巴基斯坦N-J水电站引水隧洞地应力估算

巴基斯坦N-J水电站TBM隧道所处区域构造复杂,埋深大,隧道开挖过程中岩爆频繁。现场地应力是影响岩爆等级和范围的重要因素。依据钻孔饼化岩芯的断面形态和岩饼厚度,对现场地应力方向和量值进行了估算。结果表明岩芯钻孔轴线方向近似平行于其中一个主应力方位;假设岩体自重应力为其中一个主应力,则TBM隧洞轴线方向为第三个主应力方位。对应于水平侧压力系数0.8~1.5,最大主应力量值在80MPa~110MPa。开挖拱顶和拱底部位有强岩爆倾向,需要做好防护措施。     

高地应力下深部岩芯饼化裂缝发展规律及机制研究

以青海共和盆地干热岩调查井的钻孔岩芯为研究对象，首先分析现场岩芯饼化侧面破坏特征、端面断口宏观和微观特征，然后基于三维离散–连续混合方法，分析不同类型应力状态下钻孔取芯过程中的裂缝扩展行为，从细观角度探究岩芯饼化形成机制。研究结果表明：(1)饼化岩芯侧面特征可分为破碎状、薄饼状、饼化状和厚饼状；端面断口形态可分为马鞍形、平面、杯状凹面和椭圆形，大多粗糙不平，具有凹凸性；(2)岩芯饼化侧重于拉伸破坏机制；(3)高水平主应力是岩芯饼化发生的主要因素，诱发带状分布裂缝并产生杯状凹面断口；垂向应力抑制裂缝带状分布，水平应力较小时，足够大的垂向应力可使岩芯产生平面断口；(4)在应力重分布作用下，岩芯根部产生大量裂缝，岩芯上部的已有裂缝也会继续发展，加剧岩芯饼化程度；(5)水平主应力差异性既影响裂缝的分布状态，也影响裂缝走向，裂缝走向倾向于沿最大水平主应力方向，且聚集分布在岩芯边缘附近区域；(6)裂缝走向分布受应力水平的影响，沿最大水平主应力走向裂缝比例随最小水平主应力的减小或垂向应力的增大而增加，较大的最大水平主应力会削弱裂缝走向对应力方向的指示作用。     

不同洞形与加载方式对深部岩体分区破裂影响的模型试验研究

 随着地下工程开挖深度的增加,深部洞室围岩将产生不同于浅部洞室的分区破裂现象。为研究分区破裂的破坏机制和形成机制,以淮南矿区丁集煤矿高地应力深部巷道为工程背景,采用模型相似材料和数控真三维加载模型试验系统,开展圆洞、城门洞和马蹄形洞在沿洞轴向和垂直洞轴向加载条件下的洞室开挖真三维地质力学模型试验。模型试验研究表明：(1) 初始最大主应力平行于洞轴方向且其量值超过1.5倍围岩单轴抗压强度是深部岩体产生分区破裂的重要条件;(2) 洞室分区破裂的范围与洞形和洞室尺寸有关,洞室尺寸越大,分区破裂范围越大。模型试验结果有效揭示分区破裂的形成条件和破坏规律,为深入研究高地应力深部岩体的非线性变形特征与破坏机制奠定了坚实的试验基础。     

深埋岩体隧道开挖面三维非线性挤出效应分析

深埋高应力覆存环境下的岩体隧道处于复杂真三维应力状态,围岩力学行为与浅埋隧道存在明显差异,传统的强度理论与分析方法已经不能满足深埋隧道围岩稳定性精细化分析的要求;开挖面三维空间效应问题在浅埋隧道设计与施工常被简化甚至忽略,但其对深埋隧道围岩稳定和施工安全的影响必须予以重视。为此,以西部典型深埋隧道为对象,建立精细的三维地质模型和隧道数值分析模型,采用三维GZZ强度准则对不同埋深隧道的开挖过程进行三维有限元模拟,分析开挖面三维非线性空间效应,揭示开挖过程复杂应力路径及非线性挤出变形的时效演变机制。研究表明:(1)采用现场三维照相、激光扫描和三维重构等数字化采集技术,可快速、自动、准确获取岩体GSI等GZZ准则输入参数,实现了深埋隧道围岩稳定性三维分析;(2)深埋隧道开挖面挤出变形比浅埋隧道更加显著,挤出变形主要为塑性变形,变形量随埋深呈现出抛物线变化,且开挖面进入塑性屈服状态早于洞周围岩;(3)开挖面前后3～4 m范围内具有显著的三维空间效应,3个主应力均发生了巨大变化,且伴随明显的应力主轴旋转,该现象主要由开挖面处急剧增加的切应力导致;(4)开挖面中心岩石应力水平I1远低于洞周围岩,开挖卸载导致的岩体不稳定性比处于加载状态的洞周围岩更加严重;(5)深埋隧道钢拱架扭曲变形和围岩非均匀纵向变形主要与σ_3和σ_2有关,且超过一定埋深时,σ_2的作用显著增加。     

锦屏二级水电站深埋隧洞开挖损伤区特征分析

在高应力条件下,岩体强度和应力之间的尖锐矛盾将导致围岩出现损伤,损伤是不同应力条件下围岩状态的直接体现。利用声波检测和钻孔电视对锦屏二级深埋引水隧洞的一典型断面进行全断面测试,声波测试结果显示,断面上低波速带断面形态呈不对称状,与断面应力分布也并不完全对应。在每个声波钻孔中补充钻孔电视,对破裂发育深度和围岩内部实际构造特征有了更直观的认识。为对损伤区特性进行更加准确的描述,利用FLAC3D计算在洞周不同位置处关键点的应力路径,对关键点的应力状态进行分析。在UDEC泰森多边形离散的基础上增加对于节理的描述,分析节理对损伤区分布的影响,模拟结果表明,节理的存在改变了隧洞开挖后洞周的应力分布,从而导致围岩破损和破坏区域的差异。最后,借助于颗粒流程序PFC对隧洞开挖后围岩的损伤区进行模拟,所揭示出的损伤局部化特征和损伤区、破裂区分布特征与现场实际具有很好的一致性。     

高温高压下花岗岩中钻孔围岩的热物理及力学特性试验研究

 采用自主研制的20 MN伺服控制高温高压岩体三轴试验机,对f 200 mm×400mm的花岗岩体内含f 40 mm的钻孔在600 ℃以内及6 000 m埋深静水压力下钻孔围岩的热弹性变形进行深入的试验研究。根据热弹性变形试验结果反演计算出高温高压下钻孔围岩的热物理及力学特性参数,并对钻孔围岩的热物理及力学参数进行认真细致的分析。研究结果表明：(1) 高温不同埋深应力下钻孔围岩的热变形可分为3个阶段：低温热变形微弱阶段,中高温热变形快速增长阶段,高温热变形平稳阶段,且埋深(即应力大小)对于钻孔围岩的热变形具有明显的影响;(2) 高温高压下含有钻孔的花岗岩体以剪切方式破坏,花岗岩体在经历500 ℃～600 ℃的高温仍呈现出脆性特征,岩体破坏的条件为6 000 m埋深静水压力,600 ℃左右;(3) 高温下钻孔围岩的弹性模量随温度的升高呈负指数规律减小;(4) 高温下钻孔围岩的泊松比随温度的升高总体呈增大的趋势;(5) 高温不同埋深应力下钻孔围岩的热膨胀系数不同,埋深对钻孔围岩的热膨胀系数具有很大影响。研究结果可为高温岩体地热开发深钻施工及钻井围岩稳定性维护提供理论依据与技术储备。     

卸荷取芯岩柱间隔断裂机制分析与岩芯饼化机制

岩柱的间隔断裂(或岩芯饼化)是由拉应力导致的岩石破坏现象。通过将该类现象统一归结为岩石的自持续断裂,从理论上分析该类现象的发生机制;建立量纲一的饼化厚度d/D与量纲一的初始应力s0/st之间的关系式,且对表达式中各参数的物理意义进行了理论上的解释;并结合已有研究中的实测数据对所求表达式的适用性进行了初步检验。结果表明,当圆柱形花岗岩试样的初始应力为花岗岩抗拉强度的6.85倍及以上时,以一定的速率对岩柱轴向进行卸荷,将导致岩柱内产生与卸荷方向相反的断裂波和与卸荷方向垂直的断裂面;断裂波的波长与饼化厚度直接相关,初始应力越高、卸荷速率越快,断裂波的波长越短、饼化厚度也越小;断裂波随传播而衰减,由于沿岩柱轴向短距离内断裂波的波长变化并不明显,从而造成了饼化厚度在一定范围内依然看似相等的现象。从理论上讲,只要知道了岩柱的初始应力和残余应力,便可直接算出产生饼化现象的量纲一的应力阈值,而如何测量残余应力则有待进一步的研究。     

浅埋偏压软岩隧道数值模拟及方案比选

围岩的应力应变是分析隧道开挖中围岩稳定性的重要依据。文中通过大型有限元软件ANSYS,计算了不同埋深、不同坡度角、不同覆盖层厚度条件下,马鞍形浅埋偏压软岩隧道围岩的应力应变,分析其规律进行方案比选,确定了此类隧道比较合理的设计方案。分析结果表明:以2倍洞径的埋深作为偏压隧道深埋或浅埋的判断依据是合理的;隧道内壁各点的应力应变规律可以为隧道开挖中支护结构参数的选取提供参考。     

岩体动静参数测试及其关系研究

依据钻孔岩芯、钻孔电视录像可直观定性判定岩体质量,实施声波测井可准确测定岩体纵波速度,根据可靠的声波测试成果结合对岩芯、孔内录像的直观分析可判断易受多种因素影响的孔内岩体静弹模测试值的可靠度,从而去伪存真,建立有效的动、静参数相关式。对某工程钻孔原位测试所获岩体动、静参数的空间分布特征和相关性等的研究结果表明,岩体的动、静参数与岩体的岩性、结构面发育情况和风化程度等密切相关,其相关式不仅可将大量的声波测试成果转化为静弹性模量,为建基面确定、坝基岩体质量分级、稳定性评价及静弹模分区、应力分析和三维设计提供实用指标,还可供类似地质区域借鉴。     

岩芯成饼单元安全度三维数值试验及地应力反馈分析

岩芯饼化是深部及高应力地质环境特有的现象,通过岩芯饼化来推测地应力有重要的理论和工程意义。首先模拟不同应力对岩芯饼化厚度的影响,并综合三维弹性数值模拟和带拉伸的莫尔-库仑强度准则,提出岩芯饼化"潜在破坏面"单元安全度的计算方法,研究不同地应力对岩芯潜在破坏面的影响。模拟结果表明,在水平应力一定的条件下,随着竖向地应力的变化,竖直岩芯潜在破坏面单元安全度、剪切破坏和拉伸破坏模式都发生变化。单元安全度方法能够直观显示岩芯潜在破坏面安全度的分布,并量化地反映地应力与岩芯破坏模式之间的对应关系。采用单元安全度方法,对美国Williston盆地Bakken页岩储层的前期勘测地应力和观测岩芯分析,取得相一致的计算结果,说明利用该方法分析与识别地应力是可行的。     

Determination of three dimensional in situ stress from core discing based on analysis of principal tensile stress

To determine all of the components of in situ stress from core discing, both the directions and magnitudes of the principal in situ stresses must be determined for a disc of a given thickness. In this study, we analyzed the direction and magnitude of tensile stress below an HQ core stub for 11 core lengths using stress conditions under which core discing is likely to occur. First, based on an analysis of the direction of tensile stress below the core stub, we propose a method for determining directions of in situ stress from the height distribution at the periphery of the end surface of a disc. This method can be used with a disc of any thickness. Next, based on an analysis of the magnitude of tensile stress in the central part of a core, we propose a linear criterion for core discing, which can be applied to a core of any length. This criterion was in good agreement with an empirical formula obtained previously in laboratory experiments. By combining information on the direction of in situ stress and the linear criterion for core discing, we propose a method for determining all of the components of in situ stress from core discing under the assumption that vertical stress is given by the overburden stress. Finally, these methods were applied to discs obtained from a field where hydraulic fracturing was performed to measure horizontal stresses. The results showed that the azimuths of the principal stresses estimated from core discing were consistent with those of the principal horizontal stresses determined by hydraulic fracturing and that while the magnitudes of the principal horizontal stresses estimated from core discing showed a large scatter, they were similar to those determined by hydraulic fracturing.     

A strain-softening numerical model of core discing and damage

There has been a debate in the rock mechanics community regarding the mechanisms causing what is known as core discing. This phenomenon occurs when diamond drill cores are retrieved from rock masses in which high in situ stresses relative to rock strength are present. The interest in that phenomenon is to use it to estimate the in situ stresses from the shape and frequency of the failures along the core axis. In the present paper we argue that discing is only an indicator of high stresses, and that estimating in situ stresses from fracture observations is much too inaccurate.As most of the literature found on the subject has tackled the problem using elastic numerical models, it is shown that the stress distribution in the core being formed obtained from such models does not exist once failure has been reached. Numerical analyses using Flac^2D with an elasto-plastic cohesion softening friction hardening model show that for a given stress state, discing or core damage may involve tensile failure, a combination of shear and tensile failure, or only shear failure, depending on the stress state and ratio of tensile to shear strength of the rock. The numerical model used is validated by replicating core discing observed under controlled laboratory conditions. Parametric analyses involving changes in mesh density, deformability parameters, dilatancy, drill bit pressure, drilling fluid pressure and applied stress states are also performed. Finally, it is shown that drilling-induced core discing or damage store important residual stresses in the core which may explain why recovered core samples tend to show a deterioration of their mechanical properties with time.     

In situ rock stress determinations in deep boreholes at the Underground Research Laboratory

Deep in situ rock stress determinations are typically conducted using hydraulic fracturing in an existing borehole or by applying specialized overcoring techniques that require the installation of a strain-monitoring instrument in a central pilot borehole that is subsequently overcored. The unusually high horizontal to vertical stress ratio at depth in Canada's Underground Research Laboratory (URL) results in problems in the application of either of these methods. At 420-m depth, the rock in the URL is massive granite with virtually no fractures. The maximum principal stress is approximately 60 MPa and is sub-horizontal. The minimum principal stress is only 11 MPa, resulting in a horizontal to vertical stress ratio of almost 6 to 1. In these conditions, hydraulic fracturing is unable to produce axial fractures in sub-vertical boreholes or is unable to fracture the rock at all. Overcoring methods requiring a pilot hole are not feasible because of persistent core discing. To resolve these problems, AECL has applied a remote data logger/signal conditioner to the design of a Deep Doorstopper Gauge System (DDGS) for use to 1000-m depth. The DDGS is much less susceptible to core discing than most other overcoring methods because it does not require a pilot hole. This paper describes the operating principle of the DDGS and the results of field testing at the URL, as well as issues related to its use and the interpretation of results.     

The in situ stress states associated with core discing estimated by analysis of principal tensile stress

Core discing occurs due to tensile stress induced by boring within or below a core stub when the minimum principal stress is nearly in the same direction as the core axis. To determine the effects of the core length on the magnitude and direction of tensile principal stress, a finite element analysis was carried out for an HQ core of different lengths for 77 in situ stress conditions. According to the minimum value and the mean inclination relative to the core axis for ‘the maximum semi-axial tensile stresses’, 30 in situ stress conditions were identified as being stress conditions under which core discing is likely to occur, and conditions necessary for in situ stress were proposed. The critical tensile stress, which is the tensile stress that can produce a tensile fracture that propagates throughout a cross-section, was analyzed for these stress conditions and a new criterion for core discing, which can be applied to a core of any length, was proposed. The stress conditions estimated by the criterion were consistent with previous experimental results for a long core and for thin discs. According to the criterion, the relationship between the core length and the in situ stress necessary for core discing was examined. Our analysis showed that the stress field can be divided into three regions and that core discing of short length mostly occurs at great depth. The average relationship between the core length and the disc thickness was determined by assuming that the position of a fracture is given by the mean position of ‘the maximum semi-axial tensile stresses’. Our theoretical estimates reproduced previous experimental results regarding the effects of stress magnitude on the thickness of the disc. Thus, the present proposed criterion can be used to estimate the stress condition for core discing with a given disc thickness.     

Mechanism of core discing in the relaxation zone around an underground opening under high in situ stresses

Bulletin of Engineering Geology and the Environment - Core discing is a brittle rock failure during borehole drilling in intact rock subjected to high in situ stresses. This failure mode has been...     

电阻率法在深部巷道分区破裂探测中的应用

 淮南丁集煤矿西部采区南运输大巷埋深达955 m,已显现出深部开挖的特征。为研究深部巷道围岩破裂情况,在该巷道选取2个监测断面(宽5.0 m,高3.8 m),每个断面布置5个钻孔,测量围岩电阻率沿钻孔深度的变化。电阻率是岩石的重要电性参数,岩体的破碎程度对电阻率的影响较大,一般有裂纹的地方,电阻率产生突变。采用ResiTest–4000电阻率测试仪和研制的孔内探头,对钻孔内岩体电阻率进行测试。岩石破碎区电阻率基准值根据每一钻孔电阻率平均值(剔除特异点)确定,大于基准值的为破碎区。根据测试结果,绘制巷道围岩分区破裂图,与钻孔电视观测结果较吻合。结果表明,该巷道围岩有4个破裂分区;破裂分区带的半径与巷道半径基本呈线性关系;巷道周边破裂区宽度最大,平均达到3.12 m,依次分区破裂带的宽度有递减趋势。     

高分辨率声波钻孔电视及其在核废物地质处置深部岩体研究中的应用

声波钻孔电视测量是最早能获得钻孔直观图像的地球物理测井方法,新一代声波钻孔电视探头采用了先进的声波束聚焦技术、数字记录技术和数字化数据处理技术,具有精度高、分辨率高和测井速度快等特点。在高放废物深地质处置库场址预选和场址评价研究中,深部岩体的节理裂隙特征参数是场址性能评价的基础数据之一。利用高分辨率声波钻孔电视测井技术,可获得钻孔孔壁直观图像和孔壁各点的三维磁坐标和倾斜坐标参数。利用这些参数,计算钻孔的偏移量、进行钻孔裂隙统计分析、岩芯定向排列和评价局部范围内深部岩体节理裂隙和断裂构造的延伸特征等。声波钻孔电视是一种方便、快捷、精度高的深部岩体裂隙测量工具。     

深部矿区煤岩体强度测试与分析

 基于钻孔触探法原理,开发出小孔径井下煤岩体强度测定装置。在实验室对34个煤岩样品进行试验：在煤岩块上钻取标准试件,测量单轴抗压强度;在留下的钻孔中,用煤岩体强度测定装置测定探针临界载荷,分析探针破坏钻孔壁煤岩的形态;然后确定煤岩块单轴抗压强度与探针临界载荷的关系。试验表明,探针破坏钻孔壁煤岩的形状、深度及范围与煤岩性质密切相关。煤岩体强度越高,破坏范围、侵入深度越小,破坏形状越规则。结合井下实测数据,回归得出描述探针临界载荷与煤岩体单轴抗压强度关系的公式。同时,分析临界载荷的离散性及控制措施,讨论结构面对煤岩体强度的影响及测试分析方法,并在典型的深部矿区——新汶矿区进行井下原位测试。新汶矿区巷道顶板不同岩性的岩层强度相差很大,不同矿井的岩层强度也存在明显差别。煤层强度由于煤帮出现破碎区、煤层性质不均匀、煤层结构面分布不均匀等原因变化较大,出现明显的波动。基于井下煤岩体强度实测数据的巷道支护设计,符合井下环境中的煤岩体条件,设计的合理性与可靠性显著提高,巷道围岩稳定性与支护状况得到明显改善。最后分析钻孔触探法存在的问题,并提出改进建议。     

深孔空心包体法地应力测量仪及其应用实例

 介绍用于深孔地应力测量的深孔空心包体法地应力测量仪的组成结构及辅助工具,并给出该仪器的应用实例。传统的应力解除仪器只适用于浅孔,不适用于深孔。深孔空心包体地应力测量仪将原来井上应变仪、井下空心包体应变计、电子罗盘等集成于一体,制成无电缆微型探头,测量时将微型探头安装到孔内预定位置进行测量。用该仪器在上海市浦东地震监测中心张江观测孔进行实际应用,获得365和385 m深处的三维地应力实测数据,测量结果显示张江观测孔最大主应力近水平,方向为近EW向,最大主应力值为8. 5～9.0 MPa,最小主应力近于铅直,大小与上覆岩层压力相近。实测表明深孔空心包体地应力测量仪适用于深孔三维地应力测量。