Vertical Drain Consolidation Using Stone Columns an Analytical Solution With an Impeded Drainage Boundary Under Multi Ramp Loading 2015 Geotextiles An

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    Technical note

    Vertical-drain consolidation using stone columns: An analytical

    solution with an impeded drainage boundary under multi-ramp

    loading

    G.H. Lei  a ,  *, C.W. Fu  a, C.W.W. Ng  b

    a Key Laboratory of Geomechanics and Embankment Engineering of the Ministry of Education, Geotechnical Research Institute, Hohai University, 1 Xikang 

    Road, Nanjing 210098, Chinab Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

    a r t i c l e i n f o

     Article history:

    Received 8 December 2014

    Received in revised form

    22 June 2015

    Accepted 3 July 2015

    Available online xxx

    Keywords:

    Consolidation

    Pore pressures

    Ground improvement

    Embankments

    a b s t r a c t

    An analytical solution is derived to predict consolidation with vertical drains under impeded drainage

    boundary conditions and multi-ramp surcharge loading. The impeded drainage is modelled by adopting

    the third type boundary condition with a dimensionless characteristic factor of drainage ef ciency

    developed by Gray (1945) for one-dimensional consolidation. Fully drained and undrained boundary

    conditions can also be modelled by applying an innite and a zero characteristic factor, respectively. The

    combined effects of drain resistance and smear are taken into account fully. An explicit, rigorous

    analytical solution is derived using the method of separation of variables to calculate excess pore-water

    pressure at any arbitrary point in soil and to derive the overall average degree of consolidation. The

    proposed solution can also be used to analyse one-dimensional consolidation without vertical drains but

    with an impeded drainage boundary. Its validity and accuracy are veried by comparing the proposed

    solution with the solutions developed by Gray (1945) and Terzaghi (1943). Its practical applicability is

    also evaluated by analysing a case history involving a  ll embankment, which was constructed over a

    crust layer of hard soil overlying soft clay improved with stone columns. The crust layer is modelled as animpeded drainage. Reasonably good agreement is obtained between the consolidation results obtained

    from the proposed analytical solution and available three-dimensional  nite-element predictions. With

    the further consideration of smear effects, good agreement is achieved between the consolidation results

    obtained from the proposed analytical solution and  eld measurements.

    ©  2015 Published by Elsevier Ltd.

    1. Introduction

    Soft soil is often preloadedwith surcharge pressure as one of the

    most economic and effective ways to consolidate it (Qubain et al.,

    2014). Vertical prefabricated drains or sand/stone columns are

    commonly utilised to accelerate the consolidation of soft soils un-der preloading (Almeida et al., 2015; Artidteang et al., 2011;

    Cascone and Biondi, 2013; Chai et al., 2010; Indraratna et al.,

    2010; Jang and Chung, 2014; Karunaratne, 2011; Li and Rowe,

    2001; Lo et al., 2008, 2010; Rowe and Li, 2005; Rowe and

    Taechakumthorn, 2008; Saowapakpiboon et al., 2009, 2010; Shen

    et al., 2005; Suleiman et al., 2014; Van Helden et al., 2008;

    Voottipruex et al., 2014; Xue et al., 2014). Analytical solutions

    predicting the extent of consolidation in preloading play an

    important role in the preliminary design of vertical drains (Abuel-

    Naga et al., 2012; Bari and Shahin, 2014; Basu and Prezzi, 2009;

    Chung et al., 2014; Rujikiatkamjorn and Indraratna, 2009; Sinhaet al., 2009). Since the pioneering work of   Barron (1948), the

    challenge of deriving an analytical solution for cylindrical unit-cell

    consolidation with a vertical drain has capturedthe attention of the

    ground improvement community. For consolidation of a single

    layer of homogeneous soil under surcharge preloading, various

    solutions have been proposed based on different assumptions and

    considerations. A large number of analytical solutions were derived

    for the consolidation of soil with fully drained boundary conditions

    at its top and/or bottom surface (e.g.,   Conte and Troncone, 2009;

    Deng et al., 2013a, 2013b; Indraratna et al., 2011; Kianfar et al.,

    2013; Lei et al., 2015; Lu et al., 2011, 2015; Ong et al., 2012;

    *  Corresponding author. Tel.:  þ86 13 851922201,  þ86 25 83787216; fax:  þ86 25

    83786633.

    E-mail addresses:   [email protected]   (G.H. Lei),   [email protected]

    (C.W. Fu), [email protected] (C.W.W. Ng).

    Contents lists available at ScienceDirect

    Geotextiles and Geomembranes

    j o u r n a l h o m e p a g e :   w w w . e l s e v i e r . c om / l o c a t e / g e o t e x m e m

    http://dx.doi.org/10.1016/j.geotexmem.2015.07.003

    0266-1144/©

     2015 Published by Elsevier Ltd.

    Geotextiles and Geomembranes xxx (2015) 1e10

    Please cite this article in press as: Lei, G.H., et al., Vertical-drain consolidation using stone columns: An analytical solution with an impededdrainage boundary under multi-ramp loading, Geotextiles and Geomembranes (2015), http://dx.doi.org/10.1016/j.geotexmem.2015.07.003

    mailto:[email protected]:[email protected]:[email protected]://www.sciencedirect.com/science/journal/02661144http://www.elsevier.com/locate/geotexmemhttp://dx.doi.org/10.1016/j.geotexmem.2015.07.003http://dx.doi.org/10.1016/j.geotexmem.2015.07.003http://dx.doi.org/10.1016/j.geotexmem.2015.07.003http://dx.doi.org/10.1016/j.geotexmem.2015.07.003http://dx.doi.org/10.1016/j.geotexmem.2015.07.003http://dx.doi.org/10.1016/j.geotexmem.2015.07.003http://www.elsevier.com/locate/geotexmemhttp://www.sciencedirect.com/science/journal/02661144mailto:[email protected]:[email protected]:[email protected]

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    kh

    vu

    vr 

     ¼  ksh

    vusvr 

    ;   r  ¼  r s   (6)

    The hydraulic boundary conditions can be expressed as follows:

    vu

    v z  ¼  R

    u

    h  and

      vusv z 

      ¼ Rush

    ;

     z  ¼  0 for the drainage impeded top(7)

    vu

    v z  ¼

     vusv z 

      ¼ 0;   z  ¼  h   for the impervious bottom (8)

    vu

    vr  ¼ 0;   r  ¼  r e   for the impervious vertical boundary (9)

    where h   is the depth of the vertical drain.

    The initial condition is given by

    u ¼  us  ¼  u  ¼  us  ¼  0;   t  ¼  0 (10)

    Fig. 2 schematically shows the increase in total stress in soil due

    to multi-ramp surcharge loading. To facilitate the derivation of the

    analytical solution, a new single equation is constructed to accu-

    rately describe the increase in total stress:

    sðt Þ ¼XM i¼1

    F iðt Þ½si   si1   (11)

    where

    F iðt Þ ¼t   t i;0

    t i;1  t i;0H 

    t   t i;0

    1  H 

    t   t i;1

    þ H 

    t   t i;1

      (12)

    t   t i; j

     ¼

    0;

    t   t i; j

    < 0

    1;

    t   t i; j

       0; ð j ¼  0; 1Þ   (13)

    where   H ht   t i; ji   is the Heaviside step function;   M   is the total

    number of loading ramps;  t i;

    0   and   t i;

    1  are the start time and endtime of the  i-th ramp, respectively, as shown in Fig. 2; si is the in-

    crease in total stress in soil at the end time of the  i-th ramp, and

    s0  ¼  0.

    The equations above describe the unit-cell consolidation prob-

    lem to be solved.

    3. The analytical solution

    The governing Eqs.  (2) and (3)  are solved using the method of 

    separation of variables and the Fourier series, as presented in detail

    in Appendix A. Explicit, rigorous analytical solutions are obtained

    for calculating the excess pore-water pressure at any arbitrary point

    in the undisturbed soil and the smeared soil:

    u ¼ mvgw

    kv

    X∞n¼1

    (½c 1nI 0ðmnr Þ þ c 2nK 0ðmnr Þ þ 1

     ½sinðun z Þ þ cotðunhÞcosðun z Þ

    u2nX

    i¼1

    C n

    ;

    i

    ðt Þ)  (14)

    us ¼ msvgw

    ksv

    X∞n¼1

    (½c 3nI 0ðmsnr Þ þ c 4nK 0ðmsnr Þ þ 1

     ½sinðun z Þ þ cotðunhÞcosðun z Þ

    u2n

    XM i¼1

    C n;iðt Þ

    )   (15)

    where

    C n;iðt Þ ¼sn;i   sn;i1

    t i;1  t i;0

    "e

    8ðT hT hi;1Þvn

      H hT hT hi;1i e8ðT h T hi;0Þ

    vn  H hT hT hi;0i

    #

    (16)

    where   un   is the positive root of the transcendental Eq.   (A3)   in

    Appendix A, which can be solved easily using commercially avail-

    able packages like Matlab; I 0  and  K 0  are the modied Bessel func-

    tions of the  rst and second kind of zero order, respectively;  T h  is

    the time factor given by Eq.  (A21) in  Appendix A; the expressions

    for c 1n, c 2n, c 3n, c 4n, mn, msn, T hi, j and vn are given by Eqs. (A58), (A59),

    (A47), (A61), (A14), (A30), (A46) and (A22), respectively, in

    Appendix A; and e is the base of the natural logarithm.

    As usual, the overall average degree of consolidation is dened

    as follows:

    U Sðt Þ ¼ sðt Þ  uo

    sM  (17)

    where sM  is the maximum increase in total stress in soil at the end

    time t M ,1 of the M -th ramp of surcharge loading, as shown in Fig. 2;

    and uo is the overall average excess pore-water pressure, which can

    be derived based on Eqs.  (14) and (15) as

    uo ¼

    Z   h0

    264Z   r e

    r s

    2prudr  þ

    Z   r sr d

    2prusdr 

    375d z 

    p

    r 2e    r 

    2d

    h

    ¼

      1r 2e    r 

    2d

    hX∞n¼1

    ( 1u3n

    r 2e    r 2skv mvgw

    Un þ

    r 2s    r 2d

    ksv msvgwU

    sn!

    XM i¼1

    C n;iðt Þ

    )

    (18)

    where Un and Usn aregiven by Eqs. (A18) and (A33), respectively, in

    Appendix A. Thus, by substituting Eqs. (11) and (18)   into Eq. (17),

    the overall average degree of consolidation can be obtained.

    For ease of application of the proposed solution, a simple

    Fortran program that solves the modied Bessel functions with

    freeware subroutines (Press et al., 1992) has been developed. The

    results are obtained through double-precision arithmetic

    calculation.Fig. 2.  Time-dependent increase in total stress in soil under multi-ramp loading.

    G.H. Lei et al. / Geotextiles and Geomembranes xxx (2015) 1e10   3

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    It is worth noting that consolidation under fully drained top

    boundary conditions (i.e., R ¼∞ in Eq. (A3) in Appendix A) can also

    be analysed using the proposed solution by simply letting

    un  ¼ ð2n  1Þp

    2h  (19)

    Apart from this, the proposed solution can also be used to

    analyse one-dimensional consolidation without vertical drains, byapplying an extremely low r d value (e.g., 0.001 m), letting  kd, kh, kshand ksv be equal to kv, and letting  msv be equal to  mv.

    4. Verication

    In order to verify the validity and accuracy of the proposed

    analytical solution, the results calculated from the proposed solu-

    tion for one-dimensional consolidation under impeded drainage

    boundary conditions are compared with those given by the

    analytical solution of   Gray (1945).   Gray (1945)   developed an

    analytical solution to one-dimensional consolidation with an

    impeded drainage boundary under instantaneous loading.   Fig. 3

    compares the degrees of consolidation calculated from the pro-

    posed solution, the solution of  Gray (1945) and  Terzaghi's (1943)

    well-known one-dimensional consolidation solution. Excellent

    agreement is obtained. As expected, the characteristic factor of 

    drainage ef ciency has a potentially important inuence on

    consolidation.

    5. Case study 

    The proposed solution is also applied to a case history involving

    a ll embankment at NewPantai Expressway in Malaysia (Tan et al.,

    2008). The 1.8 m high embankment was constructed with sandy

    material in 9 days. The ground consisted of a 1 m thick upper crust

    layer of hard soil and a 5 m thick layer of soft clay overlying a stiff 

    clay layer (see Fig. 4(a)). Stone columns 800 mm in diameter, ar-

    ranged in a square grid with a centre-to-centre spacing of 2.4 m,

    were installed from the embankment base to a depth of 6 m above

    the stiff clay layer. A settlement plate was installed to measure the

    settlement at the centre of the embankment (measurement point

    SP1). Using the three-dimensional nite-element method, Tan et al.

    (2008) computed the settlement at SP1 and the excess pore-water

    pressure at a computation point A, which was located at a depth of 

    3.5 m below the centre of the embankment and at the centre of the

    square grid (in plan) of stone columns (see Fig. 4(a)). In the present

    study, the excess pore-water pressure at point A and the overall

    average degree of consolidation   U S(t ) below the centre of the

    embankment are calculated using Eqs. (14) and (17), respectively,

    adopting the calculation parameters presented by Tan et al. (2008)

    (see the  rst three columns of  Table 1). Although the upper crust

    layer can serve as a horizontal drainage blanket for discharging the

    water expelled from the stone columns installed through it, its

    hydraulic conductivity is in the order of 107 m/s, which is only two

    orders of magnitude higher than the hydraulic conductivity of the

    underlying soft clay. For this reason, the upper crust layer is

    modelled as an impeded drainage boundary for the consolidation

    of soft clay. The characteristic factor R  of drainage ef ciency of the

    upper crust layer is derived from Eq.  (1)  and the adopted param-

    eters, as given in Table 1. For comparison purpose, the measured

    and computed settlements presented by  Tan et al. (2008) are nor-

    malised by their corresponding ultimate values to reect the de-

    gree of consolidation. Similarly, the computed and calculated

    Fig. 3.   A comparison between the solution proposed in this study and those developed

    by Gray (1945) and  Terzaghi (1943).

    Fig. 4.   Comparisons of calculated results from the proposed solution and reported eld

    data and computed values by Tan et al. (2008).

    G.H. Lei et al. / Geotextiles and Geomembranes xxx (2015) 1e104

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    excess pore-water pressures are also normalised by their corre-

    sponding maximum values. The smear effects due to the installa-

    tion of stone columns are not considered in the three-dimensional

    nite-element analysis performed by Tan et al. (2008).   Fig. 4(b)

    compares the degrees of consolidation calculated from the newly

    proposed solution with those measured and computed reported by

    Tan et al. (2008) using the settlement data obtained at SP1. Fig. 4(c)

    compares the excess pore-water pressures at point A calculated

    from the newly proposed solution with those computed by   Tan

    et al. (2008). The dashed lines represent the calculated results for

    consolidation under impeded drainage boundary conditions

    without consideration of the smear effect. It can be seen that these

    results are in reasonably good agreement with those computedusing the three-dimensional   nite-element method, especially

    during the loading period. Nevertheless, when compared with the

    measured data, the calculated rates of consolidation are relatively

    signicantly overestimated by both the analytical solution and the

    three-dimensional   nite-element method, as shown in   Fig. 4(b).

    This is attributed to the fact that the smear effects are not consid-

    ered in both cases.

    To investigate the smear effects on consolidation, back-analysis

    using the newly proposed analytical solution is carried out with

    back-analysed parameters for smeared soil, which are listed in the

    last column of  Table 1. According to Weber et al. (2010), the radius

    of a smear zone is assumed to be 2.5 times the radius of the stone

    column, that is,  r s  ¼  2.5r d. The vertical hydraulic conductivity and

    volume compressibility of smeared soil are assumed to be the sameas those of undisturbed soil, that is,   ksv   ¼   kv   and  msv   ¼   mv. The

    horizontal hydraulic conductivity of smeared soil is assumed to be

    0.4 times that of undisturbed soil, that is,   ksh   ¼   0.4kh, which is

    within the range of 0.2khe0.8kh   derived from experiments (Hird

    and Moseley, 2000; Juneja et al., 2013; Rujikiatkamjorn et al.,

    2013; Sathananthan and Indraratna, 2006; Sharma and Xiao,

    2000). As shown in  Fig. 4(b), it is evident that the calculated re-

    sults including the smear effects (solid line) are consistent with the

    measured degrees of consolidation. This indicates that the smear

    effects are signicant and should not be ignored.

    To investigate the effect of loading conditions on consolidation,

    the degrees of consolidation and the excess pore-water pressures

    under instantaneous loading are also calculated using the proposed

    solution, as shown by the dotted lines in  Fig. 4(b) and (c). It can beobserved that the rate of consolidation and the rate of dissipation of 

    excess pore-water pressure are generally overestimated if an

    instantaneous loading condition is assumed. This indicates that a

    realistic modelling of the loading conditions is necessary for

    consolidation analysis.

    Ideally, an application of the proposed analytical solution should

    be compared with a case history involving consolidation with

    prefabricated vertical drains. However, as far as the authors are

    aware, documented case histories involving consolidation with

    prefabricated vertical drains where sand blanket is generally taken

    for granted as fully drained are not suitable for comparison here.

    The case history reported by   Tan et al. (2008)   involving consoli-

    dation of soft ground by stone columns is thus selected as it is

    relevant to an impeded drainage boundary. The calculated results

    may be considered as a   rst approximation to the analysis of 

    consolidation of soft clay with stone columns only, as the rein-

    forcement and arching effects due to the use of stone columns are

    ignored (Ali et al., 2014; Castro and Sagaseta, 2011, 2013; Elsawy,

    2013; Indraratna et al., 2013; Miranda et al., 2015; Shahu and

    Reddy, 2014; Zhang et al., 2012). It is evident that there is a lack

    of studies of in-situ permeability of sand blanket and its effect on

    consolidation of soil. Further studies on this topic appear to be

    warranted.

    6. Conclusions

    A rigorous, explicit, analytical equal-strain solution is proposed

    for a unit-cell model of consolidation with a vertical drain under

    impeded drainage boundary conditions and multi-ramp surcharge

    loading. The solution can also be used to analyse vertical-drain

    consolidation under fully drained boundary conditions and one-

    dimensional consolidation under impeded drainage and fully

    drained boundary conditions. Excellent agreement is obtained be-

    tween the calculated results from the special cases of the proposed

    solution and those from two available analytical solutions in the

    literature. The practical applicability of the solution to consolida-

    tion under impeded drainage boundary conditions is also explored

    employing a case study involving an embankment constructed over

    a crust layer of hardsoiland a layer of soft clay improved with stone

    columns. The calculated results from the proposed solution are

    shown to be in reasonably good agreement with measureddata and

    numerically-computed results, when the crust layer is modelled as

    an impeded drainage boundary and the smear effects are consid-

    ered. This suggests that the proposed solution is valid for the more

    general cases of drainage boundary conditions. Moreover, the case

    study shows that the smear effects on consolidation with stone

    columns are signicant and should not be ignored. Owing to a lack

    of  eld studies of in-situ permeability of sand blanket, further re-

    searches on its drainage effect on consolidation are needed in order

    to evaluate the practical applicability of the proposed analytical

    solution to the consolidation problem with prefabricated vertical

    drains.

     Acknowledgements

    This study was sponsored by the National Natural Science

    Foundation of China (grant number 51278171), the 111 Project

    (grant number B13024), the Fundamental Research Funds for the

    Central Universities of China (grant number 2015B06014), and the

    Chang Jiang Scholars Program of the Ministry of Education of China.

     Appendix A. Derivation of Eqs. (14) and (15)

    Consolidation of undisturbed soil

    The excess pore-water pressure of undisturbed soil can be

    expressed by introducing the Fourier sine and cosine series as

    follows:

     Table 1

    Calculation parameters adopted from Tan et al. (2008).

    Drain properties Soil properties Drainage boundary, stress and loading conditions Back-analysed parameters for smeared soil

    kd ¼  1.16   104 m/s   kv ¼  ksv  ¼  1.16   10

    9 m/s   R ¼  500   r s ¼  2.5r d ¼  1.0 m

    r d ¼  0.4 m   mv ¼  msv ¼  0.6753   103 kPa1 M  ¼  1   ksv  ¼  kv ¼  1.16   10

    9 m/s

    r s ¼  0.4 m   kh  ¼  ksh  ¼  3.47   109 m/s   s1 ¼  32.4 kPa   msv  ¼  mv ¼  0.6753   10

    3 kPa1

    r e ¼  1.2 m   t 1,1 ¼  9 d   ksh  ¼  0.4kh ¼  1.388   109 m/s

    h ¼  5.0 m

    G.H. Lei et al. / Geotextiles and Geomembranes xxx (2015) 1e10   5

    Please cite this article in press as: Lei, G.H., et al., Vertical-drain consolidation using stone columns: An analytical solution with an impededdrainage boundary under multi-ramp loading, Geotextiles and Geomembranes (2015), http://dx.doi.org/10.1016/j.geotexmem.2015.07.003

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    T h ¼  kht 

    mvgwð2r eÞ2

      (A21)

    vn ¼  2Un

    ðmnr eÞ2

      (A22)

    Based on Eqs. (A10), (A12) and (A20), Eq. (A2) can be rewritten

    as

    uðr ; z ; t Þ ¼X∞n¼1

    ½c 1nI 0ðmnr Þ þ c 2nK 0ðmnr Þ þ 1

    ane

    8T hvn   þ mvfn f n

    ½sinðun z Þ þ cotðunhÞcosðun z Þ

    (A23)

    Consolidation of smeared soil

    Again by introducing the Fourier sine and cosine series, the

    excess pore-water pressure at any arbitrary point and the average

    excess pore-water pressure at a given depth of smeared soil can be

    expressed in accordance with Eqs. (7) and (8) of the top and bottomhydraulic boundary conditions as follows:

    usðr ; z ; t Þ ¼X∞n¼1

    usnðr ; t Þ½sinðun z Þ þ cotðunhÞcosðun z Þ   (A24)

    usð z ; t Þ ¼X∞n¼1

    usnðt Þ½sinðun z Þ þ cotðunhÞcosðun z Þ   (A25)

    where usn  and  usn  are their corresponding Fourier coef cients.

    Using the method of separation of variables, the following

    equation can be written:

    usnðr ; t Þ ¼  Asnðr ÞBsnðt Þ   (A26)

    Following the same derivation procedures as above for the

    consolidation of undisturbed soil, the following solution to Eq.(A26) for the consolidation of smeared soil can be obtained:

     Asnðr Þ ¼  lsnfsn½c 3nI 0ðmsnr Þ þ c 4nK 0ðmsnr Þ þ 1   (A27)

    Bsnðt Þ ¼  1

    lsnfsn

    asne

    8T shvsn   þ msvfsn f n

      (A28)

    where lsn   is the separation constant;  c 3n,  c 4n and  asn  are the con-

    stants of integration to be determined; and

    fsn  ¼  gw

    ksvu2n(A29)

    m2sn ¼ ksvu2n

    ksh(A30)

    T sh ¼  ksht 

    msvgwð2r sÞ2

      (A31)

    vsn ¼  2Usn

    ðmsnr sÞ2

      (A32)

    Thus, Eq. (A24) can be rewritten as

    usðr ; z ;t Þ ¼X∞n¼1

    ½c 3nI 0ðmsnr Þ þ c 4nK 0ðmsnr Þ þ 1

    asne

    8T shvsn   þ msvfsn f n

    ½sinðun z Þ þ cotðunhÞcosðun z Þ

    (A34)

    In the following sections, the constants of integration in Eqs.(A23) and (A34) are determined according to the initial conditions

    and the vertical hydraulic boundary conditions, together with the

    equations of drain resistance and interface drainage.

    Initial conditions

    Without loss of generality, the initial average excess pore-water

    pressures for undisturbed soil and smeared soil are assumed to be

    uð z ; t  ¼  0Þ ¼  1

    p

    r 2e    r 2s

    Z r er s

    uðr ; z ; t  ¼  0Þ2pr dr  ¼  s0   (A35)

    usð z ; t  ¼  0Þ ¼  1

    p

    r 2s    r 

    2d

    Z r sr d

    usðr ; z ; t  ¼  0Þ2pr dr  ¼  s0   (A36)

    Substituting Eq. (A23) into Eq. (A35) and substituting Eq. (A34)

    into Eq. (A36) yield

    an ¼  sn;0

    Un mvfn

    sn;1

    t 1;1  t 1;0(A37)

    asn ¼ sn;0

    Usn msvfsn

    sn;1

    t 1;1  t 1;0(A38)

    where  sn,0 is the Fourier coef cient of Fourier series expansions of 

    the initial increase in total vertical stress  s0 as shown in Fig. 2.

    In order to ensure continuity of pore-water pressure and  owrate at all times, the time functions for the consolidation of un-

    disturbed soil and smeared soil must be the same, i.e.,

    Bnðt Þ ¼ Bsnðt Þ   (A39)

    Substituting Eqs. (A20) and (A28) into Eq. (A39) yields

    1

    lnfn

    ane

    8T hvn   þ mvfn f n

     ¼

      1

    lsnfsn

    asne

    8T shvsn   þ msvfsn f n

    (A40)

    Eq. (A40) requires that

    an

    asn¼  lnfn

    lsnfsn(A41)

    ln

    lsn¼

      mvmsv

    (A42)

    T hvn

    ¼ T shvsn

    (A43)

    Usn ¼  1 þ2c 3n½msnr sI 1ðmsnr sÞ msnr dI 1ðmsnr dÞ 2c 4n½msnr sK 1ðmsnr sÞ msnr dK 1ðmsnr dÞ

    ðmsnr sÞ2 ðmsnr dÞ

    2  (A33)

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    It can be readily proved that by Eqs.  (A37), (A38), (A41) and

    (A42), Eq. (A43) is satised.

    For the initial conditions specied in Eq.   (10)   and   Fig. 2, i.e.

    s0 ¼  0 and sn,0 ¼  0, Eq. (A37) becomes

    an  ¼ mvfnsn;1

    t 1;1  t 1;0(A44)

    By substituting Eqs. (A9) and (A44) into Eq. (A20), the followinggeneralised time function can be derived:

    Bnðt Þ ¼ mvfnlnfn

    XM i¼1

    (sn;i  sn;i1

    t i;1  t i;0

    "e

    8ðT h T hi;1Þvn

      H hT hT hi;1i e8ðT hT hi;0Þ

    vn  H hT hT hi;0i

    #)   (A45)

    where

    T hi; j ¼kht i; j

    mvgwð2r eÞ2;   j ¼  0; 1 (A46)

    Drain resistance

    Substituting Eq. (A34) into Eq. (4)  yields

    c 3n ¼  1

    D1I 1ðmsnr dÞ  I 0ðmsnr dÞ þ D2c 4n   (A47)

    D1  ¼  2

    r d

    kshkd

    msn

    u2n

    (A48)

    D2  ¼ D1K 1ðmsnr dÞ þ K 0ðmsnr dÞ

    D1I 1ðmsnr dÞ  I 0ðmsnr dÞ  (A49)

    For an ideal drain without drain resistance,  kd  ¼ ∞, and hence

    D1  ¼  0.

    Interface continuity

    Substituting Eqs.   (A23) and (A34)   into Eqs.   (5) and (6)   and

    considering Eq. (A40) yield

    lnfn½c 1nI 0ðmnr sÞ þ c 2nK 0ðmnr sÞ þ 1

    ¼ lsnfsn½c 3nI 0ðmsnr sÞ þ c 4nK 0ðmsnr sÞ þ 1   (A50)

    khlnfnmn½c 1nI 1ðmnr sÞ  c 2nK 1ðmnr sÞ

    ¼ kshlsnfsnmsn½c 3nI 1ðmsnr sÞ  c 4nK 1ðmsnr sÞ   (A51)

    Substituting Eq. (A47) into Eqs. (A50) and (A51) gives

    anc 1n þ bnc 2n þ D4  ¼  0 (A52)

    where

    an ¼  I 0ðmnr sÞ

     ffiffiffiffiffiffiffiffiffiffiffiffiffikhkv

    kshksv

    s   I 1ðmnr sÞD3   (A53)

    bn ¼  K 0ðmnr sÞ þ  ffiffiffiffiffiffiffiffiffiffiffiffiffikhkv

    ksh

    ksv

    s    K 1ðmnr sÞD3   (A54)

    D3  ¼ D2I 0ðmsnr sÞ þ K 0ðmsnr sÞ

    D2I 1ðmsnr sÞ  K 1ðmsnr sÞ  (A55)

    D4  ¼ msv

    mv

    kvksv

    D3I 1ðmsnr sÞ  I 0ðmsnr sÞ

    D1I 1ðmsnr dÞ  I 0ðmsnr dÞ  1

    þ 1 (A56)

    Vertical hydraulic boundary conditions

    Substituting Eq. (A23) into Eq. (9)  yields

    c 1nI 1ðmnr eÞ  c 2nK 1ðmnr eÞ ¼  0 (A57)

    The following can be derived from Eqs. (A52) and (A57):

    c 1n  ¼ D4K 1ðmnr eÞ

    Dn(A58)

    c 2n  ¼ D4I 1ðmnr eÞ

    Dn(A59)

    Dn ¼ anK 1ðmnr eÞ bnI 1ðmnr eÞ   (A60)

    Substituting Eqs. (A47), (A58) and (A59) into Eq. (A50) leads to

    c 4n  ¼  mvmsv

     ffiffiffiffiffiffiffiffiffiffiffiffiksvkhkvksh

    s   c 1nI 1ðmnr sÞ  c 2nK 1ðmnr sÞ

    D2I 1ðmsnr sÞ  K 1ðmsnr sÞ

      I 1ðmsnr sÞ

    ½D1I 1ðmsnr dÞ  I 0ðmsnr dÞ½D2I 1ðmsnr sÞ  K 1ðmsnr sÞ

    (A61)

    The nal solution

    Based on Eqs.   (A2), (A10), (A12) and (A45), Eq.   (14)   can be

    formulated for calculating the excess pore-water pressure of un-

    disturbed soil. Similarly, based on Eqs. (A24), (A26), (A27), (A39),

    (A42) and (A45), Eq.  (15) can be derived for calculating the excess

    pore-water pressure of smeared soil.

     Appendix B. Derivation of Eq. (A7)

    According to Eq. (A5), the following equation can be derived.

    The numerator term of Eq.  (B1) can be easily derived as

    Z h0

    si½sinðun z Þ þ cotðunhÞcosðun z Þd z  ¼  si

    un(B2)

    sn;i  ¼

    Z   h0

    si  sin   un z ð Þ þ cot   unhð Þcos   un z ð Þ½ d z 

    P∞

    n¼1

    Z   h0

    sin   un z ð Þ þ cot   unhð Þcos   un z ð Þ½  sin   um z ð Þ þ cot   umhð Þcos   um z ð Þ½ d z 

    ( )   (B1)

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    To obtain the denominator term of Eq.   (B1), the following

    triangular orthogonal relation is needed. For m s n, itcanbe shown

    that

    Z h0

    ½sinðun z Þ þ cotðunhÞcosðun z Þ

    ½sinðum z Þ þ cotðumhÞcosðum z Þd z 

    ¼

    Z h0

    cos½unðh  z Þ

    sinðunhÞ  ,

    cos½umðh  z Þ

    sinðumhÞ  d z 

    ¼

    Z h0

    cos½ðum þ  unÞðh  z Þ þ cos½ðum   unÞðh  z Þ

    2 sinðunhÞsinðumhÞ  d z 

    ¼

    1

    um þ unsin½ðum þ  unÞh þ

      1

    um   unsin½ðum   unÞh

    2 sinðunhÞsinðumhÞ

    ¼ um sinðumhÞcosðunhÞ  un cosðumhÞsinðunhÞ

    ðu

    m þ u

    nÞðu

    m  u

    nÞsinðu

    nhÞsinðu

    mhÞ

    ¼  umh tanðumhÞ   unh tanðunhÞ

    ðum þ  unÞðum  unÞh tanðunhÞtanðumhÞ

    (B3)

    According to Eq. (A3), Eq. (B3) for  m  s n  is

    Z h0

    ½sinðun z Þ þ cotðunhÞcosðun z Þ

    ½sinðum z Þ þ cotðumhÞcosðum z Þd z  ¼  0

    (B4)

    Therefore, the denominator term of Eq. (B1) can be derived as

    By substituting Eqs.   (B2) and (B5)   into Eq.   (B1), Eq.   (A7)   in

    Appendix A can be obtained.

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    P∞n¼1

    Z h0

    sin   un z ð Þ þ cot   unhð Þcos   un z ð Þ½  sin   um z ð Þ þ cot   umhð Þcos   um z ð Þ½ d z 

    8<:

    9=;

    ¼

    Z h0

    sin   un z ð Þ þ cot   unhð Þcos   un z ð Þ½ 2d z 

    ¼ unh þ sin   unhð Þcos   unhð Þ

    2un sin2unhð Þ

    (B5)

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    Please cite this article in press as: Lei, G.H., et al., Vertical-drain consolidation using stone columns: An analytical solution with an impededdrainage boundary under multi-ramp loading, Geotextiles and Geomembranes (2015), http://dx.doi.org/10.1016/j.geotexmem.2015.07.003

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