Journal of Geotechnical and Geoenvironmental Engineering

This paper extends the design philosophy for piled rafts by exploring the factors that control the design of a published case history where the piled raft was instrumented. The case history studied differs from most published case histories in that concentrated column loads, rather than average “spread” loads, were considered in the foundation design. An extensive series of backanalyses was initially carried out with this case history, in order to calibrate the numerical program adopted herein. These analyses were followed by a parametric analysis for the evaluation of different design alternatives. These alternatives adopted distinctive pile characteristics (number, location, and length), which were varied for different raft thicknesses, yielding 26 different cases for cross comparison and comparison with the “reference” case history. Each of the solutions was also assessed in terms of relative costs against the reference case, allowing the establishment of conclusions of practical interest for those involved in the design of piled raft structures.

The design of large diameter bored piles socketed into rock has received considerable attention in sedimentary rocks but has only occasionally been addressed in igneous and metamorphic rocks. Design methods based on the performance of sockets in sedimentary rocks have been proposed in literature, but it is uncertain how applicable they are to other rock types. Large diameter (>600 mm) rock-socketed piles were used in several recent developments in Hong Kong, which involved 13 large-scale pile-load tests in various decomposed rocks. The test piles varied in length from 20 to 60 m and in diameter from 1.0 to 1.3 m. The test loads were as high as 30 MN. This provides a golden opportunity for a study of the performance of piles socketed in igneous, volcanic, and metasedimentary rocks decomposed as a result of subtropical weathering. In total, 35 pile tests (including 13 in Hong Kong) in decomposed rocks together with 44 other tests in sedimentary rocks are reviewed. Side resistance is analyzed with regard to rock type and is compared to the findings and correlations published by others. The rate of mobilization of side resistance was generally scattered in the test results, as were the general shapes of the resistance-displacement relationships. A trend line was established relating the unconfined compressive strength to side resistance for granitic rocks. During bidirectional multistage tests, a reduction in the socket stiffness was observed with the second test stage. For a given displacement, the side resistance mobilized in the second stage was less than in the first.

This paper presents results of full-scale lateral load tests of one single pile and three pile groups in Hong Kong. The test piles, which are embedded in superficial deposits and decomposed rocks, are 1.5 m in diameter and approximately 30 m long. The large-diameter bored pile groups consist of one two-pile group at 6 D (D = pile diameter) spacing and one two-pile and one three-pile group at 3 D spacing. This paper aims to investigate the nonlinear response of laterally loaded large-diameter bored pile groups and to study design parameters for large-diameter bored piles associated with the p-y method using a 3 D finite-element program, FLPIER. Predictions using soil parameters based on published correlations and back-analysis of the single-pile load test are compared. It is found that a simple hyperbolic representation of load-deflection curves provides an objective means to determine ultimate lateral load capacity, which is comparable with the calculated values based on Broms' theory. Lateral deflections of bored pile groups predicted using the values of the constant of horizontal subgrade reaction, suggested by Elson and obtained from back-analysis of the single pile load test, are generally in good agreement with the measurements, especially at low loads.

This paper presents a new empirical equation for assessing liquefaction resistance of soils based on shear wave velocity V_s and the results of probabilistic analyses based on this empirical equation. A database consisting of in situ shear wave velocity measurements and field observations of liquefaction/nonliquefaction in historic earthquakes is analyzed. This database is first used to train and test an artificial neural network to predict the occurrence of liquefaction/nonliquefaction based on soil and seismic load parameters. The successfully trained and tested neural network is then used to establish the empirical equation. The concept of clean soil equivalence is introduced and used in the development of the empirical equation. The established empirical equation represents a deterministic method for assessing liquefaction resistance of a soil. Based on this newly developed deterministic method, probabilistic analyses of the cases in the database are conducted using the logistic regression approach and the mapping function approach. The results provide a basis for risk-based evaluation of liquefaction evaluation.

This paper presents a model for bonded or cemented soils within the framework of hardening plasticity. The model is based on the concepts that (1) the strength of a cemented soil can be considered to be made up of two components, the usual strength of the soil skeleton and the strength of cementation bonds; and (2) the deformation of the soil is associated with the component of stresses on the soil skeleton (excluding the bonds), as in a reconstituted soil, whereas the cement bonds offer additional resistance at any given strain level. The overall response of the soil under loading can be visualized as two stiffnesses acting in parallel for the given unit strain. Separate stress-strain relations are defined for the two components and they are then combined to give the overall response. The stress-strain relationship for the soil skeleton component is described here using the modified Cam-clay model. A simple model within the elastoplastic framework is proposed for the “cemented” component. The cemented component incorporates strain softening. The input parameters required are minimal, well defined, and easily determined. The model predictions are compared with test data of several soils under drained and undrained triaxial conditions.

Searching for the critical slip surface and the lowest factor of safety in slope stability analysis can be achieved by means of optimization techniques. A new search procedure in generating kinematically admissible slip surfaces has been introduced in this paper. Such a procedure is based, mainly, on the Monte Carlo methods, where both the critical global slip surface as well as its associated factor of safety is determined. Several practical examples, of known minimum factor of safety and its associated slip surface, have been used to demonstrate the efficiency and capability of the proposed method. The method is intended to be robust and effective to solve problems that involve extremely complicated slope geometry. It is as powerful as any other powerful optimization methods.