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Rainfall plays an important role in affecting the slope stability in a tropical country such as Singapore. The soil in Singapore is mainly residual soil with a deep groundwater table that is commonly found in the unsaturated condition in the presence of matric suction. This paper presents the unsaturated properties of soil with roots of Caesalpinia crista and their effects on slope stability during rainfall. Laboratory tests were carried out to obtain the index properties, saturated properties (i.e. permeability and shear strength) and unsaturated properties (i.e. unsaturated shear strength and soil–water characteristic curve) of soil with and without roots of C. crista. Seepage analyses were performed to obtain the variations in pore-water pressures during and after rainfall. Slope stability analyses were conducted to obtain variations in the factor of safety during and after rainfall. The results indicated that soil with roots of C. crista had a higher saturated volumetric water content, a higher air entry value, a lower saturated permeability and a higher shear strength compared with soil without roots. In addition, the soil with roots of C. crista was effective in reducing the water infiltration into the slope during rainfall. Hence, slope stability can be maintained during rainfall.

During heavy rainfalls, the surface soil on a slope may be eroded and the erosion is much dependent on the tensile strength of soil. In addition, the tensile strength of soil is also one of the governing factors which affect the cracking of soil. It is noted that the water content of soil has a significant effect on the tensile strength of soil. The experimental works for measurement of the tensile strength of soil in laboratory (especially for the unsaturated soil) are challenging, and the indirect estimation techniques draw a lot of attentions among practical engineers. In this paper, the tensile strength of soil is estimated from the soil–water characteristic curve (SWCC) by adopting the concept of pore-size distribution function. The proposed equations were verified with the experimental data from published literature. The comparison results show that the estimated tensile strengths agree well with the experimental results of the selected soils from published literatures. In addition, the variations of SWCC due to different initial densities and fine contents are also considered and the effects of the variation of SWCC on the estimated tensile strength are also investigated and discussed in this paper. It is observed that the modified SWCCs incorporating the effects of different initial densities and fine contents are able to provide a more accurate estimation of the tensile strength of unsaturated soil.

The soil-water characteristic curve (SWCC) contains information regarding the geometric pore space in a soil and is commonly used to estimate unsaturated soil properties, such as unsaturated hydraulic conductivity and unsaturated shear strength. Soil volume change can significantly affect the SWCC and the engineering properties of soil. Different SWCCs can be obtained if the soil specimens are prepared with different initial void ratios. The volumetric shrinkage curve (VSC) is commonly used to convert the SWCC in the form of gravimetric water content (w-SWCC) into a curve that is in the form of degree of saturation (S-SWCC). In this paper, a framework is developed in which different S-SWCCs are generated based on the measured w-SWCC of soil in a relatively loose condition and the VSC. The proposed framework is based on the concept of the pore size distribution function (PSDF). The estimated SWCCs corresponding to different initial void ratios from the proposed framework were verified by using experimental data from published studies.

The understanding of unsaturated soil mechanics principles is of interest to a wide spectrum of geotechnical problems associated with soils above water table and compacted soils. This paper describes the stress state variables and constitutive equations based on the unsaturated soil mechanics principles. In addition, the basic concepts for characterization of unsaturated soils and measurements of matric suction (or negative pore-water pressures) are also explained. The application of unsaturated soil mechanics theories is illustrated through the use of capillary barrier system for minimizing rain infiltration into residual soil slopes. Published version

Many shallow foundations are constructed within the soil layer above the groundwater table, where the soil remains unsaturated, and the failure of shallow foundation is mostly related to shear failure. The shear strength of the unsaturated soil is one of the main engineering properties required in geotechnical designs. Previous researchers suggested that the shear strength of the unsaturated soil depends on matric suction in the soil. The shape of the soil–water characteristic curve (SWCC) has a significant effect on the characteristics of unsaturated shear strength with respect to matric suction. In this paper, a new model was proposed for the estimation of the unsaturated shear strength from SWCC. In this new model, meniscus was considered to transfer soil suction into both additional net normal stress and additional cohesion. Based on the categorization from soil science, water in soil can be categorized into three groups: (1) gravity water, (2) capillary water and (3) hygroscopic water. The elemental analysis on the contractile skin indicated that only the capillary water in the soil can transfer stress into soil skeleton. Consequently, the SWCC is modified by considering capillary water only for the estimation of unsaturated shear strength. In the derivation, unsaturated soil is considered as four-phase material. Finally, a new mathematical equation for the estimation of the unsaturated shear strength was proposed and verified with the experimental data from the published literature. In addition, the proposed equation does not consist of any empirical parameter and can be used to predict the shear strength of unsaturated soil. Accepted version The first author would like to acknowledge the financial supports he received from the National Natural Science Foundation of China (No. 51878160), the National Key Research and development program of China (No. 2017YFC00703408) and the Fundamental Research Funds for the Central Universities (No. 2242018K41046).

During flooding, the suspended sediment transport usually experiences a wide-range of dilute to hyper-concentrated suspended sediment transport depending on the local flow and ground conditions. This paper assesses the distribution of sediment for a variety of hyper-concentrated and dilute flows. Due to the differences between hyper-concentrated and dilute flows, a linear-power coupled model is proposed to integrate these considerations. A parameterised method combining the sediment size, Rouse number, mean concentration, and flow depth parameters has been used for modelling the sediment profile. The accuracy of the proposed model has been verified against the reported laboratory measurements and comparison with other published analytical methods. The proposed method has been shown to effectively compute the concentration profile for a wide range of suspended sediment conditions from hyper-concentrated to dilute flows. Detailed comparisons reveal that the proposed model calculates the dilute profile with good correspondence to the measured data and other modelling results from literature. For the hyper-concentrated profile, a clear division of lower (bed-load) to upper layer (suspended-load) transport can be observed in the measured data. Using the proposed model, the transitional point from this lower to upper layer transport can be calculated precisely.

Soils with two subcurves of Soil-Water Characteristic Curve (SWCC) (dual porosity soils) might be found within various residual soils. Soils located in different depths have different confining pressure. Residual soils are found in the unsaturated zones due to the deep groundwater table. There is a linear correlation between the hydraulic properties of the soil in the unsaturated area and that of its unsaturated properties. This study aims to examine the influence of the confining pressure towards the SWCC of dual porosity soil. The scope of this study involves measurements of the drying and wetting SWCC using Tempe cells, pressure plates, and an advanced triaxial apparatus. In this study, the mathematical equations were developed to explain the effect of confining pressure on SWCC. The experimental results indicated that the dual porosity soil exhibits bimodal characteristics for the drying curve of SWCC and it exhibits unimodal characteristics for the wetting curve of SWCC. As the confining pressure increases, the air entry values, the inflection points, and the standard deviation of drying SWCC increase. In addition, the hysteresis of SWCC is becoming smaller with the increasing confining pressure.

Capillary barrier system (CBS) was developed as a slope protection method to prevent rainwater infiltration into the underlying soil based on the principle of unsaturated soil mechanics by harnessing the distinct difference in hydraulic properties of a fine-grained layer with those of a coarse-grained layer of soils. The CBS is commonly designed and constructed using gravel as coarse-grained material and fine sand as fine-grained material. However, due to scarcity of natural aggregates and in consideration of environmental sustainability, there is a need to utilize recycled materials in capillary barrier system. In this project, coarse and fine recycled concrete were used as the coarse- and fine-grained materials, respectively. The appearance of CBS was enhanced with an additional layer of approved soil mixture (ASM) to incorporate vegetation as green cover. CBS as a sustainable slope cover has been constructed for slope protection surrounding basement carpark in the new public housing development at Matilda, Singapore. The design, construction and monitoring system for the CBS are presented and discussed in this paper. The field measurement data provide verification of the performance of the CBS. Both field measurement and numerical analyses demonstrated that CBS performed well as designed.

The hydraulic properties of soil (i.e. soil-water characteristic curve (SWCC) and coefficient of permeability) govern the moisture flow in it. Previous research has indicated that the hydraulic properties of soil are dependent on its pore-size distribution. An improved capillary model is now proposed to explain the concept of the pore-size distribution in soil and its relationship to SWCC. A new model, the “valve model”, is also proposed as the explanation for water flow in unsaturated soil. The pore-size distribution function is incorporated in the “valve model” and is used to calculate the relative coefficient of permeability for unsaturated soil. In this paper, the role of the pore-size distribution function in the estimation of SWCC and the permeability function are explained. Equations are proposed for estimating the pore-size distribution function from the experimental data of relative coefficient of permeability. The results from the proposed equations agree with the experimental data from laboratory measurement and published data. Accepted version Project supported by the National Natural Science Foundation of China (No. 51878160), the National Key Research and Development Program of China (No. 2017YFC00703408), and the Fundamental Research Funds for the Central Universities (No. 2242018K41046), China.

GeoBarrier system (GBS) is a system that combines geobag wall with capillary barrier concept as stabilization method against rainfall-induced failure, and vegetation as green cover. This paper presents deformation behaviour of a GBS wall constructed as pilot study when subjected to rainfall infiltration. The GBS wall was 4 m high with 70° slope angle. The wall was subjected to uniform surcharge of 10 kN/m2 and rainfall of 22 mm/h for 8 h. Recycled materials were used as components of the capillary barrier. Coupled deformation-seepage finite element analyses were carried out to evaluate the stress and pore-water pressure variations within and behind the GBS wall during and after rainfall. Limit equilibrium analyses were performed at various stages during and after rainfall to assess the near vertical wall stability. The results of the coupled deformation seepage analysis showed that the reinforced zone in the GBS remained unsaturated during rainfall, indicating that the GBS wall performed well in minimizing the rainwater infiltration. However, the GBS wall face deformation pattern altered during and after rainfall, indicating that the rainfall infiltration increased the lateral stress on the top part of the wall. The variation of lateral stress with depth agrees with field measurements from earth pressure cells. Ministry of National Development (MND) Accepted version The findings published in this paper are based on the project Geobarrier System for Use in Underground Structure (Grant No. SUL2013-3) that is supported by a research grant from the Ministry of National Development Research Fund on Sustainable Urban Living, Singapore.

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.

A GeoBarrier system (GBS) is a combination system of reinforced soil walls to stabilize near-vertical cut slopes and capillary barrier principles to protect the wall from the effect of rainfall infiltration. Singapore requires construction materials that are cost-effective to support sustainable urban development. Therefore, recycled materials are utilized as GBS materials to avoid the use of high-cost materials, such as steel or concrete. GBS consists of planting geobags with unique geosynthetic pockets for sustainable plant growth as a facing layer of GBS. The negative pore-water pressure (suction) within the reinforced soil behind GBS was assured to be constant during rainfall since GBS is designed specially to minimize the rainfall infiltration into the reinforced soil. This paper presents the practical design and stability analysis of the GBS, considering the presence of suction within the reinforced soil body. The monitoring of GBS performance in the field was carried out via field instrumentation. Finite element analyses of the GBS under extreme rainfalls were also performed for evaluation of the GBS performance. The field instrumentations and numerical analysis results showed that GBS was able to protect the slope from rainfall infiltration; therefore, the stability of the slope retained by GBS was not affected by the rainfall. Results from the analytical calculation showed that the most critical mode of failure is sliding along the base, followed by the global and local slope stability. The GBS is not susceptible to local instability. Ministry of National Development (MND) Accepted version The findings presented in this paper are based on the project “Geobarrier System for Use in Underground Structure” (Grant No.SUL2013-3) that is supported by a research grant from the Ministry of National Development Research Fund on Sustainable Urban Living, Singapore.

Soil – water characteristic curve (SWCC) is an important property of unsaturated soils that can be used to estimate various parameters to describe unsaturated soil behavior. SWCC is reported to be hysteretic because the water content at a given suction in the wetting process is less than that in the drying process. In order to simulate the hysteretic characteristics of SWCC, many models have been proposed by different researchers. However, majority of the existing models are complex and their parameters are not related to the physical significances of SWCC variables. In this study, the new equations are developed to model drying and wetting SWCC. In addition, some indexes are proposed to estimate the wetting SWCC from drying SWCC. The new equations for SWCCs were evaluated with the laboratory data from published literatures. The results showed that the proposed equations performed well in modelling drying and wetting SWCC. The new equation has less parameters than the existing published equation.

The variation of groundwater table is important information for Civil Engineering and related practices. The most appropriate datasets for the groundwater table mapping are the water level measurements in borehole piezometers. The methods for development of the groundwater table distribution on a continuous surface are affected by the hydrological factors and spatial variability of the soils. In this study, the piezometer data of the groundwater table in soils from four formations in Singapore were used to predict the continuous surface of the groundwater table. The interpolation of groundwater table elevation was performed using geostatistical analyses: inverse distance weighted (IDW) and Ordinary Kriging (OK) interpolations within residual soils from Bukit Timah Granite Formation, Old Alluvium Formation, Jurong Formation, and Kallang Formation. The interpolations were completed based on 80% of dataset (1,282 piezometers for wet season and 2,567 piezometers for dry season). The evaluation of the predicted groundwater table maps was carried out using cross-validation method (CVM) based on 20% of the datasets (256 piezometers for wet season and 512 piezometers for dry season). The results show that the IDW and OK interpolations generated quite similar predictions of the groundwater table distribution ranging between -3.7 m and 58 m elevation with respect to sea level, depending on hydrogeological conditions and soil formation characteristics. The validation method indicated a very good performance of IDW and OK in predicting the groundwater table distribution in Singapore (r2 > 0.8). The proposed procedures and methodologies covering the interpretation of data from piezometers, spatial analyses and evaluation of groundwater table map via geographical information system (GIS) approach presented in this paper will benefit Civil Engineers prior to building construction. The developed groundwater table map will be useful to determine the optimum number of piezometers still required for designs, to design the low cost and effective ground improvement, foundation and retaining wall in order to ensure the stability of the building during and after the completion of the project. Building and Construction Authority (BCA) The authors would like to acknowledge the funding support from Building Construction Authority and the sharing of the data from Singapore Land Authority, who are the collaborators of the project on The Development of Slope Management and Susceptibility Geographical Information System.

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.

The multiphase flow (including liquid flow and air flow) in unsaturated soil is related to many engineering problems such as contaminant transport, rainwater infiltration, and soil-water evaporation. It is proven that water flow in unsaturated soil can be estimated using the concept of the pore-size distribution function. Many models have been proposed to estimate the water flow or water permeability function, kw, from the soil-water characteristic curve (SWCC). On the other hand, a limited model has been proposed to estimate the air flow or air permeability function, ka, from the SWCC. Most of the models used for the estimation of the air permeability functions are empirical, and they are dependent on the empirical parameters. In this paper, the relative air coefficient of permeability was estimated using the concept of the pore-size distribution function. In the method proposed in this paper, no empirical parameters were adopted, and the estimation results purely depended on the soil-water characteristic curve. The proposed method was verified against experimental data from published literature.

Rainfall-induced slope failures are commonly observed in residual soil. The slopes in Singapore are associated with deep groundwater table. As a result, residual soils in Singapore are commonly observed within unsaturated condition above groundwater table. The unsaturated soil properties (i.e. soil-water characteristic curve) are rarely used as parameters in the development of slope susceptibility map. To incorporate important unsaturated soil properties, such as air-entry value (AEV), in the slope stability analysis, the sampled soil data must be transformed into a continuous field output. Geostatistical methods of ordinary kriging (OK) and simple kriging (SK) were used to estimate the spatial variability of air-entry value for soils in Singapore. The effect of soils from different geological formations in Singapore was considered in the analyses. The first case of analysis assumed Singapore as one region whereas the second case of analysis assumed Singapore as two regions – (i) Jurong Formation (JF); and (ii) Bukit Timah Granite and Old Alluvium (BTG & OA). Leave-one-out cross validation and one additional sampling point were used to assess the performance of the kriging methods and determine a suitable method for digital soil mapping. OK and SK had similar root mean square error (RMSE), of 66.6 and 66.4 respectively, for analyses with the assumption of Singapore as one region. For analyses with the assumption of Singapore as two regions, the RMSE using OK was estimated at 32.6 and 85.6 for BTG & OA and JF, respectively. SK estimated the additional sampling point with greater accuracy and was concluded as the better kriging method to estimate the AEV distribution of soils in Singapore as compared to OK method. Accepted version

Many research works have been carried out to develop equations for shear strength of unsaturated soil. However, most of the equations are limited to soils with unimodal soil-water characteristic curves (SWCC). The objective of this study is to propose an equation for estimating the unsaturated shear strength of soils with bimodal SWCCs. Several series of unsaturated consolidated drained triaxial tests were carried out on statically compacted sand–kaolin specimens to evaluate the proposed equation. The experimental results indicated that the ϕb angle of the soil with bimodal SWCC at suctions between the first air-entry value (AEV1) and the second air-entry value (AEV2) was less than the ϕb angle at suctions below AEV1. However, the relationships between shear strength and suction at suctions between AEV1 and AEV2 were still linear. At suctions beyond AEV2, the ϕb angle of the soil with bimodal SWCC started to decrease non-linearly. The evaluation results showed that the estimated bimodal shear strength using the proposed equation in this study had good agreement with the shear strength data from the experimental works. Housing & Development Board Nanyang Technological University Accepted version This work was supported by a research grant from a collaboration project between the Housing and Development Board and Nanyang Technological University (NTU), Singapore. The authors gratefully acknowledge the assistance of the Geotechnical Laboratory staff, School of Civil and Environmental Engineering, NTU, Singapore during the experiments and data collections.

Soil–water characteristic curve (SWCC) is an important unsaturated soil property relating the water content of a soil to soil suction and it is conventionally measured using Tempe cell, pressure plate and salt solution methods. However, these tests are tedious and time consuming. The SWCC measurements using fast and efficient methods are required for engineering designs such as excavation, slope protection, retaining wall and landfill cover designs. This paper describes the testing procedures and apparatuses associated with rapid measurements of a complete SWCC of a residual soil as obtained from combined measurements using a small-scale centrifuge and dew point methods. The SWCC test results obtained using these alternative methods were compared with the SWCC data from Tempe cell, pressure plate and salt solution methods. Shrinkage tests were carried out in this study to incorporate the volume change of soil into SWCC. The experimental data from all SWCC tests were evaluated using first order analysis with 95% confidence interval for determination of upper and lower bounds of SWCC. The analysis results showed that the SWCC data obtained from tests using small-scale centrifuge and dew point methods were in good agreement with those obtained from Tempe cell, pressure plate and salt solution methods. This indicates that the combination of small-scale centrifuge and dew point methods can be used to generate a complete curve of SWCC for the residual soil. In addition, the time required to perform SWCC tests using the alternative methods is shorter than the SWCC tests using the conventional methods. Accepted version

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.

Construction and demolition of buildings together with the reconstruction of roads produce a lot of solid wastes every year. These wastes create problems related to cost and space for disposal, especially for countries with limited land area like Singapore. Therefore, it is important to choose suitable landfill management system (i.e. cover system) for optimization of landfill area in Singapore. Landfill cover offers many geo-environmental benefits, including reducing water infiltration, isolating waste and controlling landfill gases. In Singapore the cover system is located within the unsaturated zone above the groundwater table. Therefore, it is necessary to incorporate unsaturated soil mechanics principles in designing the cover system for the landfill. Soil properties in the unsaturated soil zone affect the rate of wetting front movement from the ground surface. As a result, the rates of changes in pore-water pressures during and after rainfall will vary in accordance with the characteristics of unsaturated soil properties. In this study, the performance of dual capillary barrier (DCB) in minimizing rainwater infiltration into a sanitary landfill in Singapore was investigated. The DCB consists of fine recycled asphalt pavement (RAP) and coarse RAP as the materials for the fine- and coarse-grained layers, respectively. The recycled materials were used in the DCB to reduce the cost associated with the construction of a cover system in the field and to maintain environmental sustainability. Laboratory tests were conducted to characterize the index properties and hydraulic properties (i.e. soil–water characteristic curve (SWCC) and permeability function) of the RAP for the cover system under saturated and unsaturated conditions. The SWCC and permeability function were used in the finite element seepage analyses to study the effect of climate change on the soil cover system. The results from the seepage analyses show that the DCB was effective in minimizing rainwater infiltration into the sanitary landfill. NRF (Natl Research Foundation, S’pore) Accepted Version

The failure of steep slopes during rainfall is commonly associated with a decrease in matric suction in the unsaturated soil zone above the water table. The shear strength characteristics of residual soil under water infiltration have been studied in the laboratory using unsaturated triaxial tests. This paper presents a development of a numerical model for simulating a triaxial shearing-infiltration test to investigate the shear strength characteristics of a compacted kaolin under infiltration condition. Both the hydraulic and mechanical responses of the compacted kaolin are modeled using the commercial software SIGMA/W and in-house software YS-Slope. The numerical analyses result and their validation against laboratory test results are presented and discussed in this paper. The results from the numerical analyses show good agreements with those from the laboratory tests, indicating that the proposed numerical model can be used to simulate the triaxial shearing-infiltration tests in laboratory.

A large amount of construction waste creates problems related to cost and space for disposal, especially for countries with limited land like Singapore. Therefore, it is important to choose a suitable landfill management system (e.g., cover system) for optimization of landfill area in Singapore. In this paper, the performance of a cover system for minimizing rainwater infiltration into a sanitary landfill in Singapore is presented. The cover system was designed using principles of unsaturated soil mechanics. In this study, a residual soil from Old Alluvium with a low coefficient of unsaturated permeability was used in the cover system. Comprehensive geotechnical instrumentation was installed in the landfill area to monitor the pore-water pressure changes within the cover system and landfill during and after rainfall. Laboratory tests were conducted to characterize the index and engineering properties of the residual soil for the cover system under saturated and unsaturated conditions. The finite-e...

Numerous scientific evidence has given credence to the true existence and deleterious impacts of climate change. One aspect of climate change is the variations in rainfall patterns, which affect the flux boundary condition across ground surface. A possible disastrous consequence of this change is the occurrence of rainfall-induced slope failures. This paper aims to investigate the variations in rainfall patterns in Singapore and its effect on slope stability. Singapore's historical rainfall data from Seletar and Paya Lebar weather stations for the period of 1985â2009 were obtained and analysed by duration using linear regression. A general increasing trend was observed in both weather stations, with a possible shift to longer duration rainfall events, despite being statistically insignificant according to the Mann-Kendall test. Using the derived trends, projected rainfall intensities in 2050 and 2100 were used in the seepage and slope stability analyses performed on a typical residual soil slope in Singapore. A significant reduction in factor of safety was observed in the next 50 years, with only a marginal decrease in factor of safety in the subsequent 50 years. This indicates a possible detrimental effect of variations in rainfall patterns on slope stability in Singapore, especially in the next 50 years. The statistical analyses on rainfall data from Seletar and Paya Lebar weather stations for the period of 1985â2009 indicated that rainfall intensity tend to increase over the years, with a possible shift to longer duration rainfall events in the future. The stability analyses showed a significant decrease in factor of safety from 2003 to 2050 due to increase in rainfall intensity, suggesting that a climate change might have existed beyond 2009 with possibly detrimental effects to slope stability. Keywords: Climate change, Rainfall, Seepage, Slope stability