Preliminary Results of In-situ Stress Measurements along the Longmenshan Fault Zone after the Wenchuan M_s 8.0 Earthquake

Four months after the Wenchuan Ms 8 earthquake in western Sichuan, China, in situ stress measurements were carried out along the Longmenshan fault zone with the purpose of obtaining stress parameters for earthquake hazard assessment. In-situ stresses were measured in three new boreholes by using overcoring with the piezomagnetic stress gauges for shallow depths and hydraulic fracturing for lower depths. The maximum horizontal stress in shallow depths （-20 m） is about 4.3 MPa, oriented N19°E, in the epicenter area at Yingxiu Town, about 9.7 MPa, oriented N51°W, at Baoxing County in the southwestern Longmenshan range, and about 2.6 MPa, oriented N39°E, near Kangding in the southernmost zone of the Longmenshan range. Hydraulic fracturing at borehole depths from 100 to 400 m shows a tendency towards increasing stress with depth. A comparison with the results measured before the Wenchuan earthquake along the Longmenshan zone and in the Tibetan Plateau demonstrates that the stress level remains relatively high in the southwestern segment of the Longmenshan range, and is still moderate in the epicenter zone. These results provide a key appraisal for future assessment of earthquake hazards of the Longmenshan fault zone and the aftershock occurrences of the Wenchuan earthquake.

Advance of in-situ stress measurement in China

In-situ stress is an essential parameter for design and construction of most engineering projects that involve excavation in rocks. Progress in in-situ stress measurement from the 1950s in China is briefly introduced. Stress relief by overcoring technique and hydraulic fracturing： technique are the two main techniques for in-situ stress measurement in China at present. To make them suitable for application at great depth and to increase their measuring reliability and accuracy, a series of techniques have been developed. Applications and achievements of in-situ stress measurement in Chinese rock engineering, including mining, geotechnical and hydropower engineering, and earthquake prediction, are introduced. Suggestions for further development of in-situ stress measurement are also proposed.

New development of hydraulic fracturing technique for in-situ stress measurement at great depth of mines

In-situ stress measurement using the hydraulic fracturing technique was made at Wanfu Coal Mine in Shandong Province, China. To solve problems caused by great measuring depth and extra thick overburden soil layers in the mine, a series of improved techniques were developed for the traditional hydraulic fracturing technique and equipment to increase their pressure-enduring ability and to ensure safe and flexible removal of the sealing packers with other experimental apparatus. Successful in-situ stress measurement at 37 points within 7 boreholes, which were mostly over 1000 m deep, was completed. Through the measurement, detailed information of in-situ stress state has been provided for mining design of the mine. The improved hydraulic fracturing technique and equipment also provide reliable tools for in-situ stress measurement at great depth of other mines.

Analysis on method for effective in-situ stress measurement in hot dry rock reservoir

With the rapid increase of energy demand and the increasingly highlighted environmental problems, clean, safe and widely distributed geothermal resources have become a hot spot for renewable resources development. The state of in-situ stress is a major control parameter for multiple links including well location, fracture inspiration and reservoir assessment, so how to determine the accurate state of in-situ stress in the deep thermal reservoir becomes a core problem drawing widely attention and urgent to be solved. Based on features of hot dry rock reservoir in terms of temperature and pressure and the comparison analysis, this article proposes the method of Anelastic Strain Recovery(ASR) as an effective method for determining the state of in-situ stress in the area with HDR resources distributed and explains the availability of ASR method by application examples.

Techniques for in-situ stress measurement at great depth

Reliable information of in--situ stress state is necessary for the design andconstruction of most important rock projects. As most rock projects are getting deeper and deeper,traditional techniques of in--situ stress measurement are not very suitable. The current techniquesof in--situ stress measurement and their insufficiency for use at great depth are analyzed. Somebasic ideas of the development of new techniques and the improvement of current techniques for useat great depth are provided.

Influence caused by initial tectonic stress on relative measurement of borehole groundstresinviscoelasticmedia

In this paper, we discussed the influence of the initial tectonic stress on the relative measurement value of the ground stress when a long cylindrical straingauge is used in the viscoelastic media. We simulate the whole system consisting of the stratum, cement ring and the straingauge cylinder with a three layer medium model and obtain the solution of radial displacement on the inner wall of the straingauge under the action of initial tectonic stress by using the correspondence principle. The result showes: 1) The influence of initial stress making of the radial displacement on the inner wall of the straingauge increases gradually with time at a gradually reduced rate and tends to a certain value eventually; 2) The principal stress with a magnitude of [C 0+C 1 exp (- ω 1t)+ C 2 exp(- ω 2t )] σ i0 in the opposite direction of initial stress will be superimposed to the measuring result of stress change without considering the initial stress. where C 0, C 1, C 2 and ω 1, ω 2 are related only to the mechanical property of the stratum, σ i0 (i=1, 2) are the two principal components of the initial tectonic stress. It is pointed out that the initial tectonic stress affects obviously on the measurement of the relative change of ground stress in the cases of soft strata and high initial stress.

Reliability analysis of in-situ stress measurement using circumferential velocity anisotropy

In-situ stress measurement for deep reservoir formation is difficult in terms of security, reliability and technique. Acoustic velocity anisotropy test is a basic method for stress measurement of rock cores, which is based on the distribution of acoustic velocity in different directions around rock cores. The heterogeneity of core samples, such as fractures and gravel contained, can also lead to wave velocity anisotropy. Therefore, the corresponding reliability evaluation method is established to exclude some other anisotropy factors caused by non-tectonic stresses. In this paper, the reliability of testing results is evaluated from three aspects, i.e. phase difference, anisotropy index and waveform, to remove the factors caused by non-tectonic stresses.

Analytical investigation for stress measurement with the rheological stress recovery method in deep soft rock

Due to the difficulty and weakness of current stress measurement methods in deep soft rock, a new rheological stress recovery method of the determination of the three-dimensional(3D) stress tensor is proposed. It is supposed that rock stresses will recovery gradually with time and can be measured by embedding transducers into the borehole. In order to explore the applicability and accuracy of this method, analytical solutions are developed for stress measurement with the rheological stress recovery method in a viscoelastic surrounding rock, the rheological properties of which are depicted as both the Burger's model and a 3-parameter solid model. In such conditions, explicit analytical expressions for predicting time-dependent pressures on the transducer are derived. A parametric analysis is then adopted to investigate the influences of the grout solidification time and the mechanical properties of the grout layer. The results indicate that this method is suitable for stress measurement in deep soft rock, the characteristics of which are soft, fractured and subjected to high geo-stress.

山东龙郓煤业10·20冲击地压事故区域应力背景与防控研究

矿山巷道、交通隧道等地下硐室围岩稳定与岩体所处区域地应力环境息息相关。分析区域深部地应力与地下硐室走向、形状等因素的关系,有助于提前规避硐室开挖风险。文章以山东龙郓煤业10·20冲击地压事故为背景,通过地应力测量与监测工作,初步揭示了山东西部地壳浅表层现今地应力环境,结合龙郓煤业矿区附近现今地应力场特征,探讨此次冲击地压事故产生的区域应力背景,并从地应力角度提出相应的防控建议。研究结果表明:测量深度范围内主应力大小总体上与深度成正比线性关系,最大水平主应力值为3.48~20.76 MPa,随深度增加梯度为0.0182 MPa/m;最小水平主应力值为3.44~14.95 MPa,随深度增加梯度为0.0130 MPa/m;区内最大水平主应力方位为北东43°~89°,平均方位为北东75°;地壳浅表层构造作用以水平构造作用为主,但随着深度的增加,逐渐向垂直构造作用转变;龙郓煤业10·20冲击地压事故的诱发机制主要是垂向应力大于水平主应力,现今处于拉张应力环境,尤其是巷道走向平行于最大水平主应力方向;建议龙郓煤业巷道轴线与最大水平主应力方向的夹角为60°~90°,同时巷道顶板可以采用拱形顶板,确保巷道岩体稳定。

基于现场监控量测的高地应力隧道支护方案研究

为研究高地应力状态下隧道施工围岩稳定情况,优化支护设计方案,以段家坪隧道为研究对象。根据段家坪隧道地质情况设计支护方案,进行现场试验。借助监控量测技术和FLAC3D数值模拟分析验证支护方案的可行性。研究得出高地应力隧道围岩变形分析结论及建议,为高地应力隧道施工选择合理的施工方法,制定切实可行的支护方案提供参考和依据。

水载预压下复合地基沉降及桩土受力特性分析

以广东省某高速公路T3标段为研究对象,部分路段采用预制管桩复合地基加固软土地基,采用水袋加载的方法进行等载预压,选取3个典型监测断面埋设传感器,监测分析地基土和桩身的沉降、土压力、桩身轴力的变化规律。结果表明:复合地基中桩体为主要的受荷体,设置褥垫层使其产生刺入变形,可有效调动地基土的承载力;荷载在地基中的传递存在滞后性,提升加载速率可有效改善荷载的传递速率,分级加载有助于达到更好的预压效果;过大的加载速率会导致土压力与桩身轴力出现波动,通过荷载重分配实现桩土共同承担上部荷载。采用水载预压联合预制桩复合地基处理软土路基,可降低30%的施工成本,且可有效提升地基工后的稳定性。

中国西部深埋隧洞地应力研究与实践

深埋隧洞高地应力岩爆、大变形灾害已经成为制约地下工程施工的重要影响因素,对地应力的研究了解深部岩体破坏机理及相应的防治对深埋隧洞的设计和施工具有实际意义。依托西安市引蓝济李引水工程高地应力测试成果结合施工期深部岩体的破坏形式进行分析研究,并提出相应的防治处理措施,可为类似工程的勘察设计施工提供经验与指导。

Principle of in-situ 3D rock stress measurement with borehole wall stress relief method and its preliminary applications to determination of in-situ rock stress orientation and magnitude in Jinping hydropower station

As a main constituent of geological body, the rock masses have distinct differences from other materials, one of which is that rock masses are initially stressed in their natural states. Hence, it is an extremely challenging and significant research project to know the present residual stress of the rock masses in the earth's crust. Although some regularities of distribution of in-situ rock stresses can be deduced, the basic means to study the state of rock stress is in-situ stress measurement. After a brief review of several measuring methods of in-situ 3D rock stress, a new one, borehole wall stress relief method (BWSRM) to determine the in-situ 3D rock stress tensor in a single drilled borehole was proposed. Based on the principle of in-situ rock stress measurement with BWSRM, an original geostress measuring instrument was designed and manufactured. Preliminary experiments for determination of in-situ stress orientation and magnitude were carried out at an experimental tunnel in Jinping Ⅱ hydropower station in China, where the buried depth of overburden was about 2430 m. The results showed that it was feasible to measure the in-situ 3D rock stresses with BWSRM presented in this paper. The BWSRM has a broad prospect for in-situ 3D rock stress measurements in practical rock engineering.

Stress rise precursor to earthquakes in the Tibetan Plateau

Earthquake prediction thus far has proven to be a very difficult task, but changes in situ stress appear to offer a viable approach for forecasting large earthquakes in Tibet and perhaps other continental regions. High stress anomalies formed along active faults before large earthquakes and disappeared soon after the earthquakes occurred in the Tibetan Plateau. Principle stress increased up to ~2 -?5 times higher than background stress to form high stress anomalies along causative faults before the Ms 8.1 West Kunlun Pass earthquake in November 2001, Ms 8.0 Wenchuan earthquake in May 2008, Ms 6.6 Nimu earthquake in October 2009, Ms 7.1 Yushu earthquake in April 2010 and the Ms 7.0 Lushan earthquake in April 2013. Stress near the epicenters rapidly increased 0.10 - 0.12 MPa over 45 days, ~8 months before the Ms 6.6 Nimu earthquake occurred. The high principle stress anomalies decreased quickly to the normal stress state in ~8 -?12 months after the Ms 8.1 West Kunlun Pass and the Ms 8.0 Wenchuan earthquakes. These high stress anomalies and their demise appear directly related to the immediate stress rise along a fault prior to the earthquakes and the release during the event. Thus, the stress rise appears to be a viable precursor in prediction of large continental earthquakes as in the Tibetan Plateau.

Determination of mining-induced stresses using diametral rock core deformations

Knowledge of ground stresses is crucial for ground control activities such as the design of underground openings,selec-tion of support systems,and analysis for stability.However,it is a known fact that far field stresses experience changes in orientation and magnitude due to the presence of geological structures and due to the excavations created by mining activi-ties.As a result,in-situ stresses around drifts,ramps,and stopes in underground mines are quite different from far field or pre-mining stresses.The purpose of this research is to develop a simple and practical methodology for determining in-situ stresses.Stress relief occurs once the rock core is drilled off.Such relief is a function of the surrounding stress field.This study uses exploration rock cores that are drilled off for the purpose of orebody definition in the underground mine.The method measures and analyzes the diametral core deformations in laboratory.Two case studies from operating underground mines are presented for demonstration.In these case studies,rock core deformations are measured with a customized test apparatus and rock samples were prepared and tested for Young's modulus and Poisson's ratio.The differential stress,namely the difference between the local principal stresses in the plane perpendicular to the core rock axis is calculated.It is shown that this methodology is useful for determining the brittle shear ratio in the rock mass,which is of primary interest to ground control studies.

Recent tectonic stress field research in Shanxi graben system

By comprehension of earthquake focal mechanism solutions and the data of in-situ stress measurements, the tectonic stress field in Shanxi region has been summarized, which indicate that the stress state in this region is different from that of its eastern surrounding regions. The next, by fitting the measured data, the boundary forces that influenced the distribution of the stress field in this region has been studied using inversion method. The inversion results showed the following messages: the effect of the boundary force between the blocks is the main determinative factor for the recent tectonic stress field in Shanxi and the regional material and its property is a secondary factor; the horizontal main stress of tectonic stress field in Shanxi region is consistent with the stretch of fault basins.

高地应力隧道软岩大变形预测分析及控制措施

针对隧道穿越高地应力软弱围岩区段时易发生的大变形灾害,为保障隧道安全顺利施工,故在施工前准确进行大变形预测并制定合理的控制措施。依托最大埋深超过1 300 m的巫镇高速笔架山隧道,通过地应力实测数据建立有限元地质模型研究工程区地应力场;综合现有研究成果,建立大变形预测标准,基于此结合工程地质条件采用有限元开挖计算与地质分析预测法对围岩大变形类型、特征、机制及程度进行预测;根据所得围岩变形特征制定科学合理的变形控制措施。研究结果表明:隧道高程处地应力场由自重应力场主导,沿线围岩应力处于高—极高应力状态;开挖断面变形以拱顶、仰拱竖向位移为主,预测其将发生应力控制型大变形,最终围岩因拱顶下沉或边墙内鼓变形发生剪切破坏;隧道沿线约56.3%区段围岩会发生大变形,K25+631~K25+909、K26+422~K26+602区段在地下水软化作用影响下,出现严重大变形风险较大;根据围岩大变形预测结果针对笔架山隧道开挖提出相应的变形控制措施。

高地应力隧道支护参数优化设计研究

依托某在建高速公路项目,开展高地应力隧道支护参数优化设计研究。对试验段隧道拱顶沉降以及水平收敛进行现场监测,结果表明:采用以工字钢为支护骨架的单层初支不能控制高地应力隧道的大变形,主要表现为拱架破坏、混凝土剥落;采用“双层初支”在一定程度上可控制大变形;增加内层支护的刚度在一定程度上减小“双层支护”的累计变形量,但无法大幅度缩短变形稳定周期;适当增加内外层初支之间的预留变形量支护效果最佳,累计变形量较小同时变形稳定周期缩短。

桑树坪煤矿地应力测试过程及分布规律

地应力是诱发煤矿安全生产事故的主要因素,亟需准确测量并掌握矿井地应力的分布特征。以桑树坪煤矿南一采区为例,介绍了应力解除法测量地应力基本原理以及测试过程——利用煤岩的发生膨胀变形特点,通过地质钻孔、安装空心包体、应力测试等步骤对矿区的2个测点进行了应力测试。测试结果表明,2个测点主应力的方向基本相同,最大主应力方位角均接近110°,且均接近于水平方向;水平主应力为垂直应力的1.07~3.39倍,说明水平应力对巷道掘进的影响具有明显的作用。采用巷道轴向与最大水平主应力平行布置方式,有利于巷道顶底板的稳定。该研究成果有助于优化桑树坪矿井等同类矿区采掘布置,达到矿井的突出危险性防治的目的,为后续矿井布置及安全开采提供借鉴。

银山矿区深部铜铅锌矿地应力测试研究

江西银山铜铅锌矿浅部资源已经开采殆尽,为保证矿山产能,逐渐向深部矿体进行规模回采,将面临深部岩体应力扰动、岩爆等关键难题。为进一步探明银山铜铅锌矿深部岩体应力分布规律,基于水压致裂测量法,结合9个地质钻孔,在银山区、九龙上天区、西山区、北山区和九区5个区域开展地应力测试分析。结果表明,九龙上天区、西山区在深部标高880 m时即进入高应力开采区域,而银山区、北山区和九区在深部标高550 m时即进入高应力开采区域,此时水平地应力明显增高,测压系数可达到1.5。研究结果可为银山铜铅锌矿深部开采岩体应力演化规律提供参考依据。