Abstract
Numerous field monitoring programs have been conducted to investigate the performance of an unsaturated soil slope subjected to rainfalls in wet seasons. Most case histories focus on the response of matric suction, which is one of the two stress-state variables governing unsaturated soil behaviour. However, effects due to another variable, net normal stress, are often ignored. Also, slope performance under alternative wet and dry seasons is rarely reported and analysed. In this study, a saprolitic hillslope situated in Hong Kong was instrumented heavily to investigate its seasonal movement due to changes of the two variables and also groundwater flow mechanism. Two-year seasonal variations of matric suction and net normal stress were monitored by tensiometers together with heat dissipation matric water potential sensors and earth pressure cells, respectively. During heavy rainstorms in wet season, there was a substantial recharge of the main groundwater table, causing a significant increase of positive pore-water pressure in deeper depths. Rupture surface likely developed at depths between 5.5 and 6 m, hence resulting in a “deep-seated” mode of downslope movement. The downslope movement resulted in a peak increase of horizontal stress. In dry seasons, matric suction of up to 190 kPa was recorded, and the associated soil shrinkage led to substantial upslope rebounds. The stress built up in wet seasons hence reduced. After monitoring period of 2 years, downslope ratcheting is identified. Up to 40 % of the downslope displacements were recovered by the upslope rebounds.
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References
Alonso EE, Gens A, Josa A (1990) A constitutive model for partially saturated soils. Geotechnique 40(3):405–430
Brackley IJA, Sanders PJ (1992) In-situ measurement of total natural horizontal stresses in an expansive clay. Geotechnique 42(2):443–451
Brand EW (1984) Relationship between rainfall and landslides in Hong Kong. In: Proc. of the 4th International Symposium of Landslides, Toronto, 1, pp 37–384
Cheuk J, Ng A, Endicott J, Ho K (2009) Progressive slope movement due to seasonal wetting and drying. Invited paper. Proc. Seminar on “The State-of-the-art Technology and Experience on Geotechnical Engineering in Malaysia and Hong Kong”. The HKIE Geotechnical Division, 25 February 2009, Hong Kong, pp 105–114
Chiu CF, Ng CWW (2003) A state-dependent elasto-plastic model for saturated and unsaturated soils. Geotechnique 53(9):809–829
Coleman JD (1962) Correspondence: stress/strain relations for partly saturated soils. Geotechnique 12(4):348–350
Durner W, Schultze B, Zurmuhl T (1999) State-of-the-art in inverse modeling of inflow/outflow experiments. In: Proc. Int. Workshop on Characterisation and Measurement of the Hydraulic Properties of Unsaturated Porous Media, 22–24 Oct, 1997, University of California, Riverside, CA
Eguchi S, Hasegawa S (2007) Determination and characterization of preferential water flow in unsaturated subsoil of andisol. Soil Sci Soc Am J 72(2):320–330
Gasmo J, Hritzuk KJ, Rahardjo H, Leong EC (1999) Instrumentation of an unsaturated residual soil slope. Geotech Test J ASTM 22(2):128–137
Geotechnical Control Office (GCO) (1987) GEOGUIDE 2—Guide to site investigation. Geotechnical Control Office, Hong Kong
Geotechnical Control Office. (GCO) (1988) GEOGUIDE 3—Guide to rock and soil descriptions. Geotechnical Control Office, Hong Kong
Geotechnical Engineering Office (GEO) (1984) Geotechnical manual for slopes. Geotechnical Engineering Office, Civil Engineering Department, The Government of the Hong Kong Special Administrative Region
Geotechnical Engineering Office (GEO) (2007) Final geological assessment report for Tung Chung Foothills study area. Landslide assessment and monitoring work at four selected sites—feasibility study. Prepared by S. W. Millis, Ove Arup and Partners Hong Kong Ltd
Hassanizadeh SM, Celia MA, Dahle HK (2002) Dynamic effect in the capillary pressure–saturation relationship and its impact on unsaturated flow. Vadose Zone J 1(1):38–57
Leung AK, Sun HW, Millis SW, Pappin JW, Ng CWW, Wong HN (2011) Field monitoring of an unsaturated saprolitic hillslope. Can Geotech J 48(3):339–353
Liang WL, Kosugi K, Mizuyama T (2007) Heterogeneous soil water dynamics around a tree growing on a steep hillslope. Vadose Zone J 6(4):879–889
Ng CWW, Menzies B (2007) Advanced unsaturated soil mechanics and engineering. Taylor & Francis, London
Ng CWW, Pang YW (2000) Experimental investigations of the soil-water characteristics of a volcanic soil. Can Geotech J 37(6):1252–1264
Ng CWW, Zhan LT, Bao CG, Fredlund DG, Gong BW (2003) Performance of an unsaturated expansive soil slope subjected to artificial rainfall infiltration. Geotechnique 53(2):143–157
Ng CWW, Wong HN, Tse YM, Pappin JW, Sun HW, Millis SW, Leung AK (2011) A field study of stress-dependent soil–water characteristic curves and permeability of a saprolitic slope in Hong Kong. Geotechnique 61(6):511–521
Pagano L, Picarelli L, Rianna G, Urciuoli G (2010) A simple numerical procedure for timely prediction of precipitation-induced landslides in unsaturated pyroclastic soils. Landslide 7(3):273–289
Smethrust JA, Clarke D, Powrie W (2006) Seasonal changes of pore water pressure in a grass-covered cut slope in London clay. Geotechnique 56(8):523–537
SMO (1995) Explanatory notes on geodetic datums in Hong Kong. Survey and Mapping Office (SMO), Lands Department, the Government of the HKSAR, HK
Take WA, Bolton MD (2004) Identification of seasonal slope behaviour mechanisms from centrifuge case studies. Advances in Geotechnical Engineering: The Skempton Conference, London, 2. Thomas Telford, pp 992–1004
Tsaparas I, Rahardjo H, Toll DG, Leong EC (2003) Infiltration characteristics of two instrumented residual soil slopes. Can Geotech J 40(5):1012–1032
Tu XB, Kwong AKL, Dai FC, Tham LG, Min H (2009) Field monitoring of rainfall infiltration in a loess slope and analysis of failure mechanism of rainfall-induced landslides. Eng Geol 105(1–2):134–150
Acknowledgments
The authors would like to acknowledge the Geotechnical Engineering Office, Civil Engineering and Development Department of the Government of the Hong Kong Special Administrative Region (Ir H. N. Wong and Dr H. W. Sun) for funding the field monitoring work presented in this paper. The research fund HKUST9/CRF/09 provided by the Research Grants Council of the Government of the Hong Kong SAR is also acknowledged.
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Leung, A.K., Ng, C.W.W. Seasonal movement and groundwater flow mechanism in an unsaturated saprolitic hillslope. Landslides 10, 455–467 (2013). https://doi.org/10.1007/s10346-012-0343-7
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DOI: https://doi.org/10.1007/s10346-012-0343-7