CHARACTERISTICS OF TECTONIC STRESS FIELD OF THE XIAOWAN RESERVOIR BEFORE AND AFTER THE IMPOUNDMENT

Using the seismic waveform data of Xiaowan seismic network and Yunnan seismic network, we determined the focal mechanisms of 36 earthquakes(M_L ≥ 3.0)from Jun. 2005 to Dec. 2008 and 51 earthquakes(M_L ≥ 2.5)from Jan. 2009 to Dec. 2015 by generalized polarity and amplitude technique. We inverted tectonic stress field of the Xiaowan reservoir before impounding, using the focal mechanisms of 36 earthquakes(M_L ≥ 3.0)from Jun. 2005 to Dec. 2008 and CAP solutions of 58 earthquakes(M_L ≥ 4.0)collected and the solutions in the Global Centroid Moment Tensor(GCMT)catalog; We inverted local stress field of the reservoir-triggered earthquake clustering area, using 51 earthquakes(M_L ≥ 2.5)from Jan. 2009 to Dec. 2015. Focal mechanisms statistics show that, the Weixi-Qiaohou Fault is the seismic fault. Focal mechanisms were strike-slip type in initial stage, but normal fault type in later stage. Focal depths statistics of 51 earthquakes(M_L ≥ 2.5)show that, the average value of focal depths in period Ⅰ, period Ⅱ and period Ⅲ are 8.2km, 7.3km and 7.8km respectively and the standard deviations are 4.3km, 3.5km and 6.0km respectively. The average value of focal depths is basically stable in different period, only the standard deviation is slightly different. Therefore, there is not positive connection between focal depth and deviation of focal mechanisms. What's more, there are 2 earthquakes(number 46 and number 47 in Fig.5 and Table 3)with almost the same magnitude, epicenter and focal depth, but they have different faulting types as normal and strike-slip. The focal mechanism of event No.46 is strike:302°, dip:40° and rake:-97° for plane Ⅰ, however, the focal mechanism of event No.47 is strike:292°, dip:82° and rake:140° for plane Ⅰ. Likewise, earthquake of number 3 and number 18 have similar characteristic. Therefore, the obvious focal mechanism difference of similar earthquake pair indicates the complexity of Weixi-Qiaohou Fault. Considering the quiet-active character of reservoir-triggered earthquakes, we discussed the change of local stress field in different period. The σ₁ of tectonic stress field was in the near-south direction, with a dip angle of 14° before the impoundment, however, the direction of σ₁ of local stress field changed continuously, with the dip angle getting larger after the impoundment. The direction of σ₁ of local stress field of reservoir-triggered earthquake clustering area is close to the strike of Weixi-Qiaohou Fault, and reservoir impoundment increased the shear stress in the fault, so the weakening of fault was beneficial to trigger earthquakes. Comprehensive analysis suggests that fluid permeation and pore pressure diffusion caused by the water impounding, and the weakening of fault caused by local stress field are the key factors to trigger earthquake in the Xiaowan reservoir.     

豫北地区视应力时空变化特征

利用2008年以来豫北地区49次M_L 2.5以上地震资料,通过Brune模型,结合遗传算法,反演计算得到豫北地区地震视应力范围为0.029 9 MPa—0.642 MPa,平均视应力为0.174 8 MPa,分析该区视应力时空变化特征,讨论震级和能量的关系,结果表明,豫北地区目前处于应力积累阶段,应力高值集中在聊兰断裂濮阳—鄄城段范县地区。     

呼伦湖水位、盐度变化(1961-2002年)

为重建水文资料缺乏的呼伦湖流域的水文、水质序列,本研究基于长期的气象观测记录,采用彭曼公式估计了湖泊的水面蒸发,并建立一个两参数月水量平衡模型模拟湖周的入流,通过水量平衡计算．建立了42年(1961-2002)的呼伦湖区水量变化序列,并模拟了湖泊月水量、水位、含盐度的变化．模拟的水位、含盐度变化趋势与实际比较接近,模拟精度较好,其误差在可以接受范围内.所重建的42年呼伦湖区水文、含盐度序列,可为该区域的水资源评价管理、开发利用提供科学依据.     

夏季季风转换期间南沙群岛海域的温盐分布特征

应用2002年5月9-22日南沙群岛海域的水文观测资料,分析了季风转换期间南沙群岛海域的水文状况,并对比了调查海域不同区域的温盐分布特征。结果表明,夏季风转换期间海洋表层温度较高,基本在30℃以上,垂直结构上温度层结比较稳定。东北一西南走向界线两侧海域的温盐状况有明显的差异：西北部海域温度较高,垂向上有逆盐层现象;东南部海域温度相对较低,盐度的垂向结构上显示有双跃层现象,两海区水文特征的差异与季风结构和局地环流有密切的关系。从连续站温盐的时空剖面图可以看到,跃层以下,不规则的半日周期内潮对温盐分布有显著的影响。     

Simulation of Stress Distribution around Tunnels and Interaction between Tunnels Using an Elasto–plastic Model

This article presents a computer simulation of stress distribution around tunnels and interaction between tunnels using an elasto-plastic model. A finite element method using ANSYS software has been used for the analyses of one and two tunnels at different overburden depths with different separating distances between the tunnels. The results of numerical analyses indicate that stress distribution and stress concentration around the tunnels vary with the overburden depths. It is found that the coefficients of stress concentration for elasto-plastic medium are smaller than those for elastic one by 1.9%. Furthermore, the interaction between the two tunnels rapidly decreases with the increase of separation distance between them. In addition, for quantitatively describing the interaction between the two tunnels, a critical separation distance is introduced. The critical separation distances between the two tunnels at different overburden depths are 8 m, 12 m, and 14 m respectively. This fact is very important and essential for the design of mining tunnels and to ensure safety in tunnel engineering.     

基于声发射的岩石破裂应力场动态反演

为获取岩石在破裂过程中的动态应力场,首先进行室内物理试验获取声发射数据,在此基础上基于能量耗散原理,利用实测声发射信息建立起岩石细观损伤表征方程,并通过FLAC3D内嵌FISH语言进行二次开发,自动搜索声发射损伤影响范围内岩石单元并对其力学参数进行弱化,计算得到较为真实的岩石破裂动态应力场。结果表明,计算得到的岩石破裂应力场与实测结果有良好的对应关系,验证了岩石细观损伤表征方法的合理性。同时,获得的岩石动态破裂应力场不仅可以从岩石力学的角度解释破坏产生的原因,也对预测岩石下一步破坏具有指导意义。但这种方法仍具有一定的局限性,需要进一步的探索研究。     

应力波斜入射对岩体节理端部的影响

岩体节理在受到应力波扰动时端部受力会发生不同程度的连续性变化,为具体分析这种动态变化与应力波入射角度之间的关系,引用岩石非线性法向本构关系与线性切向本构关系的组合模型以及相应的在P波斜入射节理的应力波传播方程,结合岩石断裂力学中Ⅰ、Ⅱ型裂纹尖端应力和位移场的相关理论,得出组合形式下的节理端部应力场和位移场随质点振速变化的计算公式。通过不同入射角的模拟脉冲信号作用对端部应力位移的变化分析：应力波斜入射节理导致节理端部上、下两侧应力与位移场非对称分布,随着入射波质点振速的增减变化,应力集中位置会出现变化;计算并整理模型中节理端部上、下两侧0.005 m位置的数据,节理法向刚度由入射波质点振速带来的变化直接影响到应力波的透射与反射,进而会导致端部的应力与位移出现滞后效应;节理端部横向位移值与入射角度并非单调变化,而竖向位移会随着入射角的增大呈现下降趋势。     

定向剪切应力路径下冻结黏土变形特性试验

为研究复杂应力路径下冻土的强度与变形特征,采用冻土空心圆柱仪（FHCA-300）对不同负温状态下的饱和冻结黏土开展定向剪切试验,基于不同剪切方向下冻结黏土的轴向和扭剪分量的应力-应变关系,探讨土样的剪切变形特征、各向异性属性以及剪切带的演变规律,并考察温度、大主应力方向角、平均主应力以及中主应力系数等因素对冻结黏土强度的影响。结果表明：平均主应力p值对冻结黏土的应力-应变关系影响显著,尤其是p=4.5 MPa时具有较高的剪切强度,且该值可能为压融临界p值;大主应力方向变化会诱发冻结黏土的各向异性,随着大主应力方向角的增加,冻结黏土剪切强度逐渐降低,并有明显的剪切带产生;中主应力系数的增加使得轴向强度有逐渐降低的趋势,但对剪切强度影响不明显;随着温度的降低,冻结黏土强度逐渐增大,试样发生脆性破坏并出现剪切破裂面,其剪切强度主要取决于冰颗粒和土颗粒的胶结力。     

岩石空心圆柱扭剪仪研制的重点问题及研究进展

随着深部岩体工程的发展,开挖卸荷导致的应力主轴旋转问题引起了广泛关注。首先,分析总结了岩体工程中存在的应力主轴旋转问题及其对岩体工程稳定性的影响;其次,介绍了土体空心圆柱扭剪仪的发展现状及其特点,从荷载的施加方式、试样尺寸两个方面指出了研制岩石空心圆柱扭剪仪的关键问题,并与土体空心圆柱扭剪仪进行了对比,提出了新的扭矩施加技术和合理的岩石空心圆柱试样尺寸;最后,总结概括了考虑应力主轴旋转的土体本构模型的研究成果,并对考虑应力主轴旋转的岩石本构关系的研究进行了展望。该工作将对岩石空心圆柱扭剪仪的研制和相关理论的研究提供一定的参考和指导。     

刚性桩复合地基中性面深度与桩土应力比计算

刚性桩复合地基中性面深度及桩土应力比的简化计算主要基于桩侧摩阻力线性分布假设,当桩身较长时,桩端侧摩阻力的计算值会远大于实际值,致使中性面深度及桩土应力比的计算结果与实际差别较大,故有必要对线性分布模式予以修正。据此将桩侧摩阻力分布简化为分段线性模式,考虑负摩阻力作用及桩上、下刺入变形,根据褥垫层-桩-土变形协调关系推导了刚性桩复合地基中性面深度、桩顶面桩土应力比、中性面桩土应力比计算公式。最后通过模型试验与工程实例验证,计算值与实测值吻合较好。