Horn-Da Lin

A thermal consolidation model for pelagic clays

Abstract An analytical model has been developed for the prediction of deformations resulting from thermally induced primary consolidation and secondary compression of pelagic clays. The model is based on the results of an extensive laboratory testing program which included triaxial and one‐dimensional consolidation of clays over a temperature range of 4° to 200°C. Predictions of thermally induced strains have been made using computer programs which were modified to include the newly developed thermal consolidation model. Very good agreement was obtained between predicted and measured values.

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.

Evaluation of Soil Variability Influence on Deep Excavation Analysis - Simplified Approach

A simplified procedure is developed to provide a tool for assessing the variation of the computed excavation responses, including wall and ground deformations, induced by input parameter uncertainties through a finite element solution. A well-documented case history is employed for demonstration. The proposed scheme appears accurate and easy-to-use in the propagation of parameter uncertainties. Moreover, this procedure is capable of assessing the variability effect of either a single or multiple input parameters. Further, the sensitivity of the uncertainty effect on the excavation characteristics is also examined. The proposed procedure enables a valuable insight regarding how much variation of wall and ground movements that can be expected as a result of parameter uncertainties.

Analyses of braced excavation considering parameter uncertainties using a finite element code

This study presents a simplified procedure to quantify the effect of parameter uncertainty with consideration of soil spatial variability in analysis of braced excavations. A well-documented case history is adopted in conjunction with PLAXIS, a proprietary finite element code. The simplified approach based on first-order approximation is easy to use, requires less computational work and provides a practical tool to estimate the variation of the wall and ground responses caused by the uncertainty in input parameters. Moreover, to facilitate the numerical analysis, this study also documents a successful implementation of user-defined soil models, including the hyperbolic and the modified pseudoplasticity soil models, into PLAXIS. The results show that PLAXIS, with these user-defined soil models, can capture the laboratory stress–strain relationships and predict, accurately, the wall and ground responses in a braced excavation.

Calibrating Resistance Factors of Single Bored Piles Based on Incomplete Load Test Results

This paper addresses a problem often encountered in calibrating resistance factors for the ultimate capacities of piles based on load test databases. In practice, many pile load tests are not conducted to failure but only to a multiple (e.g., 2) of the design load. This leads to a difficult situation of incomplete information: for these test results, the ultimate bearing capacities of the test piles are unknown. How can these test results still be used to calibrate resistance factors of piles? A full probabilistic framework is proposed in this research to resolve this problem. A local pile test database of Taipei (Taiwan) is presented for demonstration. The analysis results show that the inclusion of the incomplete pile load test data enhances the stability of the calibrated resistance factors. For a target reliability index of 3, the calibrated resistance factor is in the range of 0.4–0.66 for two design models adopted in the current Taiwan design code. Moreover, it is found that the safety factor adopte...

Influence of Soil Suction on Small-Strain Stiffness of Compacted Residual Subgrade Soil

The small-strain stiffness of compacted residual lateritic subgrade soil was determined. A bender element apparatus and the filter paper method were integrated to develop a systematic testing procedure to investigate the influence of matric suction on small-strain stiffness, including small-strain shear wave velocity and small-strain shear modulus, with different wetting paths. Soil specimens were compacted at three moisture contents (optimum moisture content, dry of optimum, and wet of optimum), then wetted to various moisture contents by using an environmental simulation apparatus developed to simulate in-service moisture conditions. Test findings indicate that specimen moisture contents increase gradually and finally attain equilibrium in the wetting process. Suction decreases with increasing moisture content and degree of saturation; consequently, small-strain shear wave velocity and small-strain shear modulus decrease. The specimens compacted at dry of optimum have a much lower small-strain shear wave velocity and small-strain shear modulus than specimens compacted at wet of optimum. Nevertheless, the small-strain shear modulus exhibits higher linearity with matric suction than with moisture content. Results indicate that matric suction is a key parameter for evaluating the dynamic properties of compacted soil.

Reliability-Based Code Calibration for Axial Ultimate Bearing Capacities of Single Bored Piles in Taipei Basin

In Taipei, many pile proof load tests were not conducted to failures but only to a multiple ( e.g. , 2) of the design load. This leads a difficulty of incomplete information: For these test results, the ultimate bearing capacities of the test piles are unknown. This paper addresses the issue of calibrating resistance factors of piles for the axial ultimate bearing capacities based on the incomplete information from these tests. A simplified probabilistic method is proposed to resolve this issue. A local pile test database of Taipei is presented, and the analysis results show that the inclusion of the incomplete pile load test data helps in calibrating the resistance factors. Moreover, it is found that the calibrated resistance factors for the axial ultimate bearing capacities are consistent to the safety factors that are adopted in the current Taiwan design code. This paper also addresses another important issue for the pile design in Taipei: the end bearing capacity in the sandstone layer is very uncertain. A maximum likelihood method is taken to identify the best estimate for this end bearing capacity. The conclusion of this paper may be useful for reliability-based designs for the axial ultimate bearing capacities of single bored piles in the Taipei region.

How Small Strain Stiffness and Yield Surface Affect Undrained Excavation Predictions

AbstractThis paper discusses how small-strain stiffness and some aspects of the yield surface affect the finite-element predictions of excavation responses, including wall and ground deformations. By using the in-house-developed constitutive model SC1SS, it is possible to isolate and quantify impacts brought about by selected aspects of the soil behavior. This soil model has been validated to reasonably model Taipei silty clay and deep excavations in such soil. This study shows that ignoring small-strain stiffness can overestimate deformations by as much as 80%, leading to conservative and costly design. Inclined yield surface and Lode-angle dependency (on deviatoric planes) have lesser effects on the prediction results. In addition, the mechanisms behind these impacts are investigated through numerical triaxial tests.

Assessment of 3D Excavation and Adjacent Building's Reponses with Consideration of Excavation-Structure Interaction

This paper investigates the 3D excavation-induced ground and buildings settlements by means of 3D numerical simulations. The effects of the excavation responses are studied with and without considering the nearby building. This paper proposes a simplified simulation technique (capturing the plastic behavior of the structure to enhance the realistic modeling of the structure) in which the excavation model and the structure model are decoupled and considered by two separate numerical programs. An iteration scheme is developed to bridge the two models. The results show that the proposed analytical scheme is feasible and the convergence can be reached within several iterations. It allows more in-depth investigation on the structure response such as where the plastic hinge or high potential of damage may occur in the structure. This study also provides valuable insights not only on the interaction characteristics of the soil and the structure adjacent to an excavation site, but also on the effect on the excavation behavior due to inelastic behavior of the adjacent structure.

Anisotropic strength characteristics of composite soil specimen under cubical triaxial conditions

This paper provides 41 cubical triaxial test results to examine the influence of the stress path angle and the improvement ratio on the anisotropic strength of the composite soil specimens consisting of remolded soft clay and grout columns. Pictures of failed samples shown in this paper are especially enlightening in demonstrating failure mechanisms. Results from this study can be summarized as follows. The composite soil specimens exhibited different failure patterns depending on the stress path angle, axial compression failure for 0°, 30° and 60°; lateral compression for 120° and 150°; and axial extension for 90° and 180°. Consistently, diagonal shear cracks through the grout column were observed for axial compression failure samples. On the other hand, tension cracks were observed for samples which failed due to lateral compression and axial extension. The composite soil specimens exhibited apparent anisotropic behavior. In general, the anisotropic strength ratio increased with the improvement ratio. The equivalent strength formula commonly used in practice may give erroneous results, especially when the stress paths are those of tension failure. In such a case, the anisotropic strength ratio suggested in this paper can significantly improve its accuracy.