Numerical investigation into ground treatment to mitigate the permanent train-induced deformation of pile-raft-soft soil system

Abstract

The piled-raft foundation in soft clay ground for high-speed trains has suffered serious settlement due to cyclic train loading in recent years. It was urgent to implement countermeasures to improve the natural ground. In this research, firstly, a ground treatment technique was proposed to reduce permanent train-induced deformation. Permeation grouting was injected into the bearing strata of group piles with constant pressure. Then numerical simulations based on sophisticated constitutive model and soil–water coupled finite element-finite difference (FE)-(FD) compound arithmetic, were carried out to explore the mitigation effect of the proposed technique. The employed numerical method was validated by using the in-situ monitoring data of lateral ground displacement and surface settlement. The calculation results indicated that reinforcement in the bearing strata of group piles could efficiently increase the tip resistance of piled-raft foundation. Consequently, the surface settlement and sectional forces of group piles can be reduced to a large extent. The proposed ground treatment method was useful for reinforcing pile-raft-soft soil system and had great potential to be applied in practical situations.

Introduction

For high-speed railways constructed on soft subsoil, the vibration induced by train loading may give rise to degradation of subsoil and fatigue breakdown of railway structure and then affect the safety of trains [12], [2]. For high-speed railways, it required a smooth and stable foundation of track beneath the surface, indicating that high stability of settlement deformation was indispensable. Piled-raft composite foundations, with relatively low engineering costs and high strength and rigidity, were gradually being applied to high-speed railway subgrade engineering. However, after the trial period of operation of high-speed railway, the piled-raft foundation settled significantly. In some parts of the Hu-Hang railway lines, non-uniform settlement occurred and the rail surface had become tilted, which adversely affected running safety [26]. Accordingly, it was urgent to conduct research centered on the reinforcement and subsidence reduction measures of high-speed railways in the local subsidence areas of soft subgrade without interruption of traffic.

Many scholars have studied the mechanical properties of piled-raft foundations through physical tests or numerical methods. Centrifuge model tests [33], [18] were performed to explore the mechanical behavior of laterally loaded piled-raft foundation, and effect of group size on vertical displacement, acceleration response and bending moments was analyzed. Fukumura et al. [6] made a comparation between piled-raft and group piles due to dynamic loading in the shaking table test, and it was observed that their horizontal stiffness was basically the same. Chang et al. [5], Small et al. [15], Huang et al. [9], Tashiro et al. [20] and Moayed et al. [14] conducted three-dimensional (3D) FEM to investigate the subsidence deformation property of piled-raft foundations over time. Gandhi et al. [8] conducted 3D non-linear FEA for piled-raft foundations and raft which was loaded till to destruction. Yamashita et al. [28] studied the static-dynamic characteristics of piled-raft composite foundation bearing the base-isolated structures on soft soil foundation, and proposed the ground reinforcement of cement mixing walls with grid-pattern was highly effective to improve its bearing capacity. For deep understanding the mechanical properties of piled-raft foundation due to traffic loading, accumulation of the observed data was significant, while study concentrated on the settlement characteristics of vertically dynamically loaded piled-raft foundations was still limited. Additionally, the dynamic properties of piled-raft foundation in natural subsoil was mainly dependent on the nonlinear properties of ground soil. Much of the research about the subsidence property of piled-raft foundation was classified into a vital classification that the dynamic characteristics of subsoil was described by a primitive constitutive model. Further research of dynamic property of piled-raft foundation under train loading in soft soil areas by numerical simulations or case histories, combined with soil-water coupling analysis was needed.

With massive construction and operation of high-speed railways, the problems of permanent train-induced deformation in deep, soft ground have gradually been revealed. There has been considerable research on the mitigation of soft soil subgrade settlement [24], [3], [21], [23], [12]. Wang et al. [22] carried out centrifuge tests to research on the availability of subsidence control by geogrid-improved loess ground under high-speed train loading. Although some researches had been done on the control method for ground subsidence, there were few reports on the settlement control of pile-raft-soft soil system due to high-speed railways. Therefore, further study on the quantitative evaluation of the ground treatment scheme to reinforce piled-raft-soft soil composite foundations under long-term high-speed train loading was urgently needed.

In this study, through rigorous finite element analysis, a research design was carried out to look into the countermeasure to decrease permanent deformation of piled-raft-clay system in deep soft-soil areas. A kinematic hardening elastoplastic constitutive model, which could describe in a unified manner the mechanical properties of soft soil with various densities due to various types of loads (monotonic or cyclic) under different draining conditions, was applied. The subsidence property of piled-raft foundation was studies by soil-water coupled finite element-finite difference (FE-FD) code DBLEAVES [27], which was validated by the field recordings of trial operation. This numerical analysis is a blind one, all of the soil parameters are determined based on element tests without being influenced or modified by the observed behavior of a boundary value problem. Then, the effectiveness of permeation grouting in the supporting layer of group piles for mitigation of high-speed train-induced vibration was predicted and explored. Special attention was paid to variations of settlement deformation, pore water pressure, acceleration response and sectional forces of group piles during high-speed train loading.