International Journal of Geomechanics

Herein, a methodology is presented for the calibration of the triaxial hyperbolic Mohr-Coulomb (HMC) elastoplastic model on a set of experimental data of conventional rock mechanics tests on intact rocks. It is demonstrated here that based on uniaxial compression, triaxial compression, and direct or indirect tension tests on intact rock specimens in the laboratory, 15 parameters of the elastic and hardening plastic HMC model pertaining to Carrara (Lorano) marble may be deduced in a straightforward manner. This procedure may be applied to any other rock type as well. Furthermore, an extension of the five-parameter HMC failure criterion to a six-parameter HMC criterion with a cap is proposed for the better representation of intact rock behavior in multiaxial loading conditions.

Heterogeneity is an important factor controlling fracture initiation, accumulation, and propagation within polycrystalline rock. Internal spatial variability in terms of mineralogy, grain size, and anisotropy affect the yielding process. To investigate these factors, a texture-generating algorithm integrated within a numerical model was developed to create realistic rock analogs and provide user control over geological characteristics including mineral type, grain size, and anisotropic crystal shape. A mineral-specific constitutive model was created and calibrated using published values and real laboratory strength values. Brazilian tensile strength and unconfined compressive strength (UCS) model tests were developed using the finite-difference modeling software FLAC to perform parametric analysis of a series of geological characteristics. The results show that the methodology is capable of realistically reproducing damage propagation and failure behavior similar to that observed during laboratory testing. The strength results show trends similar to those found during laboratory testing. This methodology was applied to the simulation of tunnel-boring machine (TBM) cutter excavation to investigate the effect of geomechanical characteristics on the chipping process.

Cataclastic rocks and fault gouge present highly variable properties, from those of soil-like materials to those of competent rock mass. In most cases, the entire rock mass, or internal major shear zones, can be represented as a block-in-matrix rock (bimrock), i.e., rock fragments of different shape and size, embedded in a finer matrix. In this paper, a tectonized metamorphic rock mass at the site of a planned gravity dam is investigated. The dam foundation is crossed by numerous major discontinuities, often grouped to form wider shear zones and characterized by high persistency, waviness, and abundant cataclastic filling. Systematic measurements of local orientation and filling thickness along the discontinuity traces made a detailed morphological analysis possible. The most frequent type of filling is a lightly cemented matrix-supported gouge. Undisturbed sampling of filling materials was successful only in some special cases. Laboratory tests, as well as in-situ mechanical tests, were therefore difficult and not exhaustive. Micromechanical distinct-element method (DEM) modeling was instrumental in obtaining a more comprehensive framework of deformation and failure mechanisms of cataclastic materials. The mechanical properties of the particulate model were first analyzed by numerical simulation of biaxial and direct shear tests. The relationship between the overall constitutive parameters and micromechanical properties, such as interparticle friction, contact stiffness, cement bond strength, volumetric block proportion, and block shape, was determined. The complex interplay between the geometric features of a major discontinuity (joint-wall roughness and waviness) and the properties of the infilling bimrock was then investigated, and finally, the overall strength of the infilled discontinuity was analyzed.

In this paper, an innovative modeling approach is presented for the simulation of underground construction with the emphasis on tunneling. The boundary element method (BEM) is used, and the theoretical background is discussed first. This is followed by test examples, where the results are compared with results from available software, showing the efficiency and accuracy of the new method. Finally, a practical example considering nonlinear material, sequential excavation, and installation of ground support is presented.

The purpose of this paper is to present Y-Geo, a new numerical code for geomechanical applications based on the combined finite-discrete element method (FDEM). FDEM is an innovative numerical technique that combines the advantages of continuum-based modeling approaches and discrete element methods to overcome the inability of these methods to capture progressive damage and failure processes in rock. In particular, FDEM offers the ability to explicitly model the transition from continuum to discontinuous behavior by fracture and fragmentation processes. Several algorithmic developments have been implemented in Y-Geo to specifically address a broad range of rock mechanics problems. These features include (1) a quasi-static friction law, (2) the Mohr-Coulomb failure criterion, (3) a rock joint shear strength criterion, (4) a dissipative impact model, (5) an in situ stress initialization routine, (6) a material mapping function (for an exact representation of heterogeneous models), and (7) a tool to incorporate material heterogeneity and transverse isotropy. Application of Y-Geo is illustrated with two case studies that span the capabilities of the code, ranging from laboratory tests to complex engineering-scale problems.

During the design and construction of shield-driven tunnels, a reliable analysis of the construction process is required for the prognosis of the process-induced surface settlements, changes in soil stresses, and changes in groundwater conditions, as well as for the determination of the loads acting on the tunnel tube and on the tunnel-boring machine. In this context, numerical simulation methods like the finite-element method allow for a realistic description of the construction process and its impact on the surrounding underground. The investigated problem is governed by the interactions between the tunneling process and the surrounding underground and its constituents—soil grains, groundwater, and pore air. The tunnel-construction process interacts with the surrounding underground via the heading face support, by frictional contact between shield skin and soil, and because of grouting of the annular gap. Considering these interactions, a holistic simulation model is presented for the process-oriented simulation of shield-supported tunnel advance and its interactions with fully saturated, partially saturated, or nonsaturated soft soil. Its applicability is demonstrated by selected simulations of real-scale examples. Parametric studies are performed to investigate the influence of soil conditions and of process parameters on the time-variant settlements and groundwater conditions, showing its capabilities with respect to the simulation of the soil-process interactions in front, above, and behind the tunnel-boring machine.

The numerical analysis of tunnels in rock masses, which exhibit a squeezing behavior, is considered, with reference to the use of two elastoviscoplastic constitutive laws implemented in finite-difference and finite-element codes. Because of the fact that in the most severe squeezing conditions time dependence is observed during excavation and may last for a long time, both the reinforcement of the tunnel face and the support of the tunnel section are to be considered at the design and construction stages. As a consequence, the need arises to analyze the tunnel excavation-construction sequence in detail and to account for time-dependent behavior. Numerical analyses have been carried with the Saint Martin La Porte access adit, along the Lyon-Turin Base Tunnel, taken as a case study. The interest stems from the performance monitoring data available for a significant tunnel length in the carboniferous formation, which exhibits a severely squeezing behavior. Also of interest is the adoption of a novel yielding support system, which revealed to be successful in coping with the difficult conditions encountered. Following a series of preliminary back analyses, carried out in axisymmetric conditions and meant to calibrate the rock mass parameters, attention has been focused on the support system. The actual excavation-construction sequence has been simulated in detail to investigate the tunnel response and the stress changes in the support. The numerical results are compared with the monitoring data, and the potential of the two different elastoviscoplastic constitutive models in describing the tunnel behavior is assessed in detail.

This paper describes the application of the combined finite-discrete element method (FDEM) for the simulation of two different rock slides observed at the Alpetto Mine in Cesana Brianza (Italy). Previous numerical modeling conducted to study the stability conditions of the high rock cut are used as comparison for the results obtained. The paper presents how the FDEM simulation was performed and discusses the ability and limits of the currently available combined method to simulate rock slide problems for practical purposes.

A study is presented on the damages that developed in the segmented precasted concrete liner of a deep tunnel excavated by means of a tunnel boring machine (TBM). Various possible causes are considered i.e., the pore-water pressure acting on the liner; the excessive mountain pressure; the anisotropic in situ stress field; and the incomplete filling of the rock-lining gap. The in situ investigation, carried out to quantitatively assess the level and the extent of the damages, ruled out the pore pressure as the main source. In fact, minor water leakages were observed at the fractured sections. The remaining alternatives were analyzed through a series of nonlinear finite-element calculations in the plane strain regime. They account for the strain softening behavior of the lining and for the lack of tensile stresses at the rock-lining interface. The results give insight into the rock-liner interaction and show that the observed damages are likely to depend on the incomplete filling of the gap. On this basis some conclusions are drawn on the stresses developing within the liner and on the procedures suitable for its structural rehabilitation.