Golden Jubilee Conference of the IGS Bangalore Chapter, Geo-Innovations, 30-31 October 2014 Finite Element Analysis of Flexible Anchored Sheet Pile Walls: Effect of Mode of Construction and Dewatering Naveen Kumar 1, Arindam Dey 2* 1 Post Graduate Student, Department of Civil Engineering, IIT Guwahati, Assam-781039, India naveenk88@gmail.com 2 Assistant Professor, Department of Civil Engineering, IIT Guwahati, Assam-781039, India, arindam.dey@iitg.ernet.in Abstract. This paper reports the finite element (FE) study to assess the behavior of a flexible sheet pile wall. The effect of excavation and backfilling process on the behavior of the sheet pile wall have been thoroughly investigated and the results are presented in terms of the wall deformations and bending moments, developed anchor forces, and the earth pressures developed on both active and passive side of the wall. Development of failure mechanism in terms of soil displacements and incremental deviatoric strains, slip lines and plastic points formations have been illustrated. Such structures in the field are often associated with high water table, which renders significant hindrance in the construction of sheet pile wall, and hence, dewatering is adopted in such cases. The dewatering scheme substantially alters the stress conditions on the sheet pile wall, and if not accommodated in the design, might render the behavior as unrealistic. This aspect has been addressed in the present article. It has been observed that the sheet pile wall systems in the dewatered excavation cases are failing before the desired excavation depth, and hence, forms a crucial part of the analysis. Keywords: Flexible anchored sheet pile wall, Installation technique, Excavation, Backfilling, Dewatering, PLAXIS FE modeling. Introduction Flexible retaining walls are frequently used for earth retaining purposes as required for deep excavations and tunnelling, waterfront structures, beach and river bank protection, stabilization of ground slopes, shoring walls of trenches and construction of cofferdams. Based on the site requirements and field conditions, the two widely used installation techniques adopted are excavation and backfilling, which exhibit significant influence on the behavior of the sheet pile wall. Site conditions may govern the adoption of a tie-back mechanism as well for such structures. The problems related to flexible anchored sheet pile wall being associated to the complex interaction with soil, analytical techniques to obtain solution becomes quite cumbersome. Under such conditions, resort is taken to the numerical models such as finite element method (FEM) for the investigation of such structures. This paper utilizes PLAXIS 2D v2012, commercial geotechnical FE software, to simulate the behavioural response of sheet pile wall depending on the installation technique and construction sequences, subjected to the effect of fluctuation of the ground water table due to dewatering. Very limited studies related to behaviour of sheet pile walls subjected to the above mentioned conditions have been earlier investigated [Bilgin 2010]. The present article illustrates in detail the effect of mode of construction and fluctuation of water level on the response of of the sheet pile wall under static condition. PLAXIS FE Modelling Fig. 1 shows a schematic diagram of an anchored sheet pile wall. For the present study, such a wall retaining 6m height of soil is considered. Considering the sheet pile wall to be extended in the longitudinal direction, a plane strain model has been developed with model boundary 1
Golden Jubilee Conference of the IGS Bangalore Chapter, Geo-Innovations, 30-31 October 2014 having dimension 48m x 15m (The dimension has been so chosen that any boundary effects on the response of the sheet pile wall is eliminated [Bilgin 2010]). The soil was modeled using 15-noded triangular elements and its stress strain behavior represented by the elasticperfectly plastic Mohr-Coulomb model [Bilgin (2010), Hsiung (2009), Krabbenhoft et al. (2005)]. Two different types of soil has been considered for the present study, namely medium dense and loose sand which have been designated as D and L respectively and the properties of which are as given in Table 1. The whole soil domain is divided into two main layers, namely foundation soil (medium dense or loose sand) and backfill soil (always medium dense sand) as shown in Fig. 1. The soil-structure interface behavior has been described using an elastic-plastic model, whose interface strength is defined by R int = (tanδ/tan φ'), where δ is the interface friction angle, and φ' is the angle of internal friction of the adjacent soil mass. The sheet pile wall is modeled using 6-noded elastic plate element, while the anchor is modeled as an elastic spring element with the far end having a fixed node. The length of the anchor was so chosen that the anchor penetrates the resisting zone beyond the active zone behind the sheet pile wall. Table 2 enlists the properties of the sheet pile wall. Figure 1. Schematic diagram of a typical section The construction procedure has been modeled utilizing the staged construction sequence of the software. The backfill soil was divided into eight layers, each of 0.75m depth, on one side of the sheet pile wall. The anchor was installed when two layers of either excavation or backfill was completed. The anchor level has been fixed at 25% of wall height based on [Bilgin and Erten (2009)]. For backfilling process, two cases are considered: (a) Backfilling with initial excavation - The ground is first excavated to the depth of initial dredge line, sheet pile is installed, and then the backfilling is done. Such a condition reproduces the effect overconsolidated soils (b) Backfilling without initial excavation The existing ground level is considered as the dredge level, and hence the construction starts with the direct installation of sheet pile wall followed by backfilling process. Such a condition replicates the normally consolidated soils. In all these cases of excavation and backfill, the water level has been considered to be at the level of the anchor throughout the construction process. Each case has been designated as DL6 or DD6, where initial letter represents the type of backfill soil, the second letter the foundation soil and 6 the height of the soil retained in metres. Six cases have been analyzed with constant water level: two for excavation and four for backfilling. In the next set of analysis, the effect of dewatering during the construction process has been investigated. For the excavation case, dewatering is done after each layer of excavation so that the water level is kept 0.75m below the immediate excavated level. In the backfilling case, water level is kept 0.75m below the dredge line by dewatering, before the installation of sheet pile wall and is maintained at the same level till the backfilling is complete. After the completion of backfilling, it is assumed that dewatering is stopped and water level is allowed to reach the initial level. The effect of seepage pressure is neglected and it is assumed that due 2
Golden Jubilee Conference of the IGS Bangalore Chapter, Geo-Innovations, 30-31 October 2014 to high permeability of sand the steady state condition reaches quickly. The option of steady state nullifies the effect of permeability of the soil taken i.e. for any value of permeability the results remain the same. In the present analysis a permeability of 10-4 m/s has been taken, which is an average value for sand. Table 1. Soil properties adopted in the present study Soil Properties Medium dense sand Loose sand Saturated unit weight (γ s ) 18 kn/m 3 16 kn/m 3 Unsaturated unit weight (γ d ) 17 kn/m 3 16 kn/m 3 Angle of internal friction (φ ) 36 30 Cohesion (c) 0.3 kpa 0.3kPa Dilatancy angle (ψ) 6 0 Modulus of Elasticity (E) 35000 kpa 15000 kpa Poisson s ratio (υ) 0.28 0.20 Interface strength (R int ) 0.65 0.67 Table 2. Sheet pile wall and anchor properties adopted in the present study Sheet Pile Wall Anchor Material Steel EA 500x10 3 kn/m Elastic Modulus, E 2x10 5 MPa Spacing 2.5m EI 2.3x10 4 knm 2 /m Length 8m EA 2.738x10 6 kn/m Poisson s Ratio (υ) 0.15 Results and Discussions Six numerical simulations each have been analyzed under constant water level and subjected to dewatering conditions. The effect of construction mode and dewatering on the wall deflection, wall bending moment and anchor force developed are investigated and compared with those obtained in [Bilgin 2010]. In addition, the change in lateral earth pressure with distance from the wall and the failure surface formation will be discussed.. Figure 2. Deflection of sheet pile wall under constant water table condition Wall Deflection: Before the installation of anchor, a rigid body rotation was observed during the initial phases of excavation or backfill. Installation of the anchor resulted in the rotation of wall about the anchor leading to maximum lateral deflection occurring in between the anchor level and the bottom of the wall. Fig. 2 shows a comparison of the wall deflection profile at the final two stages of the backfilled DL6 case obtained in [Bilgin 2009] and in the present 3
Golden Jubilee Conference of the IGS Bangalore Chapter, Geo-Innovations, 30-31 October 2014 study. Reasonable agreement has been found between the results for the comparison depicted and other cases as well, which have not been presented here for the sake of brevity. Fig. 2 also reveals that the wall deflection at the end of the construction is found to be higher for backfilling cases than for excavation cases. Effect of overconsolidation and pre-existing overburden pressure is portrayed by the fact that the backfilling cases with initial excavation revealed lower deflection than other backfilling without initial excavation. Figure 3. Lateral wall deflection obtained for dewatered cases Fig. 3 shows the deflection of sheet pile wall under dewatering conditions. In comparison to the constant water level condition, higher deflections are observed due to dewatering pertaining to the removal of water load. Collapse of the wall is observed in both the cases of excavation for DD6 and DL6; The DD6 case failed during the dewatering phase after an excavation of 5.25m while the DL6 failed during the excavation of final soil layer due to excessive lateral movement. As observed for constant water table, in comparison to backfilling without initial excavation, dewatering also led to larger settlement for backfilling without initial excavation. Moreover, for all dewatered conditions, backfill cases revealed larger wall bottom displacement than that obtained for excavation conditions. Bending Moment: Fig. 4 depicts comparison plots for the results of bending moments obtained from the present numerical modeling to that recorded experimentally [Bilgin 2010]. The results show an agreeable match for the two typical cases presented. Similar observation has been made with other simulation cases as well. As observed for wall deflection, backfilling method results in higher bending moment than that obtained for excavation methods. An obvious finding is that walls embedded in loose foundation soil portrayed higher bending moment than those embedded in denser soil. 4
Golden Jubilee Conference of the IGS Bangalore Chapter, Geo-Innovations, 30-31 October 2014 Figure 4. Bending moment of sheet pile wall under constant water table condition Fig. 5 shows the comparative of the bending moment obtained for constant water table and dewatered cases. It is observed that the bending moment values obtained from the latter are higher than those with fixed water table. A significant increase in wall bending moment at the anchor level is also observed for the walls constructed by excavation method. Figure 5. Bending moment of sheet pile wall under constant water table and dewatered cases Anchor Force: The anchor forces obtained in all cases with fixed water table have values almost near to the values obtained in [1] which have been given in Table 3. The walls constructed in loose foundation soil result in higher anchor force than those in denser soil. Moreover, the anchor forces developed in all dewatered cases are much higher than those for the fixed water table cases. Lateral Earth Pressure: Fig. 6 shows the variation of effective active lateral earth pressure with the increasing distance from the face of the wall (Final stage of DL6 backfilling case having fixed water level). It is observed that the earth pressure is low wherever the wall, and hence soil, displacements are higher (which releases the lateral restraint and results inlowering of lateral stress). Moreover, the lateral earth pressure is observed to have a transition from active state at-rest state as the distance from the wall increases. It can be noticed that there is a sudden decrease in earth pressure (near the wall) below the anchor level due to the presence 5
Golden Jubilee Conference of the IGS Bangalore Chapter, Geo-Innovations, 30-31 October 2014 of water table. The variation in passive earth pressure during the construction stages for typical excavation and backfill cases are shown in Fig. 7 and Fig. 8 respectively. For walls constructed by excavation method, initially the passive resistance is provided by a larger height of soil. Towards the final stages this height resisting soil decreases, which leads to sudden increase in the passive pressure in the remaining soil; whereas for the walls constructed by backfill method, the passive resistance increases gradually as the height of resisting soil is same throughout the construction process. Table 3. Anchor forces Case Anchor Force (kn/m) (Present study) (Bilgin 2010) DD6 Excavation DL6 Excavation DD6 Backfilling without initial excavation DL6 Backfilling without initial excavation DD6 Backfilling with initial excavation DL6 Backfilling with initial excavation Dewatered DD6 Backfilling without initial excavation Dewatered DL6 Backfilling without initial excavation Dewatered DD6 Backfilling with initial excavation Dewatered DL6 Backfilling with initial excavation 44.056 45.572 40.629 54.741 40.334 44.930 58.829 87.607 57.385 86.601 ~ 40 ~ 40 ~ 40 ~ 40 Deformations in soil and failure surfaces: The deformations in soil at the final stage of walls constructed by different construction modes have been shown as a contour diagram in Fig. 9. Soil heaving is observed in the excavation case and for backfilling case with initial excavation due to the releasee of overburden stress. Distinct wedge formation on the active side can be observed in all the cases. Fig. 10 presents some typical failure surfaces formed in the final stage of construction. Most of the cases show similar failure pattern with a clear formation of failure surface on the active side and an obscure one on the passive side indicating the development of passive failure wedge in case of large deformations of the sheet pile wall. Plastic points in soil: Fig. 11 presents the plastic points generated in soil at different stages of construction of DD6 backfilled case. The formation of plastic points indicates that the corresponding zone of soil has failed or is at the verge of failure. Thus the zone of plastic points represents the surface of rupture. The rupture zone can be clearly noticed on both the sides of the wall from the same figure. Figure 6. Variation of effective earth pressure away from the wall on the active side 6
Golden Jubilee Conference of the IGS Bangalore Chapter, Geo-Innovations, 30-31 October 2014 Figure 7. Variation of passive earth pressure away from the wall for an excavation case Figure 8. Variation of passive earth pressure away from the wall for an excavation case Figure 9. Total displacement in soil at the final stage for various cases Figure 10. Incremental deviatoric strain at the final stage of various cases Summary and Conclusions Based on the numerical analyses conducted and the results reported, various conclusions that have been drawn are as follows: The wall deflections are affected by the mode of construction and the effect is significant when the foundation soil comprises of loose sand. Significant wall deflection is noted when the dewatering phenomenon is considered during the excavation process. It is to be 7
Golden Jubilee Conference of the IGS Bangalore Chapter, Geo-Innovations, 30-31 October 2014 mentioned that failure of wall occurred in both the dewatered excavation cases, with loose and dense foundation soil, before the final stage of construction was reached. Figure 11. Plastic points generated in soil at various stages of construction Bending moment of the sheet pile wall was also affected by the mode of construction and dewatering, the effect is similar in trend as of wall deflection. The anchor forces generated for the fixed water level cases were negligibly affected except for that of backfilled case with loose foundation soil, which had relatively higher anchor force. The anchor forces for the dewatered cases were significantly higher, with dewatered excavation cases having highest values. The dewatered excavations cases had high wall base deflection. Further study on the soil movement and the strain developed in soil showed a basal failure of the system rather than failure due to wall bending or rotation. The finite element modeling results gave a good idea on the stress and strain developed, strength mobilization, development of slip surface and failure pattern of the soil wall system through diagrams of earth pressure developed, relative shear stress, formation of plastic points and incremental deviatoric strains. References Bilgin O (2010) Numerical studies of anchored sheet pile wall behavior constructed in cut and fill conditions. Computers and Geotechnics 37: 399 407. Hsiung B-CB (2009) A case study on the behavior of a deep excavation in sand. Computers and Geotechnics 36(4): 665 75. Krabbenhoft K, Damkilde L and Krabbenhoft S (2005) Ultimate limit state design of sheet pile walls by finite elements and nonlinear programming. Computers and Structures 83: 383-393. Bilgin Ö and Erten MB (2009) Analysis of anchored sheet pile wall deformations. In Contemporary topics in ground modification, problem soils, and geo-support (GSP 187). International foundation congress and equipment expo, Florida 137 44. 8