Ground settlements and drawdown of the water table around an excavation

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  • Ground settlements and drawdown of the water table around an excayation J. P. HSI AND J . C. SMALL

    School of Civil and Mining Engineering, The University of Sydney, Sydney, NSW 2006, Australia Received December 16, 1991

    Accepted April 7, 1992

    In the vicinity of an excavation in a saturated soil, ground settlements are often caused by the combined effects of stress release and drawdown of the water table. These settlements may be crucial if the excavation is carried out in a congested area. A case history of excavation adjacent to closely constructed blocks of buildings is discussed in this study. Ground settlements and water-surface levels were monitored during the excavation period, as the settlement of the surrounding area was of concern. The authors have previously developed a fully coupled numerical method that allows the computation of the displacements and pore pressures in a soil taking account of the drawdown of the water table which may accompany excavation. This method is used here for back-analysis of a full-scale excavation that has been comprehensively documented. Comparisons between the field measurements and the calculated results are given in this paper.

    Key words: consolidation, excavation, finite element, seepage, transient unconfined flow.

    A proximite d'une excavation dans un sol sature, des tassements du sol resultent souvent des effets combines du relschement des contraintes et de l'abaissement de la nappe phreatique. Ces tassements peuvent 6tre cruciaux si l'excavation est realisee dans des zone achalandees. Une Ctude de cas d'une excavation adjacente a des blocs de bgtiments dense- ment construits est discutee dans cette Ctude. Les tassements et les niveaux de la nappe phreatique ont ete mesuris durant la piriode de l'excavation puisque l'affaissement de la zone environnante etait prioccupante. Les auteurs ont dkveloppe anttrieurement une methode numerique complktement appariee qui permet le calcul des deplacements et des pressions interstitielles dans un sol et qui prend en compte le rabattement de la nappe phrCatique qui peut resulter de l'excavation. Cette mkthode est utiliske ici pour l'analyse a rebours d'une excavation a pleine Cchelle qui a CtC pleine- ment ducomentke. Des comparaisons entre les mesures sur le terrain et les resultats calculCs sont donnes dans cet article.

    Mots C/&S : consolidation, excavation, Clement fini, infiltration, Ccoulement transitoire non confine. [Traduit par la redaction]

    Can. Geotech. J . 29, 740-756 (1992)

    Introduction Damage to buildings, roads, and utilities is frequently seen

    around an excavation site. It is usually caused by excessive ground movements due to excavation. Therefore care needs to be taken when an excavation is to be executed in a con- gested area. An accurate prediction of the behaviour of the soil surrounding an excavation can lead to an improved design and limit any damage around the cut. While the excavation is proceeding, a monitoring system is often needed to provide information about the response of the excavation and give early warning if any unexpected behav- iour is encountered.

    Two main factors are known to induce ground settlement around an excavation. Firstly, there is stress relief due to overburden removal. The soil that is being removed initially serves as a support to the boundary of the excavation. When it is removed, the soil around the cut starts moving inwards due to the loss of this support, and ground settlement is then generated. Secondly, if the excavation is carried out below the groundwater table and if the soil is permeable or the elapsed time is long enough to allow the water table to drop, an additional ground settlement may occur due to the increase in effective stress in the soil generated by the fall of the groundwater table.

    Currently most of the methods for excavation analysis in saturated soils have been based on the assumption that the water table remains at a constant level and that pore pres- sures and deformations of the soil can be calculated by use of effective-stress approaches (Osaimi and Clough 1979; Banerjee et al. 1988; Tsui and Cheng 1989; Yong et al. 1989; Bolton et al. 1989; Cheng and Tsui 1991; Finno and Prin~cd in Canada / In~pr~nlC au Canada

    Harahap 1991). As drawdown of the water table is one of the major factors that contributes to the ground settlement, it should be taken into account in excavation analysis where a drawdown does occur (Walker and Morgan 1977; Debidin and Lee 1980; Moran and Cherry 1981 ; Schroeder et al. 1986; Forster et al. 1991).

    The authors have successfully developed a fully coupled finite element method (Hsi and Small 1992a, 1992b; J.P. Hsi and J.C. Small, in preparation) for excavation analysis which incorporates aspects of consolidation theory (Biot 1941, 1956; Sandhu and Wilson 1969; Christian and Boehmer 1970; Hwang et a/. 197 1 ; Small et al. 1976), tran- sient free-boundary flow (Desai 1976; Rushton and Redshaw 1979; Bathe 1982; Bathe et al. 1982; Desai and Li 1983; Cividini and Gioda 1984; Gioda and Desideri 1988), and overburden removal theory (Ghaboussi and Pecknold 1984; Brown and Booker 1985). Deformations and pore pressures generated in a soil around an excavation can be evaluated by this method, and since it is fully coupled, changes in water pressure will cause deformation in the soil, and changes in soil deformation can cause changes in water pressure. A vir- tual work formulation is used to determine the out-of- balance forces applied along the boundary of the excava- tion so that overall stress equilibrium is maintained during the excavation process. As presented in the previous work (Hsi and Small 1992a, 1992b; J.P. Hsi and J.C. Small, in preparation), this method is implemented by a finite element program, EXCAZ, that is able to solve plane strain and axisymmetric problems and can be used for elastic or elasto- plastic soils.

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  • FIG. 1. Permeability-pressure relationship. k,, permeability of saturated soil.

    A case history of an excavation for which the ground sur- face settlement and drawdown of the water table were monitored is discussed in this study. The excavation was car- ried out to a depth of about 27 m mainly in silty sands inter- bedded with some thin layers of silty clay. The elastic model was used here to simulate the soil behaviour during excava- tion. Sufficient soil data from site investigations and labo- ratory tests were provided so that reliable soil parameters could be determined for use in the analysis.

    Fully coupled numerical method The fully coupled method for excavation analysis pre-

    sented here may be used to compute the movement of the soil due to stress relief and due to water table drop. In the numerical analysis the tractions removed from the finite ele- ment mesh are calculated by using a modified form of Ghaboussi and Pecknold's (1984) or Brown and Booker's (1985) method. This method can correctly evaluate the forces to be removed, unlike many methods used previously (Christian and Wong 1973; Clough and Mana 1976; Osaimi and Clough 1979; Banerjee et al. 1988; Tsui and Cheng 1989; Cheng and Tsui 1991) where the tractions were calcu- lated by integrating only the stresses in the removed soil. The numerical procedure can also predict how the water table will drop as the excavation proceeds and how this will cause deformation in the soil. Governing equations

    The finite element equation used for excavation analysis taking account of drawdown of the free surface is given by (Hsi and Small 1992a, 1992b; J.P. Hsi and J.C. Small, in preparation)

    where

    All terms used here are listed in the appendices, and details are given by Hsi and Small (1992~) and Hsi (1992).

    SMALL 741

    TABLE 1. Soil profile and description

    Layer Thickness (m) Description I 8.8-12.5 Silty fine to medium sand; grey;

    containing small amounts of shells; loose to medium dense

    2 0.7-3.1 Silty clay or clayey silt; grey; soft to stiff

    3 2.5-12.3 Silty fine to medium sand; grey; loose to dense

    4 0.0-7.5 Silty clay and clayey silt; grey; soft to stiff

    5 16.3-24.0 Silty fine sand with thin layers of sandy silt; grey; medium dense to dense

    6 6.9-10.3 Fine sandy silt; grey; with silty clay or clayey silt occasionally; medium dense to dense

    7 Unknown Silty clay; grey; stiff to very stiff

    Equations [I] and [2] are an extension of the incremental approach for solving consolidation problems using an incre- mental time marching scheme proposed by Small et al. (1976). As both stress removal and drawdown of the free surface are considered for the excavation analysis, the vector - Ag is incorporated for simulating tractions applied along the excavation boundary due to soil removal, and the term CFS is incorporated so as to take account of water flowing from the soil as the water table falls. The significance of these two terms - Ag and CFS is described in the following.

    By consideration of overall force equilibrium being main- tained in the soil while excavation proceeds, an extended form of Brown and Booker's method (1985) or the method of Ghaboussi and Pecknold (1984) was developed in which the vector of forces - A g to be removed from the finite ele- ment mesh is given by

    These forces are applied to the nodes of the finite element mesh when elements are removed. The vector is evaluated by integrating pore pressures and effective stresses at the ( j - 1)th stage over the remaining (i.e., the elements that have not been excavated) domain at the jth stage. It is noted that the coupling matrix L involves an integration over the volume of the soil mass, and it is defined in Appendix 1.

    As drawdown of the water table is considered, the con- cepts of the residual flow procedure proposed by Desai and Li (1983) and Bathe et al. (1982) were adopted. The term CFS may be written as

    [41 CFS = jr aaT Sy cos p d r This term can be used to calculate the imposed flow along

    the free surface where the pore pressure is zero. This approach can only be applied, however, when there is no negative pore-pressure zone generated by the excavation above the level of the base of the cut. When a negative pore- pressure zone is produced due to excavation, there may be several zero pore-pressure contours, and so the free surface contour cannot be precisely determined. An alternative method proposed by Cividini and Gioda (1984) has t o be

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  • CAN. GEOTECH. J . VOL. 29, 1992 I

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  • 20 30 SPT N

    HSI AND SMALL

    10

    1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 UNIT WEIGHT (tlm 3)

    -80 I I I I 0 10 20 30 40 50

    WATER CONTENT (%) FIG. 3. Relationship of elevation to (a) SPT

    -80 I I I I I 0.20 0.30 0.40 0.50 0.60

    POROSITY unit weight, (c) water content, and (d) porosity.

    used when this occurs. With this method, the free surface and the size of the time step chosen and is not as accurate is assumed to correspond to the sides of the elements, and as the residual flow procedure, but it has the advantage of its position is calculated from the velocities of the free sur- not depending on the position of the zero pore-pressure con- face. It is therefore dependent on the fineness of the mesh tour. The method is only used if excess negative pore pres-

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  • 744 CAN. GEOTECH.

    sures exist above the base level of the excavation, otherwise table drops. The porosity n of a saturated soil is the sum the method of Bathe et al. (1982) is used. of the specific yield and the specific retention; for sandy

    In [4], Sy is the specific yield that defines the amount of soils, S,, is slightly smaller than n, whereas for clayey soils, water which can be yielded from the soil when the water Sy is much smaller than n (Walton 1970).

    GI &pa) FRICTION ANGLE FIG. 3. (continued). Relationship of elevation to (e) plastic and liquid limits for CL/ML, ( f ) plasticity index for CL/ML, (g) c,

    for CL/ML, and (h) friction angle. CD, consolidated drained triaxial test; CU, consolidated undrained triaxial test; DS, direct shear test.

    (h) 10 0

    -10

    -20

    h a w

    3-30 F u > W -40 a W

    -50

    -60

    -70

    - 80 90

    (9) 10

    0

    -10-

    -20

    - a w

    2-30 2 2 > 112-40 cl w

    -50

    -60

    -70

    - 80 40

    00 00 on q

    q " q qJuou ,, q - q

    -

    -

    -

    q q 00

    -

    q -

    I I I I I I I

    10 20 30 40 50 60 70 80

    C I '

    A+ X

    - x + +xunt +

    X -

    A 0 ++ - 4

    -

    + Lu

    0

    q CU(CUML) - 0 CD(CUML)

    A CD(SM) - X DS (CUML)

    + CD(SM) I I

    S q

    I I I

    10 15 20 25 30 35

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  • HSI AND SMALL 745

    CONST. HEAD (SM) 0 TRIAXIAL (SM)

    FIG. 3. (concluded). Relationship of elevation to (i) m, and ( j ) permeability. CONST. HEAD, constant head permeability test; FIELD, field permeability test; TRIAXIAL, triaxial permeability test.

    0 10 2Om -

    Scale

    FIG. 4. Layout plan of the excavation.

    To eliminate the flow above the free surface, the perme- ability of the soil is reduced according to the levels of negative pore pressure which exist there (Bouwer 1964; Freeze 1971; Cathie and Dungar 1975; Desai and Li 1983; Li and Desai 1983) as shown in Fig. 1.

    Limit values of permeability klimi, and negative pore pressure Plimi, are defined as the lowest values that the soil can reach. The permeability and pore pressure are reduced

    in the zone above the free surface according to the perme- ability - pore pressure relationship until the negative pressure reaches Plimi,. Once suctions reach Plimi, they are held at this value and not allowed to become more negative, and the permeability is set to a value of klimit. Finite element programming

    A finite element program, EXCAZ, was developed by the authors to solve the governing equations. An eight-noded isoparametric element is used in the program. It is able to perform plane strain and axisymmetric analyses in elastic or elastoplastic materials.

    To ensure the equilibrium of stresses within each incre- ment of excavation and to determine the location of the free surface, iterations are required within each time step so that equilibrium can be achieved. Generally the program was found to give convergent results within five iterations if the implicit Euler backward method (a = 0) was used, and con- vergence of the solution was deemed to have occurred if the change in total water head at each node was less than 1000th of the height of the solution domain.

    Case history Job nature

    An excavation was to be carried out in Kaos...

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