No abstract

No abstract

Depending on shear strength and soil permeability, an excavation may initially be stable but fail after some time, thus raising the practically important issue of stand-up time. This paper uses coupled hydraulic–mechanical analyses to investigate how delayed failure manifests itself. A relatively simple geotechnical problem is considered and provides valuable insights into the mechanism of delayed failure. The results presented are thus also relevant to other geotechnical problems, such as the stand-up time of tunnel headings. Emphasis is placed on the effects of plastic dilatancy as it affects ground response not only quantitatively but also qualitatively. Under the frequently made simplifying assumption of a constant positive dilation angle, coupled analyses inevitably lead to a constant deformation rate at failure, whereas failure is (commonly) associated with accelerating displacements. Models that allow for shearing under constant volume lead to accelerating displacements but inherently exhibit numerical stability problems close to the failure state. In order to determine stand-up time, it is essential to evaluate the numerical results in their entirety, including the time development of the displacement, stress and pore pressure fields.

This paper presents the pre‐construction evaluation of the ground for the Lake Mead Intake No 3 Tunnel and the experience gained from the project, which had the aim of maintaining water supplies for the Las Vegas greater metropolitan area. The construction of this tunnel using a dual‐mode slurry shield represents a major engineering achievement, which pushed back the boundaries of closed shield tunnelling: Overcoming an extended fault zone under high hydrostatic pressure in the metamorphic rocks and mucking‐out difficulties associated with high rates of water inflow in the sedimentary rocks necessitated closed‐mode operation over about 2 km at unprecedentedly high face pressures of up to 14 bar.

No abstract