PHYSICAL AND NUMERICAL INVESTIGATION ON CONTIGUOUS PILE WALLS

Abstract

The recent expansion in the number of subterranean metro tunnels, subways, and parking lots necessitates massive excavations near utilities and buildings. An excavation without adequate lateral support will most likely threaten nearby structures, especially those near service routes or buildings (Ou et al., 2013, Tan and Wei, 25 2012). To reduce excavation-related damage, many kinds of retaining systems are used. Contiguous pile walls (CPW) as a cost-effective retention technique have emerged as a solution to land shortage due to urbanization. Many investigations have been conducted to date using empirical equations, analytical solutions, or different case studies. Only limited investigations have been conducted on experimental and numerical solutions. The stability of a pile wall is mainly governed by the cumulative influence of site geometry, backfill stiffness, wall stiff-ness, loading type and intensity. Based on detailed literature study on excavation support systems by using pile wall and other retaining walls, an experimental and numerical study has been planned to examine the response of contiguous pile wall due to staged excavation. Based on preliminary study a mitigation measure was adopted by using anchor pile wall. An experimental setup has been developed by assembling a test tank, data logger system, Linear Variable Differential Transformer (LVDT) and pile wall with two piles well-instrumented with strain gauges and calibrated to measure the center and quarter response. Two different stiffness (K1 and K2) piles were used to establish three configuration walls (S1, S2 and S3) to support sandy backfill with different relative densities subjected to surcharge loading. The lateral deflection, vertical settlement of surcharge, and bending moment response of the CPW were investigated using an extensive series of plane strain model experiments for various surcharge distances. Further numerical model has been developed by using PLAXIS 2D and PLAXIS 3D and validated with experimental model. The constitutive behavior of backfill soil has been compared by using both Mohr-coulomb (MC) and Hardening soil model (HS). Further HS model with PLAXIS 2D model has been adopted for the entire analysis. Pile material was modelled as linear elastic model. Noticeable improvement in vertical settlement, lateral deflection, and bending moment was observed with increment of stiff-ness of pile wall, soil density and surcharge distance. The field possibilities of unembedded and embedded surcharge footing were also modeled under variable loading intensity based on different factors of safety The response of the model wall clearly indicates the loss of passive resistance over the wall due to an increase in excavation. This can be seen from the increase in lateral displacements and bending moments of the wall with a downward shift in the position of the maximum bending moment. The unembedded footing is comparatively detrimental to the stability of the wall as the influence region intersects the wall along the active region without providing any passive confinement of soil causing significant lateral displacement. The stiffer configuration of the pile wall shows the least deflection even for a high magnitude of surcharge loading. The position of the footing in terms of depth and distance from the pile wall is an important factor. Further a novel 2D plane strain numerical modeling technique incorporating gap correction factor ‘k’ has been proposed. It is validated to simulate the field response of a scaled-up prototype model. The parametric study examined the effect of excavation depth, surcharge distance, surcharge magnitude, diameter, length of piles, and gap spacing between piles on the overall response of the contiguous pile walls. An optimization study has been performed to check the viability of mitigation measures using anchor-supported contiguous pile walls with the presence of a capping beam. Measures have shown improvement in the performance of wall by 85 % respectively. During earthquakes, the retaining wall may experience both vertical and horizontal inertial forces that may either be directed towards or away from it, as well as upwards or downwards. It is necessary to study the impact of vertical acceleration as importantly as horizontal acceleration for earth structures. Due to time consumption in pure dynamic analysis, pseudo-static analysis may be used as an alternative to assess geotechnical structural stability against earthquake-induced forces. Further, the potential impact of vertical (kv) and horizontal accelerations (kh) on the seismic stability of unanchored and anchored contiguous pile walls has been investigated. This will be done by considering a broad spectrum of vertical and horizontal acceleration magnitudes by conservatively determining the optimum excavation depth. The bending moment in the wall, shear force, lateral displacement, earth pressure, and vertical settlement in pseudo-static conditions with and without anchors have been investigated. The effect of friction angle, surcharge load, surcharge distance and wall stiffens has been investigated and concluded.