• Energy Research

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.

Soil suction plays an important role in governing the stability of slopes. Environmental sustainability could be jeopardized by hazards, such as slope failures (forest destruction, landscape alteration, etc.). However, the quantification of the suction effect on slope stability is a challenging task as the soil suction is usually affected by the precipitation and evapotranspiration. Numerical simulation plays an important role in the estimation of contour in soil suction due to rainfall and evapotranspiration as long-term and widespread monitoring is rarely conducted. The result of numerical simulation is highly dependent on the accuracy of the input parameters. Hence, suction monitoring plays an important role in verifying the result of numerical simulation. However, as a conventional tensiometer is limited to 100 kPa soil suction, it is hard to verify the performance of numerical simulation where suction is higher than 100 kPa. The osmotic tensiometer developed by Nanyang Technological University (NTU) can overcome this problem. It is now possible to monitor changes in soil suction higher than 100 kPa (up to 2500 kPa) for an extended period in the field. In this study, a procedure was proposed to estimate suction changes in residual soil based on rainfall and evapotranspiration data. Numerical simulation was carried out based on the soil properties and geometry of a residual soil slope from Jurong Formation Singapore. Changes in soil suction due to rainfall and evaporation were simulated and compared with the readings from the NTU osmotic tensiometers installed at 0.15 m and 0.50 m from the slope surface in the field. It was observed that numerical simulation was able to capture the variations of suctions accurately at greater depths. However, at shallow depths, erratic suction changes due to difficulties in capturing transpiration.

Rainwater infiltration is primarily governed by the soil-water characteristic curve (SWCC) and hydraulic conductivity function (HCF) of soil. Both the SWCC and the HCF are hysteretic during the drying and wetting processes. In a numerical simulation, different seepage results can be obtained by incorporating different hydraulic conductivity functions of soil. In practice, the wetting HCF is commonly estimated from the wetting SWCC using the statistical method, which is named HCFswcc,w in this note. However, there is no study that has verified the results from seepage analyses using HCFswcc,w. Therefore, the objective of this study is to investigate the influence of wetting SWCC and wetting HCF on 1-D water infiltration. The results from the numerical simulations were verified with the instrumentation reading from a soil column. It was observed that the results from the model using wetting HCFPSDF, which defines the wetting HCF estimated using the concept of pore-size distribution function, gave better agreement with the instrumented data. Therefore, both wetting SWCC and wetting HCFPSDF are advised to be used as input information for the numerical simulation of rainwater infiltration.

This study investigates the face stability of small curvature shield tunnels during excavation and its relationship with various excavation parameters. The stability of the excavation face is critical to the safety and efficiency of underground construction projects. Despite the increase in the use of small curvature shield tunnels in urban areas, research works on this type of tunnel are limited and the existing literature focuses only on straight shield tunnels. This study addresses this research gap through numerical simulations, analyzing the effects of different excavation parameters such as jacking force, cutting speed, and soil conditioning on face stability. The results of the study show that the excavation parameters significantly affect face stability. The findings can be used to optimize the performance of small curvature shield tunnels and support their continued development in urban areas.

Post-grouting is a widely used technology for ground improvement and the strengthening of bored pile foundations. In situ experiments have shown that post-grouting is effective in improving the bearing capacity of bored piles. However, due to the cost and complexity of the in situ tests, the total number of test piles is limited. Therefore, it is not surprising that uncertainty is always associated with the collected data. This may significantly affect the conclusion of analyses, and most current studies have neglected this effect. In this paper, statistical analyses were carried out in order to investigate the effect of uncertainties in the data and the impact of post-grouting on the bearing behaviors of bored piles. The results show that post-grouting can significantly improve the bearing behavior of piles. This study is expected to provide technical guidance for post-grouting in foundation engineering.

It is known that rainwater infiltration is crucial for rainfall-induced slope failure because the infiltrated water can significantly weaken the shear strength of unsaturated soil. Horizontal drains are commonly used to provide appropriate drainage for the rainwater that percolates out of a slope. However, the effects of the length and location of the subsoil pipe on the performance of horizontal drains have not been extensively investigated. In this paper, a parametric analysis by using a numerical model was adopted to investigate the distribution of pore-water pressure in a slope. The results reveal that an inclination angle of 10–15 degrees and strategic placement at the slope toe and mid-slope provide optimal drainage performance, as compared to the effect of pipe length. Multi-layers of horizontal drain (based on 10 m length) are recommended for slopes with a height of more than 15 m.

The van Genuchten (1980) (VG) model is one of the most popular and widely used equations to achieve the best fitting of experimental data of the soil-water characteristic curve (SWCC) test. As the best-fitted SWCC is determined from regression analysis, there are various possible solutions for the best-fitted curve (or the determined fitting parameters). The residual error, which is obtained from regression analyses, can be considered as an index indicating the incapability of the SWCC equation in representing the measured SWCC experimental data. As a result, there is always uncertainty associated with determined SWCC from the regression analysis. The residual error from the regression analysis can be used for the quantification of the uncertainty of determined SWCC. In this paper, Taylor’s expansion is adopted, and only the first-order term is considered for the VG model. Subsequently, the variances of the fitting parameters in the VG model are estimated from the residual error. Consequently, the uncertainty of SWCC using the VG model is quantified by using the upper and lower bounds with a 95% confidence level. The artificial soil, composed of mixed sand and kaolin, was prepared for the SWCC measurement. This paper shows that the proposed method can be used to quantify the uncertainty of SWCC from different specimens (a total of 12 specimens).

The presence of unsaturated soil is critical in geotechnical engineering since the matric suction may aid in accommodating the pile shaft capacity. The design of piles can be optimized by incorporating unsaturated soil mechanics principles. Hence, the amount of waste materials can be reduced, the duration of pile installation can be expedited, and the amount of energy used for casting the pile can be optimized, resulting in more sustainable design and construction of piles. Conventional α, β, and λ methods and modified α, β, and λ methods are the common models that are used for calculating the shaft capacity by incorporating soil–water characteristic curves (SWCCs). However, in our opinion, we feel that the investigation of the influence of seepage infiltration due to rainfall on the shaft capacity of piles, calculated using both analytical means and numerical analysis, has been dealt with inadequately in past studies. The objective of this study is to investigate changes in the shaft pile capacity according to suction changes due to rainwater infiltration for the greater reliability of the pile design, using both analytical and numerical studies with the finite element method (FEM). Sand and kaolin, which are typical components of coarse-grained and fine-grained soil, are used in this study. The laboratory results were incorporated into PLAXIS 3D (Version 22), and a coupled analysis was carried out, utilizing the meteorological conditions in Astana. The results showed that the decreases in matric suction in sand and kaolin are similar after their subjection to rainfall, yet sand produces a higher shaft capacity compared to kaolin. The modified β method offers a higher shaft capacity compared to the other methods due to the effective stress factors being taken into account. The modified α and λ methods are recommended for short piles because they are more sustainable, whilst the modified β method is preferable for long piles. Overall, unsaturated soil conditions should be applied to optimize the foundation design since they generate a higher shaft capacity.