K. Y. Yong

PROPERTIES OF SINGAPORE MARINE CLAYS IMPROVED BY CEMENT MIXING

Stabilization of soft ground by the deep cement mixing (DCM) method has become an increasingly popular method to improve stability in an excavation in soft clay and to limit movement in adjacent sub-structures. The desired increase in strength and stiffness to fulfil the intended functions can be achieved provided that the right mix proportion is adopted. To proceed with this kind of soil improvement, prediction of the strength and stiffness of the improved soil is necessary. Due to a short history of the DCM method in Singapore, there is limited data on the improved properties of local clays. This study is conducted to bridge that gap and also extends its usefulness to clays elsewhere. In the paper, the influences of three main constituents of the mixture, namely clay, water, and cement on the strength development of Singapore marine clays improved by cement mixing are investigated. Based on the experimental results, it is shown that a convenient normalization can produce a consistent pattern for evaluation of improved strength of clays from different parts of Singapore. This normalization is also shown to work for one Japanese clay. Correlations between strength and stiffness of the improved clay are also obtained. Lastly, it is shown that for a cement mixed clay there is a continual increase in strength and stiffness with time. This will help to reduce ground movement, and it will also increase the bending moment in the retaining wall. Both aspects must be considered in a design.

A variational approach for vertical deformation analysis of pile group

A variational approach for the analysis of vertical deformation of pile groups is presented. The method assumes that the deformation of piles can be represented by a finite series. The method applies the principle of minimum potential energy to determine the deformation of piles. Using this method, an analytical solution for pile groups in soil modelled by the theoretical load-transfer curves can be obtained rigorously. Analysis of field tests indicates that the method can predict the performance of pile groups reasonably well.

ELASTO-PLASTIC CONSOLIDATION ANALYSIS FOR STRUTTED EXCAVATION IN CLAY

Abstract Ground movements and strut loads in strutted excavations in clay have been observed to change with time. In this paper, the time-dependent behaviour of excavation support system is studied by comparing the results of undrained and consolidation analyses with data from an instrumented excavation project. Dissipation of excess pore pressure is modelled using a fully coupled consolidation analysis while the soil is assumed to be an elastic-perfectly plastic material obeying the Mohr-Coulomb yield criterion. The results of the study show that the undrained analysis underestimates the sheet pile wall movement and fail to reflect the progressive movement of the sheet pile. In contrast, these effects are well-predicted by the consolidation analysis, thereby indicating that dissipation of excess negative pore pressure can indeed account for much of the observed progressive ground movement and build-up of strut loads with time. The elasto-plastic consolidation model can also simulate excavation sequence including uneven excavation and time delays in excavation and strutting.

Monitoring of dynamic compaction by deceleration measurements

Abstract A method of estimating the degree and depth of improvement in the dynamic compaction of loose granular soil by matching the computed decelerations of the pounder from a numerical model to the measured decelerations is presented. This method uses a simple one-dimensional wave equation model in which the soil beneath the pounder is represented by an elastic laterally confined soil column while the surrounding soil is represented by a series of springs and dashpots. The spring simulates the dynamic soil stiffness while the dashpot accounts for the radiation damping effect. Despite the simplifying assumptions in the model, the computed results for the degree and depth of improvement show an encouraging measure of agreement with available data from a laboratory test and field measurements from a dynamic compaction site. The proposed method is potentially useful for the monitoring of dynamic compaction of loose granular soil. And like any new technique, further verifications of the approach are necessary to develop it into a reliable method.

A method for the analysis of large vertically loaded pile groups

An iterative method is described for the analysis of vertically loaded pile groups with a large number of vertical piles. The individual pile response is modelled using load-transfer (t–z) curves while pile–soil–pile interaction is determined using Mindlins solution. The present method not only keeps all the advantages of the so-called ‘hybrid method’, but also makes it possible for practising engineers to solve problems of large non-uniformly arranged pile groups in a time-saving way using a personal computer. Good agreement between the present method of analysis and the direct method is observed. A case history is analysed and the computed response of a large pile group compares favourably with the field measurement. Copyright

A NEW FINITE ELEMENT MODEL FOR PILE-SOIL INTERACTION

This paper describes the formulation and validation of a new finite element model for 3-D analysis of pile-soil interaction problems. The proposed element is constructed by wrapping 4 slip elements around a 2-nodes flexural element. The 4 slip elements are to model the interface between soil and pile and the 2-node flexural element simulates the pile segment. This element will lead to an ease of use in the 3-D numerical analysis of soil-pile interaction problems with slip elements being built in one element and a significant reduction of numbers of system Degree of Freedom comparing to the conventional way of modeling pile with 20-node brick elements. The element stiffness formulation is based on the weak formulation of virtual work principle with the internal equilibrium between pile and soil being governed by Eulers beam theory in lateral reaction. The FEM program using the proposed model was developed and verified and validated with analytical solutions and a simple case study.