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Background: Drying and rewetting process, frequently occurred during climatic changes, is an important process in soil aggregate slacking and dissolution. The severer interference of human activities on global climate makes the extreme climate scenarios like drought and rainstorm occur frequently. Therefore, there is necessity to further our understanding on the impact of the drying–wetting cycles and initial water content on the breakdown of soil aggregates. The typical yellow–brown earth composed of water–stable and water–unstable aggregates is selected. Variations of water-stable aggregate size distributions after drying-wetting cycles are measured by wet sieving, under variable initial water content and cycles respectively. Results: Drying-wetting cycles cause a significant aggregate slaking, especially within the first two cycles. After that, most aggregates show more slacking resistant. The variation curves of the proportion of water-stable aggregates with the size 1–5 mm shows a coexistence of slaking process and supplement. The critical initial water content (about 24%) and turning point (with the aggregate size of 0.3 mm) are proposed to describe the effects of initial water content on size distribution of water-stable aggregates. Overall, the increase of initial water content strengths the water stability. In addition, the mathematical model for the relative leakage ratio based on the drying–wetting cycle, initial water content and size distribution are established. Conclusions: The findings reported in this paper may be capable of supporting the intensive study for the breakdown mechanism and assessing the leakage potential under the influence of climate change. However, there exists a certain mismatch between the drying-wetting cycles in the tests and in practice, mainly in the frequency and intensity, which should be paid more attention. Keywords: Drying–wetting cycles, Yellow–brown earths, Water stability, Aggregate breakdown, Relative leakage ratio Background central and southern Europe, eastern North America Karst topography is a landscape formed from the dissol- and southwest China. (Fig. 1). ution of soluble rocks such as limestone, dolomite, and The karstification can induce land degradation, vege- gypsum. It is vulnerable to climate change and human tation coverage loss, excess soil erosion, and the karst activities (Yuan, 1996), and thus is considered as one of rocky desertification eventually (Wang et al., 2004). All the main eco–sensitive zones. Karst topography covers these conditions lead to severe pressure on geo–disaster an area of approximately 22 million square kilometers, reduction and social resilience to disasters. As the most accounting for about 15% of the land area. The karst predominant process during the formation of karst rocky topography mainly locates in lower latitudes, among desertification, large amounts of studies have docu- which the centralized karsts are mainly distributed in mented the aspects of soil erosion. Lal (2003) described the soil erosion process as a four-stage process involving detachment of particles, breakdown of aggregates, trans- port and redistribution of sediments over the landscape * Correspondence: firstname.lastname@example.org and deposition in depressional sites or aquatic ecosys- Department of Geotechnical Engineering, Tongji University, Shanghai tems. Foster (1982), Hogarth et al. (2004), Rouhipour et 200092, China © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Xu et al. Geoenvironmental Disasters (2017) 4:20 Page 2 of 13 Fig. 1 Global distribution of major outcrops of carbonate rocks (Sweeting, 1973; Wikipedia: Karst) al. (2006), Walker et al. (2007), An et al. (2012) and Par- holding capacity, unstable structures, and even lean crop lange et al. (2012) have conducted model, mathematical productivity (Gao and Li, 2007). As the basic unit of soil and simulated methods to investigate the impacts of structure of the yellow–brown earth, the stability of ag- rainfall on soil erosion. However, literatures concerning gregate is one of the most important factors controlling the mechanisms of water-induced soil erosion are not soil erodibility (Duiker et al., 2001). Soil aggregates are expressive. Tabeda and Tomita (1972) studied the mech- formed by the primary soil particles and soil cemented anism of soil erosion by correlating soil erosion with materials with the coagulation, cementation and cohe- rainfall intensity based on the energy of raindrops. sive actions, the stability of which is an important indi- Fujiwara and Yamamoto (1981) searched the mechanism cator for the soil quality evaluation. According to the of soil erosion under simulated rainfall. Owoputi (1994) different ways to destroy the soil aggregates, the stability considered mechanism of sediment detachment in the of soil aggregates can be divided into mechanical stabil- soil erosion process. Relevant indicators of vast soil deg- ity, water stability, chemical stability, acidic and basic radation and erosion include texture and porosity spoil- stability and biostability (Domżł et al., 1993). Water sta- age (Figueiredo et al., 1999; Jankauskas et al., 2008), bility can be defined as the property and capacity of soil water retention capacity decline (Ye et al., 2011) and soil aggregates to resist hydraulic damage, and this stability leanness accompanied by chemical and biologic charac- may include wet aggregate stability (macro-aggregate teristics degradation (de Paz et al., 2006) have also been stability >250 μm) and dispersible clay (micro-aggregate studied. Based on the analysis of the indicting parame- stability <250 μm) (Amezketa, 1999). ters, several attempts at assessing and evaluating soil As the important indicator of soil erosion–resistance erosion have been made including those by Olson and in the areas with rich runoff, the water stability of soil Wischmeier (1963), Rose et al. (1983), Smith et al. aggregates has been received considerable attention. (1999), Kheir et al. (2008), Febles-González et al. (2012), Two main groups of factors affecting soil aggregate sta- Tang et al. (2015), and by Nearing et al. (1990), Shelton bility can be considered: soil primary characteristics or and Wall (1998) and Feeser and O’Connell (2009) refer- internal factors like clay mineralogy (Reichert et al., ring to some predictions. 2009) and organic matter (Jozefaciuk and Czachor, The yellow–brown earths, forming on the Quaternary 2014), and external factors to the soils such as drying– red clay or weathered material of limestone in hot, wetting cycles, freezing-thawing cycles (Dagesse, 2012), humid and rainy climate conditions in Guizhou karst biological factor (Jastrow and Miller, 1991), tillage areas, are viewed as the main body of soil erosion inher- (Bartlová et al., 2015) and so on. In nature, most surficial ently and considered to be problematic in poor water- soils are subjected to drying–wetting cycles due to Xu et al. Geoenvironmental Disasters (2017) 4:20 Page 3 of 13 alternative rainfall and evapo-transpiration. During these of landforms, valleys, underground rivers (such as cycles most of the properties of soils, especially their Chenqi and Changchong underground river) are obvi- strength and hydraulic stability, are severely affected and ously affected and controlled by above tectonic charac- as a result crack propagation and stability failure occur teristics. The joints and fissures are widely distributed in (Kemper et al., 1985; Denef et al., 2001a,b; Peng et al., variously lithologic strata, with close relations to the 2007). However, the effects of drying-wetting cycles on folds, faults and stress fields. The widely formed X– water stability of soil aggregates have been considered to shaped joints are 35°–50° and 305°–320° with the dip be variable. Pires et al. (2007) suggested that wetting- angle greater than 60°. drying cycles can be used to repair some structurally The strata in the research area are mainly composed damaged soils. Soulides and Allison (1961), Tisdall et al. of post–Triassic limestone, dolomite and loose deposits. (1978) found that drying-wetting cycles decreased the Soil parent material is Quaternary red clay. The soil type water-stability of soil aggregates. Hofman (1976) ob- contains yellow–brown earth, red–brown earth and tained a contrary result that the aggregate instability of limestone soil with soil texture of silty clay, silty clay aggregates sieved immediately after sampling was greater loam and clay loam. The surface soil is thin and poor in than that obtained when the soil was first air-dried and continuity, the average thickness of which is only 10– then rewetted to its original water content. In the re- 30 cm (Fig. 2d). It lacks intermediate transition layer be- search of Utomo and Dexter (1982), wetting and drying tween the lower parent rock and the surface soils. The first increased the water stability of aggregates disturbed geological structure shows the two–layer structure by tillage to a maximum value and then decreased it (carbonate rock and residual cohesive soil). with the further wetting and drying. Due to the lithology and tectonics of carbonate rocks, Through the above literature review, the following the groundwater system in the research area is fully de- potentialproblemsarise.The effectsof dryingand veloped in limestone (T g ), and is mainly along the rewetting process on soil aggregate stability are incon- northwest tectonic fissure. The thickness of the vertical sistent, largely depending on soil type, percent of seepage zone is small. The groundwater depth is gener- stabilizer, test methods and curing conditions, as fore- ally 3–10 m and the average hydraulic gradient is about mentioned. Thus, the effects on the erodible earths in 7‰. The surface water system is not complete, and is karst areas, especially in the slacking process, the mostly composed of closed depressions, sinkholes and changes of the aggregates with different sizes and the karst springs. Generally, the lakes and springs are the water stability with further cycles, need to be studied. seasonal ones. In addition, the role of the initial water content in the effects of drying–wetting cycles on the water sta- Materials – Physical and mechanical properties of yellow– bility has not been studied. For the quantitative as- brown earths sessment of soil leakage potential, Igwe et al. (1995) In this research, the fine–grained soil was obtained listed six aggregate indices to predict soil leakage po- from a site near Chenqi of Puding town in Guizhou tential. However no attempts have focus on the soil province, China (26°15′36″– 26°15′56″ N; 105°43′30″– leakage potential considering the drying–wetting cy- 105°44′42″). According to Köppen climate classification cles and the initial water content. the climate of the research area is of the Cf type – humid Therefore, the objective of this study was (i)to subtropical monsoon climate. Average values for air describe the slaking behavior of a yellow-brown earth temperature, rainfall, and relative humidity are 14.2 °C; depending on variable drying and rewetting processes, 1336 mm per year; and 80%, respectively. The dry season (ii) to present the effect of the initial water content on covers November – April and the rainy season covers size distribution of water-stable aggregates and (iii) to May – October with the rainfall accounting for more than relate leakage potential to the drying–wetting cycles and 80% of the whole year, forming the climatic characteristic initial water content. of drought in spring/autumn and flood in summer. Enter- ing the twenty-first century, the number of days of rain- Methods storm and frequent of short-time heavy rain both have Regional geological and hydrogeological conditions increased in summer. In spring and autumn, the The study area is located in the upper Yangtze fold belt temperature increases obviously, causing greater evapora- of Yangtze paraplatform. It belongs to the southeastern tions and severer drought. Fig. 3 shows the precipitation limb of Puding synclinorium and northwestern limb of (wetting) and evaporation (drying) between May and Guandingzhuang anticline, the hinges of which are wide September in 2008, 2013 and 2015. It can be seen from and flat. The dip angles of the strata range among 6°– the figure that the intensity of precipitation and evapor- 25°, and the tectonic lines are in the northeastern direc- ation increase and the frequent of drying-wetting variation tion and are almost parallel (Fig. 2c). The development becomes higher with time. Xu et al. Geoenvironmental Disasters (2017) 4:20 Page 4 of 13 Fig. 2 Location of study area: (a) Location of Guizhou province in China; (b) Location of research area; (c) Geological structure of research area; (d) Karst geological map of research area The soil texture is yellow–brown earth (yellow soil- Wet sieving method like, slightly red and named yellow–brown earth there- In this paper, the water stability of soil aggregates was inafter) with high clay content, low organic matter measured considering the effects of drying–wetting cy- content and rich inorganic colloid. It has the proper- cles and the initial water content. Therefore, the soil ties of strong bonding capacity, poor permeability and samples were divided into two series. Series I were the poor resistance to erosion. The mineralogy is predom- samples with the field–moist content affected by dry- inantly consisted of primary mineral (quartz–35.65%, ing–wetting cycles of different cyclic numbers in five feldspar–14.55%) and secondary clay mineral (illite– replicates. Series II were the samples with different ini- 26.4%, montmorillonite–15.0%, Vermiculite–5.5%) (Chen tial water contents. In series I, 100 g of field–moist soil et al., 2010). Samples were taken at a depth of about aggregates were subjected to drying–wetting cycles with (1.5–3.0 m) below the ground surface. Laboratory the cyclic period of 192 h respectively. In series II, the tests such as Atterberg limits, specific gravity, hy- equivalent of 100 g of soil aggregates with the initial drometer analysis and compaction were performed. water contents of 5.08%, 14.51%, 24.73%, 29.68%, 37.52% Table 1 presents some indices and engineering prop- were prepared by the air–dried method. erties of the soil. The grain size distribution was A modification of the method of Yoder (1936) was 9.82% sand, 47.55% silt and 42.63% clay (Fig. 4). used to measure the water stability of aggregates by wet Based on the Casagrande plasticity chart and accord- sieving. 50 g of pre–prepared soil aggregates from series ing to the Unified Soil Classification System (USCS), I or II were broken up to pass through a set of five the soil was classified as silty clay. sieves having 5.0, 2.0, 1.0, 0.5 and 0.25 mm aperture Xu et al. Geoenvironmental Disasters (2017) 4:20 Page 5 of 13 Fig. 3 Precipitation and evaporation in 2008, 2013 and 2015 (after online of Guizhou weather) mesh respectively. The water level was adjusted so that tenderly so as to decrease the influence at most. In the aggregates on the upper sieve were just submerged rewetting process, we apply the immersing method, at the highest point of oscillation. The oscillation rate which is not as violent as in nature. So we reasonably was 30 cycles per minute, the amplitude of the sieving extend the rewetting duration, which should be im- action was 4.0 cm and the period of sieving was 10 min. proved in the following research in order to restore The sieved materials remained in the five sieves and the the real field conditions as far as possible. The wet part through all the sieves were selected and oven–dried sieving was applied to the subsamples in the wetting respectively for at least 8 h. By weighting the six parts of state of the first, second, fifth and tenth cycles to ob- dried materials, the mass percentage of soil aggregates in tain the mass contents of the water–stable aggregates. the range of >5, 5–2, 2–1, 1–0.5, 0.5–0.25, and <0.25 mm can be determined respectively. Results and discussion Water stability of soil aggregate Drying and wetting cyclic test Based on the previous researches on the water stability To describe the effects of the drying–wetting cycles on of aggregates, the following characteristic parameters are aggregate breakdown and water stability of aggregates, applied to describe the water stability of the soil aggre- the yellow–brown earth samples were subjected to sev- gates in this study. Generally, the mass percent contents eral drying–wetting cycles. In the present study, 10 dry- of soil aggregates with the size greater than 0.25 mm or ing–wetting cycles were applied. Each cycle was 5 mm after wet sieving are used as the evaluation index consisted of 96 h drying of the soil samples by air dry for the water stability, which can be symbolled as oven with 40 °C (Relative Humidity – 34%), and 96 h WSA and WSA . 0.25 5 wetting by immersing the soil samples in water at room WSA ¼ W =W 100% ð1Þ 0:25 d>0:25 0 temperature of 20 °C. From the climatic changes in Fig. 3 we can know that the interval between two pre- WSA ¼ W =W 100% ð2Þ 5 d>5 0 cipitations is approximately 7–10 d. Thus, we deter- mine the duration of each drying-wetting cycle as 8 d. where, W > 0.25 and W > 5 are the mass of the water– d d In drying period, the surface temperature under direct stable aggregates with the size greater than 0.25 mm and sunshine is about 40 °C and the air dry oven is used 5 mm respectively; W is the mass of the total to make the air-drying occur sufficiently slowly and aggregates. Table 1 Physical property indexes of intact yellow silty clay Property Water content Density Void ratio Liquid limit Plastic limit Saturation w (%) ρ (g/cm ) e w (%) w (%) S (%) L P r Max 38.86 1.837 1.45 40.12 24.42 85.68 Min 33.24 1.540 1.02 35.64 20.10 73.84 Ave 37.52 1.684 1.34 39.87 22.18 80.89 Xu et al. Geoenvironmental Disasters (2017) 4:20 Page 6 of 13 aggregates with the size greater than 5 mm by dry and wet sieving respectively. Impact of drying–wetting cycles In order to explore the effects of drying–wetting cycles on the water-stable aggregates, the particle size distribu- tions of water–stable aggregates without cycles and after 1, 2, 5 and 10 times of drying–wetting cycles are listed in Table 2. The corresponding characteristic parameters of water–stable aggregates are listed in Table 3. The size distributions of water–stable aggregates after varied drying–wetting cycles are shown in Fig. 5a. It can be seen from the figure that the peak of the size distribu- tion curves moves in the direction of size decreasing with the increasing of drying–wetting cycles, which Fig. 4 Distributions of the grain size of the yellow-brown earth in means that the water–stable aggregates crack into finer karst areas ones. Except for the sample without affected by the dry- ing–wetting cycles, the size distributions of water–stable aggregates have the similar shapes, namely the gradation Mean weight diameter (MWD) and geometric mean of aggregates will not change thoroughly with the in- diameter (GMD), reflecting the size distribution of the crease of cycles. water–stable aggregates after the slaking behavior, are The difference in the percent content of oven–dried calculated by the weighted average of the mass and aggregates in different soil samples may not be sig- diameter parameters of soil aggregates with different ag- nificant, but the contents of water–stable aggregates glomerates (Van Bavel, 1949; Gardner, 1956). Among vary a lot. Therefore, the proportion of water–stable which, the geometric mean diameter is based on the as- aggregates at each size range can better reflect the sumption that the size distribution of the water–stable quality of soil aggregates. The relationships between aggregates conforms to the logarithmic normal the mass percent content of water–stable aggregates distribution. in different size range and the drying–wetting cycles are illustrated in Fig. 5b. With the increase of dry- X X n n MWD ¼ x w = w ð3Þ ing–wetting cycles, the percentage of the water–stable i i i i¼1 i¼1 aggregates in the size range of 5–10 mm decreases X X n n obviously and the initial two cycles has the most vio- GMD ¼ Exp w lnx = w ð4Þ i i i i¼1 i¼1 lent effect on the disintegration of aggregates, which is consistent with the result by Denef et al. (2001a,b). th where, x is the average diameter of the i aggregates; w i i After 10 times cycles, the aggregates in the size range th is the mass percent content of the i aggregates. of 5–10 mm remain almost no residue. Due to the The slaking ratio of the water–stable aggregate is de- supplement from the disintegration of the aggregates fined as the ratio of the water–unstable aggregates and (5–10 mm) in the first cycle, the proportion of the the total aggregates, measured by dry and wet sieving aggregates (2–5 mm) increases slightly. After that jointly. Generally, the slaking ratios of the water–stable period, the proportion of the aggregates (2–5 mm) aggregates greater than 0.25 mm (SR )and 5mm 0.25 decreases and remains almost no residue eventually. (SR ) are applied to evaluate the degree of breakdown For the aggregates in the diameter of 1–2mm, dry- during wet sieving. ing–wetting cycles first increase the proportion slightly; after this maximum, further drying–wetting m −m ddðÞ >0:25 wdðÞ >0:25 SR ¼ 100% ð5Þ 0:25 cycles decrease the proportion. On the whole, with ddðÞ >0:25 the further drying–wetting cycles, the proportion of m −m the aggregates (1–2 mm) shows almostly no change. ddðÞ >5 wdðÞ >5 SR ¼ 100% ð6Þ We speculate that there are two possibilities: one is ddðÞ >5 that the supplement from slaking of greater aggre- where, m and m are the mass percent gates and the slaking of aggregates with the size of d(d > 0.25) w(d > 0.25) content of the aggregates with the size greater than 1–2 mm reach the equilibrium state; another is the 0.25 mm by dry and wet sieving respectively; m coexistence of breakdown of larger water–stable ag- d(d >5) and m are the mass percent content of the gregates and reunion of smaller water–stable aggregates w(d >5) Xu et al. Geoenvironmental Disasters (2017) 4:20 Page 7 of 13 Table 2 The distribution of grain size of soil aggregates under drying–wetting cycles D–W cycles Mass percent content of soil aggregates in each size range (%) 5–10 mm 2–5mm 1–2 mm 0.5–1 mm 0.25–0.5 mm <0.25 mm 0 33.63 22.14 11.53 7.81 9.76 15.13 1 9.72 22.92 17.12 13.74 13.12 23.39 2 6.86 16.03 15.88 19.22 16.49 25.52 5 3.07 5.93 15.91 26.32 20.22 28.56 10 0.25 2.29 11.69 29.92 23.79 32.06 (Degens and Sparling, 1995). Unfortunately, the results of The figure directly shows that the drying–wetting cycles this research are unable to distinguish. cause the breakdown of soil aggregates gradually. The The aggregates in the diameter of 1–2 mm are in the proportions of WSA and WSA reduce in the loga- 5 0.25 dynamic equilibrium state between the two reverse pro- rithmic form with the increase of drying–wetting cycles, cesses. It can also be seen from Fig. 5b that the propor- and SR and SR increase in the logarithmic form with 5 0.25 tions of water– stable aggregates in the size range of the increase of drying–wetting cycles accordingly. After 0.5–1 mm, 0.25–0.5 mm and <0.25 mm increase with 10 times of cycles, there are almostly no aggregates lar- the increase of the drying–wetting cycles mainly due to ger than 5 mm remained. The results show that the the breakdown of the aggregates with the greater sizes. water stability of soil aggregates has been weakened with In the initial 1–2 cycles, the proportions increase sharply the increase of drying–wetting cycles. The more the dry- and then slow down to be stable. During the drying– ing–wetting cycles are, the higher the slaking ratio of wetting cycles, the supplement from the breakdown of water–stable aggregates is and the less the larger water– larger aggregates holds the dominant position. stable aggregates are. Figure 6a shows the variations of the mean weight The breakdown degree of water–stable aggregates by diameter (MWD) and geometric mean diameter (GMD) exogenic actions has a significant effect on corrosion re- with the drying–wetting cycles. It can be seen that the sistance of earths. Generally, the breakdown mechanism MWD and GMD reduce in the logarithmic form with of water–stable aggregates are as follows: the degrees of the increase of drying–wetting cycles. In the violent water intruding into the pores or cracks are unbalanced, breakdown period, the reduction trends are obvious and making the thickening speed of diffusion layer among then tend to be stable. In the first cycle, MWD and the particles and the condition of repulsive force exceed- GMD both drop by 50%. After 10 times of drying–wetting ing the suction to be unbalanced. These unbalance stress cycles, the MWD and GMD remain only 1/6 and 1/4 of relationships will furtherly cause the stress concentra- the initial values. The stable state of the trend after 10 tion. The breakdown will occur in the surface with the times cycles indicates that the refinement of water–stable maximum difference between the repulsive force and aggregates caused by drying–wetting cycles will not go on van der Waals attractive force (Chinchalikar et al., 2012; endlessly but will be restricted by the ultimate cementa- McBride and Baveye, 2002). Hu et al. (2015) suggested tion stress inside the aggregates. two steps in aggregate breakdown when dried aggregates Figure 6b shows the proportion of water–stable aggre- were re-wetted: (1) separating soil particles in aggregates gates with the diameter greater than 5 mm (WSA ) and to a distance of 1.2–1.4 nm between two adjacent par- 0.25 mm (WSA ) under the drying–wetting cycles. ticle surfaces by the surface hydration forces (the repul- 0.25 The variations of the corresponding slaking ratio of sive force); (2) breaking soil aggregates in a way of water–stable aggregates with the diameter greater than explosion or dispersion when the repulsive force was 5mm (SR ) and 0.25 mm (SR ) are also illustrated. greater than the attractive force. 5 0.25 Table 3 Characteristic parameters of soil aggregates under drying–wetting cycles D–W cycles WSA / % WSA /% SR /% SR / % MWD / mm GMD / mm 5 0.25 5 0.25 0 33.63 84.87 0.00 0.00 3.584 1.766 1 9.72 76.61 71.10 9.73 1.969 0.903 2 6.86 74.48 79.60 12.24 1.552 0.709 5 3.07 71.44 90.87 15.82 0.985 0.513 10 0.25 67.94 99.26 19.95 0.628 0.405 Xu et al. Geoenvironmental Disasters (2017) 4:20 Page 8 of 13 ab Fig. 5 (a) Size distribution of water–stable aggregates under drying–wetting cycles; (b) Mass percent content of soil aggregates in each size range under drying–wetting cycles Impact of initial water content By wet sieving method, the particle size distribution of The initial water content of soil, affected by climate, water-stable aggregates after the breakdown of initial ag- temperature and humidity, has important influence on the gregates with different initial water content is listed in stability of aggregates and slope runoff. The initial water Table 4. The corresponding characteristic parameters of content correlates to the thickness of hydrated films sur- aggregates are listed in Table 5. rounding the soil particles, namely the diffusion layer. The Figure 7a shows the size distribution of water–stable diffusion layer has a weakening effect on the suction aggregates with the initial water content. For the initial among the soil particles, which means the increase of ini- water content of 5.08%, 14.51% and 24.73%, the smaller tial water content will weaken the matrix suction among the aggregate diameter is, the mass percent content the the soil particles and reduce the infiltration of water. In aggregates is. However for the initial water content of addition, the diffusion layer will weaken the structural 29.68% and 37.52%, there is a peak in the size range of links among soil particles or micro–aggregates, causing 0.5–1 mm. There exists a turning point in the diameter the micro–aggregates to be more susceptible to external of approximately 0.3 mm. For the aggregates with the force. In addition, the increase of initial water content will diameter > 0.3 mm, the contents of the water–stable ag- decrease the effective void ratio, thereby narrowing the gregates increase with the increase of initial water content. seepage space and weakening the intrusion effects. By contrast, for the aggregates with the diameter < 0.3 mm, ab Fig. 6 (a) Characteristic diameters of water–stable aggregates under drying–wetting cycles; (b) variations of water stability and slaking ratio of water–stable aggregates Xu et al. Geoenvironmental Disasters (2017) 4:20 Page 9 of 13 Table 4 The distribution of grain size of soil aggregates under different initial water contents Initial water Mass percent content of soil aggregates in each size range (%) content w/% 5–10 mm 2–5mm 1–2 mm 0.5–1 mm 0.25–0.5 mm <0.25 mm 5.08 0.00 0.00 0.74 3.32 4.17 91.77 14.51 0.46 1.58 4.66 7.84 17.26 68.20 24.73 1.23 4.89 12.67 16.28 20.25 44.68 29.68 1.90 5.20 15.82 23.89 17.52 35.67 37.52 3.07 5.93 15.91 26.32 20.22 28.56 the contents of the water–stable aggregates decrease After wet sieving, the proportions of water–stable ag- with the increase of initial water content. Figure 7b gregates with the grain size greater than 5 mm (WSA ) shows the mass percent content of soil aggregates in and 0.25 mm (WSA ) with the initial water content are 0.25 each size range with the variations of initial water con- illustrated in Fig. 8b. The variations of the corresponding tent. It can be seen from the figure that in the soils slaking ratio of water–stable aggregates with the diameter circularly dried and wetted, the water-stable aggregates greater than 5 mm (SR ) and 0.25 mm (SR ) are also il- 5 0.25 with the size smaller than 2 mm play the predominant lustrated. The proportions of WSA and WSA increase 5 0.25 role. We try to explore the effect of initial water con- linear with the increase of initial water content, and SR tent on the distribution of predominate aggregates. We and SR increase linearly with the increase of initial 0.25 define the criterion that if the mass percent content of water content accordingly. The results show that the smaller aggregates (<0.25 mm) is greater than the sum water stability of soil aggregates enhances with the in- of the mass percent content of medium aggregates crease of initial water content. The increase of initial (0.25–1 mm), the soil can be viewed as predominantly water content can obviously strengthen the resistance composed by smaller aggregates. By this criterion, a ability of soil aggregates on rainfall and runoff erosion, hypothesis has been introduced from the figure that reducing the degree of disintegration of aggregates. for any given soil composition and physical environment history, there is a critical water content (about 24%). Soils Mathematical model to estimate leakage ratio which have the initial water content less than the critical After the drying and rewetting by the runoffs, the ori- water content are predominantly composed by smaller ag- ginal aggregates undergo the following breakdown and gregates. However, when the initial water content is movement: (a) the original aggregates crack into smaller greater than the critical water content, the soils are pre- ones or reunite to larger ones; (b) water–unstable aggre- dominated by the medium aggregates. gates are dissolved and washed away by runoffs; (c) the After the breakdown of the soil aggregates by wet siev- water–stable aggregates have the possible to leak ing, the variations of MWD and GMD with the initial through the karst channels by the hydraulic forces water content are showed in Fig. 8a. It can be seen from (Fig. 9). the figure that MWD and GMD increase with the initial The size distribution of water–stable aggregates after water content in the form of quadratic polynomial. breakdown in procedure (a) can be symbolized as follow: When the initial water content is less than 10%, the vari- fdðÞ∈fg P jd ∈fg 7:5; 3:5; 1:5; 0:75; 0:375; 0:125 ations of MWD and GMD with the initial water content i d i are unobvious. When w > 10%, the characteristic diame- ð7Þ ters increase with a higher speed. The higher the initial water content is, the MWD and GMD of the residual where, P is the mass percent content of the water– water–stable aggregates are greater. stable aggregates with the diameter of d . The probability Table 5 Characteristic parameters of soil aggregates under different initial water contents Initial water content w/% WSA / % WSA /% SR /% SR / % MWD / mm GMD / mm 5 0.25 5 0.25 5.08 0.00 8.23 100.00 90.30 0.174 0.140 14.51 0.46 31.80 98.39 61.68 0.382 0.184 24.73 1.23 55.32 95.69 30.12 0.791 0.346 29.68 1.90 64.33 93.32 22.18 0.913 0.428 37.52 3.07 71.44 90.87 15.82 0.985 0.513 Xu et al. Geoenvironmental Disasters (2017) 4:20 Page 10 of 13 ab Fig. 7 (a) Size distribution of water–stable aggregates with different initial water content; (b) Mass percent content of soil aggregates in each size range η of the water–stable aggregates with a diameter of d normalized relative leakage ratios are illustrated in being eroded through the leakage channel can be de- Fig. 10. fined as Eq. (8). Analyzed from the data, the relative leakage ratio (R ) with the drying–wetting cycles (N) can be fitted as follow: η ∝ 1=d ð8Þ Thus, the relative leakage ratio (R) can be evaluated by R ¼ ð10Þ Eq. (9). a þ bN R ¼ ηfdðÞ ð9Þ where, a means the difficulty level of the initial cycle in- creasing the relative leakage ratio; b means the ultimate Selecting the specimen with 5.08% initial water con- leakage ratio with the increase of drying–wetting cycles; tent after 5 drying–wetting cycles as the reference, the c means the rate of rise of the relative leakage ratio. In ab Fig. 8 (a) Characteristic diameters of soil aggregates; (b) variations of water stability and slaking ratio of water stable aggregates under different initial water contents Xu et al. Geoenvironmental Disasters (2017) 4:20 Page 11 of 13 Fig. 9 Breakdown and leakage model for the aggregates in karst areas this research, a equals to 9.7737; b equals to 0.5599; c Conclusions equals to 0.3574. In general, our results suggest that the drying-wetting The relationship between the relative leakage ratio (R ) cycles cause a significant aggregate slaking, especially and initial water content (w) can be fitted as follows: within the first two cycles. However, the extent of the slaking process is significantly reduced with repeated drying-wetting cycles, showing that most aggregates be- Aw þBw < w critical come more slacking resistant. The variation curves of R ¼ ð11Þ Cw þDw > w critical the proportion of water-stable aggregates with the size 1–5 mm firstly increase by drying and wetting to a max- imum value and then decrease, showing a coexistence of where, w is the critical water content, which is con- critical slaking process and supplement. sistent to the value mentioned earlier and equals to 28% We have found that there exists the critical water con- approximately; A, B, C and D are the parameters equal tent (about 24%), with the initial water content less than to −0.0247, 1.1239, −0.0093 and 0.6922 respectively. the critical value soils are predominantly composed by smaller aggregates. A turning point has also been ob- tained that the contents of the water–stable aggregates larger than 0.3 mm increase with the increase of initial water content. Overall, the increase of initial water con- tent strengths the water stability. The mathematical model for the relative leakage ratio based on the drying–wetting cycles, the initial water con- tent and the size distribution of water–stable aggregates may help propose more precise leakage model of loams in karst areas to assess the leakage potential. Further investi- gations are required to consider more reasonable match between the drying-wetting cycles in the tests and in prac- tice, especially in the frequency and intensity. Acknowledgements The work presented in this paper was supported by the National Natural Science Foundation of China (Grant No. 41572285). Authors’ contributions Fig. 10 The relative leakage ratio with the drying–wetting cycles The work presented here was carried out in collaboration between all and initial water content and the fitting curves authors. TYQ, ZJ and XJ defined the research theme. XJ and ZJ designed Xu et al. 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Geoenvironmental Disasters – Springer Journals
Published: Dec 1, 2017
Keywords: Environment, general; Earth Sciences, general; Geography, general; Geoecology/Natural Processes; Natural Hazards; Environmental Science and Engineering
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