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Rainfall-induced slope failures commonly occur in residual soil slopes. Slope failures are triggered by the reduction in soil strength. This is attributed to the decrease in soil suction due to rainwater infiltration. Slope covers like capillary barrier system and vegetative cover are effective methods that can be used to prevent rainfall-induced slope failures. The capillary barrier system is able to limit the rainwater infiltration, and vegetation can contribute to the increase in soil strength. Vetiver grass is widely planted in tropical and subtropical areas of the world for soil and water conservation. This study investigates the characteristics of unsaturated soil slope covered with capillary barrier system and Vetiver grass in comparison with the original slope through numerical analyses and field measurements. The analyses were carried out under the advanced, normal, and delayed rainfall patterns. The results of the analyses indicated that the capillary barrier system played a more significant role than the Vetiver grass in maintaining slope stability, although both the capillary barrier system and Vetiver grass contributed to the slope stability. In addition, both numerical analyses and field measurements showed that under the delayed and normal rainfall patterns, when antecedent rainwater infiltration could increase the soil moisture, the capillary barrier system performed much better compared to Vetiver grass in maintaining soil matric suctions and slope stability. Housing & Development Board Published version This study was supported by the Housing and Development Board and Nanyang Technological University, Singapore.

Excessive rainwater infiltration can be an important causal agent of both slope and whole tree uprooting failures. Early warnings or stabilization measures on high-risk slopes or trees are critically important. To identify the high-risk areas, it is necessary to conduct seepage, slope and tree stability analyses over a large region. Given the spatial variability of soil properties, a soil database is therefore required before performing distributed or Geographical Information System (GIS) -based water balance and stability analyses. Considering that the unsaturated soil properties could be very different from saturated soil properties, in this study, a soil database containing both saturated and unsaturated hydraulic and mechanical soil properties was developed for the first time. Machine learning methods were used to predict the unknown soil properties. Based on the predicted soil properties, spatial distributions of different saturated and unsaturated soil properties were generated using the ordinary kriging method. Then the soil database was developed with Singapore island being divided into 97 zones, with each zone having similar soil properties. In this study, the importance of different input variables in soil properties prediction was also investigated. In addition to soil plasticity (i.e., Liquid Limit (LL), Plastic Limit (PL) and Plasticity Index (PI)) and grain size distribution (i.e., gravel, sand, and fines fractions), location (i.e., longitude and latitude) was found to be of high importance as well and are recommended to be used as input variables to predict soil properties, especially when data volume is relatively limited. For those soil properties that cover a large range of values, model performance is better when logarithm values were used as the outputs. Moreover, given the possible correlation between some output parameters, the prediction of the Soil-water Characteristic Curve (SWCC) from a multi-output model is recommended after comparing its performance with a single output model. Furthermore, the performance of two commonly used machine learning methods (i.e., random forest regression and artificial neural network) in soil properties prediction were compared and the prediction error resulting from the random forest regression method was generally smaller. The developed database includes the mean values of saturated permeability, saturated and unsaturated shear strength parameters, and SWCC in each zone. The database can be applied in regional GIS-based water balance and slope stability analyses to account for the spatial heterogeneity instead of assuming constant soil properties. National Parks Board The authors would like to acknowledge the support from National Parks Board, Singapore through the “Eye-on-Trees” project.

Many rainfall-induced landslides are reported to be shallow. Therefore, when regional slope stability analysis, or landslide hazard mapping is carried out, simple approaches, such as the infinite slope model, are often used. However, the infinite slope model is known to underestimate the factor of safety due to the absence of boundary effects. More sophisticated methods that account for the boundary effects at the toe and head of the landslide are much more computationally expensive. In this paper upper bound limit analysis (UBLA) is presented with a novel failure mechanism which consists of a translational parallelogram in the middle slope and two log-spiral components at the slope crest and slope toe to capture the boundary effect. The new approach is derived for a full range of pore water pressure conditions and validated by finite element limit analyses. For shallow landslides the translational UBLA is found to outperform the conventional log-spiral UBLA. The results of a large parametric study using the translational UBLA are then used to develop a novel analytical shallow landslide model which retains the simplicity of the traditional infinite slope model, but yet improves accuracy considerably, making this an attractive alternative for routine analysis such as landslide hazard mapping.

The GeoBarrier System (GBS) is designed to improve the urban sustainability with the planting bags within it. The GeoBarrier System (GBS) is a man-made 3-layered cover system comprising an exposed vegetative layer combined with hidden 2-layered unsaturated covered system, which harnesses the distinct difference in unsaturated hydraulic properties between a non-cohesive fine-grained layer and a coarse-grained layer. Previous research works indicated that GBS could be used as an earth retaining structure and slope stabilization system against rainfall-induced slope failures. However, the differential settlement was observed along the approved soil mixture (ASM) geobag layers in the previous study. The objective of this project is to investigate the appropriate modification of ASM layers in order to reduce the deformation of planting GeoBags. The research works involved the mixing of ASM with different percentages of recycled concrete aggregate (RCA) (called structured soil mixture or SSM) to improve the modulus elasticity of ASM layer within the planting geobag. The scope of this study includes the laboratory experiments for saturated and unsaturated soil characterization, numerical analyses and loading tests. The results indicated that the soil mixture with a ratio of 50% ASM and 50% coarse RCA can be used to provide improved resistance to deformation of GBS. Submitted/Accepted version

The liquid water movement is commonly considered in the seepage analysis related to the unsaturated soil (e.g, residual soil above the ground water table). The hydraulic conductivity of unsaturated soil is commonly considered to decrease with the increase in soil suction (or decrease in the degree of saturation). As a result, the estimated hydraulic conductivity of unsaturated soil from the conventional method is very low in the high suction range (i.e., beyond the residual suction). However, the experimental results from recent studies indicate that in the high suction zone, the hydraulic conductivity of unsaturated soil may increase with an increase in soil suction. In addition, recent studies also indicate that in the high suction zone (i.e., greater than 3100 kPa), water in soil moves mainly in the vapour form. As a result, the estimation of the hydraulic conductivity of unsaturated soil due to vapour flow in soil is crucial for engineers in the estimation of water transmission through a relative dry soil. In this note, the theoretical method is proposed for the estimation of vapour conductivity from the soil-water characteristic curve. The proposed method is rational and it has the theoretical basis without any empirical parameter. It is observed that the estimated results from the proposed method agree well with the results reported in the published literature. The authors would like to acknowledge the financial supports he received from the National Natural Science Foundation of China (No. 42030714, 51878160, 52078128), China Huaneng Group Co. Ltd. (No. HNKJ19-H17), and the key laboratory of geotechnical and underground engineering (Tongji university) of Ministry of Education, (No. KLETJGE-B2003).

Soil-water characteristic curve (SWCC) defines the relationship between water content and suction in soil. Many unsaturated properties can be estimated from SWCC such as permeability function and unsaturated shear strength. Therefore, SWCC is considered as the key information of unsaturated soil properties. Permeability function, which defines the relationship between hydraulic conductivity and matric suction, is essential information for seepage analysis in understanding water flow in unsaturated soil. The permeability function is commonly determined indirectly from the SWCC rather than from direct measurement in the laboratory because the direct measurement is time consuming and costly. On the other hand, it has been reported that SWCC can have unimodal and bimodal characteristics. Many models have been proposed for the estimation of permeability function from a unimodal SWCC while limited numbers of model have been proposed for a bimodal SWCC. In this paper, experimental works in laboratory were carried out for measurements of the unsaturated permeability of soils with bimodal SWCC. Zhai and Rahardjo's (2015) equation was used to estimate the permeability function of soils with bimodal SWCC. The experimental results show good agreement with the estimated permeability function. Therefore, Zhai and Rahardjo's (2015) equation was recommended for the estimation of the permeability function from bimodal SWCC. This study also shows that the variability in bimodal SWCC has significant effect on the estimation of the permeability function.

Soils with a bimodal grain-size distribution (gap-graded soils) can be associated with unimodal or bimodal soil-water characteristic curves (SWCCs). Many equations have been developed to estimate SWCCs using grain-size distribution curves to overcome the high cost and long duration of SWCC experiments. Most of the equations are limited to the estimation of the SWCCs of soils with a unimodal grain-size distribution. Few studies have been conducted on the estimation of unimodal SWCCs for gap-graded soils. In this paper, procedures, equations and computer codes are proposed for estimating the unimodal SWCCs of gap-graded soils. The proposed equations are found to perform well in estimating the unimodal SWCCs of gap-graded soils.

Soil shear strength is a critical parameter in slope stability. Shear strength properties may vary significantly over large areas. Thus, the spatial estimates of shear strength properties are necessary for deterministic slope susceptibility mapping over large areas. However, measurements of shear strength parameters are often limited as compared to other soil properties such as Atterberg limit, bulk density and grain size distribution. Multivariate methods have been shown to improve prediction accuracy, but these methods have rarely been used to predict shear strength. In this study, attempts were made to evaluate the effectiveness of using the aforementioned soil properties in predicting the spatial variation of shear strength properties: effective cohesion (c’) and effective friction angle (ϕ’). The performance of ordinary kriging (OK), Random Forest (RF) and regression kriging (RK) in predicting c’ and ϕ’ of residual soils in Singapore were compared and evaluated. In addition, the sensitivity of the three methods to the sample size was investigated. The results of RF analysis revealed that the northing coordinate and percentage of fines were the most important variables for predicting ϕ’. The spatial coordinates and ϕ’ were also important variables for predicting c’. The predicted c’ and ϕ’ using RF and RK resulted in higher spatial heterogeneity than OK. Overall, RF had the smallest error as compared to OK and RK in predicting c’ and ϕ’ at all sample sizes, except for the prediction of ϕ’ using the largest sample size. This study also showed that RF and RK were more sensitive to sample size than OK. These results highlight the benefits of using auxiliary variables when mapping shear strength properties. Submitted/Accepted version

The occurrence of rainfall-induced slope failures is mainly due to the infiltrated rainwater which reduces the shear strength of soil. The slope is initially observed in an unsaturated condition and it becomes wetted during rainfall. In other words, the infiltration of rainwater into the soil slope is a wetting process. As a result, the amount of the infiltrated rainwater is mainly governed by the hydraulic conductivity of the unsaturated soil under the wetting process. Direct measurement of the hydraulic conductivity of unsaturated soil under the wetting process is time-consuming and costly. On the other hand, there are various available methods or models for the estimation of hydraulic conductivity of unsaturated soil under the drying process. In this paper, the methodology for the estimation of hydraulic conductivity function (HCF) of unsaturated soil under the wetting process based on the drying soil-water characteristic curve (SWCC) is proposed. The proposed method is based on the concept of the pore size distribution function. Both “ink-bottle” and “rain-drop” effects on the hysteresis of SWCC are incorporated in the estimation of the HCF of unsaturated soil under the wetting process. The estimated results were verified with the experimental data from published literature.

Residual soil covers most of the land in Singapore and is commonly found in unsaturated conditions due to the deep ground water table. Matric suction in residual soil plays an important role on the stability of residual soil slopes against rainfall. Matric suction in the soil will change due to infiltration during rainfall, and infiltration is mainly controlled by the unsaturated hydraulic properties (i.e., soil-water characteristic curve (SWCC) and hydraulic conductivity) of the residual soil. Variability in SWCC and hydraulic conductivity of residual soils in Singapore, such as Jurong Formation, Bukit Timah granite and Old Alluvium, were quantified and compared with the uncertainty of saturated soil properties. A new framework for the estimation of SWCC from the saturated hydraulic conductivity, ks, is proposed in this paper. The upper bound from the proposed framework is recommended for the infiltration analyses and the lower bound from the proposed framework is recommended for the slope stability analyses.

It is known that the soil-water characteristic curve (SWCC) of unsaturated soils is hysteretic. The engineering properties of unsaturated soils during the drying process are different from those during the wetting process, and the difference can be related to the hysteresis of the SWCC. Therefore, understanding the hysteresis of the SWCC is crucial in engineering practices. In this paper, both the “rain-drop” and “ink-bottle” effects on the hysteresis of the SWCC are quantified with mathematical equations. In the proposed equations, it is assumed that soil volume changes during the drying and wetting processes are negligible, which is a reasonable assumption for most sandy soils. Based on the proposed theory, the wetting scanning curves were estimated from the main drying curves. The results from the method proposed in this paper have been compared with experimental data from the published literature. Satisfactory agreements were found between the measured and estimated wetting scanning curves. Consequently, the method for estimating the wetting scanning curves from the fitting parameters of the SWCC by using the equations proposed in this paper is recommended.

Coarse-grained soil, such as aggregate and coarse sand, is commonly selected to construct a capillary barrier system (CBS). The barrier effect in the CBS is mainly dependent on the hydraulic properties of the coarse-grained soils. It is noted that direct methods (such as experimental measurement) for the determination of the hydraulic properties (including the soil-water characteristic curve (SWCC) and coefficient of permeability) of soil are time-consuming and costly. As a result, indirect methods for the determination of the SWCC and permeability function of soil are commonly adopted by researchers and engineers. In this paper, a new method is proposed for the estimation of the SWCC from the grain-size distribution (GSD) of coarse-grained soils. The proposed method is based on the assumptions that the soil does not experience any volume change and the hysteresis of the SWCC is ignored. A contact angle larger than zero is adopted in this study. The results computed from the proposed model agree well with the experimental data from published studies.

Green technology, an integrated design approach that combines vegetation and engineering design methods, can be applied to improve slope stability. Orange Jasmine is a small tropical evergreen shrub which has deep root systems and is considered to be a drought-tolerant plant that adapts well to a wide range of climatic and soil conditions. It can also grow in infertile soils, limestone soils or loam. Vetiver grass has been widely cultivated in many tropical and subtropical regions of the world for soil and water conservation, land rehabilitation, and embankment stabilization. Vetiver grass has deep roots (2–4 m) and adapts well under extreme conditions of temperature, soil, moisture, soil acidity, and alkalinity. The role of Orange Jasmine and Vetiver grass in minimizing rainwater infiltration, for improving the stability of slopes, was investigated on a soil slope in Singapore with its tropical climate. Two slope sections, covered with Orange Jasmine and Vetiver grass, were instrumented with tensiometers, installed at different depths within the slope, and a rainfall gauge. The instruments were connected to a real-time monitoring system to study the pore-water pressure, the rainfall, and the groundwater level in the slope throughout a one-year period. The pore-water pressure characteristics within the slope sections covered with Orange Jasmine and Vetiver grass are analyzed and presented in this paper. The analyses indicate that both Orange Jasmine and Vetiver grass played a significant role in reducing rainwater infiltration into the slope, minimizing the loss of matric suction, and hence, the shear strength of the soil during rainfall and, as a result, maintained the stability of the slope. Vetiver grass and Orange Jasmine appeared to be similar in effectiveness in terms of reducing the rainwater infiltration into the slope.

Many geotechnical analyses commonly ignore the flux boundary conditions at the ground surface to avoid the complexity in determining the actual flux values at the soil-atmosphere interface. However, actual flux boundary conditions can be an important input to many geotechnical analyses, especially those related to unsaturated soils. Therefore, the effects of flux boundary conditions on pore-water pressure distribution in a residual slope are investigated in this paper. The study focuses on infiltration and evaporation processes. The characterization of climatic data in Singapore is presented in this paper to illustrate the quantification of flux boundary conditions. Evaporation rate for the seepage analyses is calculated using Penman's method. The classification of Singapore climate is carried out using Thornthwaite index. The typical differences between air and soil temperatures in Singapore are presented in this paper using case study from a residual soil slope at Yishun, Singapore. Finite element seepage analyses were conducted to investigate the effect of rainwater infiltration and evaporation on pore-water pressure distribution of a residual soil slope at Jalan Kukoh, Singapore. The numerical analysis results show good agreement with those obtained from field measurements if the evaporation is applied in the analyses during the drying process. This indicates that rainfall and evaporation play important roles in pore-water pressure changes within soil slope.

Rainfall-induced slope failures are commonly observed in residual soil. Due to weathering, the residual soil properties vary with depths, especially in tropical countries, such as: Singapore. Therefore, it is important to characterize the properties of residual soil with depth. Index properties, soil–water characteristic curve and saturated and unsaturated shear strength tests were carried out on residual soils from sedimentary Jurong Formation, Bukit Timah Granite and Old Alluvium in Singapore. The variations of residual soil properties in Singapore were determined from the laboratory test results and evaluated as a function of soil inherent variability. Typical, upper and lower bounds of soil properties for the residual soils in Singapore were described using confidence interval approach and coefficient of variation (COV) in this paper. The variations in residual soil properties can be incorporated in design based on risk or reliability approach. The COV of index and engineering properties of residual soils in Singapore indicate that residual soils from Bukit Timah Granite and Old Alluvium are coarser than residual soil from sedimentary Jurong Formation. The particle size distribution of residual soil from Old Alluvium is more uniform than that from Bukit Timah Granite. On the other hand, the particle size distribution of residual soil from Bukit Timah Granite is more uniform than that from sedimentary Jurong Formation. The shear strengths of residual soils from Bukit Timah Granite and Old Alluvium are higher than that from sedimentary Jurong Formation.