Effects of coarse-grained material on hydraulic properties and shear strength of top soil H. Rahardjo a, ⁎, I.G.B. Indrawan a , E.C. Leong a , W.K. Yong b a School of Civil and Environmental Engineering, Nanyang Technological University, Singapore b Research Division, National Parks Board, Singapore ABSTRACT ARTICLE INFO Article history: Received 17 May 2007 Received in revised form 25 April 2008 Accepted 13 May 2008 Available online 21 May 2008 Keywords: Top soil Coarse-grained material Soil mixture Hydraulic properties Soil–water characteristic curve Permeability function Shear strength Sidewalk failures associated with top soil of low shear strength are a common problem in urban areas. Mixing top soil with granite chips can be used to increase its permeability and shear strength. The effects of mixing granite chips with top soils on the hydraulic properties and shear strength under saturated and unsaturated conditions were investigated in this study. The results showed that the mixing top soils with granite chips caused changes in several key parameters of the soil–water characteristic curve (e.g., the air- entry value, the residual matric suction, and the residual volumetric water content) and the unsaturated permeability of the top soils. The saturated permeability and shear strength of the soil mixture increased with increasing content of granite chips. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The island of Singapore consists mainly of three rock formations: (a) the Bukit Timah formation; (b) the Jurong formation; and (c) the Old Alluvium (Public Works Department, 1976; Pitts, 1984a,b; Leong et al., 2002). The Bukit Timah formation is a lower to middle Triassic intrusion and consists of granitic igneous rocks. The Jurong formation consists of a series of sedimentary rocks (e.g., sandstone, mudstone, shale, tuff, conglomerate and limestone) formed in the late Triassic to early Jurassic period. The Old Alluvium is a quaternary alluvial deposit of the ancient river system in the eastern part of Singapore and it consists mainly of clayey quartzo-feldspathic medium-grained sands and fine-grained gravels with some coarser gravels and lenses of silt and clay. High temperature and large precipitation provide optimum conditions for the rapid in-situ chemical and mechanical weathering of the Bukit Timah and the Jurong formations. As a result, thick re- sidual soil profiles develop and cover almost two-thirds of Singapore's land area. The residual soil is often mixed with organic materials to form top soil, which is commonly used as a medium for tree growth. In urban areas, the top soil is often added to the tree pits adjacent to sidewalks, which are generally well compacted and have high bearing capacity. The abundance of top soil in the tree pits, however, often causes sidewalks to experience differential settle- ments. This is due to the fact that top soil is often not compacted and also has low permeability and shear strength. The low permeability of the top soil will cause the area around sidewalks to have poor drainage. The water clogged in the soil reduces its shear strength and consequently induces sidewalk damage. In order to improve permeability and shear strength, top soil can be mixed with a coarse-grained material, such as granite chips. The soil mixture can be used as a planting medium and it also provides a high bearing capacity. The concept of soil mixing using coarse-grained materials has received increasing attention as an alternative method for im- proving fine-grained soils. A number of research studies have been conducted to investigate the effects of soil mixing on the index properties (e.g., void ratio and dry density), saturated permeability, and shrink–swell potential of transported soils (e.g., Holtz and Lowitz, 1957; Holtz, 1985; Shakoor and Cook, 1990; Shelley and Daniel, 1993). Indrawan et al. (2006) investigated the effects of soil mixing on the hydraulic properties and shrinkage characteristics of Engineering Geology 101 (2008) 165–173 ⁎ Corresponding author. Tel.: +65 6790 5246; fax: +65 6791 0676. E-mail address: chrahardjo@ntu.edu.sg (H. Rahardjo). URL: http://www.ntu.edu.sg/cee/staff/infrastructure/academic/chrahardjo.asp (H. Rahardjo). 0013-7952/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2008.05.001 Contents lists available at ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo