Geobarrier system for use in underground structure

Abstract

Rainfall-induced slope failures are a common problem in many tropical areas that are covered by residual soils such as Singapore. Tropical residual soils are known to be complex and their behavior does not follow the classical saturated soil mechanics because these soils are often unsaturated in nature. The negative pore-water pressure in unsaturated soil is highly influenced by the flux boundary condition changes (i.e., infiltration, evaporation and transpiration) resulting from the variation in climatic conditions. On the other hand, the negative pore-water pressure contributes additional shear strength to the unsaturated soil. As water infiltrates into the slope, pore-water pressure in the slope increases (matric suction decreases), and the additional shear strength due to matric suction will decrease or even disappear, causing the slope to be more susceptible to failure. Singapore is a land scarce country with a critical need to optimize land utilization. Retaining walls are important part of urban development related to the creation of new spaces for urban development where land space is very limited. Conventional concrete retaining walls are usually associated with high cost, poor aesthetic and lengthy construction time. In this study, NTU collaborated with Housing and Development Board (HDB) to develop new earth retaining structure and slope stabilization system against rainfall-induced slope failures, named Geobarrier system (GBS). A GBS is a man-made three-layer cover system designed as a vegetative layer combined with a two-layer unsaturated system which harnesses the distinct difference in unsaturated hydraulic properties between a fine-grained layer and a coarse-grained layer. GBS consists of recycled materials and does not use steel nor concrete and is hence more cost effective, thereby making it economical for use in high rise building. Geobag for vegetative layer is supported by specially designed pockets for planting different types of sustainable plant species. This monograph presents the overall design, construction procedures, finite element analyses and typical results of GBS slope. In addition, apparatuses, methodology and results of the unsaturated hydraulic properties and the unsaturated shear strength are discussed briefly in this monograph.