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Effectiveness of geocell wall, geogrid and micropile anchors for mitigation of unstable slopes

Effectiveness of geocell wall, geogrid and micropile anchors for mitigation of unstable slopes Slope failure mitigation practices are well developed in recent years. Recently, geosynthetic, geocell, and geogrid combined with micropiles are being used extensively in various slope stabilization works. But integrated approaches are still lacking. In this study, a method of slope stabilization is proposed by integrated use of micropile, geocell and geogrid from an engineering and economical point of view. The study was done on slope failure located at Chandragiri Hill, south west of Kathmandu, Nepal. Geotechnical problems of the site, the design of geocell foundation, micropile and geogrid are done on the based on numerical analysis using Phase-2 software with field data. The results of analytical studies revealed that, the use of a combination of geocell, micropile and geogrid is beneficial in increasing slope stability. As per numerical analysis, in the slope failure site, geocell gravity walls each of 3.8 m, is constructed in different step. Beneath the geocell wall, different layers of geogrid were placed filled with granular materials. The geocell wall is connected with micropile from inside. The micropile works as an anchorage and support for geocell wall, which increases the stability of a failed slope. Keywords: Slope stability, Geocell, Geogrid, Reinforcement, Micro pile, Granular material Introduction defined as a planar product manufactured from polymer The modifications in the geomorphic, hydrological and used with soil, rock, earth, or other geotechnical engin- geological conditions of the area that is mainly facilitated eering related material as an integral part of a man- by geodynamic processes, vegetation, and land use prac- made project, structure or system. The use of geocell is tices, human activities, seismicity, rainfall are the factors the most recent advancement in soil reinforcement that trigger slope instability. Dealing with the problem of where the materials are confined in three-dimensional instability of slopes has always been interesting, import- pockets (Dash et al. 2001). Micropiles are widely used to ant and challenging in the field of geotechnical engineer- stabilize slopes especially for slopes located in steep, ing. Slope failures and instability are encountered in hilly, or mountainous areas; as they are simple, fast, eco- various stages and sectors of engineering such as during nomical and environmentally friendly. Micropiles are de- cutting, construction of hill roads, railway lines, reser- fined as a small diameter (generally less than 300 mm) voirs and damns among others (Soeters and Van Westen non displacement pile, generally reinforced, which are 1996). There are various methods for slope failure miti- driven into the soil and grouted (Sun et al. 2014). Many gation. In recent years, geosynthetic, geocell, and geogrid researchers have found that the use of piles is one of the combined with micropiles are being used extensively in effective methods in the stabilization of slopes. Lee et al. various slope stabilization works. A geosynthetic can be (1995) and Li et al. (2012) studied stabilization slopes using a simple approach by means of row of piles driven into the slope. Ausilio et al. (2001) has studied stability * Correspondence: gsgroup.sanju@gmail.com Lincoln University College, Kota Bharu, Malaysia of slopes that are reinforced with piles using the Full list of author information is available at the end of the article © The Author(s). 2021 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://creativecommons.org/licenses/by/4.0/. Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 2 of 15 kinematic approach of limit analysis. Geocell is three- as well as decreasing the footing displacement. (Chen dimensional cell made from high-density polyethylene and Chiu 2008) confirmed that geocell in an increased (HDPE) or polyethylene (PE) strips ultrasonically welded length perform similar to geogrid layers and provide along the width. Geocells are better alternatives to con- reinforcement to the soil. Ling et al. (2009) showed that ventional slope stability measures like use of concrete geocell can perform well in gravity walls as well as panels. Also geocell perform better than the concrete reinforcement walls. Cancelli et al. (1993) tested that in panels in cold weather conditions (Dash et al. 2003). steep slopes and areas of heavy surface runoff, where The main properties of geogrid consist of uniformity, vegetation is not effective in controlling soil erosion. stability, light, anti-corrosive, anti-aging, high tensile Geocell can be used as it has good tensile strength and strength and flexibility. Geogrids reduces the joggling of confines infill material as well as reduce the velocity of filling materials, reduce the inhomogeneous settlement surface runoff. of soil to the largest possible degree and improve stabil- Palmerton (1984) and Pearlman and Wolosick (1992), ity. These are obvious advantages of using geogrid as suggested that in case of soft or weak soil, to transfer the reinforcement in retaining walls. With the rising prob- axial and lateral load to more stable strata, micropiles are lem of global warming and environmental concerns, the perfect solution. Meantime, Pokharel et al. (2010)sug- geosynthetic materials as a measure for reinforcing soil gested that the three-dimensional geocell provided lateral and preventing soil erosion caused by runoff water have confinement, base acts as a mattress to restrain the soil gained a worldwide acceptability (Yadav et al. 2014). Wu from moving upward outside the loading area. and Austin (1992) reported the use of geocell for slope This research has attempted to validate the least popu- stability and for erosion control, as well as the walls of lar but economical solution for mitigation and control of geocell controlled the downward movement of materials critical slopes. For this purpose, geocell and micropiles as they are confined. along with the combination of geogrid reinforced soil Dash et al. (2003), Krishnaswamy et al. (2000), Mad- were used. In many cases, micropile, geocell and geogrid havi Latha et al. (2006), Mehrjardi et al. (2012), Sireesh are suitable options to mitigate slope instability issues et al. (2009), Tafreshi and Dawson (2010) and (Tafreshi and this research work attempted to evaluate the effect- and Dawson 2012); Yang et al. (2012), Zhang et al. iveness of geocell, geogrid and micropiles against retain- (2010), Zhou and Wen (2008) studied beneficial effects ing walls in critical slope and mitigation of unstable of geocell on increasing the load bearing capacity of soil slopes. Futhermore, the site at Chandragiri hills of Fig. 1 Location map of the study area Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 3 of 15 Kathmandu Valley, Nepal has been selected for the re- is an intermountain basin with valley floor surrounded by search and design and implementation of all necessary four mountain ranges from Shivapuri, Phulchoki, Nagarjun slope stabilization works. and Chandragiri located in North, South-East, North-West and South- West respectively. The geology of the Study area Kathmandu Valley and its surroundings can be put into two The study area lies on South-West of Kathmandu Valley groups, unconsolidated-slightly consolidated sediment de- at Chandragiri Municipality, Kathmandu district of Bag- posits (Quaternary deposits) and hard rocks of Precambrian mati Province, Nepal (Fig. 1). Physiographically, the study to Devonian (Stöcklin and Bhattarai 1977). The central part area belongs to the part of the Mahabharat Range of of the valley has lacustrine and fluvial deposits containing Central Nepal representing a strongly dissected range of peat, clay, carbonaceous clay, sand, gravel, and boulders topographic variations with moderate to a very steep which overlie uncomfortably on the rocks of the Phulchauki slope, ridge, spur, saddle and valley (Dahal et al. 2008). and Bhimphedi groups (Fig. 2). Geologically, it lies in the Lesser Himalaya Zone of cen- Hard rock geology around the Kathmandu Basin is tral Nepal (Stöcklin and Bhattarai 1977). The research comprised of various sedimentary, meta-sedimentary area lies within longitude 85°12′30.98“Eto85°12’36.99”E and metamorphic rocks. The rock formation surround- and latitude 27°39′57.12“Nto 27°39’59.54”N and elevation ing the basin belongs to the Phulchauki Group of the of 2300 m–2450 m. The research was mainly focused on Kathmandu Complex. The Phulchauki Group is divisible the landslides that occurred on the northern slope (Fig. 2). into five formations: Tistung Formation, Sopyang For- There is a resort that lies near the landslide area and has a mation, Chandragiri Limestone, Chitlang Formation, and famous panoramic view of the Himalayas as well as the Devonian Limestone of Phulchauki. Sheopuri Gniess Kathmandu city. (Fig. 2) is present on the northern hills (Stöcklin and Bhattarai 1977). Regional geological setting The study area lies in the vicinity of the contact of the Geologically it lies in the Lesser Himalaya Zone of central Chandragiri Limestone and Chitlang Formation on Nepal south of great Himalayan Range (Dahal et al. 2008; Chandragiri mountain range. Residual/colluvial deposit Hagen 1969; Stöcklin and Bhattarai 1977). Kathmandu valley has covered the underlying bedrock of Chandragiri Fig. 2 Study area on regional geological map of Kathmandu basin (modified after Stöcklin and Bhattarai 1977 and Dahal et al. 2009) Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 4 of 15 Limestone and Chitlang Formation (Fig. 2). The Chan- Site and soil characteristic dragiri Limestone of Middle Cambrian age consists of The site has a four storey building of the Chandragiri grey, thin to thick bedded argillaceous, laminated lime- Hills Resort with an isolated foundation (Fig. 3). There is stone with minoramount of white, ortho-quartzite, light a topographic map shown in Fig. 4. There was a stone grey with orange weathered argillaceous phyllite and masonry wall constructed around periphery of the build- greyish, micaceous metasandstone. The upper part of ing without proper drainage system for surface runoff. the succession comprises partings of dark grey to light The downward slope angle of slope is more than 42°, grey phyllite subordinated with white quartzite (about width 30 m and length 20 m. The pore water pressure 150 m) band and greyish leachate on limestone. The on the slope during rainy season cannot be released Elephant-skin type weathering on limestone is observed through the retaining structure. in some parts. The Chitlang Formation of Silurian age The site is located on a moderately steep slope that is consists mainly of white quartzite, some beds of argilla- a landslide prone zone. The down slope is bare and run- ceous limestone, dark bluish grey, violet shale, slate and off and rainfall can infiltrate easily due to the fracture of dolomitic limestone. The lower part of this unit consists rock underlain by a thin veneer of red soil. The soil is of violet, shale and slate, white, muddy quartzite, grey, unstable both in rainy as well as dry season due to its fine-to coarse grained meta sandstone, thick-to massive, silty nature. The slope is north facing. As a result the fine-to medium-grained limestone and phyllite in some moisture remains on the slope for a long time after rain- parts. Similarly, the upper-part of succession comprises fall and snowfall in winter. The area was well vegetated intercalation between yellowish grey shale and dark grey before the resort construction (Dahal et al. 2009) but the limestone minorly of dolomite with precipitated calcite vegetation was removed for construction purpose in the in which wave marks are observed. surrounding area that resulted erosion and failure prob- The study was focused on the landslide mass consist- lems. The slope consists of red silt soil from 1 m − 1.5 m ing of fine, reddish brown, moist, sandy silt to silt with depth with rock fragments and the yellowish fractured pebbles, cobbles as overburden deposit and dark grey, rock is present from 1.5 m to 6 m. The weathered slate weathered and fractured slates as exposed bedrock only and calcareous rocks are mainly present after 6 m in the a few locations. site. The soil has specific gravity 2.69 and friction angle Fig. 3 Four stories building and landslide in the slope just below the building Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 5 of 15 Fig. 4 Topographic map of study area (φ)36 and the moisture content of the soil varied from slope surface and wall to greatly improve resistance to 9% to16% with a bulk density of 2.07 g/cm . erosive forces such as rainwater run-off on steep or un- stable slopes, or slopes exposed to severe hydraulic or Materials and methods mechanical stresses (Wu and Austin 1992). In this research, effectiveness of geocells and micropiles along with the combination of geogrid reinforced soil Filling materials was explored for the slope stabilization. At first, numer- For filling, the specific gravity of filling material was ical analysis was performed for the project site using 2.66. Likewise, liquid limit and plastic limit of the clay geocell, geogrid and micropiles. Then the field applica- were 40% and 19%, respectively. The maximum dry tion of micropiles, geogrid and geocell were done to density, optimum moisture content, Standard Proctor mitigate the slope instability as per the numerical result. test were 18.2 kN/m and 13.2%, respectively. The effect- Following - materials and methods are used in the re- ive particle size (D ) was 0.26 mm. The angle of internal search process. friction was 40 degree. Poorly graded sand was used and it was SP according to unified soil classification system Geocell (USCS). Average size of the gravel was 12 mm according Geocell is a honeycomb three-dimensional cell structure to unified soil classification system, graded gravel (GP). (Fig. 5) that confines the filled compacted materials, de- Normally, select fill materials are more expensive than creases the lateral movement of soil particles and distrib- lower quality materials. The gradation requirements for utes the applied loads to a wider area. Geocell is granular reinforced fill, gradation 4 in – 100% passing, generally used in the construction of canals, embank- 40 mm – 60% passing, 200 mm – 50% passing and plas- ments, retaining walls, railways and roads, slope stability ticity index ≤20 (AASHTO T-27, T90). (Bathurst and Jarrett 1988; Dash et al. 2003). Geocell is a For this study, the filling material used was GM (silty blanket of three dimensional cell structures applied to a gravel) as per ISC and USC system. Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 6 of 15 Fig. 5 Schematic drawing of typical geocell and physical, mechanical and hydraulic parameters Geogrid specified size 150 mm (diameter). The cement grouting Geogrids are manufactured by polymers like PET (as per was done in the drilled hole under a pressure with perfo- ASTM D2455, ASTM 4603 as per ASTM D1248), they rated pipe to spread the slurry into the surrounding soil. have apertures in various sizes between individual ribs in After completing grouting process, the reinforcement is the transverse and longitudinal directions. PET and lowered into the hole. HDPE Geogrids have minimum UV resistance as per ASTM D4355. Geogrids are (a) either stretched in one, Methods of analysis two or three directions for improved physical properties, In this research, Phase 2 (2002), a Rocscience FE pro- (b) made on woven/knitted machinery by standard tex- gram was used to simulate and analyze a complex multi- tile manufacturing methods. stage model (Fig. 7) for slope stability analysis. Material In this study, biaxial geogrid is used with the charac- properties of model is taken as; Elastic modulus 15,000 teristics shown in (Fig. 6). kPa, Poisson’s ratio 0.3, tensile strength 5 kPa, friction angle 30°, cohesion 5 kPa shown in Table 1. For vertical Micropile boundary, u = 0 and u is free and for horizontal bound- x y The micropile was used in an unstable slope with geo- ary; ux = uy = 0. Mohr Coulomb failure criterion is used cell. The micropile has a small diameter and it is easy to to simulate the model. The shear strength reduction transport and install even by semi-skilled person. Micro- (SSR) technique of Finite Element (FE) and the simpli- pile bears the axial loads and lateral load therefor it can fied Bishop method was used to analyze the slope stabil- be constructed in any type of soil/rock/sand conditions. ity problem to gain insight into the soil mass behavior, Micropile depends on location, slope, cross section, progressive failures and explicit modelling of discontinu- length, group spacing and concrete cap beam of micro- ities. In both methods, at first the existing failed slope pile (Lizzi 1982). In this case study, micro piles are cast- conditions were analyzed and checked for their stability in-situ with 101 mm MS medium pipe drilled hole of the (FoS < 1 or FoS > 1). When FoS < 1, to improve the slope Fig. 6 Grid used in the field construction and properties of geogrid Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 7 of 15 Fig. 7 Schematic diagram of slope for FEM based simulation stability and increase the factor of safety the existing soil feature of this linear failure model is the fact that it can was reinforced with the use of a combinations of micro- be simply and explicitly expressed in both principal (σ - pile, geocell and geogrid. σ ) stress space and shear-normal (τ-σ ) stress space. 3 n The shear strength reduction (SSR) technique of Finite The simplicity, explicit representation in both principal Element (FE) slope stability analysis is a simple approach and shear-normal stress space, an adequate description that involves a systematic search for a stress reduction of strength behavior for a wide range of materials, and factor (SRF) or factor of safety value that brings a slope easy to obtain parameters of the Mohr-Coulomb criter- to the very limit failure. The SSR technique assumes ion account for its popularity. For Mohr-Coulomb ma- Mohr Coulomb strength for slope materials. The Mohr terial the factored or reduced shear strength can be Coulomb strength envelope is the most widely applied determined from the equation failure criterion in geotechnical engineering. A unique τ c tanφ’ Table 1 Modeling parameters ¼ þ ð1Þ F F F Modeling parameters This equation can be re-written as Elastic Modulus 15,000 kPa Poisson’s ratio 0.3 ¼ c þ tanφ ð2Þ Tensile strength 5 kPa Friction angle 30° Where, F = factor of safety; c’ = effective parameter of Cohesion 5 kPa cohesion; τ’ = effective shear strength; φ’ = effective angle Constitutive model Mohr coulomb failure criterion of internal friction Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 8 of 15 the slope model and recomputed. Record the 0 0 c tanφ maximum total deformation. c ¼ and φ ¼ arctan F F: Step 3: Repeat step 2, using systematic increments of F, until the FE model does not converge to a solution, i.e. The steps for systematically searching the critical fac- Continue to reduce material strength until the slope tor of safety value F that brings a previously stable slope fails. The critical F value just beyond which failure (F ≥ 1) to the verge of failure are as follows: occurs will be the slope factor of safety. Step 1: Develop an FE model of a slope, using the For a slope with a factor of safety less than 1, the pro- appropriate materials deformation and strength cedure is the same except fractional F values will be sys- properties. Compute the model and record the tematically decremented (translating into increments in maximum total deformation. the factored strength parameters) until the slope be- Step 2: Increase the value of F (or SRF) and calculate comes stable. factored Mohr Coulomb material parameters as The principal advantage of the SSR technique is its use described above. Enter the new strength properties into of factored strength parameters as input into models, Fig. 8 a Installation of micropile, b Geocell wall, c Laying of geogrid inside the micropile in slope area for stable of back fill materials soil, d Laying of geocell wall inside micropile Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 9 of 15 which enable the technique to be used with any existing 0 0 0 τ ¼ðÞ c þ σ tanϕ FE analysis software (Fig. 7). All the approach requires ð3Þ of a slope analyst is computation of factored Mohr Cou- lomb strength parameters. To find σ' resolve forces in the vertical direction to The simplified Bishop method (Bishop, 1955) has obtain been widely used in slope stability analysis and is regarded as the best method of limit equilibrium for 0 0 0 0 calculating the factors of safety of circular slip W−ðÞ c þ σ tanϕ ΔX tanα−ðÞ σ þ u ΔX ¼ 0 ð4Þ surfaces. In this method, the inter slice forces are as- sumed to be horizontal, or the vertical inter slice W−uΔX− c ΔX tanα forces are neglected, the vertical force equilibrium 0 F ∴σ ¼ ð5Þ and the moment equilibrium about the center of the ΔXðÞ 1 þðÞ tanϕ tanα =F circular slip surfaces are satisfied, but the horizontal force equilibrium is not considered. Now F = sum (maximum resisting forces around arc)/ The simplified analysis is as follows: sum(moving forces around arc) Fig. 9 Micropile, geocell wall layout plan Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 10 of 15 0 0 0 methods involving consideration of the forces acting ðÞ c þ σ tanϕ ΔX secα ¼ P ð6Þ on the sides of slices shows that the simplified Bishop W sinα Method yields answers to factors of safety that are close to the correct answer. ½ c ΔX þðÞ W−uΔX tanϕ M We have numerically modelled the project site ¼ P ð7Þ using FEM in the static condition considering it as a W sinα continuum by SSR approach. By determining the fac- sinα tanϕ Where, M ¼ cosα þ tor of safety of failed slope, post disaster analysis is τ = shear strength carried out. While, by using FEM methodology, stress σ = normal stress developedinthe slopeis determinedtofocus on ϕ = angle of friction probable failure. The analysis was performed using W = Weight of slice Phase2 software. FEM, a widely accepted method of ΔX = width of slice numerical modelling of slopes works on the principle u = pore pressure of discretization of whole design into a fixed number To facilitate the analyses of slope stability for a of elements through which continuous variation in large number of potential failure surfaces and a var- material properties takes place. A 2D, three nodded iety of conditions, computer programs are used. The triangular plane strain elements have been used to Bishop Method yields factors of safety that are higher discretize the slope design. The SSR approach with than those obtained with an ordinary method of non-failure criteria has been adopted. Since the max- slices. Furthermore, the two methods do not lead to imum shear strain of the failure zone coincides with the same critical circle. It has also been found that the rupture surface, it is thus assumed that failure the disagreement increases as the central angle of a mechanism of slope is directly related to the develop- critical circle increases. Analysis by a more refined ment of shear strain. Fig. 10 Schematic diagram at section B-B, showing installation of micropile, geocell wall and geogrid reinforcement Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 11 of 15 Sequence of construction method geocells were anchored to the ground using a J hook of The construction methods with reinforce soil using geo- 20 mm dia at 0.5 m c/c on both edges. Prior to execution grid are well explained by Simac (1990). Similarly, Dash of later geocell wall, the slope between the prior and the et al. (2007) have explained how geocell acts as a rigid later geocell wall was maintained by filling with granular mattress and can distribute the applied load to a larger materials reinforced using geogrid at an interval of 1 m surface area. Further, Zhang et al. (2010) explained how vertical spacing. After the completion of micropiles, the geocell can reduce the settlement and increase the load geocell wall and slope maintenance reinforced with geo- carrying capacity. (Bush et al. 1990)explained about the grid and a layer of geocell were laid throughout the slope construction of geocell and its installation in the field. along its length which was anchored with 20 mm dia J Elarabi and Soorkty (2014) has explained about micropiles hook @ 0.75 m c/c both ways. Later, bioengineering (use and suitable drilling techniques for the reinforcement with of vegetation) was done along the slope. the micropile. In this research also, like execution of any other civil Analysis and result engineering work, at first the site was cleared, excessive The slope was evaluated for a factor of safety as men- debris of the failed slope was removed and the path was tioned in earlier sections. Figure 11 illustrates that the constructed for the commencement of slope protection factor of safety of the existing slope before failure was work. The protection method made use of driven micro- found to be 0.86 that was analyzed as per Shear Strength piles with the combination of geocell and geogrid as Reduction (SSR) method. The factor of safety of 0.882 shown in (Fig. 8). The slope protection work com- for existing slope before failure analyzed as per Bishop’s menced from the toe of the slope and micropiles were method was also noticed during simulation (Fig. 12). driven of varying depth on the ground (Fig. 9). In total Since both SSR and Bishop’s method showed factor of 139 micropiles with different lengths from 10 m to 20 m, safety less than 1, so slope was prone to fail which was 2 m c/c distance were driven throughout the length of seen in the site. To overcome this issue, geocell, micro- the slope as shown in Fig. 10. As seen in section, total of pile and geogrid were applied in the simulations and FoS four geocells were constructed with base of the geocell was evaluated again. wall of 2.1 m wide, which tapered to 0.7 m at the top, It was found that the factor of safety for the slope was the total height of each geocell wall was 3.8 m. The increased from 0.882 to 1.076 form limit equilibrium Fig. 11 Maximum shear stress for slope without reinforcement Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 12 of 15 Fig. 12 Slip surface as per Bishops Fig. 13 Maximum shear strain for reinforced slope with micropile, geocell and geogrid reinforcment Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 13 of 15 Fig. 14 In left, slip surface as per Bishops for overall fos of reinforced soil with micropile, geocell wall and geogrid, in right, slip surface as per Bishops for factor of safety of individual materials method. Bishops (1995), suggested that to calculate the Discussion factor of safety of slope, the whole slope is divided into There are many methods for the stability analysis of slope. vertical slices and each of them is individually analyzed These are categorized into conventional and numerical using circular failure analysis to get the individual slice methods. For the existing slope before failure it was ob- factor of safety and summarized for overall factor of served that the factor of safety was less than 1 for Bishops safety of slope. as well as SSR method and the angle of slope was 42 . So, The factor of safety according to SSR method was it was prone to failure as observed practically. After the found to be 0.86 for the natural unreinforced slope and failure of slope the angle of slope was increased to 47 and the factor of safety was 1.13 after reinforcement work stability was more critical. So, the failed slope was miti- with a combination of micropile, geogrid and geocell gated with the introduction of reinforcement combination (Fig. 13). From Fig. 14, it can be observed that the factor of micropile, geogrid and geocell. Then it was analysed of safety according to the Bishops method was found to using Phase-2 for SSR method and Slide for Bishop be 1.076 after the slope was mitigated with the combin- Method. Both analysis results showed that the factor ation of micropile, geocell and geogrid, which was 0.882 safety was increased to more than 1. for the natural unreinforced slope (Table 2). The factor There are various methods for slope failure mitiga- of safety was in the range of 1 to 4 where the micropiles tion. In recent years, geosynthetic, geocell, and were driven into the slope. geogrid combined with micropiles are being used ex- The failure slope was analyzed being mitigated with tensively in various slope stabilization works. Micro- the combination of micropile, geocell and geogrid and it piles are widely used to stabilize slopes especially for was observed that the factor of safety was improved and slopes located in steep, hilly, or mountainous areas; obtained to be greater than 1 (Fig. 13 and Fig. 14). as they are simple, fast, economical and environmen- The result of the FE analysis were compared to an- tally friendly. This research has attempted to validate swers obtained from the Bishop limit equilibrium the least popular but economical solution for the method computed in Slide software, a slope stability mitigation and control of critical slopes. The study program developed by Rocscience. The FE factor of results show that reinforcement of slope using safety result agreed very well with the factor of safety by micropile, geogrid and geocell shows better results the limit equilibrium method. in slope stabilization work. In which, micropile was used in the analysis to resist the lateral and shear force and act as anchorage as well as the grouting filled up the fissures present in the soil mass. Geo- Table 2 Difference of factor of safety by finite element method cell wall was used to act as a retaining structure at and limit equilibrium method intervals and confine the filled soil material as well FoS of Natural FoS of Reinforced as the porous nature of geocell facilitated the seep- Slope slope age of water through the soil without soil loss. Geo- Shear reduction Method 0.86 1.13 grid was used in the backfill of soil in layers to (SSR) maintain the slope and increase the shear strength Bishop Method 0.882 1.076 of soil. Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 14 of 15 Conclusion Author details 1 2 Lincoln University College, Kota Bharu, Malaysia. ERT Tech Pvt. Ltd, The slope stability analysis is a challenging work in geotech- Kathmandu, Nepal. Tribhuwan University, Pulchowk campus, Kirtipur, Nepal. nical engineering. Formerly, the limit equilibrium method of 4 Wuhan University of Technology, Wuhan, China. analysis was widely used due to its clear physical meaning Received: 13 September 2020 Accepted: 21 April 2021 and simple calculation. Now, with the development of the fi- nite element method, the strength reduction method is grad- ually recognized to determine the factor of safety of slope. In References this paper, the factor of safety of slope is firstly calculated by Ausilio E, Conte E, Dente G (2001) Stability analysis of slopes reinforced with Bishop’s method, which is then compared with the safety fac- piles. Comput Geotech 28(8):591–611. https://doi.org/10.1016/S0266-352X(01 tor obtained from the strength reduction method by FEM. )00013-1 Bathurst RJ, Jarrett PM (1988) Large-scale model tests of geocomposite And, from the both limit equilibrium and finite element mattresses over peat subgrades method, a factor of safety was found greater than 1. After Bush D, Jenner C, Bassett R (1990) The design and construction of geocell that construction has been done and stabilized. foundation mattresses supporting embankments over soft grounds. Geotext The following concluding remarks can be drawn by Geomembr 9(1):83–98. https://doi.org/10.1016/0266-1144(90)90006-X Cancelli A, Rimoldi P, Montanelli F (1993) Index and performance tests for this work. geocells in different applications, in Geosynthetic soil reinforcement testing procedures, edited. ASTM International The main cause of failure for this slope was water Chen R, Chiu Y (2008) Model tests of geocell retaining structures. Geotext Geomembr 26(1):56–70. https://doi.org/10.1016/j.geotexmem.2007.03.001 logging, natural slope angle and surcharge load of a Dahal RK, Hasegawa S, Nonomura A, Yamanaka M, Dhakal S, Paudyal P (2008) building. Predictive modelling of rainfall-induced landslide hazard in the lesser The mitigation measure required a sustainable and Himalaya of Nepal based on weights-of-evidence. Geomorphology 102(3–4): 496–510. https://doi.org/10.1016/j.geomorph.2008.05.041 economical solution that reinforces the soil as well Dahal RK, Hasegawa S, Yamanaka M, Dhakal S, Bhandary NP, Yatabe R (2009) as allows the flow of seepage water without erosion Comparative analysis of contributing parameters for rainfall-triggered of soil. For such conditions of slope, the landslides in the lesser Himalaya of Nepal. Environ Geol 58(3):567–586. https://doi.org/10.1007/s00254-008-1531-6 combination of reinforcement of soil with geocell, Dash SK, Krishnaswamy N, Rajagopal K (2001) Bearing capacity of strip footings micropile and geogrid turns out to be the best supported on geocell-reinforced sand. Geotext Geomembr 19(4):235–256. solution. https://doi.org/10.1016/S0266-1144(01)00006-1 Dash SK, Rajagopal K, Krishnaswamy N (2007) Behaviour of geocell-reinforced The factor of safety was increased after sand beds under strip loading. Can Geotech J 44(7):905–916. https://doi. reinforcement using the combination of micropile, org/10.1139/t07-035 geogrid and geocell. Dash SK, Sireesh S, Sitharam T (2003) Model studies on circular footing supported on geocell reinforced sand underlain by soft clay. Geotext Geomembr 21(4): Slope stabilization with a combination of 197–219. https://doi.org/10.1016/S0266-1144(03)00017-7 reinforcement of soil with geogrid, geocell and Elarabi H, Soorkty AA (2014) Construction of micropiles using pressure micropile is a new technology for Nepal that shows techniques. J Civil Eng Architecture 8(1):74 Hagen, T. (1969), Report on the Geological survey of Nepal, Vol. 1: preliminary good results to mitigate slope failure issues in the reconnaissance, Denkschriften der Schweizerischen Naturforschenden dynamic Himalayan slope. Gesellschaft Memoires de la Societe Helvetique des Sciences Naturelles, 84(1), 185 Acknowledgements Krishnaswamy N, Rajagopal K, Latha GM (2000) Model studies on geocell The authors would like to thank Chandragiri Hills Ltd., for funding and supported embankments constructed over a soft clay foundation. Geotech trusting to implement new method of slope stabilization in Nepal, the first Test J 23(1):45–54 of its kind. Authors would like to acknowledge team GS Soil and Materials Lee C, Hull T, Poulos H (1995) Simplified pile-slope stability analysis. Comput for testing of sample of project area. Geotech 17(1):1–16. https://doi.org/10.1016/0266-352X(95)91300-S Li X, Pei X, Gutierrez M, He S (2012) Optimal location of piles in slope stabilization by limit analysis. Acta Geotech 7(3):253–259. https://doi.org/10.1007/s11440- Authors’ contributions 012-0170-y Sanjaya Kumar Jain was involved in the data collection in field, data analysis, Ling HI, Leshchinsky D, Wang J-P, Mohri Y, Rosen A (2009) Seismic response of simulations and writing the manuscript. And all other author contributed in geocell retaining walls: experimental studies. J Geotech Geoenviron 135(4): data analysis, simulations and writing the paper. The author(s) read and 515–524. https://doi.org/10.1061/(ASCE)1090-0241(2009)135:4(515) approved the final manuscript. Lizzi F (1982). The pali radice (root piles)” symposium on soil and rock improvement techniques including geotextiles reinforced earth and modem Funding piling Methods Bangkok D3 Chandragiri Hills Ltd. has funded the project for analysis and construction. Madhavi Latha G, Rajagopal K, Krishnaswamy N (2006) Experimental and theoretical investigations on geocell-supported embankments. Int J Geomechanics 6(1):30– Availability of data and materials 35. https://doi.org/10.1061/(ASCE)1532-3641(2006)6:1(30) The most of the data is collected from field work and simulations. Some of Mehrjardi GT, Tafreshi SM, Dawson A (2012) Combined use of geocell the data generated or analyzed during this study are included in the reinforcement and rubber–soil mixtures to improve performance of buried published articles which is mentioned in the paper. pipes. Geotext Geomembr 34:116–130. https://doi.org/10.1016/j. geotexmem.2012.05.004 Declarations Palmerton J (1984) Stability of moving land masses by cast-in-place piles (R). Federal Highway Administration, Washington DC Competing interests Pearlman S, Wolosick J (1992) Pin piles for bridge foundations, paper presented The authors declare that they have no competing interests. at Proceedings of the 9th Annual International Bridge Conference Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 15 of 15 Pokharel SK, Han J, Leshchinsky D, Parsons RL, Halahmi I (2010) Investigation of factors influencing behavior of single geocell-reinforced bases under static loading. Geotext Geomembr 28(6):570–578. https://doi.org/10.1016/j. geotexmem.2010.06.002 Simac M (1990) Design methodology for miragrid reinforced soil retaining walls. Mirafi Inc., Charlotte, p 110 Sireesh S, Sitharam T, Dash SK (2009) Bearing capacity of circular footing on geocell–sand mattress overlying clay bed with void. Geotext Geomembr 27(2):89–98. https://doi.org/10.1016/j.geotexmem.2008.09.005 Soeters R, Van Westen C (1996) Slope instability recognition, analysis and zonation. Landslides Investigation Mitigation 247:129–177 Stöcklin J, Bhattarai K (1977) Geology of the Kathmandu area and central Mahabharat range. Nepal Department of Mines and Geology, Himalayan Report, Nepal Sun S-W, Wang W, Zhao F (2014) Three-dimensional stability analysis of a homogeneous slope reinforced with micropiles. Math Probl Eng 2014:1–11. https://doi.org/10.1155/2014/864017 Tafreshi SM, Dawson A (2010) Comparison of bearing capacity of a strip footing on sand with geocell and with planar forms of geotextile reinforcement. Geotext Geomembr 28(1):72–84. https://doi.org/10.1016/j.geotexmem.2009.09.003 Tafreshi SM, Dawson A (2012) A comparison of static and cyclic loading responses of foundations on geocell-reinforced sand. Geotext Geomembr 32: 55–68. https://doi.org/10.1016/j.geotexmem.2011.12.003 Wu, K., and D. Austin (1992), Three-dimensional polyethylene geocells for erosion control and channel linings, in Geosynthetics in filtration, drainage and Erosion control, edited, 275-284, Elsevier Yadav M, Agnihotri AK, Priyadarshee A, Dhane G (2014) Application of geocells in reinforcement of soil: a review. J Civ Eng Environ Technol 1(5):60–64 Yang X, Han J, Pokharel SK, Manandhar C, Parsons RL, Leshchinsky D, Halahmi I (2012) Accelerated pavement testing of unpaved roads with geocell- reinforced sand bases. Geotext Geomembr 32:95–103. https://doi.org/10.101 6/j.geotexmem.2011.10.004 Zhang L, Zhao M, Shi C, Zhao H (2010) Bearing capacity of geocell reinforcement in embankment engineering. Geotext Geomembr 28(5):475–482. https://doi. org/10.1016/j.geotexmem.2009.12.011 Zhou H, Wen X (2008) Model studies on geogrid-or geocell-reinforced sand cushion on soft soil. Geotext Geomembr 26(3):231–238. https://doi.org/10.1 016/j.geotexmem.2007.10.002 Publisher’sNote Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geoenvironmental Disasters Springer Journals

Effectiveness of geocell wall, geogrid and micropile anchors for mitigation of unstable slopes

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Abstract

Slope failure mitigation practices are well developed in recent years. Recently, geosynthetic, geocell, and geogrid combined with micropiles are being used extensively in various slope stabilization works. But integrated approaches are still lacking. In this study, a method of slope stabilization is proposed by integrated use of micropile, geocell and geogrid from an engineering and economical point of view. The study was done on slope failure located at Chandragiri Hill, south west of Kathmandu, Nepal. Geotechnical problems of the site, the design of geocell foundation, micropile and geogrid are done on the based on numerical analysis using Phase-2 software with field data. The results of analytical studies revealed that, the use of a combination of geocell, micropile and geogrid is beneficial in increasing slope stability. As per numerical analysis, in the slope failure site, geocell gravity walls each of 3.8 m, is constructed in different step. Beneath the geocell wall, different layers of geogrid were placed filled with granular materials. The geocell wall is connected with micropile from inside. The micropile works as an anchorage and support for geocell wall, which increases the stability of a failed slope. Keywords: Slope stability, Geocell, Geogrid, Reinforcement, Micro pile, Granular material Introduction defined as a planar product manufactured from polymer The modifications in the geomorphic, hydrological and used with soil, rock, earth, or other geotechnical engin- geological conditions of the area that is mainly facilitated eering related material as an integral part of a man- by geodynamic processes, vegetation, and land use prac- made project, structure or system. The use of geocell is tices, human activities, seismicity, rainfall are the factors the most recent advancement in soil reinforcement that trigger slope instability. Dealing with the problem of where the materials are confined in three-dimensional instability of slopes has always been interesting, import- pockets (Dash et al. 2001). Micropiles are widely used to ant and challenging in the field of geotechnical engineer- stabilize slopes especially for slopes located in steep, ing. Slope failures and instability are encountered in hilly, or mountainous areas; as they are simple, fast, eco- various stages and sectors of engineering such as during nomical and environmentally friendly. Micropiles are de- cutting, construction of hill roads, railway lines, reser- fined as a small diameter (generally less than 300 mm) voirs and damns among others (Soeters and Van Westen non displacement pile, generally reinforced, which are 1996). There are various methods for slope failure miti- driven into the soil and grouted (Sun et al. 2014). Many gation. In recent years, geosynthetic, geocell, and geogrid researchers have found that the use of piles is one of the combined with micropiles are being used extensively in effective methods in the stabilization of slopes. Lee et al. various slope stabilization works. A geosynthetic can be (1995) and Li et al. (2012) studied stabilization slopes using a simple approach by means of row of piles driven into the slope. Ausilio et al. (2001) has studied stability * Correspondence: gsgroup.sanju@gmail.com Lincoln University College, Kota Bharu, Malaysia of slopes that are reinforced with piles using the Full list of author information is available at the end of the article © The Author(s). 2021 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://creativecommons.org/licenses/by/4.0/. Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 2 of 15 kinematic approach of limit analysis. Geocell is three- as well as decreasing the footing displacement. (Chen dimensional cell made from high-density polyethylene and Chiu 2008) confirmed that geocell in an increased (HDPE) or polyethylene (PE) strips ultrasonically welded length perform similar to geogrid layers and provide along the width. Geocells are better alternatives to con- reinforcement to the soil. Ling et al. (2009) showed that ventional slope stability measures like use of concrete geocell can perform well in gravity walls as well as panels. Also geocell perform better than the concrete reinforcement walls. Cancelli et al. (1993) tested that in panels in cold weather conditions (Dash et al. 2003). steep slopes and areas of heavy surface runoff, where The main properties of geogrid consist of uniformity, vegetation is not effective in controlling soil erosion. stability, light, anti-corrosive, anti-aging, high tensile Geocell can be used as it has good tensile strength and strength and flexibility. Geogrids reduces the joggling of confines infill material as well as reduce the velocity of filling materials, reduce the inhomogeneous settlement surface runoff. of soil to the largest possible degree and improve stabil- Palmerton (1984) and Pearlman and Wolosick (1992), ity. These are obvious advantages of using geogrid as suggested that in case of soft or weak soil, to transfer the reinforcement in retaining walls. With the rising prob- axial and lateral load to more stable strata, micropiles are lem of global warming and environmental concerns, the perfect solution. Meantime, Pokharel et al. (2010)sug- geosynthetic materials as a measure for reinforcing soil gested that the three-dimensional geocell provided lateral and preventing soil erosion caused by runoff water have confinement, base acts as a mattress to restrain the soil gained a worldwide acceptability (Yadav et al. 2014). Wu from moving upward outside the loading area. and Austin (1992) reported the use of geocell for slope This research has attempted to validate the least popu- stability and for erosion control, as well as the walls of lar but economical solution for mitigation and control of geocell controlled the downward movement of materials critical slopes. For this purpose, geocell and micropiles as they are confined. along with the combination of geogrid reinforced soil Dash et al. (2003), Krishnaswamy et al. (2000), Mad- were used. In many cases, micropile, geocell and geogrid havi Latha et al. (2006), Mehrjardi et al. (2012), Sireesh are suitable options to mitigate slope instability issues et al. (2009), Tafreshi and Dawson (2010) and (Tafreshi and this research work attempted to evaluate the effect- and Dawson 2012); Yang et al. (2012), Zhang et al. iveness of geocell, geogrid and micropiles against retain- (2010), Zhou and Wen (2008) studied beneficial effects ing walls in critical slope and mitigation of unstable of geocell on increasing the load bearing capacity of soil slopes. Futhermore, the site at Chandragiri hills of Fig. 1 Location map of the study area Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 3 of 15 Kathmandu Valley, Nepal has been selected for the re- is an intermountain basin with valley floor surrounded by search and design and implementation of all necessary four mountain ranges from Shivapuri, Phulchoki, Nagarjun slope stabilization works. and Chandragiri located in North, South-East, North-West and South- West respectively. The geology of the Study area Kathmandu Valley and its surroundings can be put into two The study area lies on South-West of Kathmandu Valley groups, unconsolidated-slightly consolidated sediment de- at Chandragiri Municipality, Kathmandu district of Bag- posits (Quaternary deposits) and hard rocks of Precambrian mati Province, Nepal (Fig. 1). Physiographically, the study to Devonian (Stöcklin and Bhattarai 1977). The central part area belongs to the part of the Mahabharat Range of of the valley has lacustrine and fluvial deposits containing Central Nepal representing a strongly dissected range of peat, clay, carbonaceous clay, sand, gravel, and boulders topographic variations with moderate to a very steep which overlie uncomfortably on the rocks of the Phulchauki slope, ridge, spur, saddle and valley (Dahal et al. 2008). and Bhimphedi groups (Fig. 2). Geologically, it lies in the Lesser Himalaya Zone of cen- Hard rock geology around the Kathmandu Basin is tral Nepal (Stöcklin and Bhattarai 1977). The research comprised of various sedimentary, meta-sedimentary area lies within longitude 85°12′30.98“Eto85°12’36.99”E and metamorphic rocks. The rock formation surround- and latitude 27°39′57.12“Nto 27°39’59.54”N and elevation ing the basin belongs to the Phulchauki Group of the of 2300 m–2450 m. The research was mainly focused on Kathmandu Complex. The Phulchauki Group is divisible the landslides that occurred on the northern slope (Fig. 2). into five formations: Tistung Formation, Sopyang For- There is a resort that lies near the landslide area and has a mation, Chandragiri Limestone, Chitlang Formation, and famous panoramic view of the Himalayas as well as the Devonian Limestone of Phulchauki. Sheopuri Gniess Kathmandu city. (Fig. 2) is present on the northern hills (Stöcklin and Bhattarai 1977). Regional geological setting The study area lies in the vicinity of the contact of the Geologically it lies in the Lesser Himalaya Zone of central Chandragiri Limestone and Chitlang Formation on Nepal south of great Himalayan Range (Dahal et al. 2008; Chandragiri mountain range. Residual/colluvial deposit Hagen 1969; Stöcklin and Bhattarai 1977). Kathmandu valley has covered the underlying bedrock of Chandragiri Fig. 2 Study area on regional geological map of Kathmandu basin (modified after Stöcklin and Bhattarai 1977 and Dahal et al. 2009) Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 4 of 15 Limestone and Chitlang Formation (Fig. 2). The Chan- Site and soil characteristic dragiri Limestone of Middle Cambrian age consists of The site has a four storey building of the Chandragiri grey, thin to thick bedded argillaceous, laminated lime- Hills Resort with an isolated foundation (Fig. 3). There is stone with minoramount of white, ortho-quartzite, light a topographic map shown in Fig. 4. There was a stone grey with orange weathered argillaceous phyllite and masonry wall constructed around periphery of the build- greyish, micaceous metasandstone. The upper part of ing without proper drainage system for surface runoff. the succession comprises partings of dark grey to light The downward slope angle of slope is more than 42°, grey phyllite subordinated with white quartzite (about width 30 m and length 20 m. The pore water pressure 150 m) band and greyish leachate on limestone. The on the slope during rainy season cannot be released Elephant-skin type weathering on limestone is observed through the retaining structure. in some parts. The Chitlang Formation of Silurian age The site is located on a moderately steep slope that is consists mainly of white quartzite, some beds of argilla- a landslide prone zone. The down slope is bare and run- ceous limestone, dark bluish grey, violet shale, slate and off and rainfall can infiltrate easily due to the fracture of dolomitic limestone. The lower part of this unit consists rock underlain by a thin veneer of red soil. The soil is of violet, shale and slate, white, muddy quartzite, grey, unstable both in rainy as well as dry season due to its fine-to coarse grained meta sandstone, thick-to massive, silty nature. The slope is north facing. As a result the fine-to medium-grained limestone and phyllite in some moisture remains on the slope for a long time after rain- parts. Similarly, the upper-part of succession comprises fall and snowfall in winter. The area was well vegetated intercalation between yellowish grey shale and dark grey before the resort construction (Dahal et al. 2009) but the limestone minorly of dolomite with precipitated calcite vegetation was removed for construction purpose in the in which wave marks are observed. surrounding area that resulted erosion and failure prob- The study was focused on the landslide mass consist- lems. The slope consists of red silt soil from 1 m − 1.5 m ing of fine, reddish brown, moist, sandy silt to silt with depth with rock fragments and the yellowish fractured pebbles, cobbles as overburden deposit and dark grey, rock is present from 1.5 m to 6 m. The weathered slate weathered and fractured slates as exposed bedrock only and calcareous rocks are mainly present after 6 m in the a few locations. site. The soil has specific gravity 2.69 and friction angle Fig. 3 Four stories building and landslide in the slope just below the building Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 5 of 15 Fig. 4 Topographic map of study area (φ)36 and the moisture content of the soil varied from slope surface and wall to greatly improve resistance to 9% to16% with a bulk density of 2.07 g/cm . erosive forces such as rainwater run-off on steep or un- stable slopes, or slopes exposed to severe hydraulic or Materials and methods mechanical stresses (Wu and Austin 1992). In this research, effectiveness of geocells and micropiles along with the combination of geogrid reinforced soil Filling materials was explored for the slope stabilization. At first, numer- For filling, the specific gravity of filling material was ical analysis was performed for the project site using 2.66. Likewise, liquid limit and plastic limit of the clay geocell, geogrid and micropiles. Then the field applica- were 40% and 19%, respectively. The maximum dry tion of micropiles, geogrid and geocell were done to density, optimum moisture content, Standard Proctor mitigate the slope instability as per the numerical result. test were 18.2 kN/m and 13.2%, respectively. The effect- Following - materials and methods are used in the re- ive particle size (D ) was 0.26 mm. The angle of internal search process. friction was 40 degree. Poorly graded sand was used and it was SP according to unified soil classification system Geocell (USCS). Average size of the gravel was 12 mm according Geocell is a honeycomb three-dimensional cell structure to unified soil classification system, graded gravel (GP). (Fig. 5) that confines the filled compacted materials, de- Normally, select fill materials are more expensive than creases the lateral movement of soil particles and distrib- lower quality materials. The gradation requirements for utes the applied loads to a wider area. Geocell is granular reinforced fill, gradation 4 in – 100% passing, generally used in the construction of canals, embank- 40 mm – 60% passing, 200 mm – 50% passing and plas- ments, retaining walls, railways and roads, slope stability ticity index ≤20 (AASHTO T-27, T90). (Bathurst and Jarrett 1988; Dash et al. 2003). Geocell is a For this study, the filling material used was GM (silty blanket of three dimensional cell structures applied to a gravel) as per ISC and USC system. Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 6 of 15 Fig. 5 Schematic drawing of typical geocell and physical, mechanical and hydraulic parameters Geogrid specified size 150 mm (diameter). The cement grouting Geogrids are manufactured by polymers like PET (as per was done in the drilled hole under a pressure with perfo- ASTM D2455, ASTM 4603 as per ASTM D1248), they rated pipe to spread the slurry into the surrounding soil. have apertures in various sizes between individual ribs in After completing grouting process, the reinforcement is the transverse and longitudinal directions. PET and lowered into the hole. HDPE Geogrids have minimum UV resistance as per ASTM D4355. Geogrids are (a) either stretched in one, Methods of analysis two or three directions for improved physical properties, In this research, Phase 2 (2002), a Rocscience FE pro- (b) made on woven/knitted machinery by standard tex- gram was used to simulate and analyze a complex multi- tile manufacturing methods. stage model (Fig. 7) for slope stability analysis. Material In this study, biaxial geogrid is used with the charac- properties of model is taken as; Elastic modulus 15,000 teristics shown in (Fig. 6). kPa, Poisson’s ratio 0.3, tensile strength 5 kPa, friction angle 30°, cohesion 5 kPa shown in Table 1. For vertical Micropile boundary, u = 0 and u is free and for horizontal bound- x y The micropile was used in an unstable slope with geo- ary; ux = uy = 0. Mohr Coulomb failure criterion is used cell. The micropile has a small diameter and it is easy to to simulate the model. The shear strength reduction transport and install even by semi-skilled person. Micro- (SSR) technique of Finite Element (FE) and the simpli- pile bears the axial loads and lateral load therefor it can fied Bishop method was used to analyze the slope stabil- be constructed in any type of soil/rock/sand conditions. ity problem to gain insight into the soil mass behavior, Micropile depends on location, slope, cross section, progressive failures and explicit modelling of discontinu- length, group spacing and concrete cap beam of micro- ities. In both methods, at first the existing failed slope pile (Lizzi 1982). In this case study, micro piles are cast- conditions were analyzed and checked for their stability in-situ with 101 mm MS medium pipe drilled hole of the (FoS < 1 or FoS > 1). When FoS < 1, to improve the slope Fig. 6 Grid used in the field construction and properties of geogrid Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 7 of 15 Fig. 7 Schematic diagram of slope for FEM based simulation stability and increase the factor of safety the existing soil feature of this linear failure model is the fact that it can was reinforced with the use of a combinations of micro- be simply and explicitly expressed in both principal (σ - pile, geocell and geogrid. σ ) stress space and shear-normal (τ-σ ) stress space. 3 n The shear strength reduction (SSR) technique of Finite The simplicity, explicit representation in both principal Element (FE) slope stability analysis is a simple approach and shear-normal stress space, an adequate description that involves a systematic search for a stress reduction of strength behavior for a wide range of materials, and factor (SRF) or factor of safety value that brings a slope easy to obtain parameters of the Mohr-Coulomb criter- to the very limit failure. The SSR technique assumes ion account for its popularity. For Mohr-Coulomb ma- Mohr Coulomb strength for slope materials. The Mohr terial the factored or reduced shear strength can be Coulomb strength envelope is the most widely applied determined from the equation failure criterion in geotechnical engineering. A unique τ c tanφ’ Table 1 Modeling parameters ¼ þ ð1Þ F F F Modeling parameters This equation can be re-written as Elastic Modulus 15,000 kPa Poisson’s ratio 0.3 ¼ c þ tanφ ð2Þ Tensile strength 5 kPa Friction angle 30° Where, F = factor of safety; c’ = effective parameter of Cohesion 5 kPa cohesion; τ’ = effective shear strength; φ’ = effective angle Constitutive model Mohr coulomb failure criterion of internal friction Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 8 of 15 the slope model and recomputed. Record the 0 0 c tanφ maximum total deformation. c ¼ and φ ¼ arctan F F: Step 3: Repeat step 2, using systematic increments of F, until the FE model does not converge to a solution, i.e. The steps for systematically searching the critical fac- Continue to reduce material strength until the slope tor of safety value F that brings a previously stable slope fails. The critical F value just beyond which failure (F ≥ 1) to the verge of failure are as follows: occurs will be the slope factor of safety. Step 1: Develop an FE model of a slope, using the For a slope with a factor of safety less than 1, the pro- appropriate materials deformation and strength cedure is the same except fractional F values will be sys- properties. Compute the model and record the tematically decremented (translating into increments in maximum total deformation. the factored strength parameters) until the slope be- Step 2: Increase the value of F (or SRF) and calculate comes stable. factored Mohr Coulomb material parameters as The principal advantage of the SSR technique is its use described above. Enter the new strength properties into of factored strength parameters as input into models, Fig. 8 a Installation of micropile, b Geocell wall, c Laying of geogrid inside the micropile in slope area for stable of back fill materials soil, d Laying of geocell wall inside micropile Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 9 of 15 which enable the technique to be used with any existing 0 0 0 τ ¼ðÞ c þ σ tanϕ FE analysis software (Fig. 7). All the approach requires ð3Þ of a slope analyst is computation of factored Mohr Cou- lomb strength parameters. To find σ' resolve forces in the vertical direction to The simplified Bishop method (Bishop, 1955) has obtain been widely used in slope stability analysis and is regarded as the best method of limit equilibrium for 0 0 0 0 calculating the factors of safety of circular slip W−ðÞ c þ σ tanϕ ΔX tanα−ðÞ σ þ u ΔX ¼ 0 ð4Þ surfaces. In this method, the inter slice forces are as- sumed to be horizontal, or the vertical inter slice W−uΔX− c ΔX tanα forces are neglected, the vertical force equilibrium 0 F ∴σ ¼ ð5Þ and the moment equilibrium about the center of the ΔXðÞ 1 þðÞ tanϕ tanα =F circular slip surfaces are satisfied, but the horizontal force equilibrium is not considered. Now F = sum (maximum resisting forces around arc)/ The simplified analysis is as follows: sum(moving forces around arc) Fig. 9 Micropile, geocell wall layout plan Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 10 of 15 0 0 0 methods involving consideration of the forces acting ðÞ c þ σ tanϕ ΔX secα ¼ P ð6Þ on the sides of slices shows that the simplified Bishop W sinα Method yields answers to factors of safety that are close to the correct answer. ½ c ΔX þðÞ W−uΔX tanϕ M We have numerically modelled the project site ¼ P ð7Þ using FEM in the static condition considering it as a W sinα continuum by SSR approach. By determining the fac- sinα tanϕ Where, M ¼ cosα þ tor of safety of failed slope, post disaster analysis is τ = shear strength carried out. While, by using FEM methodology, stress σ = normal stress developedinthe slopeis determinedtofocus on ϕ = angle of friction probable failure. The analysis was performed using W = Weight of slice Phase2 software. FEM, a widely accepted method of ΔX = width of slice numerical modelling of slopes works on the principle u = pore pressure of discretization of whole design into a fixed number To facilitate the analyses of slope stability for a of elements through which continuous variation in large number of potential failure surfaces and a var- material properties takes place. A 2D, three nodded iety of conditions, computer programs are used. The triangular plane strain elements have been used to Bishop Method yields factors of safety that are higher discretize the slope design. The SSR approach with than those obtained with an ordinary method of non-failure criteria has been adopted. Since the max- slices. Furthermore, the two methods do not lead to imum shear strain of the failure zone coincides with the same critical circle. It has also been found that the rupture surface, it is thus assumed that failure the disagreement increases as the central angle of a mechanism of slope is directly related to the develop- critical circle increases. Analysis by a more refined ment of shear strain. Fig. 10 Schematic diagram at section B-B, showing installation of micropile, geocell wall and geogrid reinforcement Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 11 of 15 Sequence of construction method geocells were anchored to the ground using a J hook of The construction methods with reinforce soil using geo- 20 mm dia at 0.5 m c/c on both edges. Prior to execution grid are well explained by Simac (1990). Similarly, Dash of later geocell wall, the slope between the prior and the et al. (2007) have explained how geocell acts as a rigid later geocell wall was maintained by filling with granular mattress and can distribute the applied load to a larger materials reinforced using geogrid at an interval of 1 m surface area. Further, Zhang et al. (2010) explained how vertical spacing. After the completion of micropiles, the geocell can reduce the settlement and increase the load geocell wall and slope maintenance reinforced with geo- carrying capacity. (Bush et al. 1990)explained about the grid and a layer of geocell were laid throughout the slope construction of geocell and its installation in the field. along its length which was anchored with 20 mm dia J Elarabi and Soorkty (2014) has explained about micropiles hook @ 0.75 m c/c both ways. Later, bioengineering (use and suitable drilling techniques for the reinforcement with of vegetation) was done along the slope. the micropile. In this research also, like execution of any other civil Analysis and result engineering work, at first the site was cleared, excessive The slope was evaluated for a factor of safety as men- debris of the failed slope was removed and the path was tioned in earlier sections. Figure 11 illustrates that the constructed for the commencement of slope protection factor of safety of the existing slope before failure was work. The protection method made use of driven micro- found to be 0.86 that was analyzed as per Shear Strength piles with the combination of geocell and geogrid as Reduction (SSR) method. The factor of safety of 0.882 shown in (Fig. 8). The slope protection work com- for existing slope before failure analyzed as per Bishop’s menced from the toe of the slope and micropiles were method was also noticed during simulation (Fig. 12). driven of varying depth on the ground (Fig. 9). In total Since both SSR and Bishop’s method showed factor of 139 micropiles with different lengths from 10 m to 20 m, safety less than 1, so slope was prone to fail which was 2 m c/c distance were driven throughout the length of seen in the site. To overcome this issue, geocell, micro- the slope as shown in Fig. 10. As seen in section, total of pile and geogrid were applied in the simulations and FoS four geocells were constructed with base of the geocell was evaluated again. wall of 2.1 m wide, which tapered to 0.7 m at the top, It was found that the factor of safety for the slope was the total height of each geocell wall was 3.8 m. The increased from 0.882 to 1.076 form limit equilibrium Fig. 11 Maximum shear stress for slope without reinforcement Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 12 of 15 Fig. 12 Slip surface as per Bishops Fig. 13 Maximum shear strain for reinforced slope with micropile, geocell and geogrid reinforcment Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 13 of 15 Fig. 14 In left, slip surface as per Bishops for overall fos of reinforced soil with micropile, geocell wall and geogrid, in right, slip surface as per Bishops for factor of safety of individual materials method. Bishops (1995), suggested that to calculate the Discussion factor of safety of slope, the whole slope is divided into There are many methods for the stability analysis of slope. vertical slices and each of them is individually analyzed These are categorized into conventional and numerical using circular failure analysis to get the individual slice methods. For the existing slope before failure it was ob- factor of safety and summarized for overall factor of served that the factor of safety was less than 1 for Bishops safety of slope. as well as SSR method and the angle of slope was 42 . So, The factor of safety according to SSR method was it was prone to failure as observed practically. After the found to be 0.86 for the natural unreinforced slope and failure of slope the angle of slope was increased to 47 and the factor of safety was 1.13 after reinforcement work stability was more critical. So, the failed slope was miti- with a combination of micropile, geogrid and geocell gated with the introduction of reinforcement combination (Fig. 13). From Fig. 14, it can be observed that the factor of micropile, geogrid and geocell. Then it was analysed of safety according to the Bishops method was found to using Phase-2 for SSR method and Slide for Bishop be 1.076 after the slope was mitigated with the combin- Method. Both analysis results showed that the factor ation of micropile, geocell and geogrid, which was 0.882 safety was increased to more than 1. for the natural unreinforced slope (Table 2). The factor There are various methods for slope failure mitiga- of safety was in the range of 1 to 4 where the micropiles tion. In recent years, geosynthetic, geocell, and were driven into the slope. geogrid combined with micropiles are being used ex- The failure slope was analyzed being mitigated with tensively in various slope stabilization works. Micro- the combination of micropile, geocell and geogrid and it piles are widely used to stabilize slopes especially for was observed that the factor of safety was improved and slopes located in steep, hilly, or mountainous areas; obtained to be greater than 1 (Fig. 13 and Fig. 14). as they are simple, fast, economical and environmen- The result of the FE analysis were compared to an- tally friendly. This research has attempted to validate swers obtained from the Bishop limit equilibrium the least popular but economical solution for the method computed in Slide software, a slope stability mitigation and control of critical slopes. The study program developed by Rocscience. The FE factor of results show that reinforcement of slope using safety result agreed very well with the factor of safety by micropile, geogrid and geocell shows better results the limit equilibrium method. in slope stabilization work. In which, micropile was used in the analysis to resist the lateral and shear force and act as anchorage as well as the grouting filled up the fissures present in the soil mass. Geo- Table 2 Difference of factor of safety by finite element method cell wall was used to act as a retaining structure at and limit equilibrium method intervals and confine the filled soil material as well FoS of Natural FoS of Reinforced as the porous nature of geocell facilitated the seep- Slope slope age of water through the soil without soil loss. Geo- Shear reduction Method 0.86 1.13 grid was used in the backfill of soil in layers to (SSR) maintain the slope and increase the shear strength Bishop Method 0.882 1.076 of soil. Kumar et al. Geoenvironmental Disasters (2021) 8:11 Page 14 of 15 Conclusion Author details 1 2 Lincoln University College, Kota Bharu, Malaysia. ERT Tech Pvt. Ltd, The slope stability analysis is a challenging work in geotech- Kathmandu, Nepal. Tribhuwan University, Pulchowk campus, Kirtipur, Nepal. nical engineering. Formerly, the limit equilibrium method of 4 Wuhan University of Technology, Wuhan, China. analysis was widely used due to its clear physical meaning Received: 13 September 2020 Accepted: 21 April 2021 and simple calculation. Now, with the development of the fi- nite element method, the strength reduction method is grad- ually recognized to determine the factor of safety of slope. In References this paper, the factor of safety of slope is firstly calculated by Ausilio E, Conte E, Dente G (2001) Stability analysis of slopes reinforced with Bishop’s method, which is then compared with the safety fac- piles. 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