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In the conventional method for the estimation of the hydraulic conductivity of unsaturated soil, only the capillary water is considered in the estimation. However, it is known that a water film can be formed around soil particles due to the adsorptive force. The water may migrate in the soil through the water film. As a result, both water flow in the capillary water and water film should be considered in the estimation. In this paper, a new equation was proposed for the estimation of the hydraulic conductivity of unsaturated soil by considering the film flow. The proposed method was verified with the experimental data from the literature. It is recommended that both capillary water flow and film flow should be considered in the estimation of the hydraulic conductivity of unsaturated soil.

Many high-rise buildings and bridges in the worlds are constructed using bored pile. To provide a safe design of this type of deep foundation, high-quality soil data are needed which are normally obtained from laboratory testing. However, the contractor and the consultant often conduct only a limited field testing without performing laboratory tests. As a result, the capacity of bored pile may deviate from the real value. This study aims to develop a framework for analysis and design of axial and lateral capacity of bored piles when only a few soil parameters are known. The proposed methodology uses analytical methods backed by numerical calculations according to relevant code of practice. The results indicate that as long as the analytical results are supported by the numerical calculations, a robust design can be achieved with only a few soil parameters: a standard penetration test and a cone penetration test.

The soil–water characteristic curve (SWCC) is one of the most crucial and fundamental soil properties in unsaturated soil mechanics. Many theories and equations have been developed to describe and best fit SWCC with unimodal or bimodal characteristics. In this study, a general best-fitting equation for SWCC with multimodal characteristics (bimodal and trimodal SWCC) is proposed. The parameters in the proposed equation are closely related to the properties of soil and variables in the SWCC. To evaluate the performance of the proposed equation in best-fitting multimodal SWCC that has more than two modalities, a trimodal SWCC soil mixture that consists of sand, kaolin and activated carbon has been prepared and tested in the laboratory in this study. The proposed equation has been evaluated with data from published literature and the newly developed soil mixture of trimodal SWCC. The proposed equation has shown high accuracy for best-fitting bimodal SWCC and trimodal SWCC. Nanyang Technological University Submitted/Accepted version This study was supported by a research scholarship from the Nanyang Environment & Water Research Institute (NEWRI) and Nanyang Technological University, Singapore.

Soil-water characteristic curve (SWCC) correlates the water content of a soil to its soil suction, which is a very important property of unsaturated soils. Osmotic tensiometers (OTs) have shown their capability in high soil suction measurement. This study tried to use OT combined with a pressure plate and WP4C dewpoint potentiometer to determine the SWCC of a residual soil. Three OTs with different measuring ranges (900 kPa, 1200 kPa, 2300 kPa) were prepared for soil suction measurement. The temperature effect on the pressure variations of OTs was illustrated based on Flory-Huggins polymer theories and an appropriate calibration equation was proposed to eliminate the temperature effect on the accuracy of soil suction measurement using OT. The OT showed a fast response in soil suction measurement and the equilibrium can be established in 10–15 min during SWCC measurement. Comparison of the SWCC data obtained from the pressure plate, OT, and WP4C dewpoint potentiometer proves that the OT had good performance in the determination of SWCC, especially for the transition zone (i.e., 10–1500 kPa suction range), by providing more data points to define the SWCC in a shorter period. The results show that the usage of OT will shorten the time required for the determination of SWCC and could be considered as a new and reliable technique in SWCC measurement. Submitted/Accepted version

Negative skin friction (NSF) effect on pile foundation design attracts the attention of geotechnical engineers and requires comprehensive research. However, the ways of consideration of NSF for the pile foundation design vary with different design codes. This study aims to compare design code requirements adhering to Construction Codes and Regulations (i.e., SNiP), Eurocode 7 (EC7), Canadian Highway Bridge Design Code (CHBDC), and AASHTO for the pile foundation design subjected to NSF and provide insights for the understanding effect of NSF on the behavior of pile foundation. The overall objective is achieved by designing three cases of a driven pile for given design conditions in Astana city. Then, a sensitivity analysis was performed to estimate the effects of the dimensions of the pile foundation and the shear strength of soils on the bearing resistance. The comparative analysis shows that pile foundation design adhering to CHBDC and AASHTO results in a more conservative strategy than SNiP and EC7 when considering Kazakhstani soil engineering conditions.

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.

A slope susceptibility map which incorporates unsaturated soil mechanics is imperative to accurately locate areas susceptible to rainfall-induced slope failures. This study aimed to develop slope susceptibility maps for zonations within Jurong Formation residual soils in Singapore with the incorporation of unsaturated soil parameters. Slope susceptibility maps were developed through 1-D seepage analyses using Transient Rainfall Infiltration and Grid-Based Regional Slope-Stability Model (TRIGRS) and 3-D slope stability analyses using Scoops3D. Laboratory testing of soil properties and 2-D seepage and slope stability analyses were conducted to evaluate the results from the slope susceptibility map. Comparison between the slope susceptibility map and the results from 2-D analyses showed that the factors of safety from the slope susceptibility map were consistently lower than those of 2-D analyses. Therefore, the proposed method incorporating unsaturated soil in the development of a slope susceptibility map was concluded to be reasonable on the conservative side. Building and Construction Authority (BCA) Submitted/Accepted version The authors would like to acknowledge the funding support from the Building Construction Authority and the sharing of data from the Singapore Land Authority, who are collaborators in the project on ‘The development of slope management and susceptibility geographical information system’.

Residual soils are commonly found in unsaturated condition, and they are associated with high variability. It is necessary to develop a soil database for simplicity of geotechnical analyses and design with respect to cost optimization. This study focused on developing a framework for creating a soil database based on boreholes from different projects, locations and time in Singapore. The soil database was developed utilizing user-friendly and low-cost platforms, i.e. Microsoft Access and Visual Basic Application (VBA). The developed soil database was used to generate saturated and unsaturated soil properties envelopes in Singapore. The developed soil properties envelopes were evaluated by the results from experimental works in this study. The five-parameter logistic curve was chosen as the best-fitting function to develop envelopes with reference to the boxplot. The analyses indicated that soils located at deeper depths have lesser percentage of fine particles which results in a higher saturated permeability, effective friction angle and unsaturated ϕb angle. The results from statistical analyses indicate certain upper limit, typical and lower limit for air-entry value (AEV) for residual soils from Jurong Formation, Bukit Timah Granite and Old Alluvium. Residual soils from each formation located at the ground surface produced the highest AEV while soils at deeper depths exhibit a lower AEV. The First Author appreciates the financial support from the Nazarbayev University, Collaborative Research Project (CRP) Grant No. 11022021CRP1512.

Compared with conventional water-filled tensiometers, polymer-filled osmotic tensiometers (OTs) can measure a higher matric suction by increasing the osmotic pressure of the water inside the tensiometer. The measuring range of OT is equal to the osmotic pressure of water inside the OT. All previous research used commercial polymers in developing OTs and the mechanism of long-term pressure decay of OTs was not found. In this study, polymers including polyacrylamide (PAM) and sodium polyacrylate (NaPA) with different degrees of crosslinking were synthesised by UV polymerisation to develop new OTs. Long-term pressure variations of OTs indicated that the pressure decay was more likely due to the stress relaxation of polymer materials instead of polymer leakage. PAM-filled OTs have a faster stress relaxation behaviour than NaPA-filled OTs so that they can keep constant pressure (i.e., constant measuring range) for a long time and there is no need for pressure correction based on the pressure decay rate. However, the measuring range of PAM-filled OTs is lower than that of NaPA-filled OTs. The result of matric suction measurements demonstrated that the proposed OTs can be used to measure matric suction both in low and high ranges with fast response and high accuracy. Nanyang Technological University National Parks Board Submitted/Accepted version The authors would like to acknowledge the financial support from Nanyang Environment and Water Research Institute (NEWRI), Interdisciplinary Graduate School, Nanyang Technological University, Singapore, as well as the National Parks Board, Singapore.

This paper presents the spatial distribution, variation and trend of 5-day antecedent rainfall in Singapore based on rainfall data from 22 meteorological stations. The effect of climate was analyze...

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).

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.

This study presents a development of a multi-layer perceptron (MLP) model to spatially estimate and analyze the variability of effective cohesion for residual soils that are commonly associated with rainfall-induced slope failures in Singapore. A number of soil data were collected from the various construction sites, and a set of qualified Nanyang Technological University (NTU) data were utilized to determine a criterion for data selection. Four index properties (i.e., percentage of fines and coarse fractions, liquid and plastic limits) were used as training parameters to estimate the effective cohesion of residual soils from different geological formations. Ordinary kriging analyses were carried out to analyze the spatial distribution and variability of effective cohesion. As a result, the appropriate effective cohesions can be estimated using the MLP model with the incorporation of the selected values of measured effective cohesion as training data and four index soil properties as input data. In the combination of estimated and measured effective cohesions, the spatial analysis using Kriging method can clearly differentiate the variations in effective cohesion with respect to the different geological formations. Building and Construction Authority (BCA) Submitted/Accepted version 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 collaborator of the project on The Development of Slope Management and Susceptibility Geographical Information System.

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

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.

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.

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.

Rainfall-induced slope failure is a natural hazard that occurs in many parts of the world. To identify areas where rainfall-induced slope failure may potentially occur in Singapore, a slope susceptibility map of needs to be developed. Majority of the slope failures occurred in residual soils during or after torrential rainfall. In Singapore, residual soils are generally unsaturated because of the deep groundwater table. Hence, it is important to incorporate unsaturated soil properties (i.e. soil–water characteristic curve) in the development of slope susceptibility map in Singapore. In this study, spatial distributions of saturated and unsaturated soil properties were estimated using ordinary kriging. Seepage analysis of zonation in the residual soil from Bukit Timah Granite was carried out using Transient Rainfall Infiltration and Grid-Based Regional Slope-Stability Model (TRIGRS), a publicly available software. The resulting pore-water pressures from the seepage analyses were incorporated into Scoops3D for three-dimensional slope stability analyses. Most areas in the investigated zone were found to have high factors of safety. Only 0.054% of the investigated zonation was associated with very low factors of safety. Nanyang Technological University Accepted version This work was supported by Nanyang Technological University under the Undergraduate Research Experience on CAmpus (URECA) programme.

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.

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.

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

Real-time monitoring can be used to assess the effect of climatic conditions on slope stability. However, this can be costly. There is therefore need for a methodology to select critical slopes for stability monitoring using comprehensive field instrumentation throughout the year. In this study, the Transient rainfall infiltration and grid-based regional slope stability (TRIGRS) model was used to generate a slope susceptibility map (SSM) of Singapore. This model was selected as it can incorporate unsaturated soil properties in spatial analyses of water infiltration and factor of safety (FoS) calculations. Available studies on establishing SSMs by incorporating unsaturated soil mechanics principles are limited and most focus on mountainous areas. This paper presents relatively new research on the development of a SSM for a densely populated area like Singapore. The study area was limited to residual soil from Bukit Timah Granite in Singapore. TRIGRS helped to determine zones that are highly susceptible to failure; the instrumentation can be used to monitor high-risk areas during dry and rainy periods. A selected slope was instrumented with tensiometers, moisture sensors, piezometers and a rain gauge, all connected to a data logger for real-time monitoring. The changes in pore-water pressure and soil moisture with time during wet and dry periods were used for slope stability analyses. FoS variations over time can be used to assess slope stability, especially during periods of heavy rainfall. Building and Construction Authority (BCA) Accepted version The work was supported by research funding from Building Construction Authority (BCA), Singapore.

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.

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.

Rainfall, hydrological condition, and geological formation of slope are important contributing factors to slope failures. Parametric studies were carried out to study the effect of groundwater table position, rainfall intensities, and soil properties in affecting slope stability. Three different groundwater table positions corresponding to the wettest, typical, and driest periods in Singapore and four different rainfall intensities 9, 22, 36, and 80 mm/h were used in the numerical analyses. Typical soil properties of two main residual soils from the Bukit Timah Granite and the sedimentary Jurong Formation in Singapore were incorporated into the numerical analyses. The changes in factor of safety during rainfall were not affected significantly by the groundwater table near the ground surface due to the relatively small changes in matric suction during rainfall. A delay in response of the minimum factor of safety due to rainfall and a slower recovery rate after rainfall were observed in slopes from the sedimentary Jurong Formation as compared to those slopes from the Bukit Timah Granite. Numerical analyses of an actual residual soil slope from the Bukit Timah Granite at Marsiling Road and a residual soil slope from the sedimentary Jurong Formation at Jalan Kukoh show good agreement with the trends observed in the parametric studies. Accepted version

Construction and demolition of buildings together with the reconstruction of roads and pavements produce a lot of crushed concrete and asphalt waste. Problems associated with the disposal of these waste materials can be solved by recycling them for geotechnical engineering purpose. Recycled concrete aggregate and reclaimed asphalt pavement can be used as replacement for natural aggregates in road constructions, landfill capping systems, retaining walls, drainage works and slope protections. Many studies have been carried out to study the characteristics of recycled materials under saturated conditions. However, only few studies have been conducted to study the water characteristic curve (WCC) and permeability of recycled materials under unsaturated conditions. In addition, the shear strength of recycled materials under unsaturated conditions has not been fully investigated. Therefore, this paper focuses on the unsaturated properties of recycled materials, in particular, wetting WCC, wetting permeability function and unsaturated shear strength of recycled materials. In addition, the drying WCC, drying permeability function and saturated shear strength of recycled materials are also discussed. The experimental data of wetting WCC of recycled materials are compared with the predicted wetting WCC obtained using published equations. The experimental data of the saturated and unsaturated shear strength of recycled materials are fitted with nonlinear shear strength equations. In general, the investigated recycled materials have similar characteristics with natural aggregates under saturated and unsaturated conditions. The finer recycled concrete aggregate and reclaimed asphalt pavement have smaller particles sizes, higher water-entry values, lower permeability, lower values of minimum ϕ’ at failure and maximum ϕ b as compared with the coarser recycled concrete aggregate and reclaimed asphalt pavement.

Soil–water characteristic curve (SWCC) is the most fundamental and important soil property in unsaturated soil mechanics. It has been used for analyzing slope stability due to the infiltration of rainfall into slopes and water flow in unsaturated embankments. Generally, SWCC is obtained by laboratory tests. However high cost, long duration and difficulty of the tests impede the application of unsaturated soil mechanics to practical design or analysis. Therefore, several equations have been developed to predict the SWCC using grain-size distribution (GSD) curve. However, most of the equations were limited to soils with unimodal characteristics and the parameters of the equations are not related to the physical properties of the soil. In this paper, an equation to predict SWCC for soils with bimodal characteristics is proposed. The parameters of the proposed equation are related to the physical properties of soil and the variables of SWCC closely. The proposed equation is evaluated with data from the literature and laboratory tests carried out in this study. In addition, the computer codes for the computation of the predicted bimodal SWCC are presented.

Soil–water characteristic curves (SWCCs) of compacted mixtures of residual soil and gravel with different particle sizes were determined from laboratory tests. The study involved compacted mixtures of 50% residual soil by dry mass and 50% gravel of different particle sizes. The compacted mixtures were gap-graded in grain–size distribution with bimodal characteristics, but their SWCCs did not exhibit a bimodal characteristic. The gap-graded nature of the grain–size distribution of the mixtures did not affect the conventional sigmoidal shapes of the SWCCs. The pore-size distribution of the mixtures indicated that large pores within the compacted mixtures were filled with fine-grained soils. The air-entry value of the compacted mixtures was found to be high due to the high dry density of the specimens. The different particle sizes of the gravel used in the mixtures resulted in a variation of air-entry value of the mixtures. The dry density and coefficient of uniformity of the grain–size distributions were found to affect the air-entry values of the mixtures and the SWCC fitting parameters n and m, respectively. The variation of SWCC fitting parameters n and m showed significant correlations between grain–size distribution and pore-size distribution of the mixtures, whereas the SWCC fitting parameter a was not the air-entry value of soil.