深部硬岩剪切边界下应力诱发片帮的机制研究

深部工程开挖现场发现硬岩的剪切破坏过程对地下工程稳定性具有重要影响。深埋硬岩隧洞边墙处片帮或板裂主要为压致拉裂的结果,可通过室内单轴压缩试验来模拟;而拱肩和底角处的片帮或板裂多数是在剪切边界下形成的,需通过室内完整硬岩直剪试验来再现。针对不同类型硬岩压致拉裂型片帮,已进行了相当深入的研究,但对完整硬岩剪切边界下应力诱发型片帮的直接研究还存在着不足。为了深入研究深埋硬岩隧洞片帮或板裂的形成机制,采用FLAC3D软件模拟隧洞不同位置处围岩受开挖扰动的应力调整过程,根据中国锦屏地下实验室二期工程区域的片帮统计结果,将深埋硬岩隧洞片帮划分为压致拉裂型和剪切边界应力诱发型两类,提出剪切边界应力诱发型片帮形成过程的概念模型;利用完整大理岩设计了不同正应力下压剪破裂到剪切滑移的连续性直剪试验,试样典型片帮破坏结果验证了概念模型的合理性。     

预应力格构锚固结构受力特征模型试验研究

格构锚固是边（滑）坡工程常用的防治技术之一,为了研究格构锚固结构实际支挡碎石土边坡的受力特征,利用现场大比例尺物理模型试验,填筑粘土方式模拟滑坡体,设计相似比为1:2.5的格构梁,通过后缘加载模拟了格构锚固体系承载受力直至失稳破坏的全过程,监控格构梁后部土压力、格梁应力、格梁位移等参数,并分析了格构梁受力、位移特征和变形破坏模式。结果表明:格构边界处锚固点处变形较内部更为明显,跨梁呈"拱桥式"变形,最终在锚固点附近折断破坏;各横纵梁受力特征类同,梁交叉锚固点处应力集中,梁中部受力最小,应力呈近倒三角形分布;模型中锚固点受力和位移自顶到底呈现线性递减的应力分配现象。上述研究可以为工程优化设计提供一定参考。      

In-situ Rock Spalling Strength near Excavation Boundaries

It is widely accepted that the in-situ strength of massive rocks is approximately 0.4 ± 0.1 UCS, where UCS is the uniaxial compressive strength obtained from unconfined tests using diamond drilling core samples with a diameter around 50 mm. In addition, it has been suggested that the in-situ rock spalling strength, i.e., the strength of the wall of an excavation when spalling initiates, can be set to the crack initiation stress determined from laboratory tests or field microseismic monitoring. These findings were supported by back-analysis of case histories where failure had been carefully documented, using either Kirsch’s solution (with approximated circular tunnel geometry and hence σ _max = 3σ ₁ ?σ ₃) or simplified numerical stress modeling (with a smooth tunnel wall boundary) to approximate the maximum tangential stress σ _max at the excavation boundary. The ratio of σ _max /UCS is related to the observed depth of failure and failure initiation occurs when σ _max is roughly equal to 0.4 ± 0.1 UCS. In this article, it is suggested that these approaches ignore one of the most important factors, the irregularity of the excavation boundary, when interpreting the in-situ rock strength. It is demonstrated that the “actual” in-situ spalling strength of massive rocks is not equal to 0.4 ± 0.1 UCS, but can be as high as 0.8 ± 0.05 UCS when surface irregularities are considered. It is demonstrated using the Mine-by tunnel notch breakout example that when the realistic “as-built” excavation boundary condition is honored, the “actual” in-situ rock strength, given by 0.8 UCS, can be applied to simulate progressive brittle rock failure process satisfactorily. The interpreted, reduced in-situ rock strength of 0.4 ± 0.1 UCS without considering geometry irregularity is therefore only an “apparent” rock strength.     

Spalling Experiments on Large Hard Rock Specimens

Specimens of coarse-grained Äspö diorite were axially compressed to observe stress-induced spalling. The specimens had a novel design characterized by two manufactured large radius notches on opposite sides. The tangential stress occurring in the notches aimed to represent the tangential loading around a circular opening. Fracture stages were monitored by acoustic emission measurements. Rock chips were formed similar to those found in situ, which indicates a similar fracture process. Slabs were cut out from the specimens and impregnated using a fluorescent material to visualize the cracks. The cracks were subsequently examined by the naked eye and by means of microscopy images, from which fracture paths could be identified and related to different minerals and their crystallographic orientations. The microscopy analyses showed how the stress field and the microstructure interact. Parallel cracks were formed 2–4 mm below the surface, sub-parallel to the direction of the maximum principal stress. The crack initiation, the roles of minerals such as feldspar, biotite and quartz and their grain boundaries and crystallographic directions are thoroughly studied and discussed in this paper. Scale effects, which relate to the stress gradient and microstructure, are discussed.     

A numerical and laboratory investigation into the behaviour of a dry underclay beam

A simple and inexpensive laboratory device has been developed and used to simulate the behaviour of a dry underclay beam as the floor of an underground roadway subjected to a uniformly distributed uplift pressure. The Airy stress function method proved to be a useful means of expressing and predicting of the uplift of such beams. This method has been related to both classical beam theory and laboratory investigations. It was found that the Airy stress function method, utilizing polynomial equations, provides a closer fit to the mechanism of beam deformation and stress developments under dry laboratory conditions.     

基于围岩片帮形迹的宏观地应力估计方法探讨

基于深部地下勘探隧洞围岩应力型片帮破坏是岩体初始地应力作用的最直观结果的认识,建立借助围岩片帮形迹分析工程区岩体宏观地应力的估计方法。该方法通过对比分析地应力作用下多个交叉隧洞围岩的片帮空间分布特征和烈度强弱关系,建立岩体3个初始主应力的空间大小和方向关系;采用片帮劈裂纹方位角统计获得岩体最大主应力方向;依据片帮的应力门槛值和起裂应力门槛值估算出3个主应力量值。现场实测地应力数据和多工况数值计算分析都有力地表明,借助该分析方法获得的白鹤滩右岸厂址区域三维地应力是科学、合理的,这也为深入认识工程区地应力特征提供了一条新思路     

Modeling of a Multiply Jointed Voussoir Beam in the Centrifuge

Summary The mechanics and kinematics of a multiply jointed Voussoir beam have been studied based on the response of small scale beams tested in the centrifuge. The model beams were made up of six blocks, confined between fixed abutments. The tests included monitoring of beam deflection, development of axial thrust and distribution of strain within the beam. The models were subjected to accelerations of up to 90 g, depending upon the beam geometry. Two different beam geometries were considered. The test data were analysed in the light of pseudo analytical algorithms accepted in the analysis of singly jointed Voussoir beams. The data show that small translations and rotations of the block at the beam-abutment interface are required for a stable compression arch to set up in the beam. The computations and physical models indicate that the development of thrust is in linear proportion with the applied acceleration and is independent of beam thickness. The distribution of strains along the beam leads to the conclusion that the depth of the compression arch varies from the total beam thickness, to approximately one half of the beam thickness, under stable conditions. The data should be of considerable use to researchers looking to calibrate/validate numerical techniques used in describing the response of discontinuous rock masses.     

动力沉桩桩基拉应力分布及其施工控制试验研究

采用“现场沉桩试验、室内桩基模型试验、现场沉桩实例验证”的技术路线,通过对16根桩的动测试验结果的全面介绍和分析,揭示了打桩过程中桩身拉应力的分布规律,即桩身最大拉应力通常发生在桩的上部,特别是打桩初期,桩尖位于软土层或桩周为软土地基时,桩身最大拉应力点发生在距桩顶1/4l（l为桩长）附近,拉应力占同点压应力的50 %左右,这是桩身产生垂直桩轴线方向横向裂缝乃至发生桩身断裂现象的根源。试验亦表明,采用碟簧桩帽沉桩能有效地减小锤击拉应力,克服了采用老式桩帽辅助沉桩时预应力混凝土（管）桩桩身拉应力过大而造成桩身断裂这一缺陷,控制实测拉应力在允许拉应力（5 MPa）范围之内,以确保沉桩桩身完整,这是防治桩身拉裂的有效措施。     

Crack damage stress as a composite function of porosity and elastic matrix stiffness in dolomites and limestones

Twenty-five uniaxial compression tests were performed to determine stress at onset of dilation, referred to herein as “the crack damage stress,” in heterogeneous dolomites and limestones. A simplified model for crack damage stress (σ_cd) is developed here using porosity, elastic modulus, Poisson's ratio and three empirical coefficients. The model shows that when porosity decreases and elastic modulus increases, σ_cd rapidly increases and approaches its maximum value. On the other hand, when porosity increases and elastic modulus decreases, σ_cd rapidly decreases and approaches its minimum value. The proposed model is validated for six heterogeneous limestone and dolomite formations which are widely distributed in Israel.     

深部厚顶煤巷道让压型锚索箱梁支护系统布置方式对比研究

针对深部厚顶煤巷道围岩变形特点,以“先控后让再抗”为支护理念,设计了让压型锚索箱梁支护系统横梁、纵向单梁、纵向双梁及纵横组合4种不同布置方式的支护方案。数值试验结果表明,纵横组合方案具有最好的围岩控制效果,横梁方案支护效果相对较差。对让压型锚索箱梁支护系统的4种方案进行了现场试验,监测结果显示,纵横组合方案作用下的围岩变形量最小,其两帮移近量和顶板沉降量仅为120 mm和90 mm,同时该方案支护反力最大;锚杆、锚索受力与巷道围岩控制效果相对应,且锚索让压环作用明显;设计制作的测力箱梁监测表明,箱型支护梁受力合理协调,材料性能发挥充分。对让压型锚索箱梁支护系统及其不同布置方案的作用机制进行了探讨。综合分析表明,深部厚顶煤巷道中,优先选用让压型锚索箱梁支护系统中的纵横组合与纵向单梁方案,可有效地控制巷道围岩变形,满足支护要求。     

碳纤维布用于格构缺陷修复技术的模型试验研究

格构锚固是边（滑）坡工程常用的防治技术之一,长期服役的格构梁通常会产生局部破损的现象,影响边（滑）坡长期稳定性,而贴附碳纤维布是修复格构梁缺陷最行之有效的手段。采用室内模型试验槽填筑黏土方式模拟滑坡体,设计相似比为1:10的格构梁,通过千斤顶施加滑坡推力,开展碳纤维布用于格构缺陷修复技术的模型试验,监控格构梁后部土压力、格梁应力、格梁位移等参数,分析碳纤维布修复前后格构梁和碳纤维布两者的荷载分担比、钢筋应力改善情况和格梁弯矩的前后变化规律。试验数据分析表明,碳纤维布和格构梁荷载分担比得到了改善,滑坡推力作用在碳纤维布和格构梁复合体时,碳纤维布先于格构梁受力,并且受力远远大于格构梁,碳纤维布所承受的荷载分担比约为83%;格梁破坏首先为受拉破坏,然后为受压破坏;模型试验中,在钢筋表面粘贴了应变片,监测到钢筋的应变情况,碳纤维布修复后钢筋受力得到改善,修复后的最大钢筋应力比修复前降低了8倍多;对修复前后,格构梁弯矩变化进行了研究,碳纤维布修复后格梁弯矩显著降低,约为修复前的1/10-1/3。鉴于以上研究结果,说明碳纤维布用于修复、加固缺陷格构梁工程效果显著。     

大直径盾构隧道扩挖地铁车站的力学性能研究

日益复杂的地铁建设环境使得地铁线路布置困难、施工风险加大,同时对施工方法也提出了更为严格的要求。采用大直径盾构建造地铁单洞双线区间并在盾构隧道基础上小规模扩挖形成车站是解决复杂环境下地铁建设的一种新思路。以北京地铁14号线高家园站为背景,提出了在外径10 m的大直径盾构隧道基础上采用CRD(Cross Diaphragm)法扩挖地铁车站的两种方案,利用“地层-结构”相互作用有限元法模拟了车站扩挖施工过程,研究了结构体系的受力转换规律。结果表明：在扩挖施工中,结构受力转换频繁;结构体系的最大轴向应力位置由管片环转移到初期支护,最大剪应力位置转移到封顶块管片;管片环由受压状态为主转向受剪状态为主,初期支护、中隔板、梁柱及临时支撑以受压状态为主;封顶块管片和顶梁上部翼缘处的应力较大,应对这些位置进行加强处理。     

Influence of Sample Height-to-Width Ratios on Failure Mode for Rectangular Prism Samples of Hard Rock Loaded In Uniaxial Compression

Surface-parallel slabbing is a failure mode often observed in highly stressed hard rocks in underground excavations. This paper presents the results of experimental studies on slabbing failure of hard rock with different sample height-to-width ratios. The main purpose of this study was to find out the condition to create slabbing failure under uniaxial compression and to determine the slabbing strength of hard rock in the laboratory. Uniaxial compression tests were carried out using five groups of granite specimens. The mechanical parameters of the sample rock, Iddefjord granite from Norway, were measured on the cylindrical and Brazilian disc specimens. The transition of the failure mode was studied using rectangular prism specimens. The initiation and the propagation of slabbing fractures in specimens were identified by examining the relationship among the applied stress, strain and the acoustic emission. The stress thresholds identified were compared to those reported by other authors for crack initiation and brittle failure. It is observed that the macro failure mode will be transformed from shear to slabbing when the height/width ratio is reduced to 0.5 in the prism specimens under uniaxial compression. Micro σ ₁-parallel fractures initiate when the lateral strain departs from its linearity. Slabbing fractures are approximately parallel to the loading direction. Labotatory tests show that the slabbing strength (σ _sl) of hard rock is about 60% of its uniaxial compression strength. It means that if the maximum tangential stress surrounding an underground excavation reaches about the slabbing threshold, slabbing fractures may take place on the boundary of the excavation. Therefore, the best way to stop or eliminate slabbing failure is to control the excavation boundary to avoid the big stress concentration, so that the maximum tangential stress could be under the slabbing threshold.     

A study of the behaviour of brittle rocks subjected to confined stress based on the Mohr–Coulomb failure criterion

In this study the relationship between brittle rocks’ behaviour and uniaxial compression stress is obtained based on the Mohr-Coulomb failure criterion and the behavioural characteristics of brittle rocks under uniaxial compression stress are investigated. According to the laboratory uniaxial compression tests on granite rocks, and also available results in the literature, required parameters in the study are obtained. It is indicated that the behaviour of brittle rocks which is affected by uniaxial compression is function of cohesion. In the other words, cohesion changes as the uniaxial compression increases. Also, in yield point where its stress is equivalent to the uniaxial compressive strength, the cohesion maximises. By suggesting a new viewpoint to the Mohr’s circle, normal stresses on failure plane, shear stresses tangential to the failure plane and hydrostatic stresses are investigated. Results show that, normal stress on the failure plane in yield point of the behavioural curve equals zero and shear stress tangential to the failure plane is maximum which is equal to the maximum rocks’ natural strength. Also, in this point the strength is equi-pressure, therefore the stress is of hydrostatic type.     

地下洞室围岩脆性破坏时的应力特征研究

在高应力作用下,岩爆、钻孔崩落、片帮都是地下空间硬脆围岩中常见的破坏现象,这三类现象本质上均可归于完整岩体的脆性破坏,它们分别反映了高应力作用下完整岩体不同的破坏程度。通过对前人关于岩爆判据、钻孔崩落判据和片帮应力强度比判据研究成果的类比分析可知,这些脆性破坏现象在破坏时具备相同的应力背景条件。脆性破坏的应力条件可以用地下空间周边切向最大应力与岩石单轴抗压强度之比（ / ）或者工程区最大主应力与岩石单轴抗压强度之比（ / ）来描述,两种指标本质上反映了相同的应力背景条件。对于 / , / = 0.4 ± 0.1是发生脆性破坏的应力临界条件;对于 / , / = 0.15 ± 0.05是发生脆性破坏的应力临界条件。大量的工程实例和基于Hoek-Brown强度准则的力学分析也证明了这一背景条件的正确性。这里两种指标都取了一个范围,主要是由于不同的岩体分级、岩性和工程地质条件会对指标的界定产生较为显著的影响。     

Deformation of a rock in different tensile tests

Strength and deformation of a typical homogeneous sandstone under tensile stresses, have been studied in the laboratory, using standard tensile tests, viz. uniaxial tension, the Brazilian and beam bending tests. In addition to the parameters such as uniaxial tensile strength, modulus of deformation in tension and Poisson's ratio in tension obtained directly by the uniaxial tension test, the Brazilian tensile strength, the modulus and Poisson's ratio have also been determined by the Brazilian test. Bending tensile strength has been obtained by the beam bending test using the four-point loading system and the tensile modulus of elasticity has been estimated from the observations of beam bending and uniaxial compression tests, using “double elasticity” relationships.The rock under high tensile stresses is found to be much less “stiffer” than under high compressive stresses and is therefore likely to result in a relatively less violent tensile failure. The nature of the tensile deformation in the region of maximum tensile stress is found to be strikingly similar in all the three tensile tests under study. Although the initial, the 50% and the final tensile modulus values differ in each tensile test, the general trend of the tensile modulus with increasing stress is found similar in all these tensile tests.     

Delayed failure in rock loaded in uniaxial compression

Summary The long-term response to sustained compressive loading of two crystalline hard rocks of the Canadian Shield has been investigated. Static fatigue tests conducted on granite and anorthosite have shown that in a humid environment the long-term strengths of these crystalline igneous rocks could be less than 60 percent of their dry instantaneous strengths. Such reduction in strength has implications for the design and construction of deep tunnels, mines and other underground installations. The particular case of a nuclear fuel waste vault located at a depth of one kilometer is considered.Considering a service life of 1,000 years, a vault one kilometer deep in granite could suffer time-dependent spalling in highly stressed regions of the rock where the maximum principal stress exceeds about 130 MPa. Anorthosite has a lower instantaneous strength than granite and it is sensitive to static fatigue as well. A vault in anorthosite would be subject to time-dependent spalling of the perimeter rock in places where the maximum principal stress is above 80 MPa.     

Size and Geometry Effects on Rock Fracture Toughness: Mode I Fracture

In this paper, the effects of specimen size and geometry on the apparent mode I fracture toughness (K _c) of an Iranian white marble (Neyriz) are studied. A number of fracture tests were conducted on center-cracked circular disk (CCCD) specimens with different radii to investigate the size effects on K _c. The experimental results demonstrate that the apparent fracture toughness increases in bigger specimens. In order to explain the experimental results, the modified maximum tangential stress (MMTS) criterion is used, where higher order terms of the Williams’ series expansion are included in the maximum tangential stress criterion. It is shown that the MMTS criterion provides good estimates for the apparent fracture toughness of Neyriz marble, obtained from fracture tests of edge-cracked triangular specimens. It is, therefore, concluded that the proposed criterion is able to account for the size and geometry effects on the fracture resistance of rocks simultaneously.     

考虑基础刚度影响的风机梁板式桩筏基础模型试验研究

梁板式桩筏基础作为新型风机基础,前景广阔,适当优化结构,可节约成本。采用大型室内模型试验,研究了梁板式桩筏基础在不同筏板刚度情况下的受力变形特性,同时讨论了桩土荷载分担比的关系。研究表明,在工作荷载范围内,适当增大筏板刚度,桩平均轴力增加,外圈桩承担的荷载增大,内圈桩的轴力减小,环梁及肋梁弯矩减小,但继续增加刚度,对桩身轴力、环梁弯矩、肋梁弯矩的影响并不大;随着荷载的增大,土体承担的荷载比逐步增大,趋于稳值。随着梁板式基础刚度增加,基础内外差异沉降减少,但是达到一定刚度后再增加刚度对变形影响较小。梁板式基础刚度的变化会影响基底反力分布的均匀性,且存在一个最优刚度,使基础底部反力分布最为均匀,本次试验中12梁的梁板式基础基底反力比6梁和实体基础更均匀。桩基的分布和梁的分布存在相互影响,改变梁的分布会影响桩顶力,而桩基在梁底下又会使梁的应力产生重新分布。研究成果可为设计桩筏基础提供参考依据。     

Modeling the behaviour of alluvial and blasted quarried rockfill materials

Two types of modeled rockfill materials were collected from Renuka dam site, Himachal Pradesh, India and Salma dam site, Afghanistan. The rockfill material collected from Renuka dam site is rounded to sub-rounded in shape and the rockfill material collected from Salma dam site is angular to sub-angular in shape. The prototype gradation rockfill material consists maximum particle size larger than 1,000 mm. Therefore, for carrying out laboratory testing and modeling the bahaviour, the prototype rockfill material is scaled down to the maximum particle size (d_max) of 25, 50 and 80 mm for both projects material using parallel gradation technique. Triaxial compression and Index properties tests were conducted on both project rockfill materials and are presented. From the triaxial behaviour, it is observed that the stress–strain behaviour is non-linear, inelastic and stress dependent for both the materials. The material compresses during the initial shearing and shows dilation effect with further shearing. It is observed that the ?-value for alluvial rockfill material increases with increase in d_max and reverse trend is observed for blasted quarried rockfill material which shows the importance of the type of material. The stress–strain-volume change behaviour of both projects modeled rockfill material was predicted by using hierarchical single surface (HISS) model based on elasto plasticity and compared with the laboratory test results. From the comparison, it is observed that both results match closely. It is, therefore, suggested that the behaviour of both types of rockfill materials can be characterized successfully using HISS model.