EFFECT OF DREDGING AND ADJACENT FOOTING ON CANTILEVER AND ANCHORED SHEET PILE WALLS IN SAND

Abstract

Sheet pile walls are generally used to support the excavation. These are comprised of sections of sheet material with interlocking edges. Unlike retaining walls which are permanent structures, sheet pile walls are constructed for temporarily supporting the soil mass laterally. Proper design of the support system is essential as it can cause potential damage to the adjacent structures if not designed properly. This has been an area of research for more than six decades. Model studies on embedded walls have been performed by Rowe (1951), Bransby & Milligan (1975), Lyndon & Pearson (1985) and Seok et al. (2001), etc. Rowe (1952) studied the influence of different parameters like surcharge, position of anchors, dredge level, type of soil, flexibility of pile and distribution of soil pressure on the flexible retaining wall. Hanna and Kurdi (1974), Milligan (1983), Rajgopal and Sri Hari (1998) had carried out experimental studies on anchored flexible retaining walls in sand by considering certain primary factors like anchor geometry, initial design assumption, soil density, wall flexibility and initial stress state. Clough and Reed (1984) studied different parameters like wall movements, wall settlements and pore pressures in sheet pile wall supported excavation. Ohori et al. (1988) proposed a calculation model for static analysis of a double sheet pile wall structure which had been subjected to horizontal force. Clough and O’Rourke (1990) studied the effect of excavation depth on ground settlements and lateral movement of different type of walls. Bica and Clayton (1993) have developed empirical charts for the design of cantilever walls. Georgiadis and Anagnostopoulos (1998) had conducted experiments on sheet pile wall supported excavation. The effects of surcharge load and surcharge distance on the bending moment of the wall and lateral earth pressure were studied. This shows that previous studies conducted on sheet pile wall covered a major area including the effect of surcharge (Rowe, 1952; Rajgopal and Sri Hari, 1998; Georgiadis and Anagnostopoulos, 1998), method of construction (Milligan, 1983; Bilgin, 2010), depth of excavation (Wong and Broms, 1989; Tan and Tan, 2004; Finno et al., 2007; Goh et al.,2017; Konai et al., 2018), type of soil (Rowe, 1952; Rajgopal and Sri Hari, 1998; Bilgin, 2010; Bilgin 2012), stiffness of sheet pile wall (Wong and Broms, 1989; Finno et al., 2007; Goh et al.,2017; Konai et al., 2018), factor of safety against basal heave (Clogh and Reed, 1984; Finno and Roboski, 2005; Finno et al., 2007), distance of loading from the wall (Georgiadis and Anagnostopoulos, 1998; Soek et al., 2001; Nasr, 2014), flexibility of the sheet pile (Rowe, 1952; Hanna and Kurdi, 1974; Milligan, 1983), etc. by conducting experimental or numerical analysis on both cohesive and cohesionless soils. A number of studies had been conducted on anchored sheet pile wall (Rowe, 1952; Mc Rostie et al., 1972; Hanna and Kurdi, 1974; Rajgopal and Sri Hari, 1998; Finno and Roboski, 2005; Laefer et al., 2009; Bilgin 2010; Bilgin, 2012; Zhao et al., 2019) covering the area like single anchored wall, multi anchored wall, effect of size of anchors, shape of anchors, inclination of anchors, number of anchors and length of anchors etc. But, the work on the effect of depth of footing on sheet pile wall behaviour has not been studied yet. The comparison between the stagewise excavation and full excavation has also not been found in the literature. So, a detailed experimental investigation has been carried out to further study the behaviour of cantilever and anchored GI sheet pile walls. PLAXIS 3D, a finite element code has also been used for the numerical modelling of the work. The experimental study has been broadly divided into two cases, Case A and Case B. Effect of adjacent footing on the behaviour of the cantilever and anchored sheet pile wall supported excavation is studied in Case A. Effect of pre-existing excavation on the adjacent footing is studied in Case B. Four types of anchored sheet pile wall with varying number of anchors and anchor rod length have been studied. Anchored wall 1 is having two anchors of 400 mm length. Anchored wall 2 is having four anchors with 400 mm length. Anchored wall 3 is having two anchors with 600 mm length. Anchored wall 4 is having four anchors with 600 mm length. All the experiments are carried out by using the dredging method. Various factors affecting the sheet pile wall system have been varied. The distance between the footing and sheet pile wall face (X), Depth of footing (Df) and the excavation depth (H) are varied in cantilever sheet pile wall related model tests. The length of tie rods (L) and no. of anchors (N) along with the factors of cantilever sheet pile wall are varied in anchored sheet pile wall. The deflection of the sheet pile wall, lateral load on the sheet pile wall and settlement of the footing are the main findings of the tests conducted under Case A and Case B. The effect of various factors on the bending moment of sheet pile wall is studied by conducting tests on strain gauge mounted MS sheet pile wall. The excavation has been simulated by dredging the sand in front of the sheet pile wall. A mechanical jack is used to apply load onto the footing, equivalent to a load intensity of 20 kN/m2. Dial gauges have been used to observe the deflection of sheet pile wall and settlement of footing. Proving rings have been used to measure the lateral load exerted by backfill on the sheet pile wall. Results show that excavation in front of the sheet pile wall induces both the horizontal movement of the sheet pile wall and the vertical movement of the adjacent ground. On excavating the soil in front of the sheet pile wall, the backfill soil exerts pressure on to the wall so the wall tends to move away from the backfill resulting in a decrease in the density of the backfill material causing vertical settlement to the ground. The deflection of sheet pile wall, the settlement of footing, maximum lateral load coming on the sheet pile wall and the bending moment on the sheet pile wall reduce with the increase in the distance between the wall and the footing. At higher distances from the sheet pile wall, the applied footing load transfers less lateral load to the wall causing less movement of it, less lateral load, bending moment on it and also the less settlement of the ground. The final deflection and load on the sheet pile wall observed in Case B for the same footing position has been found to be higher as compared to Case A. This may be attributed to the fact that the stagewise excavation causes lesser disturbance in the system and thereby lesser deflection and load on the sheet pile wall. The addition of anchors in the sheet pile wall reduces the amount of deflection and lateral load on it significantly. An anchored wall shows restricted wall movement with wall rotation occurring at anchor level. So, it results in a changed pattern of sheet pile wall deflection and lateral load variation as compared to cantilever retaining wall. The installation of anchors at two level results in lesser deflection of the wall and a steeper curve of deflection because of the support provided at two levels while one level anchor gives higher deflection and load on the wall compared to two level anchors. The response of the anchor depends upon the length of the anchor vis a vis the distance of failure plane from the wall. The performance of the anchor is poor when the anchor just crosses the Rankine failure wedge as compared to those anchors which fully cross the failure wedge. For the favourable response of the anchored sheet pile wall, the length of the anchor (L) should be taken as more than the width of the failure wedge and less than the footing distance from the wall: [H × ta n (45° − φ 2 )] < L < X.