Effectiveness of horizontal drains for slope stability H. Rahardjo a, * , K.J. Hritzuk a , E.C. Leong b,1 , R.B. Rezaur c,2 a School of Civil and Environmental Engineering, Nanyang Technological University, Blk N1, #1A-02, Nanyang Avenue, Singapore 639798, Singapore b School of Civil and Environmental Engineering, Nanyang Technological University, Blk N1, #1C-80, Nanyang Avenue, Singapore 639798, Singapore c School of Civil and Environmental Engineering, Nanyang Technological University, Blk N1, #B4-06, Nanyang Avenue, Singapore 639798, Singapore Received 29 August 2002; accepted 11 December 2002 Abstract Horizontal drains have been commonly used in stabilising unsaturated residual soil slopes. This study examines the effectiveness of horizontal drains in stabilising residual soil slopes against rainfall-induced slope failures under a tropical climate. The study includes field instrumentation at two residual soil slopes complemented with a parametric study relating to drain position. Field monitoring results indicate that rainfall infiltration is limited to a certain depth below which infiltration becomes insignificant. This zone tends to be unsuitable for horizontal drains. Horizontal drains were found to be most effective when located at the base of a slope. The parametric study indicated conditions under which horizontal drains are effective or ineffective in improving the stability of a slope. It was also found that horizontal drains have little role in minimising infiltration in an unsaturated residual soil slope. Benefits of using horizontal drains can be obtained through the lowering of the water table. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Rainfall-induced slope failure; Horizontal drain; Residual soils; Pore– water pressure; Tensiometer 1. Introduction Singapore is located in the tropics where heavy rainfalls and high temperatures provide conditions for rapid and thorough in-situ chemical and mechanical weathering that result in the development of deep residual soil profiles. Because of the frequent high intensity tropical rainfalls, slope failures are common in this region. Pierson et al. (1992) observed that landslides in Hawaii coincided with or followed an extremely heavy rainfall. Studies of slope failures in Hong Kong (Brand, 1992) and in Singapore (Pitts, 1985; Tan et al., 1987; Toll et al., 1999; Rahardjo et al., 2001) also showed the destabilising effects of short duration, high intensity rainfalls. Some studies have also shown the effects of rainfall on residual soil slopes by pore – water pressure measurements using piezometric and matric suction (Fredlund and Rahardjo, 1993) measuring 0013-7952/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0013-7952(02)00288-0 * Corresponding author. Tel.: +65-6790-4104; fax: +65-6791- 0676. E-mail addresses: chrahardjo@ntu.edu.sg (H. Rahardjo), cecleong@ntu.edu.sg (E.C. Leong), crezaur@ntu.edu.sg (R.B. Rezaur). URL: http://www.ntu.edu.sg/cse/. 1 Tel.: +65-6790-4774; fax: +65-6791-0676. 2 Tel.: +65-6790-6199; fax: +65-6791-0676. www.elsevier.com/locate/enggeo Engineering Geology 69 (2003) 295 – 308