Application of a modified structural clay model considering anisotropy to embankment behavior

Transcription

1 Geomechanics and Engineering, Vol. 13, No. 1 (17) - DOI: htts://doi.org/1.1989/gae Alication of a modified structural clay model considering anisotroy to embankment behavior Hao Zhang 1a, Qiushi Chen b, Jinjian Chen 1c and Jianhua Wang 1 1 Deartment of Civil Engineering, Shanghai Jiao Tong University, Shanghai, 4, China Glenn Deartment of Civil Engineering, Clemson University, Clemson, SC 9634, USA (Received January 5, 16, Revised March 31, 17, Acceted February 16, 17) Abstract. Natural clays exhibit features such as structural and anisotroy. In this work, a constitutive model that is able to relicate these two salient features of natural clays is resented. The roosed model is based on the classical S-CLAY1 model, where the anisotroy of the soil is catured through the initial inclination and rotation of the yield surface. To account for the structural of the soil, the comression curve of the reconstituted soil is taken as the reference. All arameters of the roosed constitutive model have clear hysical meanings and can be conveniently determined from conventional triaxial tests. This roosed model has been used to simulate the behavior of soft soil in the undrained triaxial tests and the erformance of Murro embankment in terms of settlement and horizontal dislacements during embankment construction and consolidation stage. Results of numerical simulations using roosed model have been comared with the field measurement data. The comarisons show that the two features significantly influence the rediction results. Keywords: constitutive model; structural; anisotroy; soft clay; embankment 1. Introduction Construction of embankment on soft soil has seen a considerable increase due to economic and societal develoment in recent years (Leroueil and Vaughan 199). As the influence of fundamental features such as anisotroy and structural, the stress-strain behavior of soft soil is very comlex and highly nonlinear. Predicting the behavior of embankment on soft soil remains a major challenge in geotechnical engineering. Anisotroy of natural soil is develoed during the long-term sedimentation, which greatly affects its mechanical behavior. Neglecting such effect may greatly undermine the accuracy of redicted behavior (Zdravkovic et al. ). To this end, a variety of constitutive models accounting for lastic anisotroy of natural soils have been develoed (Dafalias 1986, Whittle and Kavvadas 1994, Marcin et al. 1, and Pieter 4). The S-CLAY1 model (Wheeler et al. 3) introduces a rotational hardening comonent to account for the influence of lastic anisotroy in soft clays. The rotational hardening law of this model includes deendence on lastic shear strain Corresonding author, Professor, wjh417@163.com a Ph.D., zhanghao839@163.com b Ph.D., qiushi@clemson.edu c Ph.D., chenjj@sjtu.edu.cn Coyright 17 Techno-Press, Ltd. htt:// ISSN: 5-37X (Print), (Online)

2 Hao Zhang, Qiushi Chen, Jinjian Chen and Jianhua Wang increment as well as lastic volumetric strain increment. Based on the S-CLAY1 model, Yin (Karstunen and Yin 1) roosed an elasto-viscolastic model EVP-SCLAY1S, which is able to account for time-deendent large strain anisotroy, bonding and structural of naturally clays. And the model has been alied to simulate the behavior of Murro clay. Generally, the natural soils exhibit higher strength than the reconstituted soils at the same void ratio. The difference has been attributed to the structural of soils (Locat and Lefebvre 1985, Burland 199, Cotecchia and Chandler, Nakano et al. 5, Ng et al. 11, Yin 1, Chen et al. 14). Exeriment studies (Burland 199, Leroueil and Vaughan 199, Vincenzo and Ghassan 9, Cheng and Wang 16) show that when natural soils are comressed, the structural is rogressively lost and soil starts to behave as a reconstituted soil. Great rogress has been made in recent years to account for the behavior of soil with natural structural in constitutive models (Leroueil et al. 1979). Based on the MCC (Modified Cam Clay) model, Chai et al. (4) introduced structural-deendent comression behavior and discussed the effects of structural on the calculated load-settlement curve. Liu and Carter () and John et al. (5) introduced a simle redictive model for naturally structural clay. The model incororated the effect of structural on the volumetric deformation behavior and the lastic strain direction into the MCC model. The effect of structural on volumetric deformation is taken into account by a arameter called additional void ratio. Based on an intrinsic yield surface roosed by Gens and Nova (1993), Karstunen et al. (5) develoed an S-CLAY1S model for structural soils. Kimoto and Oka (5) develoed a rate-deendent model accounting for structural and inherent anisotroy. In this study, an anisotroic lasticity model for clays that is able to account for structural is develoed. The model is extended from the classical S-CLAY1 model (Wheeler et al. 3), the initial and induced anisotroy is exressed through the initial inclination and subsequent rotation of the yield surface. The structural effect was exressed through the exonential function of Δe (the additional void ratio between the structural soil and reconstituted soil) (Jirayut et al. 1). The roosed model is then imlemented into a finite element code and is used to simulate undrained triaxial tests and Murro test embankment in terms of settlement and horizontal dislacement. The results of the numerical analyzes are comared with the exerimental results and field dates to demonstrate the alicability of the roosed model and the effects of anisotroy and structural on redicted behavior.. Formulation of the constitutive model.1 Descrition of anisotroy The starting oint of the roosed constitutive model is the S-CLAY1 model (Wheeler et al. 3). The yield surface of S-CLAY1 model is a sheared ellise in -q sace, as shown in Fig. 1. The inclination of the yield curve is described with a scalar arameter, which is identical to that roosed by Dafalias (1987) f q M (1) ' ' ' ( ') ( )( m ) Where M is the stress ratio η (= q/ ) at the critical state. There are two internal variables in the S-CLAY1 model, i.e., m and α, which control the size and inclination of the yield surface resectively. The arameter is a measure of the degree of lastic anisotroy of the soil. For isotroic behavior, = and Eq. (1) reduces to the Modified Cam Clay (MCC) yield surface.

3 Alication of a modified structural clay model considering anisotroy to embankment behavior Yield surface M α Fig. 1 S-CLAY1model yield surface S-CLAY1 model incororates two hardening laws describing the evolution of two internal variables. The first hardening law, which is the same as that in the MCC model, describes changes in the size of the yield surface (d m ) caused by changes of lastic volumetric strain d v d ' m ' vmd v () where is the sloe of the ost-yield comression curve for a constant η stress ath involving no change of anisotroy ( = constant) and is the sloe of elastic swelling lines. The second hardening law describes changes in the inclination of the yield surface associated with changes in lastic volumetric strain and shear strain: 3 d dv d s 4 3 (3) where the arameter μ controls the rate at which the comonents of the deviatoric fabric tensor moves toward their current target values, which deends on the stress ath. Parameter controls the relative effect of lastic shear strain in rotating the yield and loading surfaces, the arameters can be exressed by (Wheeler et al. 3) 3(4M 4 3 ) 8( ) K K K M K (4) The exression of the control arameter μ can be obtained from (Sheng et al. ) e M 1 1 ln n M (5) To interolate M between its values M c (for comression) and M e (for extension), a modification of M by means of the Lode angle is assumed (Sheng et al. ) 4 c M Mc c (1 c )sin 3 1/4 (6)

4 Hao Zhang, Qiushi Chen, Jinjian Chen and Jianhua Wang in which c M / M (7) e c J sin 3 3/ 6 3 J 6 (8) where J and J 3 are the second and the third invariants of the deviatoric stress tensor, resectively. Table 1 Selected aroaches to account for soil structural in current constitutive models Models Internal variables for structural Key arameters Comments Gajo and Wood 1 k dr ( r 1) d d d (1 A)( d ) A( d ) d v s r is a variable describing the current degree of cementation. k and ψ are structural control arameters. is the d generalized lastic strain. Consider the influence of both lastic volumetric strain and lastic shear strain. Parameter k and ψ cannot be determined in a direct way. Liu and Carter ' yi e ei ' s b Δe is the additional void ratio, i.e., the difference in void ratio between natural and reconstituted soil at same confining stress. b is the structural control arameter. Back analysis is needed to determine the arameter b. The influence of lastic shear strain is not considered. Marcin and Pieter 4 b b a n k k k ex is the state variable reresenting bonding, a is the control k arameter, n is the normal invariant of lastic micro strain. b k The determination of arameter a is very difficult, and the simulation result is very sensitive to this arameter. The influence of lastic shear strain is not considered. Koskinen et al. d a d b d v s χ is the amount of bonding, constant a controls the absolute rate of destructuration and b controls relative rate of structuration Parameters a and b are fitting arameters without clearn hysical meanings. Kavvadas and Amorosi 1 e da a v ex( v, v ) v q q q s d s d ex(, ) Parameter a reresents the size of the yield surface of structural soil. (ζ v, η v ) are the volumetric structural degradation arameter, and (θ q, ζ q, η q ) are deviatoric structural degradation arameters. This model contains more fitting arameters and are difficult to determine in calibration rocess.

5 Void ratio,e Alication of a modified structural clay model considering anisotroy to embankment behavior. Proosed descrition of structural Based on the MCC (Modified Cam Clay) model, taking the yield surface of reconstituted soil as reference, many different constitutive models have been roosed to account for the influence of structural. The aroaches to incororate structural of these models are summarized in Table 1. In this study, based on the destructuration law roosed by Liu and Carter (), a new direct method to determine the structural arameters is roosed, which incororates the influence of lastic shear strain on the structural, as well be detailed in the following. Taking the comression curve of reconstituted soils as reference, Liu and Carte () roosed the use of the additional void ratio ( e) to reresent the difference between the structural (natural) soil and its corresonding reconstituted soil. e decreases with the increase of stress and will eventually diminish at a very high stress, as shown in Fig. (b). The yield surface for structural and corresonding reconstituted soil is shown in Fig. (a). The comression behavior during the destructuring rocess of structural soil can be equivalently catured by describing the change in the void ratio, i.e., * e e e (9) where e is the void ratio of structural soil, e* is the void ratio of reconstituted clay at the same stress state and can be exressed as * IC * * e e * ln ' (1) IC where e is the void ratio at a reference mean effective stress on the comression line of a * reconstituted soil. For instance, e IC can be aroximately taken as the initial void ratio of corresonding natural clay, λ * is the sloe of the comression line of reconstituted soil. The void ratio e for a structural soil can be exressed in terms of the corresonding void ratio for the reconstituted soil e*, the additional void ratio e as (Liu and Carter ) ' yi ' * * e e e e ei b (11) yield surface of structural soil yield surface of reconstituted soil M α Comression line of structural soil: e=e*+δe Comression line of reconstituted soil (a) Fig. Material idealization of structural soil: (a) Yield surfaces for structural soil and reconstituted soil; (b) One-dimensional comression behavior of structural soil showing the additional void ratio ( e) (b)

6 Hao Zhang, Qiushi Chen, Jinjian Chen and Jianhua Wang where yi is the yield stress and is the current mean stress, b is called the structural index due to volumetric deformation. At the yield oint, the increment of void ratio increment for a structural soil (de) and reconstituted soil (de*) can be exressed as de d ' (1) n ' yi * de* d ' (13) Combining Eqs. (11), (1) and (13), arameter b can be obtained and it is written as b ' yi e * n (14) In the roosed model, it is assumed that the yield surface of reconstituted soil has the same shae as the yield surface of structural soil. d and reresent the size of yield surface for reconstituted soil and structural soil, resectively, they can be related through the arameter Δe i e ' ' d ex * (15) where the additional void ratio Δe serves as a structural arameter and its initial value Δe i can be determined from one-dimensional conventional comression tests, as shown in Fig.. The value of Δe decreases with the develoment of lastic deformation. Eq. (15) deicts the relationshi between the strucutral arameter Δe and the corresonding effective stress. The value of Δe is changing owing to structural degradation, ultimately to zero, analogously to the S-CLAY1S model (Karstunen et al. 5). v d s d e e d d (16) Where ξ and ξ d are structural arameters, in which arameter ξ controls the absolute rate of destructuration and ξ d controls relative effectiveness of lastic deviatoric strains and lastic volumetric strain in destroying the bonding. In this study, an aroach to determine these two structural arameters is roosed as follows. Under isotroic comression condition, the lastic shear strain is zero. When the effective stress is greater than the yield stress yi, the bonding arameter increment d(δe) of structural soil can be exressed as (see Fig. 3) d e de de e d * n n r (1 ) v n (17) Where de n and de r is the lastic void ratio increment of structural soil and reconstituted soil, resectively. Let ξ = (1 + e )(λ n λ * )/(λ n κ), then the Eq. (17) can be written as

7 Alication of a modified structural clay model considering anisotroy to embankment behavior Structural soil Reconstituted soil Fig. 3 Isotroic comression curves d e d (18) Parameter ξ d can be determined from a one-dimensional comression test, for the onedimensional comression test, the initial inclination angle α remains constant, i.e., α = α K. The relationshi d / d can be derived as s v v d ( ) d s K K v M K (19) Where η K is the stress ratio of the one-dimension comression test, which can be written as K 3 1 K 1 K () The void ratio increment d(δe) can be exressed from Eq. (11) be d e d ' (1) ' Combined with Eqs. (16), (19) and (1), it can be obtained that n M K d ( K K ) b 1 e M K () In the above analysis, arameter ξ can be exressed as (3) * (1 e )( n ) ( n ) Combined with Eqs. () and (3), the exression of arameter ξ d can be obtained d * M K b n * ( K K ) n (4)

8 Hao Zhang, Qiushi Chen, Jinjian Chen and Jianhua Wang Table State arameters and soil constants of this model Grou Parameter Definition Determination Modified Cam Clay arameters Structural arameters Anisotroy arameter yi Initial size of yield surface From consolidation test on structural soil e Initial void ratio Standard manner from undisturbed samle ν Poisson s ratio Tyically ( ) κ Sloe of the swelling line From consolidation test λ n λ * Sloe of the comression line of structural soil Sloe of the intrinsic comression line From consolidation test on intact samle From consolidation test on reconstituted samle M Sloe of the critical state line From triaxial shear test in comression e i ξ ξ d α μ β Additional void ratio Absolute rate of bond degradation Relative rate of bond degradation Initial inclination angle of yield surface Absolute rate of yield surface rotation Relative rate of yield surface rotation From one consolidation test on structural soil and one isotroic consolidation test on reconstituted soil From oedometer test and i otroic consolidation or Eq. (3) From oedometer test and isotroic consolidation or Eq. (4) From one-dimensional consolidation test From Eq. (5) From Eq. (4) Considering the influence of structural, the yield surface equation can be exressed as 1/ b ' ei yi f ( q ') ( M ) ' ' e The material constants and state variables in the roosed model can be classified into three grous, which are summarized in Table. This roosed model has been imlemented into the finite element code ABAQUS as a usermaterial soil model. By switching on/off certain arameters, this roosed model can be reduced to only consider anisotroy ( e i = ξ = ξ d = ) or structural (α = μ = β = ), which can also be reduced to Modified Cam Clay (MCC) (α = μ = β = and e i = ξ = ξ d = ). (5) 3. Model arameters and element test 3.1 Determination of the structural arameter It can be seen from Eq. (4) that the structural arameter ξ d can be directly related to the

9 e e e e e e e e Alication of a modified structural clay model considering anisotroy to embankment behavior Table 3 Calculate of arameter b for 8 structural soils Clay λ n λ * κ e i yi (kpa) b = (λ n λ * )/ e i Bothkennar Sault Ste Marie Troll Pisa Osaka Lianyungang Wenzhou Shanghai Bothkennar clay Equation(11).4 Lianyungang clay Equation(11).4 Sault ste Marie clay Equation(11) ln' ln' ln' (a) (b) (c).4 Pisa clay Equation(11).4 Wenzhou clay Equation(11).4 Osaka clay Equation(11) ln' ln' ln' (d) (e) (f).4 Troll clay Equation(11).4 Shanghai clay Equation(11) ln' (g) ln' Fig. 4 Comression curves for 8 structural clays: (a) Bothkennar clay; (b) Lianyungang clay; (c) Sault ste Marie clay; (d) Pisa clay; (e) Wenzhou clay; (f) Osaka clay; (g) Troll clay; (h) Shanghai clay (h)

10 q(kpa) Hao Zhang, Qiushi Chen, Jinjian Chen and Jianhua Wang arameter b, which has a clear hysical meaning and controls the shae of the comression curve of structural soil. The arameter b can be easily calculated using Eq. (11). To illustrate this oint, consolidation test data for 8 structural soil samles are collected from the literatures (Yin 1, Nakano et al. 5, Ng et al. 11) and the arameter b is calculated and used to simulate the comression curve for the 8 structural soils according to Eq. (11). The values of comression arameters are listed in Table 3. Fig. 4 shows the comarisons between the simulated results with Eq. (11) and the exeriment results. It can be seen that for the structural clay, there is an obvious yield oint for the comression curve. The simulated results agree well with the exeriment results when the arameter b of Eq. (14) is adoted. The curve fitting is not needed to determine the structural arameter b as the original model roosed by Liu and Carter (), which simlify the calibration rocess. 3. Triaxial tests on soft clays In this section, the roosed constitutive model is emloyed to simulate the behavior of Vallericca clay (Burland et al. 1996) and Shangai clay (Huang et al. 11) in triaxial comression tests. The values of inut arameters of the two clays are listed in Table 4. The triaxial drained test on Shanghai soft clay was carried out by Huang et al. (11). The samles were anisotroically consolidated to their in-situ stress states. In the shearing stage, the secimens were loaded in drained condition following different stress aths, which are reresented by the angle between stress ath and -axis in the -q lane, Fig. 5 shows the loading stress aths, the exerimental results of SCD5 and SED56 have been selected to comare with the simulated results. Fig. 6 shows the simulated triaxial comression test results with the angle between stress ath and -axis being 5 degree. It can be seen that the simulation using roosed constitutive model Table 4 Model arameters for Vallericca and Shanghai clays Clay λ n λ* M ν e yi (kpa) ξ ξ d Δe i Vallericca Shanghai SCD9 SCD6 SCD5 SCD9 1 SCD15 θ SCD 1 '(kpa) 3 SED-15 SED-56 SED-9-1 Fig. 5 Standard consolidation stress aths

11 (KPa) q(kpa) (KPa) q(kpa) Alication of a modified structural clay model considering anisotroy to embankment behavior 5 SCD5 5 SCD5 15 Exeriment Simulated 15 Exeriment Simulated v (%) (a) - v s (%) (b) q- s Fig. 6 Simulated and observed results of SCD Exeriment Simulated SED56 5 SED v (%) (a) - v Exeriment Simulated s (%) (b) q- s Fig. 7 Calculated and observed results of SED56 catures the stress-strain behavior very well. There exists a turning oint, which is the characteristic for structured clay. Fig. 7 shows the simulated triaxial extension test results with the angle between stress ath and -axis being 56 degree. The turning oint of the simulated results under extension condition is not as obvious as the results under comression condition. This is due to the fact that structural of clay has less influence on the stress-strain behavior when loaded under extension condition comared to load under comression condition. In both comression and extension tests, the simulations cature the exerimental behavior very well. The triaxial undrained tests on Vallericca clay were carried out by Burland et al. (1996). In the tests, the clay samle was first anisotroically consolidated u to an effective stress of max that is larger than yi, and then the samle was swelled back to different OCRs. Results of the simulation using roosed model were shown in Fig. 8. It can be seen that although there are some discreancies in the effective stress ath results, in general, the simulated results agree with the exerimental results very well. The dilatancy trend was well catured. Moreover, in the overconsolidation region, the eak strength line is exceeding the critical state line, which means that the overconsolidated clays are tougher than the reconstituted clays.

12 q(mpa) q(mpa) Hao Zhang, Qiushi Chen, Jinjian Chen and Jianhua Wang 5 4 Exeriment OCR=1. Exeriment OCR=1.7 Exeriment OCR=.4 Simulated CSL Exeriment OCR=1. Exeriment OCR=1.7 Exeriment OCR=.4 Simulated '(mpa) (a) ε 1 (%) Fig. 8 Comarison between exerimental and simulated results of Vallericca clay: (a) Effective stress aths; (b) Deviatoric stress versus axial strain (b) 4. Numerical examle Murro test embankment 4.1 Murro test embankment The roosed constitutive model is alied to simulate behavior of the Murro test embankment, which was constructed on a soft clay deosit in Finland in 1993 and has been analyzed in a number of revious studies (Karstunen and Yin 1, Karstunen et al. 5). The urose of this numerical examle is to demonstrate the alicability and caability of the roosed constitutive model, in articular, the effect of anisotroy and structural on the redicted embankment resonse. The embankment is m high and 3 m long with a 1: sloe. Dimension of the embankment is shown in Fig. 9(b). The embankment was constructed within days. The instrumentation of Murro test embankment included 7 settlement lates (S1-S7), inclinometers (I1-I), 1 extensometer (E) and 8 ore ressure robes (U1-U8), the layout of which is shown in Fig. 9. More details on the embankment can be found in Karstunen and Yin (1). 4. Model arameters for Murro clay The embankment was constructed using crushed rock which was modeled using a linear elastic erfectly-lastic Mohr Coulomb model. Model arameters have been calibrated as [1] : Young s modulus E = 4 kn/m, Poisson ratio ν =.35, friction angle ϕ c = 4, dilation angle ψ = and unit weight γ = 19.6 kn/m 3, aarent cohesion c = kn/m. The roosed constitutive model will be used to model the underlying soft clay. The values of clay arameters e, λ i, λ n, κ, k h, k v, yi, α, Δe i, M and ν were estimated from laboratory results on intact and reconstituted samles. These arameters have been determined by Karstunen and Yin (1) from conventional triaxial and oedometer tests. Anisotroy arameter μ and β are obtained from Eqs. (5) and (4), structural arameters ξ and ξ d are obtained from Eqs. (3) and (4) resectively. All the model arameters are summarized in Table 5.

13 6m 1.5m 5.5m 8m 6.5m m m 3.5m m 5m 1.5m.5m 5m Alication of a modified structural clay model considering anisotroy to embankment behavior S5 m 1m A S6 U5 U6 U7 U8 U1 U U3 U4 S1 S S3 S4 E 3m I1 I A S7 (a) 1m I1 I S6 S1 S(S5,S7) S3 S4 U U5 U7 U3 U4 U1 U8 Fig. 9 Murro test embankment with details of instrumentation: (a) lan view; (b) rofile view along A-A cross section (b) Table 5 Values of arameters for Murro clay Deth (m) γ (kn/m 3 ) e M K κ λ i λ n ν Δe i Deth (m) ξ ξ d μ β k h (m/s) k v (m/s) E-9 1.9E E E E E E-9 1.5E E E E E-9

14 8m 5m 3.3m 3.7m m 1.6m Hao Zhang, Qiushi Chen, Jinjian Chen and Jianhua Wang 5m 4m 7m Fig. 1 Finite element mesh for Murro test embankment on soft clay 4.3 Finite element models In the finite element method analysis, the lain strain condition was assumed and the finite element mesh is shown in Fig. 1. The groundwater table was located at a deth of.8 m. Vertical boundaries on both sides are restrained from moving horizontally, and the bottom boundary is restrained in both directions. Due to symmetry, only half of the embankment is reresented in the finite element mesh. The finite element mesh is comosed of 156 elements with 81 nodes, the element tye above the ground table is CPE4, and CPE4P below the ground table. Several locations for reorting settlement are monitored in the simulation, which corresond to S, S4, S5, S6 and S7 in the field-monitoring lan. The location for reorting horizontal dislacements is 5 m off the centerline which corresonds to inclinometer I1 in the fieldmonitoring lan. The embankment loading was added within two days to mimic real construction. To illustrate the influence of different model features on the simulated embankment behavior, three scenarios are considered: (1) no anisotroy and structural features were turned on, which corresonds to (α = μ = β = and e i = ξ = ξ d = ); () only anisotroy was included, which corresonds to ( e i = ξ = ξ d = ); and (3) both anisotroy and structural were activated, in which case the inut arameters are the same as listed in Table Settlement and horizontal dislacement Fig. 11 shows the surface settlements under the centerline and 5 m off the centerline of the embankment by the roosed model with or without clay anisotroy and structural. It can be seen that the influence of anisotroy and structural is not very clear at the early stage. However, the difference among simulation results with or without clay anisotroy and structural become rofound in the long-term. Clay anisotroy results in larger settlement than that in the isotroic case, then the destructuration further increases redicted the settlement. It shows the influence of anisotroy and structural influence on the settlement over the long term. Fig. 1 shows the measured and simulated horizontal dislacements immediately after construction and after 8 years, resectively. The influence of the two features on the horizontal dislacement is similar to that on the settlement. Simulation without anisotroy and structural

15 Height(m) Height(m) Settlement(m) Settlement(m) Alication of a modified structural clay model considering anisotroy to embankment behavior. Time(days) 1 3. Time(days) S S5 S7 Proosed model Proosed model( ei=ξ=ξd =) Proosed model(α=μ=β= ei=ξ=ξd =) (a) Under the centerline S4 S6 Proosed model Proosed model( ei=ξ=ξd =) Proosed model(α=μ=β= ei=ξ=ξd =) (b) 5 m from centerline Fig. 11 The settlement-time curves: (a) under the centerline; (b) 5 m from centerline Proosed model 5 Proosed model ( ei=ξ=ξd =) Proosed model(α=μ=β= ei=ξ=ξd =) Measured Dislacement(mm) (a) Immediately after construction Proosed model 5 Proosed model(α=μ=β= ei=ξ=ξd =) Proosed model ( ei=ξ=ξd =) Measured Dislacement(mm) (b) After 8 years Fig. 1 The measured and simulated horizontal dislacements underneath crest of embankment sloe redicts the small horizontal dislacements, then both anisotroy and structural increase the redicted horizontal dislacement. The difference between the redicted results after 8 years is same as that at the end of construction, their differences is still obvious, but at the same time, the difference is decreasing with the increasing of the deth, at the deth of about 8 m after construction and 5 m after 8 years, the horizontal dislacement is almost the same. The deth of the maximum horizontal dislacement is well redicted in the three cases, it shows that anisotroy and structural have little influences on the deth of the maximum horizontal dislacement. Fig. 13 shows the evolution of the ratio of maximum horizontal dislacement U xmax at I1 to the maximum settlement U ymax at centerline with time, comaring the model redictions with the field

16 Hao Zhang, Qiushi Chen, Jinjian Chen and Jianhua Wang U xmax /U ymax U xmax (cm) U xmax (cm) Measured Proosed model Proosed model( ei=ξ=ξd =) Proosed model(α=μ=β= ei=ξ=ξd =) Time(days) Fig. 13 The ratio of the maximum horizontal dislacement and maximum settlement versus time data, it can be seen there is a significant increase is noticed during the construction of embankment. When the construction is comleted, the ratio between U xmax and U ymax decreases over time. Simulation without anisotroy and structural effects redicts the highest ratio, while simulations with anisotroy and structural redict the lowest ratio at the long term. The simulations both anisotroy and structural effects give the best matches to measurements in the three circumstance. In order to establish the relationshi between the maximum horizontal dislacements and maximum settlement for convenient alications, filed data of maximum settlements and maximum horizontal dislacement of 36 embankments (Saiichi and Takeshi 1996, Zhu and Yin, Mestat 1, Zdravkovic et al., Karstunen 5, Karstunen et al. 6, Abdulazim et al. 8, Abdulazim 9, Curtis et al. 9, Karstunen and Yin 1, Paulo et al. 11, Karim et al. 11) have been collected in this aer, the relationshi between maximum settlements and maximum horizontal dislacement is shown in Fig. 14. The relation between the maximum horizontal dislacements and the settlements at the end of construction can be exressed y =.379x R² = y =.1845x R² = Measured 1 Measured U ymax (cm) (a) At the end of construction 1 3 U ymax (cm) (b) Over the long term Fig. 14 Relation between horizontal dislacement and settlement

17 Alication of a modified structural clay model considering anisotroy to embankment behavior U xmax. =.379U ymax (6) Then when entering the consolidation stage, the maximum horizontal dislacements and the settlements at the end of construction can be exressed: U xmax. =.184U ymax (7) It can be seen that the simulated results have good agreement with Eqs. (6) and (7), it shows the rediction caability of this model. 5. Conclusions In this aer, an elasto-lastic model able to cature the anisotroy and structural of natural soft clay is roosed. The yield surface of the roosed model is adated from the anisotroic elastolastic S-CLAY1 model. The comression curve of reconstituted clays is taken as the reference to describe the structural of clay. There are 6 arameters which are similar to the S-CLAY1 model and 3 structural arameters. The model arameters all have clear hysical meanings and can be determined in a relatively straightforward way. The model was then imlemented into the finite element code for analyzing boundary value roblem, including triaxial tests and an Murro embankment. The simulated results were comared with the measured data in terms of settlement and horizontal dislacement. Comarisons showed the imrovement of redicted erformance by incororating structural and anisotroy, demonstrating the imortance of these two features on redicted clay behavior. The relation between the maximum horizontal dislacements and the maximum settlements at the end of construction and over the long term has also been roosed and the influence of anisotroy and structural on this ratio has been found to be less significantly as comared to the settlement and horizontal dislacements. References Abdulazim, Y., Minna, K. and Harald, K. (8), Effect of anisotroy and destructuration on behavior of Haarajoki test embankment, Int. J. Geomech., 9(4), Abdulazim, Y. (9), Numerical analyses of embankments on PVD imroved soft clays. Advances in Engineering Software, 4, Burland, J.B. (199), On the comressibility and shear strength of natural clays, Geotechnique, 4(3), Burland, J.B., Ramello, S., Georgiannou, V.N. and Calabresi, G. (1996), A laboratory study of the strength of four stiff clays, Geotechnique, 46(3), Chai, J.C., Miura, N. and Zhu, H.H. (4), Comression and consolidation characteristics of structured natural clays, Can. Geotech. J., 41(6), Chen, B., Xu, Q. and Sun, D.A. (14), An elastolastic model for structured clays, Geomech. Eng., Int. J., 7(), Cheng, X.L. and Wang, J.H. (16), An elastolastic bounding surface model for the cyclic undrained behaviour of saturated soft clays, Geomech. Eng., Int. J., 11(3), Cotecchia, F. and Chandler, R.J. (), A general framework for the mechanical behavior of clays, Geotechnique, 5(4), Curtis, K., Jitendra, S., David, H. and James, G. (9), Finite element analysis of an embankment on a soft estuarine deosit using elastic-viscolastic soil model, Can. Geotech. J., 46(3),

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