Experiments on rock joint behaviors have shown that joint surface roughness is mobilized under shearing, inducing dilation and resulting in nonlinear joint shear strength and shear stress vs. shear displacement behaviors. The Barton–Bandis (BB) joint model provides the most realistic prediction for the nonlinear shear behavior of rock joints. The BB model accounts for asperity roughness and strength through the joint roughness coefficient (JRC) and joint wall compressive strength (JCS) parameters. Nevertheless, many computer codes for rock engineering analysis still use the constant shear strength parameters from the linear Mohr–Coulomb (M−C) model, which is only appropriate for smooth and non-dilatant joints. This limitation prevents fractured rock models from capturing the nonlinearity of joint shear behavior. To bridge the BB and the M−C models, this paper aims to provide a linearized implementation of the BB model using a tangential technique to obtain the equivalent M−C parameters that can satisfy the nonlinear shear behavior of rock joints. These equivalent parameters, namely the equivalent peak cohesion, friction angle, and dilation angle, are then converted into their mobilized forms to account for the mobilization and degradation of JRC under shearing. The conversion is done by expressing JRC in the equivalent peak parameters as functions of joint shear displacement using proposed hyperbolic and logarithmic functions at the pre- and post-peak regions of shear displacement, respectively. Likewise, the pre- and post-peak joint shear stiffnesses are derived so that a complete shear stress-shear displacement relationship can be established. Verifications of the linearized implementation of the BB model show that the shear stress-shear displacement curves, the dilation behavior, and the shear strength envelopes of rock joints are consistent with available experimental and numerical results

Explicit solution techniques have been widely used in geotechnical engineering for simulating the coupled hydro-mechanical (H-M) interaction of fluid flow and deformation induced by structures built above and under saturated ground, i.e. circular footing and deep tunnel. However, the technique is only conditionally stable and requires small time steps, portending its inefficiency for simulating large-scale H-M problems. To improve its efficiency, the unconditionally stable alternating direction explicit (ADE) scheme could be used to solve the flow problem. The standard ADE scheme, however, is only moderately accurate and is restricted to uniform grids and plane strain flow conditions. This paper aims to remove these drawbacks by developing a novel high-order ADE scheme capable of solving flow problems in nonuniform grids and under axisymmetric conditions. The new scheme is derived by performing a fourth-order finite difference (FD) approximation to the spatial derivatives of the axisymmetric fluid-diffusion equation in a non-uniform grid configuration. The implicit Crank-Nicolson technique is then applied to the resulting approximation, and the subsequent equation is split into two alternating direction sweeps, giving rise to a new axisymmetric ADE scheme. The pore pressure solutions from the new scheme are then sequentially coupled with an existing geomechanical simulator in the computer code fast Lagrangian analysis of continua (FLAC). This coupling procedure is called the sequentially-explicit coupling technique based on the fourth-order axisymmetric ADE scheme or SEA-4-AXI. Application of SEA-4-AXI for solving axisymmetric consolidation of a circular footing and of advancing tunnel in deep saturated ground shows that SEA-4-AXI reduces computer runtime up to 42%-50% that of FLAC's basic scheme without numerical instability. In addition, it produces high numerical accuracy of the H-M solutions with average percentage difference of only 0.5%-1.8%. (C) 2018 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V.

Excavating a tunnel in a deep and saturated ground affects the short- and long-term hydro-mechanical (H-M) response in the ground surrounding the opening. However, the interactions between transient pore pressure behavior and the corresponding deformation and stresses in the ground ahead of and behind the tunnel face are still not well understood. This paper investigates the transient H-M response of excavating a tunnel in a deep and saturated ground using a two-dimensional axisymmetric coupled model in the computer code Fast Lagrangian Analysis of Continua (FLAC). The tunnel was advanced in a stepwise excavation procedure consisting of undrained excavation and drained consolidation until the final tunnel face was reached. The final excavated face was then left to consolidate toward the steady-state condition. The main results of the paper are as follows: (1) when simulating a tunnel excavation in deep saturated ground using the convergence-confinement method, the unloading factors should be nonlinear and should consists of the mechanical unloading factor in the form of excavation force and the hydraulic unloading factor in the form of excavation pore pressure. These two unloading factors are necessary because the induced H-M response near the tunnel face is a rather transient response instead of an initial or final response. Moreover, it is observed that the pore pressure dissipation is not linear either with time or with distance to the tunnel face, (2) a relationship between the unloading factors and the distance to the tunnel face should then be established. This relationship is vital because it will provide the timing for tunnel support installation, and (3) the extrusion and the convergence of the advance core could be related through the proposed equations capturing the linear relationships between the face extrusion and its convergence as well as between the core extrusion and its pre-convergence. Through these relationships, the tunnel engineer may be able to estimate the magnitude of the deformation ahead of the face, which will subsequently allow control of the deformation behind the face.

This report presents the results of a comprehensive study on a proposal to use cellular cofferdams as basis for the design and construction of water retaining structures to sustainably and cost-effectively harness hydropower. Previously, cellular cofferdams have been widely used mainly as temporary water exclusion devices to permit dry construction of in-water structures such as dams, locks, bridge footings and piers, and hydroelectric power plants. Design and construction requirements for cellular cofferdams are less stringent than for hydropower dams. To make cellular cofferdams suitable for permanent hydropower use, different design concepts that utilize cellular cofferdams as the main or core element of the water-retaining dam structure are proposed. One key design concept is the so-called “dry construction technique” in which the granular fill in cofferdam cells and the downstream berm are permanently kept dry in contrast to the wet construction technique for temporary use of cellular cofferdams. The viability of the proposed permanent cellular cofferdam design concepts is demonstrated using well-established structural and geotechnical design procedures and computational modeling. The improved performance of the proposed design concepts, particularly in combination with the dry construction technique, show cellular cofferdams have the potential to be used as basis for the construction ofmore » permanent hydropower dam structures that are versatile, with less impact on the environment, and will cost less to build than conventional hydropower dams. « less