Chang-Yu Ou

Deep excavation : theory and practice

Introduction Soil Properties and Lateral Earth Pressures Excavation Methods and Supporting System Lateral Earth Pressure Stability Analysis Stress and Deformation Analysis - Simplified Method Stress and Deformation Analysis - Beam on Elastic Foundation Method Stress and Deformation Analysis - Finite Element Method Dewatering of Excavations Design of Structural Components Excavation and Protection of Adjacent Buildings Monitoring System

Performance of Excavations with Cross Walls

This paper presents the performance of an excavation with cross walls based on field observations and numerical analysis results. A small strain constitutive model was used in the analysis. By comparing the movements of the real case with the same case but without cross walls installed, the effectiveness of cross walls was thus evaluated. Results indicate that the movements in the case history were substantially reduced by the installation of cross walls. The maximum wall deflection and ground settlement at a place where the cross wall was installed was reduced by about 75 and 82%, respectively. The movements near the diaphragm wall or cross wall corner were relatively small, compared with those far away from the corner. The diaphragm wall still had some deflection at the sections where cross walls were installed and the amount of deflection was close to that from linear elastic analysis. It is justified that slime between cross walls and diaphragm walls during trench excavation was fully cleaned.

Failure Investigation at a Collapsed Deep Excavation in Very Sensitive Organic Soft Clay

AbstractCollapse of a 15.7-m-deep excavation in very sensitive organic soft clay in Hangzhou, China, is presented in this paper. The collapse of the excavation caused the break and/or inclination of the retaining walls, the wracking of the supporting system, significant sinking of a major road adjacent to the excavation site, and also the break in a water main beneath the road. After the collapse, the ground disturbance due to the collapse was investigated by cone penetration tests (CPTs) and field vane shear tests (VSTs). A simplified method, based on the CPT and VST test results, was used to evaluate the strength reduction ratio (SRR) of the in situ soils. According to the SRR values of the soils at different depths, the lower boundaries of the severely disturbed soils were determined and a possible slip failure surface passing through the bottom of the severely disturbed soils was justified. The factors of safety (FOSs) against basal heave of the excavation calculated from various methods were 1.05, 0....

Evaluation of Factors of Safety against Basal Heave for Deep Excavations in Soft Clay Using the Finite-Element Method

AbstractThis study investigates basal heave stability of deep excavations in soft clay using FEM with reduced shear strength. Three failure and one excessive deformation case histories are utilized. Three methods of estimating factors of safety against basal heave are employed, including the convergence criterion method involving numerical convergence, the angle method of quantifying the kick-out phenomenon of the wall, and the intersection method of examining the behavior of nodal displacements. Results show that the convergence criterion method and the angle method overestimated the factors of safety (FS) against basal heave. The intersection method, on the other hand, resulted in good agreement with the field observations because the calculated factors of safety were close to unity. The intersection method was also verified by a parametric study wherein its factors of safety increased linearly with the undrained shear strength of soil.

Predicting Wall Displacements for Excavations with Cross Walls in Soft Clay

AbstractCross walls are commonly used in some Asian countries like Japan and Taiwan for constraining the excavation-induced ground settlements as well as mitigating the subsequent building damages. To quantify the effect of cross walls, 22 case histories, including 11 excavations with cross walls and 11 excavations without cross walls, are collected in this study. It is found that cross walls can effectively reduce ground settlements by minimizing wall displacements—the maximum wall displacements for cases with cross walls are within 0.1%He-0.35%He (He=excavation depth), compared with those for cases without cross walls being within 0.3%He-0.8%He. Case histories show that the spacing of cross walls has a major effect on wall displacements. A physical model is proposed to predict the system stiffness in the presence of cross walls. Based on the data points in the collected case histories and the proposed stiffness model, a regression model that is capable of predicting maximum wall displacements is further...

Fully Probabilistic Framework for Evaluating Excavation-Induced Damage Potential of Adjacent Buildings

This paper presents a framework for a fully probabilistic analysis of the potential for damage to buildings adjacent to an excavation. Herein, the damage potential index (DPI), which is a function of angular distortion and lateral strain, is used to assess building damage potential. A serviceability limit state is established in which the resistance is expressed in terms of the “limiting” DPI, and the load is represented by the “applied” DPI. In this context, damage to the building adjacent to an excavation is said to occur deterministically if the applied DPI is greater than the limiting DPI. For the fully probabilistic analysis, both parameter and model uncertainties of the limiting and applied DPIs are first characterized. The analysis framework is then presented and demonstrated with a case history. Finally, sensitivity analysis is performed to identify the factors to which the probability of damage is most sensitive and to analyze the effect of various assumptions of the input parameters on the compu...

Development of a Suitable Operation Procedure for Electroosmotic Chemical Soil Improvement

AbstractTo expand the improvement region, a series of electroosmotic chemical tests was performed to study the mechanism of improvement. Results showed that injection of calcium chloride and sodium silicate solutions could improve the soil near the cathode and anode, respectively. Injection of calcium chloride solution for a rational injection time, followed by the injection of sodium silicate solution, could yield a fairly good improvement result for the entire sample. The rational time for the injection of calcium chloride solution could be obtained by selecting the time between the first and second turning points on the relational curve of total drained water volume and injection time. Moreover, appropriate concentrations of injected solutions were studied and were discussed in this paper.

Soil improvement using electroosmosis with the injection of chemical solutions: laboratory tests

The electroosmotic improvement of silty clay using calcium chloride and sodium silicate solutions as injection materials was studied. A series of laboratory tests were performed on reconstituted samples in order to assess the probable effectiveness of electroosmotic treatment with injection. Results indicate that the two injection materials can be effectively introduced into silty clay by electroosmosis. Very stiff cemented soil, approximately 5–6 cm in diameter, surrounding the anode, was formed due to cementation reactions between the two injection materials and soil particles under the electric field. It is justified to assume that pozzolanic reactions were responsible for the formation of very stiff cemented soil. In addition to the area of very stiff cemented soil, the region of improvement was also expanded to the cathode, near which the undrained shear strength of soil was increased by an average of approximately 195%. It was also found that the technique of polarity reversal was unnecessary for the employment of calcium chloride and sodium silicate solutions as injection materials.

Stress-strain characteristics of the Taipei silty clay at small strain

Abstract This study first developed a triaxial apparatus with the capability of small‐strain measurement. Then a series of undrained triaxial tests were carried out to investigate the stress‐strain behavior of the reconstituted Taipei silty clay at small strain. Test results show that the non‐linear behavior of the normally consolidated Taipei silty clay at small strain (ϵ=10–5∼10–3) is significant irrespective of the initial effective stress state and the shearing rate. The peak deviator stress occurs at strain of 10–3∼4×10–3. The normalized initial elastic stiffness at strain of 10–5 can reach 1600∼2450, which is much higher than that measured by conventional triaxial tests.

Time‐dependent displacement of diaphragm wall induced by soil creep

Abstract This paper presents the time‐dependent performance of an well‐instrumented excavation case using the top‐down construction method. Studies show that this time‐dependent wall displacement is mostly likely due to undrained creep of clayey soils surrounding the excavation. In other words, soil creep may make significant contributions to diaphragm wall displacement and ground settlement in a deep excavation project in soft clay strata. The extra movement was found to be about 30% of the total displacement. This paper also presents a quantitative method to evaluate this creep effect. The essence of this method is to incorporate a time‐dependent soil modulus that can reflect the effects of soil creep. This method has been shown to give reasonable results in the field and the laboratory.