Measurement of in-situ stress in weak rocks at Mont Terri Rock Laboratory, Switzerland

In-situ stress measurements in weak rocks, such as clay shales, that respond strongly to environmental changes are particularly difficult. An extensive in-situ stress measurement program has been conducted at the Mont Terri Rock Laboratory in Northwestern Switzerland. Hydraulic fracturing, Undercoring around a 600-mm-diameter borehole using CSIRO triaxial strain cells and the Borehole Slotter were used to establish the state of stress for the Opalinus Clay at this location. Conflicting orientations and magnitudes resulted from the measurement programs. Three dimensional elastic modelling was used in conjunction with tunnel and borehole observations to establish the most likely stress tensor. A stress measurement program using a borehole deformation gauge is currently underway to check the stress tensor resulting from the observational modelling.     

Bolt length requirement in underground openings

A parametric study has been carried out using the numerical analysis code FLAC^3D to obtain the influence of various shapes of underground openings on the maximum induced boundary stress. Five shapes—viz. circular, horseshoe, rectangular, elongated D-shape and elliptical—have been considered. For each shape, four tunnel depths and five horizontal in situ stress models have been taken for the study of induced boundary stresses.The values of maximum and minimum induced boundary stresses in the roof and wall have been obtained from the analyses. This data has subsequently been used to develop correlations to estimate the normalized maximum and minimum boundary stresses, which have been subsequently compared with the strength of the rock mass obtained from the Sheorey's non-linear failure criterion for three rock masses represented by three values of Bieniawski's RMR and three values of crushing strength of intact rock material. The values of minimum factor of safety at the roof and the wall have been collected from all the plots. Using these data sets, different correlations have been developed to estimate the minimum factor of safety (f_min) in the roof and wall.Since the bolt length should be normalized with the opening size, some more computer models have been run with varying tunnel width of 5 and 20 m besides the earlier 10 m size to obtain the correlations for estimating the bolt length. The depth of factor of safety contour of 1.5 from the opening periphery has been picked up from all these models and the correlations have been developed for estimating the roof and wall bolt length for the five shapes of underground openings. The correlations for bolt length show that in addition to the shape of underground openings and in situ stress, the bolt length also varies with the rock mass type. These correlations have been verified for field cases of elongated D-shape openings.     

Causes, impact and control of overbreak in underground excavations

Drill and blast system is used in hard rock excavation due to its economics and adaptability to changing rock mass conditions. Common question during mining and tunneling operations is ‘whether overbreak has been caused by blasting practice or poor rock mass quality’. Critical evaluation of the factors influencing blast damage is required to address such questions.In order to understand the mysterious nature of blast damage prediction and control, the field work involved the small scale blasting of physical models and the assessment of blast damage during drifting operations. The damage was measured by the Half cast factor, percentage overbreak and the Blast damage index. The influence of rock mass features, explosive characteristics and blast design parameters on overbreak has been examined in this study. A new approach for the judicious design of perimeter hole pattern and charge concentration has been proposed. Implications of blast damage have also been outlined in this paper.     

An in-situ thermo-hydraulic experiment in a saturated granite I: design and results

This paper is Part I of a series of two papers concerned with an in situ Thermo-Hydraulic Experiment (THE) carried out at the Underground Research Laboratory of Atomic Energy of Canada Limited. The THE was designed to determine the thermoporoelastic parameters, which control the magnitude of the pore pressure induced by thermal loading. The experimental setup involved a heater installed in a sub-horizontal borehole drilled from an underground gallery, and piezometers and thermistors located at different distances from the heater in auxiliary boreholes drilled from an adjacent gallery. Several water injection and heater tests were conducted during this experiment. Part I focuses on the design of the experiment, which involves specifying the nature of the boundary conditions, and determining the heater power, the duration of the thermal load, and the locations of the piezometers and thermistors. Part I also reports the experimental data collected in this experiment, and in particular the significant thermally induced pore pressures measured in the heater tests.     

Estimation of the three-dimensional in situ stress field around a large deep underground cavern group near a valley

Understanding three-dimensional(3D) in situ stress field is of key importance for estimating the stability of large deep underground cavern groups near valleys.However,the complete 3D in situ stress fields around large deep underground cavern groups are difficult to determine based on in situ stress data from a limited number of measuring points due to the insufficient representativeness and unreliability of such measurements.In this study,an integrated approach for estimating the 3D in situ stress field around a large deep underground cavern group near a valley is developed based on incomplete in situ stress measurements and the stress-induced failures of tunnels excavated prior to the step excavation of the cavern group.This integrated approach is implemented via four interrelated and progressive basic steps,i.e.inference of the regional tectonic stress field direction,analyses of in situ stress characteristics and measurement reliability,regression-based in situ stress field analysis and reliability assessment,and modified in situ stress field analysis and reliability verification.The orientations and magnitudes of the3D in situ stress field can be analyzed and obtained at a strategic level following these four basic steps.First,the tectonic stress field direction around the cavern group is deduced in accordance with the regional tectonic framework and verified using a regional crustal deformation velocity map.Second,the reliability of the in situ stress measurements is verified based on the locations and depths of stressinduced brittle failures in small tunnels(such as exploratory tunnels and pilot tunnels) within the excavation range of the cavern group.Third,considering the influences of the valley topography and major geological structures,the 3D in situ stress field is regressed using numerical simulation and multiple linear regression techniques based on the in situ stress measurements.Finally,the regressed in situ stress field is further modified and reverified based on the stress-induced brittle failures of small tunnels and the initial excavation of the cavern group.A case study of the Shuangjiangkou underground cavern group demonstrates that the proposed approach is reliable for estimating the 3D in situ stress fields of large deep underground cavern groups near valleys,thus contributing to the optimization of practical excavation and design of mitigating the instability of the surrounding rock masses during step excavations.     

Infinite elements for numerical analysis of underground excavations

Infinite elements were developed to overcome an inherent limitation of the Finite Element Method. The technology of infinite elements has been refined to such an extent that the modeling of an unbounded medium is no longer a limitation of the Finite Element Method. This paper shows that the remaining problem associated with a non-uniform and non-zero far-field decay of any one of the problem variables can also be resolved without having to create nodes at infinity. A combined finite-infinite element analysis now matches the power of the Boundary Element Method in handling the unbounded analysis domains while retaining the versatility of the Finite Element Method.     

Ground behaviour and rock mass composition in underground excavations

Underground excavations like tunnels, caverns shafts, are located in a vast variety of rock mass and ground conditions with different modes of behaviour. The paper describes the main parameters and features determining the behaviour of the ground surrounding an underground excavation, namely: (1) the ground conditions (rock mass, stresses and water) and (2) the project related features, (size and shape of the opening and excavation method). Based on this a simple, qualitative division of the rock masses is presented, which together with the influence of stresses, ground water and dimension of the excavation, is used to find the probable behaviour. This may help the user to select appropriate rock engineering tool(s) for the geotechnical design as has been described by the two authors in an earlier paper.     

Improved routine estimation of the minimum horizontal stress component from extended leak-off tests

In this paper, several high-quality extended leak-off tests (XLOT) with flowback are reviewed. It is argued that a properly performed XLOT with flowback gives a precise determination of the minor in situ stress. Further, it is found that the improved stress estimate tends to predict a stress level significantly lower than that found by traditional interpretation of leak-off or extended leak-off tests.The flowback test with volume measurements should be the preferred method of performing leak-off tests for precise routine stress determination in deep wells.     

Transverse drilling-induced tensile fractures in the West Tuna area, Gippsland Basin, Australia: implications for the in situ stress regime

Drilling-induced tensile fractures (DITFs) have been interpreted on image logs from vertical wells in the Gippsland Basin, offshore southeastern Australia. Interpreted axial (vertical) DITFs have previously been well described worldwide. We also interpret transverse (horizontal) DITFs, which are horizontal fractures that are electrically conductive, non-planar, bimodal and constrained to the tensile region of the wellbore.Elasticity theory predicts formation of both transverse and axial drilling-induced tensile fractures (DITFs) in vertical wells depending on the magnitude of the principal in situ stresses, pore-pressure and mudweight. Drilling-induced tensile fractures initiate in very specific stress environments. Axial DITFs can closely constrain a lower bound to the maximum horizontal stress (S_H max) magnitude where the minimum horizontal (S_h min) stress is known. If transverse DITFs are observed, they can constrain a lower bound to maximum and minimum horizontal stress magnitudes. The observation of transverse DITFs on image logs can constrain the stress field to one on the border of strike-slip and reverse faulting () without requiring knowledge of the S_h min or S_H max magnitude. The observation of transverse DITFs in the West Tuna area combined with wireline log data, leak-off tests and pore pressure data are used to constrain the in situ stress tensor. The interpreted in situ stress tensor lies on the border of a strike-slip and reverse faulting regime (S_H max40.5 MPa/km>S_h min≈S_v21 MPa/km). Interpreted data from leak-off tests in the West Tuna area confirm that S_h minS_v.     

应力铲试验在粉土场地的应用

本文简要论述了应力铲试验（简称ＴＰＴ）在粉土场地的应用方法与成果分析，提出了利用应力铲土性指数ＩＴ来划分土类的方法和部分土类的划分标准，文中还建议在应力铲上安装孔压传感器，以进一步提高ＴＰＴ的准确性。     

The relationship between closure pressures from fluid injection tests and the minimum principal stress in strong rocks

Closure pressures measured during injection tests such as mini-fracs are normally considered an accurate measure of the minimum in situ principal stress magnitude. This paper presents stress, strength and image log data from the Australian Cooper Basin, which suggests that in reservoirs with high in situ stress, high tensile strength and weak geological fabrics, interpreted closure pressures may be significantly greater than the minimum principal stress.Closure pressures interpreted from mini-frac injection tests in the Cooper Basin, suggest the minimum principal stress varies from 12.4–27.2 MPa/km (0.55–1.2 psi/ft). To better understand the reasons for this variation in closure pressure, image logs and mini-frac data from 13 treatment zones, and core from seven of these treatment zones, were analysed. The analysis revealed that treatment zones with high measured closure pressures (18.1 MPa/km; 0.8 psi/ft), high treating pressures (>31.6 MPa/km; 1.4 psi/ft) and high measured hydraulic fracture complexity existed in reservoirs with high tensile rock strength (>7 MPa; 1015 psi) and geological fabrics (planes of weakness) including natural fractures. Conversely, treatment zones with lower measured closure stress (19 MPa/km; 0.84 psi/ft) and low hydraulic fracture complexity occurred in reservoirs with lower tensile strength and/or no geological fabrics.We suggest that closure pressures in rocks with high tensile strength and weak geological fabrics may not be representative of the minimum principal stress magnitude in the Cooper Basin where they are associated with hydraulic fracture complexity. Rather, they reflect the normal stress incident on pre-existing weaknesses that are exploited by hydraulic fluid during the mini-frac injection.     

Simulation of progressive fracturing processes around underground excavations under biaxial compression

Fractures that develop progressively around underground excavations can be simulated using a numerical code called RFPA (rock failure process analysis). This code incorporates the mesoscopic heterogeneity in Young’s modulus and rock strength characteristic of rock masses. In the numerical models of a rock mass, values of Young’s modulus and rock strength are realized according to a Weibull distribution in which the distribution parameters represent the level of heterogeneity of the medium. Another notable feature of this code is that no a priori assumptions need to be made about where and how fracture and failure will occur – cracking can occur spontaneously and can exhibit a variety of mechanisms when certain local stress conditions are met. These unique features have made RFPA capable of simulating the whole fracturing process of initiation, propagation and coalescence of fractures around excavations under a variety of loading conditions. RFPA is used herein to simulate progressive fracturing processes around three common shapes of underground excavations – circular, elliptical and inverted U-shaped. The results of the simulations show that the code can be used not only to produce fracturing patterns similar to those reported in previous studies, but also to predict fracturing patterns under a variety of loading conditions. Based on these fracturing patterns, failure mechanisms are identified for various loading conditions.     

地下室侧墙结构设计的若干问题讨论

为了较真实地反映地下室侧墙结构受力情况,结合工程设计实践经验,对地下室侧墙计算模型的简化、边界条件的确定、荷载组合及有关设计方法进行了讨论,为地下室侧墙结构的合理设计提供依据。     

Borehole breakouts and compaction bands in two high-porosity sandstones

We compare the shape and mechanism of failure around vertical boreholes drilled in blocks of two high-porosity sandstones subjected to unequal far-field principal stresses. Tablerock sandstone has a porosity of 28%, and is composed of 55% quartz and 37% weaker feldspar grains. Grain cementation is substantial through microcrystalline quartz. Critical far-field stresses induce failure around boreholes in the form of V-shaped (dog-eared) breakouts, the result of dilatant intra and trans-granular microcracking subparallel to both the maximum horizontal far-field stress and to the borehole wall. On the other hand, boreholes in Mansfield sandstone, which has similar porosity (26%), but contains mainly quartz grains (90%) held together primarily by spot-sutured contacts, fail by developing fracture-like breakouts. We show evidence suggesting that Mansfield sandstone breakouts are preceded by the development of a narrow (several grain diameters) localized compaction zone along the direction of the least horizontal stress, which is where the maximum compressive tangential stress concentration occurs. Failure mechanism here is simply the removal by the circulating drilling fluid of mainly intact grains loosened during the formation of the compaction band. The type of cementation, mineral homogeneity, grain strength, and sphericity appear to be major factors in the formation of compaction bands. Some breakout dimensions in both rocks are correlatable to the far-field principal stresses, and can be potentially used (in conjunction with other information) as indicators of their magnitudes.     

FLAC/PFC coupled numerical simulation of AE in large-scale underground excavations

Acoustic emission (AE) and microseismic (MS) events are indicators of rock fracturing or damage as the rock is brought to failure at high stress. By capturing the AE/MS events, underground excavation induced rock mass degradation or damage can be located and evaluated. A better understanding of the extent and shape of the excavation damaged zone (EDZ) or yield zone around caverns helps to arrive at safe and economic design and construction of the caverns. For this purpose, one needs to understand the AE mechanism associated with the excavation process.In the present study, a coupled numerical method is used to study AE at the Kannagawa underground powerhouse cavern in Japan. Two codes, Fast Lagrangian Analysis of Continua (FLAC), a finite difference code and Particle Flow Code (PFC), a distinct element code, are coupled. The motive to apply the FLAC/PFC coupled approach is to take advantage of each modeling scheme while at the same time minimizing the requirement for computational resources. The coupling is realized through an exchange of displacements, velocities, and forces in each cycling step. The rock mass surrounding an AE sensor is modeled using PFC and the remaining rock mass is modeled with FLAC to consider the geological complexity and the excavation sequence. In this manner, the AE activities at AE sensor locations of the Kannagawa cavern were simulated and found to be in good agreement with field monitoring results. This approach takes account of stress redistribution and provides stress and displacement patterns in the rock mass that are consistent with AE observations for excavation design. The observed AE activities in the rock mass can thus be utilized to assess the effectiveness of the rock support system and the overall stability of the cavern.     

灌注桩施工应力与配筋对工程质量影响的分析

结合工程实例分析了挤土效应,挖土,截桩,基坑位移产生的施工应力对灌注桩桩身质量的影响,并提出了减轻施工应力及其危害的措施。     

深基坑坑底地基的回弹应力与回弹变形

某长条形地铁车站深基坑,基坑开挖深度15m,通过设置数个分层回弹磁环,测得回弹变形沿坑底深度方向的分布规律;通过固结回弹试验建立回弹模量与卸荷比关系,得到回弹模量沿坑底深度方向的分布规律;结合以上两种成果,并考虑坑底土承受侧向净土压力的伸长作用,得到回弹应力沿深度方向的分布规律:作用在坑底的反向荷载相当于挖去土的自重,因受坑底土有效自重应力的抵消作用,回弹应力沿深度方向呈线性衰减,坑底土的"残余应力"可以认为就是坑底回弹影响范围内的有效自重应力。用上述确定回弹应力的方法计算另一个新近案例的回弹变形,与实测结果比较接近。     

The implications of joint deformation in analyzing the properties and behavior of fractured rock masses, underground excavations, and faults

For a given stress state, joint deformation depends on the joint’s geometry, including surface roughness, spatial geometry of the contact area, and large-scale topographical features such as dips. Under normal stress, contacting asperities compress, and the half spaces bounding the joint are deformed. A very significant consequence of half-space deformation is that it allows mechanical interaction among all contact points between the joint surfaces. As a result, the contact area’s overall spatial geometry plays an important role in determining the distribution of stress across joint surfaces and the change in geometry of the void space between surfaces that occurs with changes in stress. Mechanical interaction among contact points is important in determining normal joint stiffness: two joints with the same total contact area can have substantially different stiffnesses depending on the spatial geometry of their contact areas. Modeling results indicate that joints with small contact areas uniformly distributed across the surfaces can be nearly as stiff as a perfectly mated interface. These results have significant implications for almost any endeavor in fractured rock, including designing underground excavations, predicting the hydraulic response of a rock mass to changes in stress, understanding the deformation and failure of joints under shear stress, and analyzing the stability of faults. In underground excavations, for example, deformation of the roof and floor means that the load acting on any supporting pillar and the distribution of stress throughout the pillar depend on: the pillar’s size and shape; the size, shape, and proximity of neighboring pillars; and the spatial geometry of the pillar array. Purely elastic deformation can lead to either catastrophic or progressive failure. Similarly, to accurately predict fluid flow through jointed rock, changes in void space geometry that result from changes in stress must be considered; these changes in geometry are not predicted by methods that assume that aperture changes uniformly across the joint. For joints and faults, the nonuniform distribution of normal and shear stresses resulting from surface roughness and mechanical interaction between contact points suggests a progressive form of shear failure. Failure is initiated at points of low normal stress and propagates as stress from failed asperities is redistributed to neighboring asperities. Consistent with observations on many faults, modeling and analytical results predict that earthquakes on a fault would be clustered in time and space because of mechanical interaction between persistent asperities.