A database of some 300 case histories of wall and ground movements due to deep excavations worldwide is presented. Although recognizing the weakness in the approach, a large database is used to exa...

Database for Retaining Wall and Ground Movements due to Deep Excavations

This paper examines the strength and modulus of marine clay-cement mixes with high cement content. Although similar studies have been reported, many of these studies were conducted using soil-cement and water-cement ratios which are more applicable to deep cement mixing than jet grouting. The objective of this study is to investigate how the strength and modulus of cement-treated Singapore marine clay vary with cement and water contents at a range of cement contents, which is more representative of that used in jet grouting. To facilitate parametric studies that are relevant to jet grouting operations, a working range of the constituents was proposed for Singapore marine clay based on the liquid and bleeding limits of the soil-cement mixes. Comparison with data from some previous jet grouting studies and projects indicates that the liquid and bleeding limits can encompass most, if not all, of the parameter range normally used in jet grouting operations. The results of unconfined compression tests on cemen...

Strength and modulus of marine clay-cement mixes

A database of some 300 case histories of wall and ground movements due to deep excavations worldwide is presented. Although recognizing the weakness in the approach, a large database is used to examine general trends and patterns. For still soil sites, movements are generally less than those suggested in the well-known relationships proposed by Clough and his coworkers. However, for walls that retain a significant thickness of soft material but have a high factor of safety against basal heave, movements are similar to those calculated using the Clough charts. In these cases, when soft ground is actually present at dredge level, the Clough charts will underpredict movement and need to be used with care. For the above cases there is no discernible difference in the performance of propped or anchored systems but there is some evidence to suggest top-down systems perform better. In cases where there is a low factor of safety against excavation base heave, large movements can occur, but the Clough charts will ...

Closure of "Database for Retaining Wall and Ground Movements due to Deep Excavations"

This paper presents the results of 150 finite-element simulations conducted to define the effects of excavation geometry, i.e., length, width, and depth of excavation, wall system stiffness, and factor of safety against basal heave on the three-dimensional ground movements caused by excavation through clays. The results of the analyses are represented by the plane strain ratio (PSR), defined as the maximum movement in the center of an excavation wall computed by three-dimensional analyses normalized by that computed by a plane strain simulation. A simple equation for PSR is presented based on excavation geometry, wall system stiffness, and factor of safety against basal heave. This PSR equation reasonably represents trends in results of the 150 simulations as well as those simulations reported in literature. When the excavated length normalized by the excavated depth of an excavation wall is greater than 6, results of plane strain simulations yield the same displacements in the center of that wall as those computed by a three-dimensional simulation.

Three-Dimensional Effects for Supported Excavations in Clay

A 16–18 m deep excavation for a metro station with in-plane dimensions of approximately 20 m wide by 290 m long was constructed by using the cut-and-cover technique in soft clay in the Shanghai metropolitan area, in which many high-rise buildings and utilities exist. To investigate the performance of this deep excavation and the associated effect on the adjacent buildings and utility pipelines, a long-term comprehensive instrumentation program was conducted during construction. Field observations included deflections of diaphragm walls, vertical movements at wall tops, ground settlements, and settlements of surrounding buildings and utilities. Analyses of field data indicated that overexcavation (i.e., removing more soil than designed) and a long construction duration caused the diaphragm walls to develop substantial deflections. The base and middle floor slabs played dominant roles in suppressing postexcavation wall deflections and ground settlements. This excavation in general exhibited different behavi...

Observed Behaviors of a Long and Deep Excavation Constructed by Cut-and-Cover Technique in Shanghai Soft Clay

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.

Properties of Singapore marine clays improved by cement mixing

This paper discusses the effects of corners on wall deflection and ground movement around multistrutted deep excavations. To assess the significance of such effects in a typical-sized excavation in Singapore, a field monitoring exercise was conducted into an excavation in soft marine clay. Field data from this exercise indicate that the strengthening effect of the corner can lead to a significant reduction in wall deflection and ground settlement. Finite-element back-analyses of the excavation also showed that, where corner effects are significant, three-dimensional analysis may be able to offer significantly better predictions of movement than two-dimensional analysis. Comparison with several other excavation projects where corner effects were considered or observed suggests that the significance, or otherwise, of such effects depends on three factors: the length-to-depth ratio of the excavation, the depth to a relatively stiff stratum, and the stiffness of the strutting system.

Effect of Corners in Strutted Excavations: Field Monitoring and Case Histories

A simplified model based on the one‐dimensional wave equation, which accounts for the interaction of the pounder and the soil, and the propagation of stress wave in the soil during the dynamic compaction of loose granular soil is presented. The soil beneath the pounder is represented by a nonlinear soil column, while the surrounding soil is represented by a series of springs and dashpots. The spring simulates the dynamic soil stiffness and the dashpot accounts for the radiation damping effect. The input soil parameters of the model can be determined in the laboratory or estimated from correlations with measured field data. In spite of the various simplifying assumptions in the model and the predictive method, computed results, such as pounder penetration and degree and depth of improvement, show an encouraging measure of agreement with available field measurements from two dynamic compaction projects. The proposed model is potentially useful for the analysis of dynamic compaction of loose granular soil.

Dynamic Compaction Analysis

A method is presented to evaluate the effect of print spacing on the dynamic compaction of loose granular soils. It uses a recently developed wave-equation model together with an approach to predict the lateral extent of soil improvement around the pounder. The procedure to evaluate the effect of print spacing is demonstrated using two examples, which show that the most critical areas are the center of the grid and the middle of the side of the grid where the least improvement of soil is achieved. Three reported case histories of dynamic-compaction projects are analyzed, and the solutions are shown to be in good agreement with those obtained from the field. Two design curves, one for the center of the grid and the other for the middle of the side of the grid, are established. These curves summarize the influence of print spacing on the effectiveness of dynamic compaction in densifying the soil at these two critical locations and are useful for the selection of print spacing in dynamic-compaction projects.

K. Y. Yong

Papers

Strength and modulus of marine clay-cement mixes

Properties of Singapore marine clays improved by cement mixing

Effect of Corners in Strutted Excavations: Field Monitoring and Case Histories

Dynamic Compaction Analysis

Dynamic Compaction of Loose Granular Soils: Effect of Print Spacing