Probabilistic analysis of underground excavation stability

Abstract

The role of cementation, porosity, permeability and consolidation on acoustic properties of unconsolidated sands and poorly consolidated sandstones has been investigated. Partially and fully water saturated Ottawa sand and poorly consolidated sandstone samples were measured in a bench-top ultrasonic assembly in permafrost conditions at several ice saturation states to determine the variations in wave velocities associated with the degree of cementation. A Berea sandstone sample was also measured in permafrost conditions to investigate the role of consolidation on the measured wave velocities. Permeability and porosity of the samples were found to have significant effect on the variation of the wave velocities. Freezing water creates thin ice layers surrounding the grains resulting cement-like response in poorly consolidated sands allowing grains to be in contact even at large ice saturations. This response can be predicted using a modified cementation model presented here. The model is designed based on the cementation model of Dvorkin et al. 1994. There are two extreme schemes where the ice is located in pore space in the modified model. In the first scheme, ice acts like cementing material deposited only at the grain contacts in pendular ring form resulting high velocities that agree with the observed trends for low saturations in both low and high permeability sands. In the second scheme, ice envelopes the grains corresponding to the variations in high permeability sands at intermediate to high saturations. Ice acts as the load-bearing cementation material which strongly increase the stiffness of the sands tested. Wave velocities obtained from frozen drainage tests indicate a plateau at the intermediate saturations in all samples studied. The width of the plateau is controlled by the permeability of the sample tested. The higher the permeability, the smaller the plateau becomes and the corresponding velocity reduction is more gradual.