This study aims to investigate the feasibility of deriving in situ horizontal stresses from the breakout width and depth using the analytical method. Twenty-three breakout data with different borehole sizes were collected and three failure criteria were studied. Based on the Kirsch equations, relatively accurate major horizontal stress (σH) estimations from known minor horizontal stress (σh) were achieved with percentage errors ranging from 0.33% to 44.08% using the breakout width. The Mogi-Coulomb failure criterion (average error: 13.1%) outperformed modified Wiebols-Cook (average error: 19.09%) and modified Lade (average error: 18.09%) failure criteria. However, none of the tested constitutive models could yield reasonable σh predictions from known σH using the same approach due to the analytical expression of the redistributed stress and the nature of the constitutive models. In consideration of this issue, the horizontal stress ratio (σH/σh) is suggested as an alternative input, which could estimate both σH and σh with the same level of accuracy. Moreover, the estimation accuracies for both large-scale and laboratory-scale breakouts are comparable, suggesting the applicability of this approach across different breakout sizes. For breakout depth, conformal mapping and complex variable method were used to calculate the stress concentration around the breakout tip, allowing the expression of redistributed stresses using binomials composed of σH and σh. Nevertheless, analysis of the breakout depth stabilisation mechanism indicates that additional parameters are required to utilise normalised breakout depth for stress estimation compared to breakout width. These parameters are challenging to obtain, especially under field conditions, meaning utilising normalised breakout depth analytically in practical applications faces significant challenges and remains infeasible at this stage. Nonetheless, the normalised breakout depth should still be considered a critical input for any empirical and statistical stress estimation method given its significant correlation with horizontal stresses. The outcome of this paper is expected to contribute valuable insights into the breakout stabilisation mechanisms and estimation of in situ stress magnitudes based on borehole breakout geometries.

The aim of this study is to investigate V-shaped borehole breakout formation process and the impacts of the principal stresses and loading paths on breakout geometries. A three-dimensional bonded-particle model (BPM) is employed to simulate borehole breakout based on Tenino sandstone. Simulations are conducted by applying pre-loading condition to the synthetic sample and then gradually removing the particles inside the borehole to mimic the drilling process. The model successfully reproduced the V-shaped breakout, and the breakout geometries were found to be in better agreement with the experimental results compared to 2D BPM simulations in the literature. According to the borehole breakout simulations, it was revealed that the process of the breakout formation could be divided into two stages, (i) breakout initiation stage in conjunction with breakout width development and (ii) breakout propagation stage after the completion of the width development. Furthermore, all three principal stresses can affect the breakout geometries, among which the maximum horizontal stress has the predominant effect on the extent of borehole failure, followed by the minimum horizontal and the vertical stresses in order. This study also compared the breakout formation between the laboratory-scale pre-drilled (sample is drilled prior to loading) and pre-stressed (sample is drilled under pre-loading) conditions. At the stage where the sample undergoes plastic deformation, the pre-drilled breakout starts to differ from the pre-stress breakout, indicating the loading path induced by different test methods has a significant influence on the breakout formation.

Borehole breakout is a natural phenomenon resulting from in-situ stress redistribution around a borehole, and its orientation and geometries are critical indicators for in-situ stress direction and magnitudes. The aim of the study is to better understand the borehole breakout formation mechanism and reproduce breakout geometries using numerical simulations. A 2D bonded-particle model (BPM) is developed based on Tenino sandstone using particle flow code (PFC), and simulations are conducted by applying different maximum horizontal stress values with the constant minimum horizontal stresses to a synthetic rock sample. V-shaped failure patterns are observed, and the obtained breakout geometries are compared with the published true-triaxial experimental results on the same rock properties and loading conditions. The modelled breakout widths are generally larger than the experimental results due to the lack of vertical stress in two-dimensional BPM simulation. For breakout depth, it is found that some broken particles remain within the breakout zone, constraining the damage zone development and thus withstanding more internal loads. Consequently, the breakout propagation process is suppressed, and the simulated breakout depths are significantly lower than the experimental values. To overcome the limitations of the breakout simulation, a particle removal algorithm using the strain energy release criteria is developed and implemented into the model. With the embedding of the algorithm, the breakout propagation process in the vicinity of the borehole shows a good agreement with the experimental breakout test results, indicating effective removal of failed rock is critical for accurately simulating borehole breakout. Model results from this study can provide new insights into the formation mechanism of V-shaped borehole breakout, as well as strain energy evolution during breakout initiation and propagation.

ABSTRACT Shotcrete is a support system that is sprayed onto the surface of excavated tunnels and mines to create a self-supporting layer. It is an economic and integral solution to ground support in civil and mining construction projects. To ensure the long-term stability and safety of underground structures, it is essential to understand shotcrete's performance and failure mechanisms and to develop effective design methods. While previous studies have elucidated the performance and failure mechanisms of shotcrete, many unknowns and misinterpretations remain. This study conducted a numerical analysis replicating full-scale load-deflection tests in comparison with field test results. Parametric studies were also performed on the interaction between support elements and shotcrete adhesion under different loading configurations. The simulation revealed that the ultimate capacity of the shotcrete liner is primarily governed by flexural tensile failure mode, and the residual bonded area along the shotcrete panel periphery after the major bonding failure plays an important role in increasing the apparent flexural failure resistance of the shotcrete liner. A new alternative shotcrete design method based on the bending moment coefficient has been proposed and compared with empirical support design methods. INTRODUCTION Shotcrete refers to a ground-support system that is pneumatically sprayed, typically onto the exposed surface or an excavation face to produce a compacted self-supporting and load-bearing layer for underground structures. Today shotcrete has become an essential component of an initial ground support system or permanent structural lining that can be an economic and integral solution to a host of applications in civil and mining construction. Therefore, it is crucial to comprehend the performance and failure mechanisms of shotcrete as a ground-support system and to develop practical and effective design methods that can ensure the long-term stability and safety of underground structures. Many researchers, Barrett and McCreath (1995), Kaiser, and Tennent (2001), and Bernard (2004), have elucidated the performance and failure mechanism of the shotcrete liner. However, many unknowns and misinterpretations remain on the failure process of shotcrete panel, such as: the interactions between shotcrete and rock bolt configuration, and the effect of the shotcrete adhesion, etc. These factors play an important role in the understanding and development of a more practical and suitable shotcrete lining design. Thus, the main objective of the paper is to: 1) investigate the influence of those key factors which could affect the performance and the failure mechanism of the shotcrete, by means of replicating the full-scale load-deflection test using Finite Element Method (FEM) analysis, 2) propose a new method of shotcrete design based on the findings above.

Timely detection of hidden potential solution cavities subject to adverse subsurface conditions is a challenge, especially when located under active pavement areas. Site subsurface conditions for this study consisted of the subgrade layer reinforced by the existing geogrids overlying deeper untreated thick gypsum backfill. The tidal groundwater and/or rainfall infiltration caused dissolution of the gypsum, creating large voids in the subgrade. This paper describes how the use of Finite-Element Method (FEM) analyses provides a practical approach to help improve the prediction of performance for a unique Load-Transfer Platform (LTP) design. Considering various potential void diameters under applied vehicular live loads, parametric studies were conducted using the FEM approach to determine key design values (i.e., resulting forces on geogrid layers and a maximum settlement). Subsequently, by correlating surface settlement measurements to tensile load on each geogrid layer, project settlement criteria were developed to help facilitate scheduling of future pavement repairs of the LTP based on long-term effects on capacity of the geogrid layers.

Excavation of underground cavern and shaft was proposed for the construction of a ventilation facility in an urban area. A shaft connects the street-level air plenum to an underground cavern, which extends down approximately 46 m below the street surface. At the project site, the rock mass was relatively strong and well-defined joint sets were present. A kinematic block stability analysis was first performed to estimate the required reinforcement system. Then a 3-D discontinuum numerical analysis was conducted to evaluate the capacity of the initial support and the overall stability of the required excavation, followed by a 3-D continuum numerical analysis to complement the calculated result. This paper illustrates the application of detailed numerical analyses to the design of the required initial support system for the stability of underground hard rock mining at a relatively shallow depth.

This paper, not properly checked by the authors when receiving the proofs, is in need of corrections as indicated below.

The strength and dilation of rock joints in the field cannot be evaluated solely on the basis of parameters scaled from laboratory data, but also requires assessment of large-scale irregularities not present in the laboratory sample. A constitutive model for rock joints has been developed that considers the dilation and strength along both small-scale joint roughness scaled from laboratory data, and large-scale waviness determined from geologic observations. The model’s performance is illustrated by providing its correlation with experimental results taken from literature. The degradation in dilation and post-peak strength along small-scale irregularities is modeled using the plastic work done in shear, and the degradation along large-scale irregularities is modeled using a sinusoidal function. A dimensionless product of plastic work, rock strength, and wavelength of irregularities has been developed which fits the direct shear test results. An approach to scaling shear strength and shear displacement from laboratory to field-scale is also suggested.

A series of laboratory drilling experiments were conducted on two arkosic sandstones (Tenino and Tablerock) under polyaxial far-field stress conditions (σ h  ≠ σ H  ≠ σ v ). V-shaped breakouts, aligned with the σ h direction and revealing stress-dependent dimensions (width and length), were observed in the sandstones. The microscale damage pattern leading to the breakouts, however, is different between the two, which is attributed to the difference in their cementation. The dominant micromechanism in Tenino sandstone is intergranular microcracking occurring in clay minerals filling the spaces between clastic grains. On the other hand, intra- and transgranular microcracking taking place in the grain itself prevails in Tablerock sandstone. To capture the grain-scale damage and reproduce the failure localization observed around the borehole in the laboratory, we used a discrete element (DE) model in which a grain breakage algorithm was implemented. The microparameters needed in the numerical model were calibrated by running material tests and comparing the macroscopic responses of the model to the ones measured in the laboratory. It is shown that DE modeling is capable of simulating the microscale damage of the rock and replicating the localized damage zone observed in the laboratory. In addition, the numerically induced breakout width is determined at a very early stage of the damage localization and is not altered for the rest of the failure process.

The full scale Soil Cement Mixing (SCM) wall was constructed to investigate viability of the wall. Long term behavior of the wall resulted in viability of the wall. However, design guidelines on SCM wall are limited due to its complex geometry. In this paper, the three dimensional finite element method is employed to study the influence of various design decisions for SCM wall. The numerical model is first calibrated with an instrumented case history. Then a parametric study is constructed. The results give information on the influence of the following factors on the wall behavior: size of column, column overlapping, column embedment, existence of relieving platform, and column strength. The implications in design are discussed.

The efficiency of TBM performance affected by the specific s/p (s: spacing and p: penetration) ratio of the disc cutter is a research issue in demand. This article presents a multi-indentation simulation using discrete element method (DEM) analysis to study the optimal rock-cutting phenomena in terms of the interaction of the s/p ratio with intact rock properties. The multi-indentation simulation attempts to represent a linear cutting machine (LCM) test, which is a full-scale test for evaluating the optimal rock-cutting condition and measuring required reaction forces based on the intact rock condition in general practice. A governing equation relating mechanical rock properties with geometric characteristics for the optimal rock-cutting condition is derived by the numerical simulation, and its performance is evaluated with the result of the laboratory LCM tests. The results of simulations and real LCM tests show that the effective rock-cutting condition corresponding to the minimum specific energy can be estimated by an optimized s/p ratio, which, in turn, is linearly proportional to the square of the material brittleness, B (2), and cutter tip width, t (i.e., s/p = cB (2) t, where c is coefficient). The limitation of the numerical simulation associated with the sample preparation is also discussed.

The rock and soil cutting efficiency and reliability of cutting tools are critical factors to tunneling. However, due to its complexity, in-depth researches on some problems existing in rock fragmentation has not been conducted. This paper introduces the mechanism of cutting rock and soil by cutting tools. On the basis of this mechanism, it establishes finite element models using ABAQUS software to simulate the cutting processes when a cutter and disc cutter are cutting rock and soil masses. The results show that when the cutting force of a cutter is stronger than the shear strength of soil mass, breakage occurs at the interface between the cutting edge and the soil mass, leading to a successful cutting. When a disc cutter is pressed into a rock mass, with penetration depth increasing, the maximum Mises stress of the rock increases almost linearly, and plastic strain accumulates constantly, resulting in the stress growing as well. The plastic deformation zone in the rock mass is larger than the area compressed by the disc cutter. Along the working direction of the disc cutter, the rock mass is subject to continuous compression, producing plastic deformation until a fragment is broken off from its parent body. In this paper, a finite element model is established to simulate the process of cutting rock and soil by cutting tools, and a method for analyzing the interactions between cutting tools and the rock and soil is provided.