Civil Engineering Association

Numerical Study of the Bearing Behavior of Piled Rafts

Based on a numerical study by means of three-dimensional finite-element analysis, this paper discusses investigations of the bearing behavior of piled rafts in overconsolidated clay. It is shown that the interaction between piles and rafts is a major influence. The potential savings of an optimized foundation design are demonstrated for a simple example.

Effects of the Erosion and Transport of Fine Particles due to Seepage Flow

The gradual erosion and transport of fine particles are among the possible consequences to the subsoil of a severe seepage flow, such as the seepage induced by the artificial lowering of the water table by means of pumping wells. The erosion, in turn, could induce local effects, in terms of reductions in the soil volume and variations in the soil mechanical characteristics, that could be non-negligible when working in an urban area. In this paper, some laboratory tests are presented aimed at investigating the erosion of fine particles from soil samples subjected to a controlled seepage flow. The phenomenon of erosion and transport is then modeled by combining, in a single governing equation, the conservation of mass of moving particles with a suitable law of erosion, which is calibrated on the basis of the seepage tests. This equation, coupled with the equation governing the seepage problem, permits one to evaluate the quantity of particles eroded and transported from the soil mass. The effects of the loss of fine material on the stress-strain distribution within the soil mass are estimated by a finite-element analysis. The proposed model is then adopted for the evaluation of the surface settlements induced by the water pumping from a drainage trench.

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Seepage-Induced Erosion in Granular Soil and Consequent Settlements.

The erosion of the fine fraction of granular soils due to seepage is a possible cause of the settlements of shallow foundations. Here the problem is tackled with reference to deep pumping wells to be installed in an urban area. First, the results of laboratory tests on reconstituted soil samples are illustrated that provide the quantity of eroded particles during time depending on the imposed hydraulic gradient. Based on the experimental data, an incremental erosion law is proposed that has been implemented in a finite-element code for erosion-transport analysis. After describing the steps of the numerical solution procedure, the results of two- and three-dimensional finite-element calculations are discussed. They provide an estimation of the quantity of eroded material in the vicinity of the pumping wells and of the possible consequent settlements of nearby buildings.

Improving the Uniqueness of Surface Wave Inversion Using Multiple-Mode Dispersion Data

The use of multiple-mode dispersion data in surface wave inversion to derive shear-wave velocity (vs) profiles has increased in the past decade as the inclusion of higher mode data can improve the accuracy of the inversion results. However, the error associated with nonuniqueness in the multiple-mode inversion has not been clarified and quantified. This research focuses on the attempt to improve the accuracy of multiple-mode surface wave inversion result by optimizing the use of multiple-mode dispersion data to reduce the error associated with the nonuniqueness in inversion. In this research, an alternative approach was used where inversion of surface wave dispersion data was performed using three distinct modes. Four different vs profiles, representing regular and irregular cases, were used, and multiple-mode dispersion data were synthesized from these profiles using the dynamic stiffness matrix method as the theoretical model. The dispersion data were then inverted using the Levenberg–Marquardt method. The results demonstrated that, as expected, inclusion of higher modes did not improve the accuracy of the inversion results for the regular profiles. However, inclusion of higher modes significantly improved the uniqueness of the inversions for the irregular profiles. The results also demonstrate that regardless of the nature of the profile, the accuracy of the inversion improves when the starting profile more closely matches the true profile. Of all the inversion approaches investigated, the best approach was one where three successive inversions, using one, two, and three modes, respectively, was used, where the inverted profile from one inversion was used as a starting model for a subsequent inversion that used one additional mode.

Estimation of Footing Settlement in Sand