Field Behavior and Numerical Simulation of Coastal Bund on Soft Marine Clay Loaded to Failure Faisal Ali Department of Civil Engineering, Faculty of Engineering, National Defense University of Malaysia
[email protected]
Esam Ahmad S. Al-Samaraee Department of Civil Engineering, Faculty of Engineering, University of Malaya
[email protected]
ABSTRACT Marine clay is one of the problematic soils commonly found along the coastal area of west Malaysia. Therefore, it is very important to understand the representative characteristic and behavior of this marine clay to ensure that any construction on the clay such as embankment or bund will not fail or settle excessively. The paper presents a case study of constructing a trial coastal bund that was loaded to failure. Field instruments were installed in the soft clay beneath the bund before construction began to measure parameters such as pore pressure, settlements and lateral movements. Numerical simulations were then implemented to predict the bund behavior during construction and until failure occurred. Comparisons between measured and predicted behavior show that the settlement and the development of pore pressure could be reasonably be predicted by the numerical simulation. However the finite element analysis over-predicts the lateral movements.
KEYWORDS:
Earth bund, Correlation, Soft Clay, Consolidation, Finite Element
Analysis.
INTRODUCTION The rapid agricultural development strategies in certain parts of Malaysia especially at the coastal areas have obliged engineers to construct earth structures such as coastal bunds over soft marine clay deposits having very low bearing capacities coupled with excessive settlement characteristics. As part of Malaysian plans for coastal reclamations, the Drainage Department office (JPS) in Malaysia initiated the plans to conduct comprehensive geotechnical study and design earth structures along five kilometers coastal line at Bagan Datoh district, Perak State, which is vital for the reclamation process of more than 1000 hectares of coconut plantation land that have been lost to the sea water as a result of a failure of previously constructed earth bund. Marine clay is one of the problematic soils which are commonly found along the coastal area of west Malaysia. Therefore, it is very important to understand the representative characteristic and behavior of marine clays. However, in many situations geotechnical engineers are often expected - 4027 -
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to provide prediction of the subsoil behavior during and after construction. To provide a satisfactory prediction, geological knowledge and understanding of subsoil are essential in order to use the reliable correlations developed by the researcher based on the existing data. This research attempts to hoard information obtained from site investigation works for better understanding of the marine clay properties of this area. In addition, as geotechnical engineers are often expected to provide their estimations of soil behavior even when there are no relevant test results available, this research attempts to develop empirical correlations for estimating the engineering characteristic and shear strength properties. Such correlations included undrained shear strength (Su) and compressibility parameters with basic properties. Compressibility parameters are equally important in the design of earth embankment on soft clay, therefore some of the compressibility parameters which are correlated include compression index (Cc), coefficient of volume compressibility (mv), void ratio (eo) and compression ratio (CR) with basic properties. All these parameters would be normalized for Bagan Datoh clay and utilized for the estimation of the Soft Soil Creep (SSC) model parameters, which will be used for the finite element simulation analysis for the marine earth bund. The clays along the west coast of Peninsular Malaysia constitute coastal plain soft marine clay up to 20 m thick, with an average lateral extent of about 25 km. The site of the coastal bund is along the coastal line of Bagan Datoh District east of Perak State on the northwest coast of Malaysia. Based on the geological map prepared by the Geological Survey Department, Malaysia, the site under study is essentially unmapped. The alluvial deposit is recent and classified as Quaternary. The geotechnical investigations done earlier by other agencies and lately carried out at nearby locations confirmed the findings as a recent alluvium and the inference is that the area is a mud flat underlain by soft marine silty clay. Various sub-soil investigation works been conducted at the selected study site on the different dates; November 1996, December 1998 & August 2002, all collected data had been thoroughly studied and classified for the purpose of usage in the design process. Three different type of soil investigation had been carried out to cover the study area; rotary boring with in-situ vane shear test and undisturbed samples collection, Piezocone probe test and Geonor Vane Shear test.
SOIL PROPERTIES The subsurface geology data at the site reveal the existence of a weathered crust of about 2.0 m thick above a 16.5 m thick layer of soft silty clay. The latter layer can be further divided into an upper very soft and a lower soft silty clay. Immediately beneath this lower clay layer is a 0.3-0.5 m thick peaty soil followed by stiff sandy clay. The clayey succession ends at a dense sand layer at about 22.5 m below ground level. Although many soft clays encountered in the Southeast Asian countries are generally normally consolidated, they may exhibit light overconsolidation caused by surface desiccation and weathering. The apparent overconsolidation ratio (OCR) of such clay can be as high as 2.5, and this influences its preconsolidation pressure and undrained strength (Bjerrum 1972).
Index Properties The soil density γ, initial void ratio eo, liquidity index wL, and the plasticity index Ip are obtained from soil characterization tests, a summary of index properties are shown in Fig. 1.
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Figure 1: Basic soil properties of Bagan Datoh Clay. Compressibility and Strength Properties For numerical analysis, soil parameters are estimated based on careful sampling and laboratory testing, or on correlations with field test data. For Bagan Datoh Marine Earth Bund, undisturbed soil samples have been taken from the soft clay layers with the aid of stationary piston sampler. Summary of test results is shown in Figs. 2 and 3.
Figure 2: Compressibility parameters for Bagan Datoh Clay.
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Su (kPa) [0 to 40 kPa]
Depth, m 0 to 20m
Theoretical Skempton (1951) Lab Test Piezocone Geonor Linear (Theoretical Skempton (1951)) Linear (Lab Test) Linear (Piezocone) Linear (Piezocone Up) Linear (Geonor)
Figure 3: Variation of undrained shear strength Su with depth.
Estimation of OCR In ground settlement analysis, one of the important parameters for geotechnical design over soft clay is Overconsolidation Ratio (OCR). The lab test results did not give a good correlation trend for OCR with depth as the results were scattered. Underestimating OCR will cause overestimation of consolidation settlement magnitude and thus lead to expensive and timeconsuming geotechnical solutions. Hence, it is enormously useful if reliable correlations on OCR can be obtained from Piezocone data. Using the effective stress approach, Chen and Mayne (1995) suggested the following simplified relation to estimate OCR from Piezocones with pore pressure element located at the tip, u1 OCR k1 (
qt u1 ) vo
(k1 value of 0.81)
(1)
Secondary Compression Index Cα In geotechnical engineering practice the scholarly work of Mesri et al.(1987) is used extensively in the estimation of primary and secondary settlements. Secondary compression ratio is estimated from the work of Mesri et al. (1987) as follows,
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C C (0.04 0.01) c 1 eo 1 eo
(2)
Figure 4 shows a plot of corresponding values of Cα and Cc for lab test. The linear regression line originally has a positive intercept on the Cα axis. However, the intercept is minimal and hence the relationship between Cα and Cc is taken to pass through the origin. The slope of the best-fit line through the origin defines Cα/Cc, or
Cα/Cc = 0.032
(3)
Figure 4: Plot C versus Cc.
Earth Pressure Coefficient K0 The initial state can significantly affect the predicted soil response and, therefore, it needs to be defined carefully. In the embankment foundation, the initial vertical effective stress distribution can be obtained simply and reliably from measured values of soil unit weight, and the initial horizontal stress can be defined with the earth pressure coefficient at rest K0. For the normally consolidated (NC) soil, K0 can be related to the friction angle by Jâky (1944).
K 0 nc (1 sin )
(4)
For overconsolidated clay, the following relation is considered to give a fairly good estimation for K0 (Wood, 1990)
K 0 K 0nc OCR
(5)
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CONSTITUTIVE MODEL The Soft Soil Creep (SSC) model with Modified Cam Clay (MCC) parameters has been used to model soft soil behavior. The details regarding the model and its finite-element implementation can be found elsewhere (e.g., Wood , 1990; Gens and Potts, 1988). The original critical state of the model was formulated based on conventional triaxial tests. In this numerical analysis, a generalization to take into account the variation of the limit stress ratio in the deviatoric stress plane is accepted by using the modified yield condition.
*
Cc C 2 Cr * * , k , 2.3(1 e ) 2.3(1 e ) 2.3 1 e
(6)
This SSC model was chosen because at the location of the Bagan Datoh site the stratification of the sub-soil consists of soft layers of peat and clay. Soft marine clay shows a stress dependent non-linear behavior in which the stiffness of the material is stress dependent. Furthermore creep and consolidation plays an important role in the time dependent settlement behavior when applying a load (earth bund) on the sub soil. Therefore the soft soil creep model has been chosen for making predictions of the behavior of the Bagan Datoh marine bund. The soil profile and parameters for calculations are given in Table 1, which have been extracted, interpolated and refined from the lab and in-situ test data. Some engineering judgment was made to arrive at the SSC parameters. The pre-overburden pressure (POP) in the numerical calculations were taken as 10 for all layers, except POP=5 for the crust layer.In summary, the best-estimate values of the soil parameters, for the Bagan Datoh soil layers in the geotechnical model, are listed in Table 2. When evaluating the material properties from field and lab data, a decision has to be made whether the permeability from lab data or from field test data should be used in the calculations. The permeability from field tests were in general a factor 6 to 10 higher than the permeability from lab tests (oedometer). According to literature the values for the permeability found by field and lab tests both are within the range one should expect. In finite element calculations it was decided that the permeability from lab test would be used, as the in-situ permeability test are not available. Ground investigations revealed that the surface layer at the bund area comprises approximately 2m thick very soft marine deposits referred to as the Bagan Datoh Mud (BDM), which is overlying very soft clay material (BDS). Cone Penetration Tests (CPT) & Geonor Vane Tests (GVT) carried out in the offshore geotechnical investigation suggested that the BDS could have possessed undrained shear strength as low as 6 kPa at shallow depth. It is also revealed that occasionally there are deeper deposits of the BDM, associated with very soft clay sediments seeped into the surface of the underlying clay layers deposits by the river. An averaging procedure with respect to thickness of subsoil layers has been used to obtain the values for the second soil layer. As mentioned above, some soil parameters are sensitive to the change of environment during sampling, and may have quite different values under in situ conditions. In addition, there is considerable measurement uncertainty due to the inherent variability of the soil deposit. The effects of such uncertainty are investigated by means of the parametric study, in which one parameter is varied while the rest take the best-estimate values. The finite element computer program PLAXIS was used for the analysis of the bund. PLAXIS is a commercially available finite element package specifically designed for two
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dimensional geotechnical analyses. PLAXIS makes use of advanced constitutive models for the simulation of non-linear behavior of soils.
Table 1: The soil parameters Clay Layer Crust Upper Soft Clay Lower Soft Clay1 Lower Soft Clay2 Lower Soft Clay3
Layer Thickness m 2.0 4.0 4.0 4.0 6.0
Cr
Cc
C
0.120 0.120 0.110 0.110 0.110
0.985 0.950 0.850 0.800 0.750
0.032 0.030 0.027 0.026 0.024
m /yr
3 KN/m
eo
2.5 2.5 2.5 2.5 2.5
13.6 14.0 14.3 13.8 14.0
2.5 2.5 2.3 2.0 2.0
Cv 2
Po
Pc 2
2
kN/m
kN/m
7.2 16.0 25.8 22.8 28.0
35 45 55 40 58
Table 2: The SSC Model parameters
NUMERICAL SIMULATION A new Plaxis module has been used, in which updated mesh analysis was combined with creep and consolidation. In an updated mesh analysis the finite element mesh is updated after every displacement increment, so that every nodal point (x,y) will be updated to a new coordinate (x+Δx, y+Δy). The updated mesh analysis was chosen, as large deformations were expected. In such a case the traditional stress-strain relation will not be accurate. In the present finite-element analysis, the problem is solved as a two-dimensional (2D) plane strain consolidation problem with asymmetry condition. Due to the fact that Bagan Datoh marine bund is not symmetrical and the sea water level fluctuating at one side of the bund, the model been taken as a full scale, in order to simulate the actual construction sequence as well as phreatic line variations, full model in Plaxis is shown in Figure 5, and the 15-node mesh plot shown in Figure . Standard Plaxis boundary conditions were used, that is horizontal fixities at the left and right boundary of the finite element mesh and horizontal and vertical fixities at the bottom of the mesh. For the consolidation boundary, closed consolidation boundary has been chosen at the left and right side of the geometry.
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Figure 5: Computer model for earth marine bund (Plaxis Software).
Figure 6: Plane strain 15-Node mesh The earth bund raised on this soft Bagan Datoh marine clay formation (BDM) failed by the development of a "quasi slip circle" type of rotational failure at a critical height of approximately 4.0 m with a pronounced tension crack propagating vertically through the crust and the fill. This paper elucidates the predicted behavior of the soft Muar clay foundation with regard to the critical height of embankment at failure. Excess pore pressures, lateral and vertical displacements (including heave), as well as an attempt to propose a conceivable failure mode. The predictions are made on the basis of several deformation analyses incorporating two different constitutive soil models, based on the modified Cam-clay theory and the hyperbolic stress-strain behavior. From the numerical [finite-element method (FEM)] analysis, predictions of displacements and pore pressures were made and subsequently compared with the field measurements obtained at various locations within the subsoil and the embankment. The zones of yielding and the potential failure surface are interpreted based on the locations of critical shear-stress ratios and maximum displacement vectors. The actual failure surface was identified by the researcher with a combined topographical survey and inclinometer response within the upper clay layer.
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Figure 7: Instrumentation layout for the coastal bund. Pre-planned geotechnical instruments been installed at specific locations and depths beneath the proposed marine earth bund, as shown in Figure 7. With this instrumentation the researcher recorded and analysed all collected data from site during the construction of the of earth bund, and these instruments had been installed to measure: • Vertical settlement of the original ground level; • Vertical settlement of layers subject to deformation as a function of depth; • Porewater pressure distribution or consolidation of particularly compressible layers; • Horizontal displacement at ground level; • Horizontal displacement of layers subject to deformation as a function of depth.
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BUND GENERAL BEHAVIOUR Figure 8 represents the scenario of construction of bund versus time and the correspondence measured and calculated settlement below the bund; it is found that the settlement values obtained from the finite element analysis show similar results to the on-site settlement measurements, also it reveals the failure time, which was predicted to be after one day from the final lift of the bund, and it matches what happens on site exactly.
Figure 8: Earth filling, FEM calculated, classical calculated & measured settlements at center of the bund The construction of the marine earth bund experience landward quasi slip circle failure immediately after completing the construction of the 4m bund height, The movements take place in soft BDM layer as soon as the first layer of bund fill material (BFM) is placed.The initial loading of the bund causes a “squeezing” effect on the underlying BDM, which demonstrated a predominately horizontal movement pattern. In the subsequent consolidation, the movement pattern turns “vertical”. This assumption in movement patterns is considered reasonable and is consistent with the observation in the trial bund construction. The scenario of failure addressed in four stages is illustrated in Figs. 9 & 10.
Bund Settlement & Deformation The bund had been constructed in relatively short period of time. In the numerical modeling, the time allowed for consolidation in construction stage was 14 days. The consolidation in the BDM would take place as soon as gravity loading is applied by the placement of bund material. Figure 11 shows also comparison between calculated settlement and recorded settlement and heave, which reveals almost similar values, especially for the lift stages up to 3.5m of bund height, and the pattern of settlement and ground deformation show some also same values but
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with different trend, i.e. the measured settlement pattern moves more into landward, and this is true as the soil body collapse and push further landwards, changing dramatically the soil properties, which becomes anisotropic.
Stage 1
Stage 2
Stage 3
Stage 4
Figure 9: Progress photo for the earth bund collapse.
Figure 10: Schematic diagram for bund failure stages.
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Figure 11: Calculated and measured settlement below the bund vs time. Lateral displacement with depth been recorded against bund construction time by utilizing the inclinometers installed, a comparison been made at the bund toe location, where maximum lateral deformation detected. Figure 12 shows the lateral deformation at the base of the bund recorded against lift. It is clear that the trend of lateral movement increased dramatically after the final lift. The finite element results for the degree of consolidation revealed similar trend with the classical method but with lower degree of consolidation values against time, and this would reveal a close and realistic trend, as the finite element method take into account the soil properties changes with time as a result of consolidation, unlike the classical method which results are essentially based on initial soil parameters only. (See Figure 13)
Figure 12: Calculated lateral movement below the bund.
Figure 13: Comparison on degree of consolidation, measured, classical & FEM methods
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The lateral displacement was small before the fill thickness reached 4.0 m (failure occurred). Before failure, the simulated values overpredict the measured values, and when the failure state is reached, the simulation underpredicts the field values at near ground surface. Generally, FEM analysis yielded a poor simulation on lateral deformation. This is partially due to the fact that the adopted soil model may not fully represent the behavior of the subsoil and partially due to the limitation of the FEM analysis in simulating large deformation at the close-to-failure state. The lateral deformation at the toe of the bund against depth at Fig 14 revealed that the FEM method overpredicts measured lateral movement at all time by 130%. Figs. 15 & 16 give the measured variation of the ratio of lateral displacement – settlement versus embankment fill thickness. In this case it is clearly indicates that when the embankment approaches failure, the lateral displacement – settlement ratio increases rapidly. These results support the proposal made by Matsuo and Kawamura (1977) for assessing the stability of embankment over soft subsoil.
Measured
4
2
0
Lateral Deformation, mm -200 0 200 400 600
Lateral Displacement, m
One of key interests in the finite element analysis is the development of the excess pore water pressure in the BDM and the bund itself during the construction of the bund and placement of BFM. The materials have been assumed to be “undrained type” to allow the built-up of excess pore water pressure within the bund and the underlying BDM. In Figure 17, it can be seen that the maximum excess pore water pressure is between 60 to 64 kPa .
6
Thickness of Fill, m
18
Measured, 14days Measured, 18days Calculated, FEM 14days Calculated, FEM 18days
Lateral Displacement/Settle ment
16
14
12
Depth, m 10 8
Figure 15: Variation of lateral displacement response at 3.5m Depth Measured Calculated,…
Thickness of Fill, m Figure 16: Lateral displacement over settlement ratio.
Figure 14: Comparison between measured & calculated lateral displacement.
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Pore Water Pressure Variation The generation and dissipation of the excess pore water pressure during the initial and final stage of consolidation are illustrated in Figure 18. It is interesting to note that at the initial stage, the excess pore water pressure developed rapidly upon gravity loading of the bund material. The construction of bund was simulated by adding layer by layer (layer 1 to layer 8), which allows the accumulation of excess pore water pressure in the bund. After 60 days, the excess pore water pressure in the BDS dissipated significantly, whereas the excess pore water pressure in the BDM reduced a little. When the BFM was placed on ground, the excess pore water pressure developed under the bund mainly within the BDM and the subsequent upper soft clay layer. The pattern of the water pressure shows unsymmetrical distribution due to the bund unsymmetrical shape. Figure 19 shows the distribution of excess pore water 60 days after the placement of the BFM, where the excess pore water pressure gradually dissipated in the BFM and the BDS layers. Noticeable excess pore water still trapped in the BDM due to its lower permeability. The excess pore water slowly and gradually dissipated with time until reached 90% consolidation after a total period of approximately 5years. The model predicted that consolidation in the BDM would take approximately 25 years to complete.
Figure 17: Excess pore water pressure after construction.
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Figure 18: Excess pore water pressure change with time.
Filling Height & PWP, m
A plot between build up pore water pressure (PWP) and filling height rate at 2m below bund shown in Figure 20 with superimposing the calculated PWP obtained from FEM, both trends of building up shows similar trends with slightly higher values of PWP obtained from piezometer, and both trends shows sudden drop of PWP after 14 days, indicating the collapse of the bund after reaching full height of 4m.
Fill scheme Measured PWP Calculated PWP
Time, days
Figure 19: Predicted Excess Pore Water Pressure.
Figure 20: Variation of PWP between Measured and Calculated.
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CONCLUSIONS Bagan Datoh Clay consists of top 20m of very soft clay deposit. Index & compressibility properties had been verified and can be correlated to natural moisture contents, liquid limit and void ratio. The ratio of Cα/Cc for Bagan Datoh Clay is equal to 0.032. Bagan Datoh Clay Undrained Shear Strength (Su )ranges from 6 to 35kPa. SSC model used in the FEM analysis of the Bagan Datoh Clay shows that the calculated settlements are in agreement with the monitored settlements. In addition, the numerical model provided some insights into a history of the development of the excess pore water pressure in all soil layers and also the FEM simulation has produced a good prediction of failure of the coastal bund.
REFERENCES 1. Bjerrum, Laurits (1972). Embankments on soft ground, Proc. Specialty Conference on Performance of Earth and Earth-Supported Structures, ASCE, Purdue, 2, 1-54. 2. Chen, B.S., and Mayne, P.W. (1994). Profiling the overconsolidation ratio of clay by piezocone tests. Report: Georgia Tech Research Corporation and Georgia Institute of Technology, School of Civil and Environmental Engineering, GA. 3. Gens, A., and Potts, D. M. (1988). Critical State models in computational geomechanics. Engrg. Computations, 5. 178-197. 4. Jáky, J. (1944). The coefficient of earth pressure at rest: J. Soc. Hung. Eng. Arch., p. 355-358.Dunphy, M.P., Patterson, P.M., and Simmie, J.M. High temperature oxidation of ethanol. Part 2- Kinetic modeling. Journal of Chemical Society. Faraday Transactions, Volume 87, (1991), pp. 2549-2560. 5. Matsuo, M., and Kawamura, K. 1977. Diagram for construction control of embankment on soft ground. Soils and Foundations, 17(3): 37–52. 6. Mesri, G. and Castro, A. (1987). Cα/Cc concept and K0 during secondary compression, J. Geotech. Engrg. Div., ASCE, Vol. 113, GT. 3, pp. 230-247. 7. Wood D.M. (1990). Soil behaviour and critical state soil mechanics, Cambridge University Press.
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