Click here to load reader

Effects of Groundwater Table Position and Soil · PDF fileEffects of Groundwater Table Position and Soil Properties ... Permeability; Shear strength; Slope stability. ... and soil

  • View
    215

  • Download
    0

Embed Size (px)

Text of Effects of Groundwater Table Position and Soil · PDF fileEffects of Groundwater Table...

  • Dow

    nloa

    ded

    from

    asc

    elib

    rary

    .org

    by

    Cur

    tin U

    niv

    of T

    echn

    olog

    y 20

    09 o

    n 10

    /06/

    12. F

    or p

    erso

    nal u

    se o

    nly.

    No

    othe

    r us

    es w

    ithou

    t per

    mis

    sion

    . Cop

    yrig

    ht (

    c) 2

    012.

    Am

    eric

    an S

    ocie

    ty o

    f C

    ivil

    Eng

    inee

    rs. A

    ll ri

    ghts

    res

    erve

    d.

    Effects of Groundwater Table Position and Soil Propertieson Stability of Slope during Rainfall

    Harianto Rahardjo1; Alfrendo Satyanaga Nio2; Eng Choon Leong3; and Ng Yew Song4

    Abstract: Rainfall, hydrological condition, and geological formation of slope are important contributing factors to slope failures.Parametric studies were carried out to study the effect of groundwater table position, rainfall intensities, and soil properties in affectingslope stability. Three different groundwater table positions corresponding to the wettest, typical, and driest periods in Singapore and fourdifferent rainfall intensities 9, 22, 36, and 80 mm/h were used in the numerical analyses. Typical soil properties of two main residualsoils from the Bukit Timah Granite and the sedimentary Jurong Formation in Singapore were incorporated into the numerical analyses.The changes in factor of safety during rainfall were not affected significantly by the groundwater table near the ground surface due to therelatively small changes in matric suction during rainfall. A delay in response of the minimum factor of safety due to rainfall and a slowerrecovery rate after rainfall were observed in slopes from the sedimentary Jurong Formation as compared to those slopes from the BukitTimah Granite. Numerical analyses of an actual residual soil slope from the Bukit Timah Granite at Marsiling Road and a residual soilslope from the sedimentary Jurong Formation at Jalan Kukoh show good agreement with the trends observed in the parametric studies.

    DOI: 10.1061/ASCEGT.1943-5606.0000385

    CE Database subject headings: Rainfall intensity; Groundwater; Water table; Residual soils; Permeability; Shear strength; Slopestability; Soil properties.

    Author keywords: Rainfall intensity; Groundwater table; Residual soils; Matric suction; Permeability; Shear strength; Slope stability.

    Introduction

    Many slope failures around the world, particularly in regions withresidual soils, occurred due to changes in unsaturated soil condi-tion as a result of frequent heavy rainfalls. Matric suction or nega-tive pore-water pressure plays an important role in the stability ofthese slopes Fredlund and Rahardjo 1993. A deep groundwatertable and a significant thickness of unsaturated zone above thegroundwater table are general characteristics of steep residual soilslopes. In tropical and subtropical areas, rainfall-induced slopefailures are closely related to the properties of the soil, the geom-etry of the slope, the position of groundwater table, and certainenvironmental factors vegetation and weathering effects.

    Previous research works have shown that rainfall is the maincontributing factor of slope failures in Singapore Brand 1984;

    1Professor and Head of Div., School of Civil and EnvironmentalEngineering, Nanyang Technological Univ., Blk N1, No. 1B-36, NanyangAve., Singapore 639798, Singapore corresponding author. E-mail:[email protected]

    2Project Officer, School of Civil and Environmental Engineering,Nanyang Technological Univ., Singapore 639798, Singapore. E-mail:[email protected]

    3Associate Professor, School of Civil and Environmental Engineering,Nanyang Technological Univ., Blk N1, No. 1C-80, Nanyang Ave.,Singapore 639798, Singapore. E-mail: [email protected]

    4Deputy Director and Professional Engineer, Dept. of Building Tech-nology, Housing and Development Board, HDB Hub 480, Lorong 6, ToaPayoh, Singapore 310480, Singapore. E-mail: [email protected]

    Note. This manuscript was submitted on April 29, 2009; approved onFebruary 19, 2010; published online on May 11, 2010. Discussion periodopen until April 1, 2011; separate discussions must be submitted forindividual papers. This paper is part of the Journal of Geotechnical andGeoenvironmental Engineering, Vol. 136, No. 11, November 1, 2010.

    ASCE, ISSN 1090-0241/2010/11-15551564/$25.00.

    JOURNAL OF GEOTECHNICAL AND GEOE

    J. Geotech. Geoenviron. Eng.

    Tan et al. 1987; Chatterjea 1989; Lim et al. 1996; Toll et al. 1999;Rahardjo et al. 2007a,b. The last heavy rainfall between Decem-ber 2006 and January 2007 caused a number of slope failures inSingapore. Almost all of the slope failures occurred in residualsoils from the Bukit Timah Granite BT and the sedimentaryJurong Formation JF Rahardjo et al. 2007a.

    Numerical analyses of rainfall-induced slope failures havebeen carried out to study the controlling parameters Gasmo et al.2000; Tsaparas et al. 2002; Rahardjo et al. 2005, 2007a,b andthe effects of antecedent rainfall on rainfall-induced slope failuresRahardjo et al. 2001, 2008. Cho and Lee 2002 indicated thatmost shallow slope failures were caused by the advancement ofa wetting front into the slope. Ng et al. 2001 conducted three-dimensional numerical analyses of groundwater response to rain-fall and found that rainfall pattern, duration, and its return periodhave major influences on the changes in pore-water pressuresin unsaturated cut slopes. Tohari et al. 2007 conducted a labo-ratory study of slopes under rainfalls and observed that shallownoncircular slip was the dominant mode for rainfall-induced slopefailures.

    In this paper, parametric studies were performed to study theeffect of position of groundwater table, rainfall intensity, and soilproperties on the stability of slope. The observation was focusedon the residual soils from the BT and the sedimentary JF in Sin-gapore. Numerical analyses of existing slopes were comparedwith the results from the parametric studies.

    Slope Observation

    Two-thirds of Singapore consist of two major formations, the BTand the sedimentary JF PWD 1976. BT underlies the Bukit

    Timah nature reserve and the central catchment area in the center

    NVIRONMENTAL ENGINEERING ASCE / NOVEMBER 2010 / 1555

    2010.136:1555-1564.

  • Dow

    nloa

    ded

    from

    asc

    elib

    rary

    .org

    by

    Cur

    tin U

    niv

    of T

    echn

    olog

    y 20

    09 o

    n 10

    /06/

    12. F

    or p

    erso

    nal u

    se o

    nly.

    No

    othe

    r us

    es w

    ithou

    t per

    mis

    sion

    . Cop

    yrig

    ht (

    c) 2

    012.

    Am

    eric

    an S

    ocie

    ty o

    f C

    ivil

    Eng

    inee

    rs. A

    ll ri

    ghts

    res

    erve

    d.

    of the island. Sedimentary rocks of the JF, which contain varia-tions of conglomerate, shale, and sandstone, are located in thesouthern, southwestern, and western parts of Singapore. Climaticconditions in Singapore are characterized by uniform tempera-ture, high humidity, and particularly, abundant rainfalls. The rainyseason can be divided into two main seasons, the wetter NortheastMonsoon season from December to March and the drier South-west Monsoon season from June to September National Environ-ment Agency 2007. During the Northeast Monsoon season,moderate to heavy rainfalls usually occur between December andJanuary. Maximum rainfall usually occurs between December andJanuary, whereas July is noted as the driest month National En-vironment Agency 2007.

    Several slopes located at the residual soil slopes from the BTand the sedimentary JF were investigated to obtain information ontypical soil properties, slope geometry, and position of ground-water table for both formations. The investigated slopes had aslope angle and a slope height varying from 15 to 40 and from5 to 42 m, respectively.

    Location and Geometry of Slopes

    Six slopes were selected from two different geological formationsin Singapore. Ang Mo Kio St. Twenty-one AMK, ThomsonRoad TR, and Marsiling Road MR slopes are located inthe BT, while Bukit Merah View BM, Jalan Kukoh JK, andHavelock Road HR slopes are located in the JF Fig. 1. Soilsampling was performed using continuous foam drilling with aMazier sampler to obtain good quality samples. Laboratory testswere then carried out to obtain saturated and unsaturated hydrau-lic properties and shear strength of the investigated soils. Soilprofiles of the observed slopes could then be constructed. Theunsaturated laboratory tests included soil-water characteristiccurve SWCC tests and unsaturated triaxial tests for obtainingunsaturated shear-strength parameters.

    Positions of Groundwater Table

    The investigated slopes were instrumented with at least two pi-ezometers to monitor the position of groundwater table. Manualmonitoring of the Casagrande piezometers for all slopes was car-ried out over a 2-year period June 2006 until September 2008.Variations of groundwater table positions for the driest and wet-test periods are shown in Figs. 2 and 3 in the residual soil slopesfrom BT and JF, respectively. The average positions of ground-water table, representing its typical position in residual soil slopesin Singapore, were calculated based on the minimum position of

    Fig. 1. Location of instrumented slopes in Singapore

    1556 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGIN

    J. Geotech. Geoenviron. Eng.

    groundwater table during the dry period and the maximum posi-tion of groundwater table during the wet period Figs. 2 and 3.The symbol in Figs. 2 and 3 represents the distance of thepiezometers P1 and P2 measured from the crest of slo

Search related