Analysis of anchored earth wall supporting soft backfill

Abstract

A parametric study using a finite element method of analysis is undertaken to investigate the behavior of a new type of anchored earth wall system supporting simultaneously constructed roadway. Although the elasto-plastic strain hardening softening constitutive models are more appropriate for soil, but for simplicity elastoperfectly plastic Mohr-Coulomb constitutive model is used in this study. The emphasis of the study is placed on the effect of various components of proposed wall system to the deformation patterns and other associated behavior. The parameters studied are (1) stiffne'ss of reinforcement, (2) elastic modulus of soft backfill, (3) elastic modulus of retained soil, (4) elastic modulus of granular soil in between backfill and retained soil, (5) rigidity of facing (6) anchor size and (7) anchor position. A 4.5 m high vertically faced anchored earth wall supporting a simultaneously constructed roadway is designed as per BS 8006 (1995). Starting from facing to centerline of roadway, the wall system consists of (i) facing wall, (ii) thin vertical layer of uncompacted coarse sand as filter and drainage, (iv) thick vertical layer of moderately compacted soft backfill, (v) thin vertical layer of compacted granular backfill (vi) anchor plates or blocks and (vii) retained soil of roadway. Anchor sizes are designed according to RajagopaJ and Hari (1996) for a surcharge loading of 100 kN/m2 on the roadway. Horizontal spacing of reinforcement is 1.0 m and length of anchor is also taken as 1.0 m so that it become a plane strain anchor of length 1.0 m and height 0.12 m in the finite element model. Large factor of safety for anchors is used for standard wall configuration to minimize the effect of anchor size during study of other parameters. Lightly steel reinforced concrete wall with a small pad below wall is used as continuous rigid facing to reduce lateral deformation of wall and hence vertical deforination of top surface of roadway. It is observed that deformation of wall decreases with increasing stiffness of reinforcement and after certain value of stiffness it has no effect on deformation. On the other hand, anchor force increases with increasing stiffness of reinforcement and after certain value of stiffness it has no effect on anchor force. Deformation decreases with increasing stiffness of backfill and retained soil. Anchor force also decreases with increasing stiffness of backfill but remain constant with variation of stiffness of retained soil. Facing rigidity has significant effect on wall deformations. Deformation decreases' with increasing facing rigidity and after certain value of facing rigidity its effect become negligible. From the study it may be concluded that locally available silty clay soil may be used in the reinforced zone as well as in the retained soil mass of roadway. Moderately compacted fill of which elastic modulus is greater than or equal to 10 MPa is sufficient for reinforced zone, provided that stiffness of reinforcement is greater than 5.0x106 N/m and F.S. for anchor size is greater than or equal to 2.0. Moderately compacted fill of which elastic modulus is greater than or equal to 20 MPa is sufficient for retained soil mass of road embankment. Continuous rigid facing must be used for the proposed wall system. To avoid differential settlement of roadway, reinforced soil and retained soil should have same stiffness and strength properties and in no case should stiffness of retained soil be less than that of reinforced soil. Formula given by Rajagopal and Han (1996) is found to be more acceptable to estimate ultimate pull out capacity of vertical anchor used in anchored earth wall. Greater length of anchorage improves the serviceability of the retained structures in addition to ensuring stability.