Variation with depth of the stress tensor anisotropy inferred from microfault analysis

In order to study the Pyrenean tectonic phase, a quantitative method of stress analysis using microfault measurements is used on a calcareous plateau located in southern France. The method developed here allows the determination of several tectonic events and the evaluation of (with σ σ₂ σ₃).The Pyrenean compression is seen to occur in two stages, confirming previous geological studies.A recent canyon allows the study of the variation with depth of the R ratio on a vertical cross-section (300 m). With a simple model of gravity and tectonic stresses, the vertical variation of R can be used to estimate quantitatively the Pyrenean tectonic stresses. For realistic values of the parameters in this model, the horizontal tectonic stresses are obtained in the following range: 50–200 bar for the maximum horizontal principal stress, 10–25 bar for the other horizontal stress.These results seem to be consistent with in situ stress measurements, but they are much lower than those predicted by experimental rock mechanics.     

On the estimation of scale of fluctuation in geostatistics

Describing how soil properties vary spatially is of particular importance in stochastic analyses of geotechnical problems, because spatial variability has a significant influence on local material and global geotechnical response. In particular, the scale of fluctuation θ is a key parameter in the correlation model used to represent the spatial variability of a site through a random field. It is, therefore, of fundamental importance to accurately estimate θ in order to best model the actual soil heterogeneity. In this paper, two methodologies are investigated to assess their abilities to estimate the vertical and horizontal scales of fluctuation of a particular site using in situ cone penetration test (CPT) data. The first method belongs to the family of more traditional approaches, which are based on best fitting a theoretical correlation model to available CPT data. The second method involves a new strategy which combines information from conditional random fields with the traditional approach. Both methods are applied to a case study involving the estimation of θ at three two-dimensional sections across a site and the results obtained show general agreement between the two methods, suggesting a similar level of accuracy between the new and traditional approaches. However, in order to further assess the relative accuracy of estimates provided by each method, a second numerical analysis is proposed. The results confirm the general consistency observed in the case study calculations, particularly in the vertical direction where a large amount of data are available. Interestingly, for the horizontal direction, where data are typically scarce, some additional improvement in terms of relative error is obtained with the new approach.     

Prediction of the tunnel displacement induced by laterally adjacent excavations using multivariate adaptive regression splines

Excavations may cause excessive ground movements, resulting in potential damage to laterally adjacent tunnels. The aim of this investigation is to present a simple assessment technique using a multivariate adaptive regression splines (MARS) model, which can map the nonlinear interactions between the influencing factors and the maximum horizontal deformation of tunnels. A high-quality case history in Tianjin, China, is presented to illustrate the effect of excavation on the tunnel deformation and to validate the FEM. The hypothetical data produced by the FEM provide a basis for developing the proposed MARS model. Based on the proposed model, the independent and coupled effects of the input variables (i.e. the normalized buried depth of tunnels H_t/H_e, the normalized horizontal distance between tunnels and retaining structures L_t/H_e, and the maximum horizontal displacement of retaining structures, δ_Rmax) on the tunnel response are analysed. The prediction precision and accuracy of the MARS model are validated via the artificial data and the collected case histories.

     

Seismic hazard evaluation for dams in northern Colorado, U.S.A.

A seismic hazard evaluation for three dams in the Rocky Mountains of northern Colorado is based on a study of the historical seismicity. To model earthquake occurrence as a random process utilizing a maximum likelihood method, the catalog must exhibit random space-time characteristics. This was achieved using a declustering procedure and correction for completeness of recording. On the basis of the resulting a- and b-values, probabilistic epicentral distances for a 2 × 10^–5 annual probability were calculated. For a random earthquake of magnitude M _L 6.0–6.5, this distance is 15 km. Suggested ground motion parameters were estimated using a probabilistic seismic hazard analysis. Critical peak horizontal accelerations at the dams are 0.22g if median values are assumed and 0.39g if variable attenuation and seismicity rates are taken into account. For structural analysis of the dams, synthetic acceleration time series were calculated to match the empirical response spectra. In addition, existing horizontal strong motion records from two Mammoth Lakes, California earthquakes were selected and scaled to fit the target horizontal acceleration response spectra.     

多年冻土区L型挡墙水平冻胀效应计算方法与分析

针对多年冻土区L型挡墙设计中水平冻胀力计算方法不完善的问题,基于利夫金地基模型及挡墙?土体之间的协同变形原理,分别建立了墙后有无换填土两种情况下的L型挡墙水平冻胀效应计算模型,利用叠加原理及有限差分法对所提出的计算模型进行了求解,并依托MATLAB自行编制了水?热?力耦合分析软件。结合工程实例,应用提出的L型挡墙水平冻胀效应计算方法得到的水平冻胀力值分别与现场实测值、修正土压力值、规范经验值及水?热?力耦合软件模拟值进行对比分析,结果表明：提出L型挡墙水平冻胀效应计算方法得到的水平冻胀力值与现场实测值及数值模拟值在大体趋势上吻合较好,修正土压力值、规范经验值低估了水平冻胀力对挡墙的作用;相比规范经验值及现场实测值,提出的L型挡墙水平冻胀效应计算方法得到的水平冻胀力沿墙高呈抛物线和梯形两种分布模式,更具有普遍性;通过多场耦合分析可知,所提出L型挡墙水平冻胀效应计算方法与水?热?力耦合方法得到冻胀力趋势相似,表明了计算方法的可行性,可为多年冻土区L型挡墙设计提供一定的理论支持和指导。     

Analysis of refracturing in horizontal wells: Insights from the poroelastic displacement discontinuity method

In this paper, a fully coupled 2‐dimensional poroelastic displacement discontinuity method is used to investigate the refracturing process in horizontal wells. One of the objectives of refracturing is to access new reserves by adding new hydraulic fractures in zones that were bypassed in the initial fracturing attempt. Pore pressure depletion in the vicinity of old fractures directly affects the state of stress and eventually the propagation of newly created hydraulic fractures. Thus, a poroelastic analysis is required to identify guidelines for the refracturing process, in particular to understand the extension of the pore pressure depletion, and eventually, the orientation of new as well as old fractures. We propose a fully coupled approach to model the whole process of child fracture propagation in a depleted area between 2 parent fractures in the same wellbore. This approach omits the need of using multistep workflow that is regularly used to model the process. The maximum tensile stress criterion (σ criterion) is used for hydraulic fracture propagation. The proposed method is verified using available analytical solutions for total stress and pore pressure loading modes on a line fracture in drained and undrained conditions. Then, test cases of multifractured horizontal wells are studied to calculate the time evolution of the stress and pore pressure fields around old fractures and to understand the effect of these fields on the propagation path of newly created fractures. Finally, the effect of the pore pressure depletion on the propagation path of the newly created fractures in the bypassed area of the same wellbore is studied. The results show that the depleted areas around old fractures are highly affected by the extent and severity of the stress redistribution and pore pressure depletion. It is observed that a successful creation of new fractures may only happen in certain time frames. The results of this study provide new insights on the behavior of newly created fractures in depleted zones. They also clarify the relationship between stress change and pore pressure depletion in horizontal wells.     

水平条分法下边坡稳定性分析与计算方法研究

极限平衡水平条分法较适用于成层岩土边坡,但较竖直条分法尚有不足之处。通过研究水平条分与竖直条分中条间力的关系,建立起水平与竖直条分法相对应的条间力假设条件;并针对水平条分法遇到滑动面存在弓形体时,采取水平条分和斜条分相结合的模式进行处理。在此基础上,推导出了瑞典法、简化Bishop法、简化Janbu法和Morgenster-Price（M-P）法在水平条分时的安全系数计算公式,其中,根据满足条件不同,得到了瑞典法的两种不同安全系数计算公式,并根据以往研究成果将一般简化Janbu法进行改进。当使用任意曲线和圆弧滑动面两种型式时,通过算例对比分析,验证了对水平条分法所做研究的可行性。同时,也表明了两种瑞典法得到的结果是一致的,改进的简化Janbu法在边坡稳定性分析上较一般简化Janbu法更优。     

European continuous active tectonic strain–stress map

This paper shows a new continuous strain–stress map for Europe obtained from the direct inversion of earthquake focal mechanisms calculated from the centroid tensor method. A total of 1608 focal mechanisms have been selected with several quality criteria from different catalogues (CMT Harvard, ETH, Med-Net, I.G.N. and I.A.G.) from 1973 to the present day. Values for the maximum horizontal shortening direction and brittle strain–stress regime defined by the k′ ratio (e_y/e_z, horizontal maximum/vertical strain) have been calculated following in Europe and Pannonian Basin the slip model of tri-axial deformation. The individual results including Dey and the shape of the active brittle strain ellipsoid have been interpolated to a final 15′ regular grid taking into account the relationship between the tectonic horizontal strain–stress value and the vertical load. Both continuous strain regime and maximum horizontal shortening (Dey) maps show a good correlation with the primary tectonic forces generated along the plate boundaries, plate kinematics and also some local perturbations related with main crustal heterogeneities and topography, as well as significant spatial variations in integrated crustal strength.     

含水层层状非均质对地下水流系统的影响

区域尺度上含水层非均质具有复杂的结构性和随机性,难以准确刻画,造成非均质对区域地下水流系统的影响机制研究不够深入。本文以鄂尔多斯盆地白垩系地下水流系统为研究实例,选择典型剖面,采用剖面二维随机数值模拟方法,通过对比不同非均质刻画方法下地下水流场的变化,探讨含水层层状非均质对地下水流系统的影响机制。结果显示,均质条件下模型各向异性（含水层水平和垂向渗透系数比值Kh/Kv）取值为1000时,地下水流场与实际条件较为接近;非均质条件下,渗透系数方差取值0.91,水平相关长度取值5000 m,Kh/Kv取值150时,接近实际条件。研究表明,在大尺度地下水流模拟研究中,采用水平相关长度、渗透系数方差和各向异性值三个变量生成的随机场能很好地刻画含水层的层状非均质特征及其对水流系统的影响控制作用。由于含水层不同尺度层状非均质的叠加效应,采用均质各向异性介质等效概化含水层层状非均质性会造成等效各向异性值偏大失真的效应。     

考虑基坑卸载影响的改进弹性抗力法及其应用

弹性抗力法是目前实际工程中基坑支护结构变形内力计算的一种主要方法,该方法借鉴的是计算水平荷载作用下竖直放置弹性桩的地基反力系数法,但没有考虑开挖卸载对土抗力系数和坑内土体的变形影响,因而导致变形计算值通常小于实测值。首先在开挖面施加反向自重应力模拟基坑的开挖卸荷,利用Mindlin解得到坑内土体的卸荷应力分布;在前人的实验研究基础上,推导了不同卸荷比下考虑卸荷影响的土体水平抗力系数;将支护结构的变形分解为开挖瞬时的卸载作用和基坑内外水土压力差作用2个阶段,考虑卸载应力路径对坑内土体水平抗力系数以及开挖瞬时变形的影响,对传统的弹性抗力法进行了改进,最后将该方法应用于典型三维基坑算例。研究发现,改进法得到的支护结构水平变形大于传统方法,而对内力结果影响不大;基坑平面尺寸越大,两者的变形结果差别越明显,改进法更能反映平面尺寸对基坑变形的影响     

砂土地基斜桩水平承载特性p-y曲线法

基于Reese建议的砂土p-y曲线,提出了适用于斜桩水平承载特性分析的修正p-y曲线及相应的有限元分析方法。根据桩前土体受力特点,假设桩前浅层土体达到极限破坏状态时桩侧被动破坏区为一个三维楔形体,采用极限平衡分析方法导出了斜桩桩侧极限土抗力表达式,对比研究了斜桩倾角、桩-土界面摩擦系数等因素对极限土抗力的影响规律,并给出了斜桩初始地基反力模量确定方法。在此基础上,修正Reese提出的直桩p-y曲线,提出了砂土地基中斜桩p-y曲线确定方法。利用虚功原理,导出了斜桩水平承载特性分析的有限单元列式,编制相应的分析程序,通过算例验证了方法的合理性和有效性。最后,采用修正的p-y曲线法对一离心机模型试验进行了模拟分析,结果表明,计算与试验结果吻合得很好     

Poroelastic model for pile–soil interaction in a half‐space porous medium due to seismic waves

In this paper, frequency domain dynamic response of a pile embedded in a half‐space porous medium and subjected to P, SV seismic waves is investigated. According to the fictitious pile methodology, the problem is decomposed into an extended poroelastic half‐space and a fictitious pile. The extended porous half‐space is described by Biot's theory, while the fictitious pile is treated as a bar and a beam and described by the conventional 1‐D structure vibration theory. Using the Hankel transformation method, the fundamental solutions for a half‐space porous medium subjected to a vertical or a horizontal circular patch load are established. Based on the obtained fundamental solutions and free wave fields, the second kind of Fredholm integral equations describing the vertical and the horizontal interaction between the pile and the poroelastic half‐space are established. Solution of the integral equations yields the dynamic response of the pile to plane P, SV waves. Numerical results show the parameters of the porous medium, the pile and incident waves have direct influences on the dynamic response of the pile–half‐space system. Significant differences between conventional single‐phase elastic model and the poroelastic model for the surrounding medium of the pile are found. Copyright © 2007 John Wiley & Sons, Ltd.     

A new approach to interpret self-potential anomaly over a two-dimensional inclined sheet using complex gradient analysis

A new approach is developed to determine the model parameters of a two-dimensional inclined sheet from self-potential anomaly. In this method, the numerical horizontal self-potential gradient obtained from self-potential anomaly is convolved using Hilbert transform to obtain the vertical self-potential gradient. The complex gradient is the sum of horizontal and vertical gradient anomalies. The horizontal and vertical gradients are plotted in one graph to form the complex gradient graph. By defining few characteristic points and distances along the complex gradient profile, procedures are then formulated using the analytical functions of the complex gradients to obtain the model parameters of sheet-like structures. The validity of the new proposed method has been tested on synthetic data with and without random noise. The obtained parameters are in congruence with the model parameters when using noise-free synthetic data. After adding 10% random error in the synthetic data, the maximum error in model parameters is 11.8%. Moreover, the method have been applied to analyze and interpret the self-potential anomaly measured on a graphite ore body at southern Bavarian woods, Germany to prove its efficiency where an acceptable agreement has been noticed between the obtained results and the other published results.     

鄂尔多斯地块南缘地应力测量研究

利用水压致裂法得到的地应力测试数据对鄂尔多斯地块南缘地壳浅部地应力分布规律及断层活动性进行研究。结果表明:（1）两个水平主应力随深度线性增长,应力梯度分别为0.032和0.021,在测量深度域内水平和垂直应力的关系为S_H > S_h > S_V,该应力状态有利于断层发生逆断层活动,与1556年华山大地震的发震正断层的性质不同;（2）研究区的最大水平主应力方向为北南-北北西向,与区域速度矢量场方向一致,与其他资料解译的区域构造应力方向有一些差异,主要是受鄂尔多斯地块周缘断层活动的影响;（3）利用Mohr-Coulomb准则及Byerlee定律,摩擦系数取0.6~1.0,对研究区的地应力状态进行分析,发现鄂尔多斯地块南缘的测点未达到或超过地壳破裂极限状态,不存在断层失稳或地震等其他形式的地壳活动,处于较稳定地壳应力状态;（4）实测数据为该区补充了新的地应力测量资料,研究结果为该区工程设计及建设、构造应力场数值模拟提供了边界条件,对于该区地质灾害评价、地壳稳定性以及大陆动力学的研究具有重要意义。      

Theoretical and numerical analyses of convective instability in porous media with upward throughflow

Exact analytical solutions have been obtained for a hydrothermal system consisting of a horizontal porous layer with upward throughflow. The boundary conditions considered are constant temperature, constant pressure at the top, and constant vertical temperature gradient, constant Darcy velocity at the bottom of the layer. After deriving the exact analytical solutions, we examine the stability of the solutions using linear stability theory and the Galerkin method. It has been found that the exact solutions for such a hydrothermal system become unstable when the Rayleigh number of the system is equal to or greater than the corresponding critical Rayleigh number. For small and moderate Peclet numbers (Pe ⩽ 6), an increase in upward throughflow destabilizes the convective flow in the horizontal layer. To confirm these findings, the finite element method with the progressive asymptotic approach procedure is used to compute the convective cells in such a hydrothermal system. Copyright © 1999 John Wiley & Sons, Ltd.     

Impact of difference accuracy on computational properties of vertical grids for a nonhydrostatic model

Starting from nonhydrostatic Boussinesq approximation equations, a general method is introduced to deduce the dispersion relationships. A comparative investigation is performed on inertia-gravity wave with horizontal lengths of 100, 10 and 1 km. These are examined using the second-order central difference scheme and the fourth-order compact difference scheme on vertical grids that are currently available from the perspectives of frequency, horizontal and vertical component of group velocity. These findings are compared to analytical solutions. The obtained results suggest that whether for the second-order central difference scheme or for the fourth-order compact difference scheme, Charny–Phillips and Lorenz (L) grids are suitable for studying waves at the above-mentioned horizontal scales; the Lorenz time-staggered and Charny–Phillips time staggered (CPTS) grids are applicable only to the horizontal scales of less than 10 km, and N grid (unstaggered grid) is unsuitable for simulating waves at any horizontal scale. Furthermore, by using fourth-order compact difference scheme with higher difference precision, the errors of frequency and group velocity in horizontal and vertical directions produced on all vertical grids in describing the waves with horizontal lengths of 1, 10 and 100 km cannot inevitably be decreased. So in developing a numerical model, the higher-order finite difference scheme, like fourth-order compact difference scheme, should be avoided as much as possible, typically on L and CPTS grids, since it will not only take many efforts to design program but also make the calculated group velocity in horizontal and vertical directions even worse in accuracy.     

Numerical investigation of soil plugging inside open-ended piles with respect to the installation method

The development of a soil plug inside an open-ended pile with respect to the installation method is examined using numerical simulations. In this paper, the penetration process of an open-ended tubular pile with diameter D = 61 cm into granular soil is investigated. The aim is to achieve a better understanding of the mechanisms of soil plugging inside open-ended piles with respect to the installation method. As an example, the horizontal stress distribution inside the tubular pile after installation shows significant increase depending on the installation method. The numerical results are compared to experimental data.     

地下水水平井流的模型及数值模拟方法—考虑井管内不同流态

评述用线汇刻画水平井管可能存在的问题, 并用理想模型证明.提出考虑井管不同流态(线性流与非线性流) 水流阻力的水平井流的模型, 并将内边界取在水平井的出水口处, 避免了采用线汇刻画水平井管时假定其流量分布和水头分布的困难.运用作者提出的等效渗透系数的概念与确定方法, 将水平井-含水层系统视为含有圆柱形透镜体的非均质含水层, 给出新的水平井流的数学模型.求解了一个理想模型, 得出河下水平井开采地下水的动态.      

水平面渗流有限元中利用接触单元模拟悬挂式阻水结构

区域性渗流场由于范围大,通常忽略掉竖向的水头损失而用水平面二维有限元进行分析,但在城区区域渗流场中悬挂式阻水结构物量多,竖向绕流现象突出,用一般水平面二维有限元进行分析将产生较大的偏差。针对这一问题在悬挂式结构体周边引入一层无厚度界面接触单元。在水平面有限元中利用它考虑因竖向绕流产生的局部水头损失。利用阻力系数的方法和二维状态下截面突变流道流动的解析解,同时考虑防渗墙本身的弱透水性,推导了接触单元的导水矩阵。应用接触单元编制了相关程序,在水平面有限元中利用接触单元对悬挂式阻水结构进行分析,并将结果与三维有限元进行比较,两者的计算结果在流量上保持一致,在水头上前者是后者在同一位置处沿高程方向平均值。提出的该计算方法,对于用水平面二维有限元分析含有悬挂式阻水结构的大区域渗流问题,是一个有效的手段。     

水平液化场地土表位移简化理论解答

水平液化场地地表往返位移的求解是基础和地下工程抗震设计的迫切需求。利用双层模型模拟实际水平场地,提出了可考虑液化层存在下的土表位移简化计算方法并给出频域理论解答,同时采用逐循环累计方法给出水平场地任意荷载下土表位移时域解答。采用适于水平场地孔压增量模型,并逐循环修正土层模量,模拟液化引起的土层的非线性过程。振动台试验结果和提出棋型的计算结果吻合,验证了所提方法能够反映土体液化对土表位移影响的基本过程。提出的方法和解答物理意义明确,可代表土体液化对土表位移影响的基本形态,并可用于频域及时域的无量纲分析。