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Numerical Analysis of Coastal Bund on Floating Piled Foundation

Faisal Ali

Faculty of Engineering, National Defence University of Malaysia Kuala Lumpur, Malaysia

fahali@gmail.com

Esam Ahmad S. Al-Samaraee

Faculty of Engineering, University of Malaya Kuala Lumpur, Malaysia

asumi13@gmail.com

ABSTRACT Marine clay is one of the problematic soils commonly found along the coastal area of west Malaysia. One of the possible methods of coastal bund construction in this material is by using piled foundation. The paper presents a numerical analysis of a coastal earth bund constructed on floating piles in soft marine clay. Numerical method (FEM) with Plaxis software exploiting soft coil creep model (SSC) and other analytical methods like Terzaghi’s, British Standard BS8006 and the German method EBGEO 2004 have been used for the assessment of the bund behavior and compared to the measured values obtained from field instrumentations. The finite element program (Plaxis) captures the overall behavior relatively well. However, the program also have tendency to under-predicts settlement at early stages after construction and over predict the settlement at later stages, while the classical method give somehow close results of early stage settlement but with slower rate of consolidation than the measured values, and then under-predicts the long term settlement in a wide range.

KEYWORDS: Pile-Supported Bund, Soil Arching, Load Transfer, Soft Soils, Finite Element Method.

INTRODUCTION The ever increasing need of coastal reclamation and development often forces engineers to

build on soft soils. Soft soils cannot sustain external loads without having large deformations. Hence, a soft soil improvement measure is habitually a prerequisite. Soft foundation soils are particularly problematic for infrastructure structures, e.g. embankments, costal bunds and abutments.

The major part of the work presented in this paper relates to the challenges in predicting settlement in marine type of soft soil and addressing soil arching effect, explicitly at a coastal area named Bagan Datoh at the state of Perak in Malaysia, where 5km long coastal earth bund on floating piles is raised for the reclamation of more than 1000 hectares of land. See Figure 1

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Piling has been used to control embankment settlements in South East Asia since the 1970's, Holmberg (1978). The conventional form of piled embankment, shown in Figure 3a,consists of piles with large size pile caps to ensure all of the embankment loading is transferred directly onto the piles (and not onto the soft foundation). In addition, raking piles are normally installed at the outer extremity of the piled area to counteract the horizontal outward thrust of the embankment fill. Since the early 1980's basal reinforced piled embankments have become increasingly common and a comparison between the two systems is shown in Figure 2.

The benefits of the basal reinforced piled embankment technique are that relatively small size pile caps can be used since the basal geosynthetic reinforcement is used to transfer the unarched vertical embankment loading onto the pile caps, and there is no need for raking piles as the geosynthetic reinforcement counteracts the horizontal outward thrust of the embankment fill,( Figure 1). This leads to considerable economies over the conventional piled embankment technique.

Figure 1: Floating piled embankment.

a) Conventional piled bund

b) Basal reinforced piled bund

Figure 2: Basal Reinforced Piled Bund

A major advantage of piled embankments is that the height of embankment construction is controlled by the group capacity of the foundation piles, not by the strength of the soft foundation soil. Furthermore, basal reinforced piled embankments can be constructed at unrestricted rates because the loading is not dependent on the consolidation rate of the soft foundation. In view of

Vol. 18 [2013], Bund. T 4295 the above two advantages, most of the applications where basal reinforced piled embankments are used are those where differential settlements are to be minimized.

The embankment can be supported by either end-bearing piles, i.e. piles that reach a hard stratum, or floating piles, which are piles that do not reach a hard stratum. When the installed piles do not reach a hard stratum due to large thickness of the soft soil, the construction is known as an “embankment on floating piles”. A well premeditated piled embankment has the advantages that it requires less construction time and yields little embankment settlement. Moreover, its construction process causes minimal disturbance or disruption to the natural environment. This technique is extremely time effective and it is considered an environmental friendly technique.

Earth embankments constructed to functional elevations are essential to create the necessary earth bund along coastal lines and prevent sea water inundation into seashore land. Such embankments, if placed on highly compressible soft clays or problematic organic soils, may experience long term settlements and edge stability problems. An economical means to improve existing soils in these cases is use of prefabricated vertical drains combined with gradual placement of the embankment fill. This well-established technique can permit construction of embankments on soft ground at a lower construction cost than by using the column-supported embankment technology. However, use of vertical drains and gradual embankment placement requires considerable time for consolidation and strengthening of the soft ground, and this approach can also induce settlement in adjacent facilities, such as would occur when an existing embankment is being widened.

Pile-supported embankments are constructed over soft ground to improve embankment stability, control total and differential settlements, and protect adjacent facilities. The columns that extend into and through the soft ground can be of several different types: driven RC or timber (Bakau) piles, vibro-concrete columns, deep-mixing-method columns, stone columns, etc. The columns are selected to be stiffer and stronger than the existing site soil, and if properly designed, they can prevent excessive movement of the embankment. Pile-supported embankments are in widespread use in Thailand, Indonesia, Japan, Scandinavia, and the United Kingdom, and they are becoming more common in Malaysia and other countries. The Pile-supported embankment technology has potential application at many soft-ground sites, including coastal areas where existing embankments are being widened and new embankments are being constructed.

The cost of pile-supported embankments depends on several design features, including the type, length, diameter, spacing, and arrangement of pies. Geotechnical design engineers establish these details based on considerations of load transfer, settlement, and stability. A report by Filz and Stewart (2005) addresses the load transfer and settlement issues. Established procedures are available for analyzing the stability of embankments supported on driven piles and on stone columns. Stability analysis methods for embankments supported on piles and presented in appendices to this report.

The pile-supported embankment, consisting of piles, pile caps, high tensile geotextile, foundation soil, and embankment fills, has been progressively used in soft soils due to its advantage of rapid construction and small settlement. Though, the mechanics of load transfer is very complex in the pile-supported embankment. Differential settlement exists between the pile and foundation soil owing to the distinct compression modulus of them. Consequently, a relative displacement would occur at the interface of the slippage within the embankment fill. The slippage would be resisted by shear stresses, and thus the overburden stress on the foundation soil is reduced while the pressure on the pile cap is increased. This phenomenon is called ‘arching effect’ (Terzaghi, 1943).

Vol. 18 [2013], Bund. T 4296

Numerous researchers have investigated the effect of soil arching in the embankment build. Russell et al. (1993) investigated soil arching in the pile-supported embankments and proposed the theoretical solution; he also presented 3D numerical analyses for soil arching to analyze the behavior of pile-supported embankments. The British Standard BS8006 (1995) introduced Marston’s formula to estimate the vertical stress on the top of the pile cap. Han and Gabr (2002) conducted numerical analysis on the embankments and the analysis indicated that the soil arching ratio decreases with an increase in the height of embankment fill. The German method EBGEO 2004-Section 9 adopts the so-called multi-shell arching theory based on the work of Zaeske (2001).

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).

CONSTITUTIVE MODEL The Soft Soil Creep (SSC) mode 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.

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 to the SSC parameters. The pre-overburden pressure (POP) in the numerical calculations were taken 10 for all layers, except POP=5 for the crust layer.

Calibration of SSC parameters was made by a back analysis, that is, by matching the model response with the laboratory test results. In summary, the best-estimate values of the soil parameters, for the Bagan Datoh soil layers in the geotechnical model, are listed in Tables 2&3.

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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 is decided the permeability from lab test will be used, as the in-situ permeability test are not available. Comparison of the results from finite element calculations and the measured settlements and excess pore water distribution as function of time, show that this engineering judgment was correct.

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 6kPa 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 dimensional geotechnical analyses. PLAXIS makes use of advanced constitutive models for the simulation of non-linear behavior of soils.

Table 1: Normalized Soil Parameters

Table 2: Calibrated SSC Model Parameters

Clay LayerLayer

Thickness m

C r C c C C v

m 2 /yr

KN/m3 e o

P o

kN/m2

P c

kN/m2

Crust 2.0 0.120 0.985 0.032 2.5 13.6 2.5 7.2 35Upper Soft Clay 4.0 0.120 0.950 0.030 2.5 14.0 2.5 16.0 45Lower Soft Clay1 4.0 0.110 0.850 0.027 2.5 14.3 2.3 25.8 55Lower Soft Clay2 4.0 0.110 0.800 0.026 2.5 13.8 2.0 22.8 40Lower Soft Clay3 6.0 0.110 0.750 0.024 2.5 14.0 2.0 28.0 58

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Table 3: Material Properties for Plaxis Analysis

NUMERICAL ANALYSIS MODEL DETAILS 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 3, and the 15-node mesh plot shown in Figure 4.

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.

The axi-symmetric model is adopted in the Plaxis analysis to perform a parametric study only. The four important materials involved in the geometry are the piles, the geotextile, the foundation soil and the embankment fill. Input parameters are shown in Table 1 & 2.

The "Mohr-Coulomb Model" was used for the bund fill, and input parameters are shown in Table 3. The geotextile is represented by a geotextile element in Plaxis. These are flexible elastic elements that represent sheet of fabric in out of plane direction. They can sustain tensile forces but not compression. The factors those are varied in the parametric study are geotextile stiffness, the height of the bund, the position of the geotextile layer and the modulus of elasticity of the pile.

When axi-symmetric model is used, the lateral movement of Geotextile Reinforced Pile Supported Bund System (GRPS), the bending moment in the support, and the actual tensile force

Vol. 18 [2013], Bund. T 4299 distribution and deformation shape of the whole system cannot be studied. Therefore, the plane strain model is more frequently used in GRPS embankment or bund design.

Figure 3: Computer Model for Earth Marine Bund.

Figure 4 (a): Plane Strain 15-Node Mesh.

Figure 4 (b): Plane Strain 15-Node Mesh Details.

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INSTRUMENTATION Pre-planned geotechnical instrumentations had been installed at specific locations and depths

beneath the proposed marine earth bund, refer to Figure 5. With these instrumentations the researcher recorded and analysis all collected data from site during the construction of the of earth bund, and these instrumentations been installed to measure:

• Vertical settlement of the original ground level;

• Vertical settlement of layers subject to deformation as a function of depth;

• Pore-water 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.

• Geotextile tensile force.

• Pile & pile cap pressure.

Figure 5: Field instruments layout on piled marine earth bund.

Vol. 18 [2013], Bund. T 4301

BUND SETTLEMENT & DEFORMATION The bund is essentially gravity loaded material and its load shall be transfer to the lower clay

layers via the Bakau Pile system. The vertical movements take place in the upper soft clay layer 2 directly below the pile tip, while minor heave occurs for the soft BDM layer as soon as the first layer of bund fill material (BFM) is placed. Due to the asymmetry condition of the marine bund, the recorded lateral movement was pushing more towards land side.

The measured settlement below bund centerline immediately after final bund lift was 55mm while the calculated settlement was 33mm (lower than measured), and similar trend was recorded for the settlement after 60days as the measured was 121mm against 73mm calculated. The gap of the difference between measured settlement and calculated settlement get closer as time advances. Nearly after 160 days the measured settlement and calculated settlement get same value of 145m. The measured settlement continue to gain with constant rate till it was recorded 396mm after 730days while the calculated settlement was 417mm for the same period. See Figure 6

Figure 6: Comparison between Measured & Calculated Settlement below Bund.

Figure 7a: Comparison Graph between Calculated & Measured Settlement vs Time.

Figure7b: Comparison Graph between Calculated & Measured Settlement vs Time.

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In summary the rate of actual settlement is higher than the rate of the calculated settlement for the first 90days after which the rate of measured settlement getting slower as compared to the calculated settlement rate.

Both measured and calculated settlement agree to have similar trends and close values, except that measured settlement values during construction and after short time of construction showed higher values as compared to the calculated ones, and this might be related to the extra load on the bund generated from the construction machinery, as the measured settlement follow the values and trends of the calculated settlement, refer to Figs. 7a & 7b.

The maximum settlement increased with an increase in the height of embankment, and the presence of the amount of calculated settlement are lightly less values than the measured values, and this might be related to the extra load from machinery and compactors that is used for the construction of the bund. See Fig. 8

Figure 9 shows similar trend recorded for the tensile force generated in the geotextile, as the tensile force in the geotextile increases as the bund height increase, also showed that the measured tensile force are higher than those obtained from finite element analysis.

Figure 8: Influence of Bund Height on Maximum Settlement.

Figure 9: Influence of Bund Height on Geotextile Tension Force

LATERAL DISPLACEMENT 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. For instance 9mm lateral movement at the bund base and toe location of the marine bund that have been recorded immediately after laying the final layer of bund fill material, which discloses very minor drift tendency of the upper BDM material and the upper soft clay layer. Another subsoil lateral movement recorded at soil layer location below pile tips (below 6m), namely at the upper soft clay2 and lower clay1 with a maximum value reach to 16.6mm.

The monitoring of lateral displacement continues and records show that top surface lateral movement double its initial value after 60days (18.9mm) , 23mm after 6months, 45mm after 1year and 50mm after 2years, indicating a slowing rate as time laps. The underground lateral movement also shows 34.9mm after 60days, 43.6 after 6months, 82.8mm, after 1year and 165.6mm after 2years, indicating larger lateral movement at the subsoil layers.

The marine bund was stable thru out 2years of monitoring, and no signs of tension cracks of failure appears, except with vertical settlement increase, which was accepted as per the design

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Vol. 18 [2013], Bund. T 4303 concept and a top up of 300mm of bund fill material were placed on the bund top surface after 2years to maintain the height of 4m.

The finite element simulation was made to verify the model behavior. Figs. 10 to 12 show similar trends of lateral deformation to that obtained and recorded from field instruments, but with higher values of about 1.83 times the field values at early stages after construction, after gap gets closer between measured and predicted lateral displacement to reach a difference of 10% only.

Several factors may cause the early stage big gap; one is mainly due to the machinery and compactors activity during the construction of the bund that is exerting additional pressure on the subsoil layers and lead to a higher movement in the soft clay layers, other factor may be related to the soft soil clay model parameters used in the FEM analysis is not fully represent the soil condition or simulate actual clay behavior

It should be noted that the magnitude of settlement calculated in the model is mainly dependent of the compression index, swelling index, creep index, initial void ratio as well as undrained shear strength adopted for the model materials. The observed settlement may be well in excess of the calculated values. The other soil parameters adopted in Table 1 were based on a broad correlation with the material description encountered in the ground investigation and the results obtained from laboratory testing as well as the CPT and GVT results.

The numerical analysis showed that the bund model was generally stable during and after the construction phases. This is principally due to the low sloping angle of the bund, counter berm, friction piles and high tensile geotextile layer that transfer bund material weight to the lower clay layers and sustainability of lower clay layers to this bund weight.

DEVELOPMENT OF PORE WATER PRESSURES One of key interests in the finite element analysis is the development of the excess pore water

pressure in the BDM and the clay layers below the tips of the bakau piles 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 underlying BDM and subsequent clay layers. It is interesting to note that at the initial stage, the excess pore water pressure is 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. It can be seen that the maximum excess pore water pressure was recorded at 7m depth with value of 45.32 kPa (Figs. 13 & 14), and the calculated pore water pressure was equal to 42.69kPa, meaning the FEM method slightly underestimates pore water pressure. The Pore water pressure after 60 days in the lower clay layers dissipated significantly. The measured PWP was 26.98kPa compared to the calculated pore water of 28kPa, whereas the excess pore water pressure in the BDM (upper clay layer) reduced a little. When the BFM was placed on ground, the excess pore water pressure was developed under the bund mainly within the BDM and the subsequent upper soft clay layer, this due to the fact being the bund material arch effect is not developed yet and the BFM weight transfers directly via geotextile layer to the BDM layer. See Fig 15.The pattern of the water pressure shows unsymmetrical distribution due to the bund unsymmetrical shape. Noticeable excess pore water still trapped in the lower clay layers 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 3.5years.The highest PWP recorded at a depth of 7m below bund base of value equal to 49kPa (Figure 16) and the calculated

Vol. 18 [2013], Bund. T 4304 PWP also show a high value of 44.38kPa at the same depth, the significance of the high PWP at this location referring to a 1m distance below the tip head of the installed bakau piles, that reflect a true behavior of highly stressed clay layers beneath pile heads. With time passes the PWP started to dissipate with trend of pushing the high PWP value to be located at deeper clay layers. Generally, the FEM analysis simulated the field data well.

Figure 10: Calculated vs Measured Lateral Movement.

Figure 11: Lateral Displacement vs Time.

Figure 12: Lateral Displacement at Bund base vs Bund Lift

Figure 13: Pore Water Pressure Built up at Last Lift

Figure 14: Earth Filling, Calculated & Measured PWP at 7m below Center of Bund.

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Figure 15: Pore Water Pressure vs Time

Figure 16: Measured & Calculated Pore water

Pressure vs Depth

STRESS CONCENTRATION AND RATIO The Stress Concentration Ratio, n is defined as the ratio of the stress on the pile caps to that

on the subsoil and/or reinforcement. This value increases as the effect of arching increases. The interactions among embankment fill, geosynthetic reinforcement, pile cap and foundation soil are complex and can be schematically described as shown in Figure 17. Since compression stiffness of the pile is greater than that of the foundation soil, the embankment fill mass directly above the foundation soil shall have a tendency to move downward. This movement is partially restrained by shear stress , from the bund fill mass directly above the pile cap. The shear stress increases the pressure acting on the pile cap but also reduces the pressure on the foundation soil. This load transfer mechanism was termed the “soil arching effect” by Terzaghi (1943). The presence of geosynthetic reinforcements complicates the load transfer mechanism. The soil arching has a significant influence on the behavior of piled embankments. Stress concentration ratio, n, is an important parameter to assess the degree of soil arching and is defined by Han and Gabr (2002) as follows:

where σp is the applied pressure on the pile cap and σs is the average pressure applied on the foundation soil. n= 1 represents no soil arching. The greater the value of n, the higher the degree of soil arching. If the degree of soil arching is not sufficient, too much bund load will be borne by the foundation soil and the pile–subsoil relative displacement, Δs will be reflected to the top of the bund and an unacceptable differential settlement may occur, which would harm normal function and durability of the embankment. However, too large value of the stress concentration ratio, n, implies that nearly all the bund load will be borne by the piles and high costs. Consequently, a thorough understanding of soil arching mechanism within piled embankments is

Exc

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, kP

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Measured PWPCalculated PWP, FEM

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th, m

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Measured, 16daysCalculated FEM, 16daysCalculated FEM, 60daysMeasured, 60days

Vol. 18 [2013], Bund. T 4306 essential for engineering design. Four empirical design method (Terzaghi, BS8006, EBGEO & FEM) been conducted on the piled marine bund, to estimate the increase in the vertical stress on top of the clay layer and estimate the tensile force generated in the geotextile layer due to bund fill material.

Figure 17: Soil Arching in Piled Embankments

Figure 18 shows a comparison between three different design methods, Terzaghi (1943) Method, BS8006, EBGEO Method and FEM Method (Plaxis Program). A stress concentration ratio n of 3.02 been obtained with Terzaghi Method, 2.24 with BS8006 method, 3.64 with EBGEO Method and 3.42 with FEM Method. And by comparing these values with the actual stress concentration ratio of 3.5, EBGEO Method and FEM Method give close value of stress reduction ratio n to the actual values (EBGEO with high side value and FEM with lower side value), while Terzaghi (1943) and BS8006 generate an underestimated ratio.

The other great benefit of this comparison is to show that the stress concentration ratio is not fixed along with time, in another word the value of n getting less as time pass, and none of the empirical method have the capability to address this phenomenon, except FEM method that show clearly the reduction in n value versus time and very close trend with the actual values, and this is true as the settlement and lateral spreading happens below the bund, and redistribution of forces keeps on going among bund fill material, geotextile, piles and clay soil.

Furthermore, Figure 19 shows another comparison of tensile force in the geotextile, the pattern of which is identical to that of the force reduction plot, only with different magnitude. It is expected when all parameters are fixed. FEM method was on the higher value side meaning an underestimated tensile force of 33.3kN/m in the geotextile layer immediately after construction as compared to the actual measured tensile force of 35.0kN/m, similarly to the force concentration ratio the tensile force generated in the geotextile increased with time laps. FEM method generates higher tensile force than measured values with time, and it is recorded to have 64.27kN/m from FEM program as compared to measured tensile force of 56.2kN/m after 1year period.

BS8006 show a tensile force of 59.51kN/m and EBGEO with 56.33kN/m (see Fig 20). Both methods seem to address higher tensile force values than FEM method, and also can not address the variation of tensile force with time.

Similar trend recorded of load increase on pile head as bund fill increased. BS8006 show the lowest load value on pile heads EBGEO method show higher values of pile head load, while FEM method showed most close values to the measured pile head load at all stages of bund fill. See Fig 21. Figure 22 shows this phenomenon of tensile force variation with time for FEM method and measured values.

Vol. 18 [2013], Bund. T 4307

Figure 19: Tensile Force in Geotextile.

Figure 20: Tensile Force in Geotextile vs Bund Height

Figure 21: Pile Vertical Load increase vs Bund Height

Figure 22: FEM method vs measured Geotextile Tensile Force with Time

Figure 26 shows the results of design with various methods. It is observed that, for different magnitudes of bund height, the stress concentration ratio versus pile-subsoil displacement, �S, had the same characteristics. With an increase in pile–subsoil relative displacement, n, stress concentration ratio increased and reached a maximum value, then decreased gradually and maintained nearly a constant value. FEM method is the only method to address and capture such behavior as compared to Terzaghi (1943), BS8006 and EBGEO, which show a constant relationship. Figure 6.44 also show that shows that EBGEO method always overpredicts the stress concentration ratio, while BS8006 method underestimate the stress concentration ratio, and Terzaghi (1943) method shows an impartial neutral value.

Str

ess

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Figure 18: Stress Concentration Ratio

Vol. 18 [2013], Bund. T 4308

Results from current FEM investigations are in close agreement with their observations. A higher value of stress concentration ratio indicates that a higher percentage of the embankment load is transferred to cap beams. The soil arching is strongly reliant on pile–subsoil relative displacement, and there exists a critical relative displacement, Δsc (=75-100mm), at which the soil arching developed most effectively and beyond which there may be less effective arching. In other words, the stress concentration ratio has upper and lower bounds. Although the stress concentration ratios obtained from the design methods are not related to the pile–subsoil relative displacement, horizontal lines represent the calculated stress concentration ratios are plotted in Figure 23 for comparison.

STRESS REDUCTION RATIO In order to compare the various design methods, a parameter called the stress reduction ratio

(SRR) has been defined (Kempton et al. 1998). The stress reduction ratio is defined as the ratio of the average vertical stress carried by the geotextile reinforcement to the average vertical stress due to the embankment fill.

As shown in Figure 24, BS8006 method gives very low stress concentration ratio as compared to Terzaghi Method (1943) and EBGEO Method, while FEM method gives the most conservative value among all methods. However, the FEM method address show a very close values to the measured values, and it also address the variation of the stress concentration ratio with time with similar trend that obtained from measured values.

INFLUENCE OF SUPPORT SPACING In order to analyze the influence of support (pile) spacing on stress reduction ratio, the

support spacing was varied from 0.5m to 2.5m, while all other parameters remain fixed. Later the stress reduction ratio been calculated by using the four analytical methods, Terzaghi (1943), BS8006, EBGEO and FEM Method, and the results are plotted in Figure 25.

The graph suggested that there is a positive relationship between stress reduction ratio and pile spacing. Terzaghi (1943) and FEM methods produce similar trend in SRR versus support spacing plot. And for the case of BS8006 and EBGEO, the calculated SRR is more sensitive to spacing changing.

PILED BUND EFFICACY The ‘Efficacy’ is the proportion of the embankment weight carried by the piles rather than the

subsoil and/or reinforcement. Figure 26 shows the efficacy values been calculated with different methods, which revealed that Efficacy increases (tending towards 1.0) as the effect of arching increases. Both BS8006 and EBGEO methods suggested low efficacy values of 0.47 and 0.58 respectively indicating to higher portion of bund fill load to be carried by the geotextile, while FEM method addresses near exact the actual efficacy values of 0.95 against actual value of 0.97, which indicates a majority of bund fill load to be carried by the piles, which agrees with the pile load and geotextile stress results.

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Figure 23: Stress Concentration Ratio.

Figure 24: Stress Reduction Ratio.

Figure 25: Influence of Support Spacing on Stress Reduction Ratio.

Figure 26: Efficacy.

CONCLUSIONS FEM program (Plaxis) captures the overall behavior very well. However, the program also

have tendency to under-predicts settlement at early stages after construction and over predict the settlement at later stages, while the classical Terzaghi 2D computational method give somehow close results of early stage settlement but with slower rate of consolidation than the measured values, and then under-predicts the long term settlement in a wide range.

The calculated settlement from Plaxis correspond very well with the time-settlement curve for the measured period, while the classical calculation under predicts the settlements after long term period.

The measured excess pore pressure tends to consolidate faster in the clay beneath the bund, probably due to the existence of sand pockets in the marine clay layers act as a release points for the excess pore water pressure and help to consolidate faster, and this non-homogeneous material was difficult to be simulated in FEM program.

The stress concentration ratio is not fixed, and with decreasing trend for long period, FEM program capture this behavior clearly with a close range of stress concentration ratio as compared to those obtained from measurements, while three analytical methods did not address this phenomena, instead it give one value at a time. EBGEO method always overpredicts the stress

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Vol. 18 [2013], Bund. T 4310 concentration ratio, while BS8006 methods underestimate the stress concentration ratio, and Terzaghi (1943) method shows an impartial neutral value.

There is a positive relationship between stress reduction ratio and pile spacing. Terzaghi (1943) and FEM methods produce similar trend in SRR versus support spacing plot. And for the case of BS8006 and EBGEO, the calculated SRR is more sensitive to support spacing changing.

Both methods BS8006 and EBGEO suggested low efficacy values indicating a high portion of bund fill to be carried by the geotextile, while FEM program method addresses near exact the actual efficacy values, indicating a full arching behavior with majority of the bund weight to be carried by the piles, and that was in agreement with the actual measured pile and geotextile loads.

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. BS 8006 : (1995). Code of Practice on Reinforced Soil and Other Fills, British Standards Institution.

3. Filz, G. M. and Stewart, M. E. (2005). “Design of Bridging Layers in Geosynthetic-Reinforced, Column-Supported Embankments.” Virginia Transportation Research Council, Charlottesville, VA.

4. Gens,A. , Potts,D.M. , Critical state models in computational geomechanics, Engineering Computations, 1988, Vol:5, Pages:178-197.

5. Gesell, Deutsche. (2011). Recommendations for Design and Analysis of Earth Structures using Geosynthetic Reinforcements - EBGEO. Wilhelm Ernst & Sohn.

6. Han, J. and Gabr, M.A. (2002). A numerical study of load transfer mechanisms in geosynthetic reinforced and pile supported embankments over soft soil. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 128(1), pp. 44-53.

7. Holmberg, S. (1978). Bridge Approaches on Soft Clay Supported by Embankment Piles. journal Geotechnical Engineering.10(1): 77-89

8. Russell, D. and Pierpoint N. 1997. An assessment of design methods for piled embankments. Ground Engineering, November, pp. 39-44.

9. Terzaghi, K. (1943). Theoretical Soil Mechanics, John Wiley & Sons, New York, 66.

10. Wood D.M. (1990). Soil behaviour and critical state soil mechanics, Cambridge University Press.

11. Zaeske, D. (2001). Zur Wirkungsweise von unbewehrten und bewehrten mineralischen Tragschichten über pfahlartigen Gründungselementen. Schriftenreihe Geotechnik, Uni Kassel, Heft 10, Februar 2001.

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