5b depicts the plasticity map of all cohesive soils used in the analysis, based on Casagrande (1947). The result ensures that fine-grained soils are mostly silt rather than clays. This finding is crucial for liquefac - tion researchers, as low plasticity silts are more vul- nerable to earthquake motion (Bray and Sancio 2006). Fig. 3. 15 times vertically exaggerated 3D lithological model of Seed et  al. (2003) proposed liquefaction susceptibility the Kathmandu Valley created in Rockworks 2016 ( The lithology requirements, represented in the same figure. descriptions are CH: high plastic clay, CL: low plastic clay, GM: silty gravel, GP: poorly graded gravel, GW: well-graded gravel, MI: intermediate plastic silt, ML: low plastic silt, SC: clayey sand, SM: silty Hydrological conditions sand, SP: poorly graded sand, and SW: well-graded gravel) Groundwater is a crucial factor for soil liquefaction and swelling of extremely fine sediments, making the depth to the groundwater table critical in assessing the extent of a coefficient of variation of 70.24%, which means most water concentration in-situ sediments (Yilmaz and Bagci parts of the valley have low FC in shallow depths. For 2006). The shallow groundwater level reduces the effec - most soils in the Kathmandu Valley, the plasticity index tive confining stress at any depth, making liquefaction (PI) values range from 10 to 20% (μ = 15.13, σ = 6.26, and more likely during an earthquake (Ayele et al. 2021; Nath COV = 41.34%) as shown in Fig.  4c. The average LL and et  al. 2014). The groundwater table study was conducted PI values are found to be 44.29% and 15.13% (Table  1), using data from two sources, i.e. borehole log informa- which clearly shows the abundance of medium plastic- tion and groundwater table map prepared by Shrestha ity soils in the Kathmandu Valley. Furthermore, the spe- et al. (2016). For this, groundwater map of monsoon sea- cific gravity (G ) of soil is between 1.89 and 2.89 (μ = 2.59, son (by Shrestha et al. 2016) and borehole locations were σ = 0.15, and COV = 5.79%) (Fig.  4d). The average value overlaid in ArcGIS. Moreover, groundwater depths for of G is 2.59 which indicates higher content of silty sand all borehole locations were extracted from the map and (which is vulnerable to liquefaction) in the valley. It is of compared with the observed groundwater depths during interest to note that the lower value of SPT-N, abundance borehole tests. Finally, the worst-case scenario for lique- of sandy and silty soils with low fines content and low faction study, i.e., shallow groundwater level, was chosen specific gravity support the soil liquefaction vulnerability for all boreholes from the above two data sources. The in the Kathmandu Valley. spatial map showing the maximum groundwater level Generally, cohesionless sands and coarse silts to of Kathmandu Valley is shown in Fig.  6. Groundwater in low plasticity fines are found to be susceptible to the valley is found at shallow depths ranging from 0.5 to Table 1 A statistical overview of the geotechnical properties of Kathmandu Valley soils Property Number of data Min Max Mean (μ) Standard deviation COV (%) (n) (σ) Specific gravity, G 302 1.89 2.89 2.59 0.15 5.79 Plastic limit, PL (%) 239 15.2 50.98 29.12 7.84 26.91 Liquid limit, LL (%) 239 22.8 69.12 44.29 11.29 25.49 Plasticity index, PI (%) 239 1.73 34 15.13 6.26 41.34 Standard penetration test, (SPT-N) 1167 0 50 15 11 71.87 Fines content, FC (%) 933 0 100 52.24 36.69 70.24 Subedi and Acharya Geoenvironmental Disasters (2022) 9:1 Page 6 of 17 Fig. 4 Depthwise distribution of a SPT-N, b fines content (FC), c plasticity index (PI), and d specific gravity (G ) showing mean (μ) and standard deviation (σ) of the data 5 m below the ground surface, which attributes the Kath- 2015 Gorkha Earthquake. After the 2015 Gorkha Earth- mandu soil to be highly susceptible to liquefaction during quake, NBC (2020) has recommended an earthquake seismic events. of M 8.4 with a PGA of 0.36  g for the Kathmandu Val- ley. All these recommended scenarios were used in the Methodology analysis of liquefaction susceptibility in terms of factor As also stated elsewhere above, the liquefaction analysis of safety (FOS), liquefaction potential index (LPI) and was conducted for three likely-to-recur scenario earth- probability of ground failure (P ). These parameters were quakes of magnitudes M 7.8, M 8.0, and M 8.4 consid- obtained using standard procedures as described in the w w w ering peak ground acceleration (PGA) of 0.18  g, 0.30  g below sections. The obtained result data were interpo - and 0.36  g respectively using existing standards and fre- lated in ArcGIS using inverse distance weighting (IDW) quently used analysis and computation methods. method and presented as spatial zonation maps in terms The PGAs and earthquake magnitudes were taken of the FOS, LPI and P . Moreover, graphical and statisti- from three standard seismic hazard assessments con- cal visualizations were prepared using OriginPro. Finally, ducted for the Kathmandu Valley. JICA (2002) reported the target SPT-N values (N ) at potentially liquefi - improved a PGA of 0.3 g for the scenario earthquake of M 8.0 with able areas were assessed using back analysis to ascertain a 10% probability of exceedance in 50  years (i.e., return no liquefaction during the aforementioned three scenario period of 475  years) in the Kathmandu Valley. A PGA earthquakes. of 0.18  g was observed in the valley during the M 7.8, w Subedi and A charya Geoenvironmental Disasters (2022) 9:1 Page 7 of 17 Determination of the factor of safety (FOS) against liquefaction Several methods based on cone penetration test (CPT), shear wave velocity and cyclic loading test (Bolton Seed et  al. 1985; Robertson and Wride 1998; Onder Cetin et  al. 2004; Moss et  al. 2006; Idriss and Boulanger 2008) are used to do accurate liquefaction potential assess- ment. However, as the CPT, shear wave velocity test, and cyclic loading test are not commonly practiced in Nepal, the liquefaction potential assessment in the valley largely relies on borehole data with SPT-N. In this regard, a method developed by Idriss and Boulanger (2008) is adopted in this study to perform an analysis of the factor of safety (FOS) against liquefaction. Idriss and Boulanger (2008) method is further modified and verified with the liquefaction cases during earthquakes. Additionally, the Iwasaki et al. (1982) method is adopted to compute lique- faction potential index (LPI) at the target locations. Idriss and Boulanger (2008) use the SPT-N data and geotechnical properties of the soil layers to predict the FOS against the liquefaction for each layer. The cyclic resistance ratio (CRR) of the soils is specified in the sys - tem, and the stress (loading) produced in the field as a result of a design earthquake that results in liquefaction is defined as the cyclic stress ratio (CSR). Equation  1 is used to determine the factor of safety against liquefaction: CRR 7.5 Fig. 5 a Particle size distribution graph of Kathmandu soils with a FOS = MSF • K (1) CSR demarcation of liquefiable soils suggested by Tsuchida and Hayashi (1972), b Casagrande’s plasticity chart with liquefaction criteria given where CRR is the cyclic resistance ratio calibrated for 7.5 by Seed et al. (2003) the earthquake of M 7.5; MSF is the magnitude scaling factor that accounts for the effects of shaking duration, and K is a factor for the presence of sustained static shear stresses, such as may exist beneath foundations or within slopes. MSF and K were calculated using Eqs. 2 and 3 4 (2) MSF = 6.9e − 0.058(≤ 1.8) K = 1 − C ln ≤ 1.1 (3) σ σ where C = √ ≤ 0.3 σ (4) 18.9 − 2.55 (N ) 60cs The SPT-N value derived from the field investigation was used to calculate the CRR, while Eq.  5 was used to correct the raw SPT-N value. (N ) = NC C C C C 1 N E B R S (5) Fig. 6 Spatial mapping of the maximum groundwater level obtained by analyzing borehole data and map by Shrestha et al. (2016) Subedi and Acharya Geoenvironmental Disasters (2022) 9:1 Page 8 of 17 Fig. 7 Box plots of the factor of safety (FOS) against liquefaction for three PGAs at depths of a 1.5 b 3 c 6 d 9 and e 15 m in the Kathmandu Valley soils where (N ) is the SPT-N normalised to an overburden the correction factor for rod length. C is the correction 1 60 S pressure of 101 kPa (i.e., atmospheric pressure) with a factor for samplers with and without liners. hammer efficiency of 60%. N is the measured SPT blow The CRR is calculated using Eq. 6. 7.5 count. C is the correction factor for overburden stress. C is the correction factor for borehole diameter. C is B E the correction factor for the hammer energy ratio. C is R Subedi and A charya Geoenvironmental Disasters (2022) 9:1 Page 9 of 17 Fig. 8 Cumulative distribution function (CDF) for FOS against liquefaction at PGA of a 0.18 g b 0.30 g, c 0.36 g against liquefaction, and d liquefaction potential index (LPI) for three seismic scenarios τ σ a 2 max vc max (N ) (N ) CSR = 0.65 = 0.65 r 1 60cs 1 60cs (9) ′ ′ CRR = exp + σ σ g 7.5 vc vc 14.1 126 (6) 3 4 where a is the peak horizontal ground acceleration max (N ) (N ) 1 1 60cs 60cs − + − 2.8 (PGA) at the ground surface, g is the gravitational accel- 23.6 25.4 eration, σ and σ are the total overburden stress and vc vc effective overburden stress, respectively, and r is the where (N ) is an equivalent clean-sand SPT blow 1 60cs stress reduction factor as given in Eq. 10. count. Equations 7 and 8 are used to calculate (N ) : 1 60cs r = exp −1.012 − 1.126sin + 5.133 11.73 (N ) = (N ) + �(N ) 1 60cs 1 60 1 60 (7) +M 0.106 + 0.118sin + 5.142 11.28 (10) 9.7 15.7 �(N ) = exp 1.63 + − where z is the depth of the soil layer in meters. FC + 0.01 FC + 0.01 (8) Estimation of the liquefaction potential index (LPI) where FC is the fines content in the soils obtained from The FOS in Eq.  1 above does not help to obtain precise sieve analysis of the borehole or split-spoon samples. information on the severity of the potential ground defor- The CSR is calculated by using Eq. 9. mation at a given depth while the liquefaction potential index (LPI) introduced by Iwasaki et al. (1982) considers Subedi and Acharya Geoenvironmental Disasters (2022) 9:1 Page 10 of 17 less than 1. The LPI is estimated by using Eq.  11 for the top 20 m or less soil profile (Iwasaki et al. 1982). LPI = F (z)W (z)dz (11) where z = depth of layer; F(z) = function of FOS against liquefaction and is defined as: F (z) = 1 for FOS ≤ 1 (12) F(z) = 0 for FOS > 1 (13) W(z) is a depth-weighting factor defined as: W (z) = 10 − 0.5z. (14) Based on the LPI value, the liquefaction sensitivity can be divided into four groups: very low, low, high, and very high. Estimation of the probability of ground failure (PG) For quantitative evaluation, the liquefaction-induced probability of ground failure (P ) was estimated using Eq. 15 (Li et al. 2006). P = (15) 4.71−0.71∗LPI 1 + e where LPI is the liquefaction potential index as in Eq. 11. Improved SPT‑N values The N is calculated to provide desired SPT-N after improved ground improvement ensuring no liquefaction condition. Improved SPT-N values were determined for all liquefi - able sites using calculations for desired FOS. Correlations in Eqs. 5, 6, 7, 8, and 9 are used to determine the amount of in-situ soil strength change needed to avoid liquefac- tion at previously defined liquefiable locations. In case any soil stratum is found to be liquefiable for a known improved value of CSR, the CRR is calculated using Eq.  16 7.5 as follows. improved CRR = CSR × FOS (16) 7.5 Then, (N ) is evaluated using Eq.  6, and Eqs.  7, 8, 1 60cs and 5 are subsequently used to obtain the final targeted Fig. 9 Spatial distribution of FOS against liquefaction in the SPT-N (i.e. N ) value. The obtained N values improved improved Kathmandu Valley for the seismic scenarios of a 0.18 g, M 7.8, b ensure no potential liquefaction with suitable methods of 0.30 g, M 8.0, c 0.36 g, M 8.4 based on Idriss and Boulanger (2008) w w ground improvement needed at liquefaction-prone sites. These values are supposed to prove highly evidential in planning and estimating the cost of ground improvement. the effect of the liquefiable soil layer’s width, depth, and FOS assuming that the severity of liquefaction is propor- tional to the thickness of the liquefied layer, its proximity to the ground surface, and the extent to which the FOS is Subedi and A charya Geoenvironmental Disasters (2022) 9:1 Page 11 of 17 exacerbated by the shallow water table of 0.5  m to 5  m in these regions. For M 8.4 (0.36 g) scenario earthquake, the FOS values were observed to be the lowest at these depths with an interquartile range of 0.3–0.6 and mini- mum value of 0.1 for both 6 m and 9 m depths. However, for the M 8.0 (0.30  g) earthquake, the FOS values were found to be the lowest for the depth of 9  m with inter- quartile range and minimum value of 0.4–0.7 and 0.2, respectively. As expected, for M 7.8 (0.18 g) scenario, the FOS values were the highest among the three scenarios with the minimum values of 0.4, 0.3, 0.3, 0.3 and 0.4 with the corresponding interquartile range of 1–1.6, 0.9–1.4, 0.7–1.1, 0.6–1.2 and 0.6–1.1 respectively for the depths of 1.5 m, 3 m, 6 m, 9 m and 15 m. Moreover, if the median value for a given depth at a particular location is less than one, soil liquefaction Fig. 10 Area in percentage of FOS against liquefaction in three is likely to occur at that location (Dixit et  al. 2012). In scenario earthquakes of M 7.8 (0.18 g), M 8.0 (0.30 g), and M 8.4 w w w Fig.  7, the median of the FOS values for a given depth (0.36 g) tends to be decreasing as the magnitude of the FOS val- ues increases. Similar results of increase in vulnerability of seismic phenomena with their intensification are also Results and discussions reported by Nath et al. (2018) and Dixit et al. (2012). To analyze the variation of factor of safety (FOS) values The FOS profiles were used to compute cumulative for each substratum encountered at all borehole sites, distribution functions (CDF) at different depths of 1.5, 3, the box plots of FOS against liquefaction are prepared 6, 9 and 15  m, which are shown in Fig.  8a–c. The prob - at depths ranging from 1.5 to 15 m for the three ground ability of liquefaction susceptibility at a depth for vari- motions of scenario earthquakes as shown in Fig.  7. Box ous earthquake magnitudes is depicted in this diagram. plots are a non-parametric representation of a data set’s Similarly, in Fig.  8d, CDF versus LPI is plotted for three statistical distribution which depict the minimum, maxi- ground motions. Using these distribution functions, the mum, mean, median, first quartile (25th percentile), and probability of occurrence of liquefaction (P(FOS < 1)) at third quartile (75th percentile) of FOS values. The red different depths can be computed. dots in the figure denote outliers in the FOS dataset. Figure  8a shows that, for the M 7.8 (0.18  g) scenario These figures show that the FOS values are estimated to earthquake, the probability of liquefaction (i.e., FOS < 1) be higher for the earthquake scenario of M 7.8 (0.18  g) is 0.25 at shallow depth and increases to 0.65 at 15  m followed by M 8.0 (0.30  g) and M 8.4 (0.36  g) for all w w depth. It is interesting to note that the probability of depths. In all three earthquake scenarios, the 6  m and liquefaction is greater than 0.5 at all depths (except for 9  m depths are assessed to be the most vulnerable. The the 1.5  m depth) up to 15  m below the ground surface liquefaction vulnerability of these depths may have been Table 2 Calculation of FOS-based liquefaction potential index (LPI) using Iwasaki et al. (1982) and probability of ground failure (P ) using Li et al. (2006) at Babarmahal (27° 41′ 39″ N, 85° 19′ 28″) Depth (m) FOS F(z) W(z) F(z) * W(z) LPI P 1.5 0.4 0.5879 9.25 5.4376 10 0.93 3 0.5 0.5257 8.50 4.4682 4.5 2.0 0.0000 7.75 0.0000 6 2.0 0.0000 7.00 0.0000 7.5 2.0 0.0000 6.25 0.0000 9 2.0 0.0000 5.50 0.0000 10.5 2.0 0.0000 4.75 0.0000 12 0.96 0.0417 4.00 0.1667 13.5 1.34 0.0000 3.25 0.0000 15 0.91 0.0889 2.50 0.2223 Subedi and Acharya Geoenvironmental Disasters (2022) 9:1 Page 12 of 17 Fig. 12 Area in percentage of liquefaction susceptibility based on liquefaction potential index (LPI) in three scenario earthquakes: M 7.8 (0.18 g), M 8.0 (0.30 g), and M 8.4 (0.36 g) w w (Gautam et al. 2017; Sharma and Deng 2019). Moreover, with the increase in magnitude and PGA of the earth- quake scenario from Fig.  8a–c, the probability of lique- faction at all depths has increased significantly, which is in agreement with the findings of Dixit et  al. (2012) and Raghu Kanth and Dash (2008). Figure 9 shows FOS against liquefaction maps for three different seismic scenarios for the Kathmandu Valley. All red-colored areas with FOS < 0.5 are highly susceptible to liquefaction. Orange-colored areas with 0.5 < FOS < 1 may also be considered susceptible to liquefaction. So, the liquefaction-susceptible areas under the three seismic scenarios are quantitatively summarized in Fig. 10, which indicates that 58.16%, 79.98%, and 96.73% of the study area may liquefy if the ground is shaken during earth- quakes with the PGA values of 0.18 g, 0.30 g, and 0.36 g respectively. Observing all the scenarios, almost 80% of the Kathmandu Valley is found to have FOS less than 1, which means a significant portion of the valley is poten - tially liquefiable. Gautam et  al. (2017), KC et  al. (2020), Piya (2004), Sharma et al. (2019), and Subedi et al. (2021) also bring it to the light that the valley has high suscepti- bility to liquefaction. Table  2 presents a typical calculation for LPI, and based on the LPI values for all the 410 boreholes, the LPI zonation maps were generated for the Kathmandu Val- ley considering the three scenario earthquake. The spa - Fig. 11 Liquefaction potential index (LPI) map of the Kathmandu tial variation of the LPI in the valley is shown in Fig.  11. Valley for the seismic scenarios of a 0.18 g, M 7.8, b 0.30 g, M 8.0, c w w 0.36 g, M 8.4 based on Iwasaki et al. (1982) Severe liquefaction is more likely to occur at locations with an LPI value greater than 15 and is improbable at locations with an LPI value less than 5 (Iwasaki et  al. 1982; Sonmez 2003). According to Sonmez (2003), liq- indicating that the soil is highly susceptible to liquefac- uefaction potential is very high for LPI > 15; high for tion under this earthquake scenario. This is consistent 5 < LPI ≤ 15; moderate for 2 < LPI ≤ 5; low for 0 < LPI ≤ 2 with the occurrence of widespread liquefaction in the and non-liquefied for LPI = 0. Kathmandu Valley during the 2015 Gorkha Earthquake Subedi and A charya Geoenvironmental Disasters (2022) 9:1 Page 13 of 17 Fig. 14 Area in percentage of the level of risks based on probability of ground failure (P ) in three scenario earthquakes of M 7.8 (0.18 g), G w M 8.0 (0.30 g), and M 8.4 (0.36 g) w w analysis indicates that 17.9%, 64.15%, and 80.4% of the sample region fall inside a zone with very high liquefac- tion potential for 0.18 g, 0.30 g, and 0.36 g PGA, respec- tively. Likewise, 55.69%, 32.02%, and 18.1% of the sample region are located within high liquefaction hazard areas for the aforementioned earthquake scenarios while 14.32%, 2.13%, and 0.8% of the region can be classified as having moderate liquefaction hazard. According to the LPI analysis, 70% of the study area in the Kathmandu Val- ley is liquefiable with LPI values greater than 5, which is in agreement with the results from FOS-based analysis in the preceding part of this study. The probability of ground failure (P ) for each target location, after calculation, interpolation, and mapping, is shown in Fig. 13. The red-colored area indicates a very high risk zone while dark green area indicates a very low risk zone. It was observed that the central and south- ern parts of the valley have higher risk of ground failure than the northern parts. Within the study area, 1.87%, 17.64%, and 41.76% of the area are respectively found to be at very high risk of liquefaction-induced ground fail- ure during the scenario earthquakes (i.e., 0.18  g, 0.30  g, and 0.36 g PGAs) while 13.06%, 0.61%, and 0% of the area were found to be at very low or no risk of ground failure Fig. 13 Probability of ground failure (P ) map of the Kathmandu (Fig. 14). Results from ground failure analysis are consist- Valley for the seismic scenarios of a 0.18 g, M 7.8, b 0.30 g, M 8.0, c w w 0.36 g, M 8.4 based on Li et al. (2006) ent with the preceding results based on FOS and LPI. Furthermore, the resulting maps of liquefaction analy- sis based on the FOS, LPI, and P were compared with the observed liquefaction cases during the 2015 Gorkha For further interpretation, the prepared maps are stud- Earthquake documented by the authors along with the ied, and the percentage of locations in the Kathmandu reports by Okamura et  al. (2015); Gautam et  al. (2017); Valley that fall under various liquefaction susceptibility and Sharma et  al. (2019) as shown in Fig.  15. The FOS, zones is comparatively shown in Fig. 12. The comparative Subedi and Acharya Geoenvironmental Disasters (2022) 9:1 Page 14 of 17 Fig. 16 Improved SPT-N (N ) distribution map of the improved Fig. 15 Liquefied sites during the 2015 Gorkha Earthquake Kathmandu Valley for FOS of 1.5 for the seismic scenarios of a 0.18 g, presented in a FOS, b LPI, and c P maps of Kathmandu Valley M 7.8, b 0.30 g, M 8.0, c 0.36 g, M 8.4 w w w prepared for earthquake scenario of M 7.8 (0.18 g) w Subedi and A charya Geoenvironmental Disasters (2022) 9:1 Page 15 of 17 Table 3 Tabular presentation of calculated FOS, LPI and P at liquefaction observed sites during the 2015 Gorkha Earthquake S.N Location Latitude Longitude FOS LPI P 1 Bagdol 27.6676° N 85.2980° E 0.5–1.0 High Medium 2 Bungamati 27.6222° N 85.2622° E 0.5–1.0 Very high High 3 Changunarayan, NEC 27.7090° N 85.4140° E 0.5–1.0 High High 4 Duwakot 27.7094° N 85.4139° E < 0.5 Very high Very high 5 Guheshwori 27.7093° N 85.3576° E < 0.5 High High 6 Gwarko 27.6670° N 85.3380° E < 0.5 Very high High 7 Harisiddhi 27.6549° N 85.3352° E < 0.5 High Medium 8 Hattiban 27.6668° N 85.3344° E < 0.5 High Medium 9 Imadol 27.6668° N 85.3383° E 0.5–1.0 High Very high 10 Itapakhe 27.6792° N 85.4289° E < 0.5 Very high High 11 Jharuwarashi 27.6151° N 85.3439° E < 0.5 Very high Medium 12 Kamalvinayak 27.6785° N 85.4370° E 0.5–1.0 Very high High 13 Khadka Gaon 27.6950° N 85.2714° E 0.5–1.0 very high High 14 Lokanthali 27.6748° N 85.3626° E < 0.5 High Very high 15 Malpokhari 27.6720° N 85.2958° E 0.5–1.0 Very high Medium 16 Manamaiju 27.7453° N 85.3007° E 0.5–1.0 Very high Very high 17 Mulpani 27.7025° N 85.7005° E 0.5–1.0 Very high High 18 Pakune Pati 27.6969° N 85.4401° E < 0.5 Very high Medium 19 Ramkot 27.7110° N 85.2622° E < 0.5 Very high Medium 20 Satdobato 27.6552° N 85.3264° E < 0.5 High High 21 Singhadurbar 27.6987° N 85.3200° E < 0.5 High High 22 Sitapaila 27.7200° N 85.2726° E 0.5–1.0 High Medium 23 Syuchatar 27.6972° N 85.2740° E 0.5–1.0 Very high High 24 Taudaha 1 27.6499° N 85.2829° E < 0.5 Very high High 25 Taudaha 2 27.6484° N 85.2811° E < 0.5 Very high Very high scenarios in the valley. As in the case of FOS, LPI, and Table 4 Improved SPT-N (N ) range for FOS of 1.5 to improved P mapping, the N values assigned to all liquefi - ascertain no liquefaction during three seismic scenarios i.e., G improved M 7.8 (0.18 g), M 8.0 (0.30 g), and M 8.4 (0.36 g) able borehole locations in the Kathmandu Valley were w w w mapped. Based on these maps, it is possible to observe S.N N range for FOS = 1.5 Seismic scenarios improved N values that ensure no liquefaction situation in improved 1 15–25 0.18 g, M 7.8 w potentially liquefiable zones for each of the three earth - 2 20–30 0.30 g, M 8.0 w quake scenarios. Moreover, Table 4 summarises the range 3 25–35 0.36 g, M 8.4 w of N values that could be obtained with each case improved with a FOS of 1.5. It is interesting to note that the range of N values obtained in this study is consistent with improved LPI, and P values were assessed for liquefied locations the results of Khan and Kumar (2020). Selective ground using the resulting maps of this study and are presented improvement methods may be used to reach the target in Table 3. It is observed that at all the 25 liquefied sites, value of N . The adoption of available approaches is improved the FOS values are less than 1, the liquefaction suscep- conditional on the site requirements and available funds. tibility varies from high to very high, and the ground failure risk level ranges from medium to very high. This Conclusion highlights that the findings of this study are coherent A larger part of the Kathmandu Valley is considered to with the field observations of liquefaction during the be highly susceptible to soil liquefaction during seis- 2015 Gorkha Earthquake. mic activities owing to its geological, geotechnical, and Figure  16 shows distribution of N values (or the improved hydrogeological conditions. In this study, we assessed improved SPT-N values), which were determined for all susceptibility, hazard, and risk of liquefaction phenom- liquefiable sites with FOS value of 1.5 to ensure no liq - enon in subsoil stratum of the valley using borehole uefaction condition, for the cases of three earthquake Subedi and Acharya Geoenvironmental Disasters (2022) 9:1 Page 16 of 17 Availability of data and materials data including SPT-N values and laboratory test param- The datasets used and/or analysed during this work are accessible upon eters. We used factor of safety (FOS) against liquefac- reasonable request from the corresponding author. tion, liquefaction potential index (LPI), and probability of ground failure (P ) as the main parameters consider- Declarations ing three likely-to-recur scenario earthquakes of M 7.8 Competing interests (0.18  g), M 8.0 (0.30  g) and M 8.4 (0.36  g). The w w The authors declare that they have no competing interests. obtained results are presented as liquefaction hazard maps showing FOS against liquefaction, assessing liq- Received: 19 August 2021 Accepted: 15 December 2021 uefaction manifestation in terms of LPI and P values. Based on these parameters, the calculated susceptibil- ity of soil liquefaction corroborates with the sites of soil References liquefactions observed during the 2015 Gorkha Earth- Ansal A, Tönük G (2007) Source and site factors in microzonation. In: Pitilakis quake. The resulting maps illustrate the quantitative KD (ed) Earthquake geotechnical engineering. Springer, pp 73–92 features of the liquefiable layers and the region where Ayele A, Woldearegay K, Meten M (2021) A review on the multi-criteria seismic hazard analysis of Ethiopia: with implications of infrastructural develop- ground failure due to liquefaction is likely. The majority ment. 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Journal

Geoenvironmental DisastersSpringer Journals

Published: Jan 3, 2022

Keywords: Liquefaction; SPT-N; Hazard; Risk; Ground improvement; Kathmandu Valley

References