Charles James Russell Coccia

A Thermo-Hydro-Mechanical True Triaxial Cell for Evaluation of the Impact of Anisotropy on Thermally Induced Volume Changes in Soils

This paper describes a new thermo-hydro-mechanical true triaxial cell used for the evaluation of the impact of stress-induced anisotropy on thermally induced volume changes in saturated soils. Specifically, details of the experimental setup, instrumentation, thermal calibration of the device, experimental procedures, and typical measurements are presented in this paper. Principal stresses were applied to the sides of a cubical specimen with a side length of 178 mm independently using flexible bladders, while the pore water pressure and temperature were controlled at the top and bottom of the specimen using rigid plates with embedded heaters and fluid control ports. In the testing program, temperatures between 25 and 65 °C were applied in stages to four different specimens of compacted bonny silt which had been consolidated to different initial anisotropic stress states under quasi-plane strain conditions. Consistent volumetric contraction was measured in each of the specimens during heating, regardless of the initial stress state. However, for specimens with a greater initial principal stress difference, the soil was observed to expand in the direction of the minor principal axis and contract in the direction of the major principal stress during heating. Relatively consistent elastic volumetric and axial contraction was noted during cooling regardless of initial stress state. The results from this preliminary investigation indicate the importance of measuring the impact of temperature changes in the directions of anisotropic stresses as part of the design of thermally active geotechnical systems.

Impact of Heat Exchange on the Thermo-Hydro-Mechanical Response of Reinforced Embankments

This paper describes a numerical investigation into the influence of heat exchange on the thermo-hydro-mechanical response of embankments with poorly-draining backfills. Heating of unsaturated soils within an embankment can lead to permanent volume changes as well as thermally-induced water flow away from the zone of heat exchange, resulting in an increase in effective stress and shear strength. Predictions of the coupled flow of heat and water in an unsaturated soil during application of high temperatures indicate that heating of the unsaturated soil layer leads to increased suction in a relatively small zone of influence, with a greater increase in shear strength for soils with initially lower degrees of saturation

High-Pressure Thermal Isotropic Cell for Evaluation of Thermal Volume Change of Soils

This paper describes a new high-pressure, temperature-controlled isotropic cell used for evaluation of the thermal volume change mechanisms of saturated and unsaturated soils under isotropic stress states. Specifically, details of the experimental setup, instrumentation, thermo-mechanical calibration of the device, experimental procedures, and typical results are presented in this paper. The thermal isotropic cell includes suction control using the axis translation technique, saturation control/monitoring using a pore water pressure flow pump, cell pressure control using a high-pressure flow pump, and a stainless steel cell to permit application of isotropic net mean stresses up to 10 MPa. The cell fluid temperature is regulated by circulating heated water through a copper heating coil within the cell, and an internal circulating fan is used to promote homogenous temperature throughout the cell chamber. Non-contact proximity transducers are used to directly measure soil deformation in the radial and axial directions, permitting assessment of thermo-mechanical anisotropic strains during changes in mean effective stress or temperature while also avoiding the need to consider complex thermo-mechanical cell deformations. The high-pressure flow pump and thermal control system are designed to apply changes in net mean stress and temperature at slow rates to characterize the full soil compression and thermal volume change curves, respectively. Along with the thermo-mechanical calibration of the cell, the results from two tests on compacted silt specimen having different initial degrees of saturation are presented that show how the cell can be used to characterize changes in volume and degree of saturation during thermo-mechanical loading. Both normally consolidated soil specimens were contracted during heating, although the specimen with a lower degree of saturation showed slightly greater thermal volume change.

Modeling of tunnel lining deformation due to face instability

Long-term settlement of tunnels has caused concerns about its influence on tunnel safety and serviceability. Aiming to investigate the long-term behaviour of tunnels against the background of Shanghai metro line, two cases of centrifuge modelling were conducted, with efforts to expose the mechanism affecting the consolidation of the ground. Evenly layered ground and transitional ground strata were set for each case separately and the settlement, lining load and pore water pressure were checked against elapsed time up to 20 years. The results verified some previous findings concerning the settlement and lining load development trend, however, it was also shown that the transitional ground made the tunnel response more complicated. The research is expected to provide some basis for further research on other affecting factors, such as lining permeability.

Centrifuge Modeling of Face Excavation in Tunnels with a Deformable Lining

This study presents the results from a series of centrifuge modeling experiments performed on tunnels in sand. The goal of these experiments was to evaluate the role of tunnel face movements on both tunnel lining deformations and collapse of the overburden soil. Specifically, a half-space tunnel was modeled, in which a strain-controlled stepper motor was used to withdraw a tunnel face at a constant displacement rate during centrifugation under a target acceleration level. In these experiments, the effects of overburden height, tunnel lining stiffness, and staged construction using the New Austrian Tunneling Method (NATM) were investigated. Relatively narrow shear bands were noticed in all of the collapse tests. Moments and shear forces measured within the tunnel lining indicate that construction staging has the greatest impact on tunnel lining behaviour. The results of the experiments are suitable for investigating whether the design approaches and partial safety factors suggested in EuroCode 7 are appropriate for ultimate state analysis of tunnel faces. Further, they are useful to identify whether modifications of the safety factors for soil and shotcrete lining are needed to achieve low and consistent failure probabilities. Introduction During construction of tunnels in soils or rocks using the New Austrian Tunneling Method (NATM), the tunnel face is the most susceptible to collapse. Collapse failures can lead to surface deflections and tunnel cave-ins, which can create a variety of problems for tunnels constructed in urban areas. Despite the high potential for loss of life and property, the safety factors currently used in tunnel design are mainly based on experience, with little basis in field measurements or validated modeling results. Previous work on centrifuge modeling of tunnels focused on face stability and face support measures (Meguid et al. 2008). With very few exceptions (Konig et al. 1991), centrifuge models used rigid tunnel lining and could not follow the soil deformation during spin-up of the centrifuge. Whereas the stress level in the lining is of minor importance in connection with excavation by means of a tunnel boring machine, the stability of the lining near the face is crucial in connection with cyclic excavation using shotcrete as primary support. To address the effects of the tunnel lining on face stability and to investigate strains and stresses in the lining, physical modeling experiments have been conducted in the geotechnical centrifuge at the University of Colorado in Boulder. Specifically, a half-space model was constructed in which a tunnel face is used to vary the pressure on the tunnel face during centrifugation under a target acceleration level. An acrylic lining was used in the model because the material properties are comparable to those of young shotcrete. The face displacements and strain distribution in the tunnel lining both during spin-up of the centrifuge to the desired g-level as well as during reduction of the face pressure were measured. Care has been taken that the boundary conditions of the lining allow displacements and deformations together with the soil while enforcing symmetry and boundary conditions close to reality. The experimental study also assessed the impacts of overburden, the length of an unlined section close to the face (where the shotcrete has not hardened yet), and the effects of staged excavation. In this paper some details of the model setup are described, and selected results are presented. Additionally, shortcomings of the current model are discussed and some improvements are suggested. The results presented in this paper will be used to validate three dimensional numerical models of soil-structure interaction in tunnels.