This study presents a coupled material point method (MPM) formulation for the analysis of cone penetration in clays. The formulation is based on the generalized interpolation material point method (GIMP) variant of MPM. A single material point is used to represent both the soil matrix and water. The governing equations are solved using an explicit scheme with the velocity of the soil matrix and the velocity of water as the primary variables. Incompressibility constraints in the soil matrix are resolved using the nonlinear B-bar method, and pore pressures are computed at element centers. The formulation is validated through problems for which analytical or numerical solutions are available. Cone penetration resistances measured at a Boston Blue Clay (BBC) test site are then computed using the coupled MPM formulation with the constitutive response of BBC captured using an advanced bounding surface model based on critical state soil mechanics. Based on the cone penetration simulation, the penetration resistances under undrained, partially drained, and drained conditions are computed by varying the hydraulic conductivity of the clay. The cone factor for undrained penetration is also calculated. Verification and validation exercises demonstrate the efficacy and robustness of the adopted formulation in the realistic simulation of cone penetration in clay.
A variety of pile design methods, either soil property-based, or in-situ test-based methods, have been proposed to estimate the ultimate bearing capacity of driven, closed-ended, pipe piles. Yet, due to the limited number of well-documented, high-quality pile load tests, efforts still need to be made to assess the performance of these methods. This paper presents case histories of three carefully-instrumented static load tests, performed in Indiana, U.S., on closed-ended pipe piles driven in multilayered soil. Several property-based and CPT-based pile design methods for both sandy and clayey soils are used to estimate the limit shaft and ultimate base resistances of the test piles. The performance of these design methods is assessed through detailed, layer-by-layer comparison between the estimated resistances and those obtained from pile load test measurements. For the cases considered, the Purdue, ICP, and UWA methods produce the most accurate, reliable, and consistent predictions.
The boundary value problems (BVPs) of geomechanics are challenging due to the complexity in modeling soil behavior and difficulties in modeling large deformations. While traditional numerical schemes have struggled in realistically simulating geomechanical BVPs, new numerical methods –such as the material point method (MPM)–are increasingly being used to tackle these problems. However, algorithms in MPM have not yet been sufficiently developed, scrutinized, and validated. This thesis focuses on the development, verification, and validation of MPM for use in geomechanical BVPs. In particular, the thesis focuses on simulation of cone penetration tests in both controlled environments and in field conditions.To efficiently simulate cone penetration, a silent boundary scheme, known as a cone boundary, was proposed in the generalized interpolation material point method (GIMP), a variant of MPM. The implementation of the cone boundary in GIMP was discussed, and the boundaries were validated by comparison against several benchmark problems. The cone boundaries were shown to be suitable in transmitting energy at the boundary. In addition, the implementation of traction boundaries in GIMP was analyzed. In GIMP, traction boundaries may be implemented either at the centroid of the material point, or at the edge of the material point domain. It was shown that the implementation of traction boundaries at the edge of the domain led to stress oscillations near the boundary, which were minimized when the traction boundaries were implemented at the edge of the domain.During cone penetration, the soil near the cone-soil interface is pushed to large strains. At large strains, soils reach critical state, a state in which the soil shears at constant volume. Simulation of incompressible materials using low-order shape functions commonly used in GIMP leads to stiffer solutions and stress oscillations. To mitigate the constraints imposed by incompressibility, the non-linear B-bar method was implemented in GIMP. The modifications required for the implementation of the B-bar method in GIMP were discussed, and the efficacy of the method in mitigating incompressibility was demonstrated by analyzing several benchmark problems.To simulate cone penetration in saturated soil, a coupled formulation was proposed in GIMP.A single material point was used to represent both the soil matrix and water. The governing equations were solved using an explicit scheme with the velocity of the soil matrix and the velocity of water as the primary variables. The formulation was validated through problems for which analytical or numerical solutions are available.Finally, cone penetration analyses were performed both in dry sand and saturated clays. Two bounding surface models –one for sand and one for clay –were used for accurately capturing the soil response. Cone penetration tests were performed on Ottawa 20-30 sand under a variety of loading conditions at a large calibration chamber. The penetration resistances were measured, and the displacement fields were captured using the digital image correlation technique(DIC). The cone penetration resistances predicted by MPM were within 25% of the measured values, and the displacement fields computed using MPM were similar to those captured using DIC. For saturated clays, cone penetration test results reported in the literature for a Boston Blue Clay (BBC) test site were used. The simulated cone resistance of 650 kPa lied within the CPT resistance range of 580-730 kPa reported in the field. The results demonstrate the capability of MPM in simulating cone penetration in both sands and clays provided that sufficiently accurate algorithms and advanced constitutive models capable of reproducing realistic soil behavior are used in the analyses.
Driven piles are commonly used in foundation engineering. The most accurate measurement of pile capacity is achieved from measurements made during static load tests. Static load tests, however, may be too expensive for certain projects. In these cases, indirect estimates of the pile capacity can be made through dynamic measurements. These estimates can be performed either through pile driving formulae or through analytical methods, such as the Case method.
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