Soil pressure and pore pressure for seismic design of tunnels revisited: considering water-saturated, poroelastic half-space

This paper describes a systematic study on the fundamental features of seismic soil pressure on underground tunnels, in terms of its magnitude and distribution, and further identifi es the dominant factors that signifi cantly infl uence the seismic soil pressure. A tunnel embedded in water-saturated poroelastic half-space is considered, with a large variety of model and excitation parameters. The primary features of both the total soil pressure and the pore pressure are investigated. Taking a circular tunnel as an example, the results are presented using a fi nite element-indirect boundary element(FE-IBE) method, which can account for dynamic soil-tunnel interaction and solid frame-pore water coupling. The effects of tunnel stiffness, tunnel buried depth and input motions on the seismic soil pressure and pore pressure are also examined. It is shown that the most crucial factors that dominate the magnitude and distribution of the soil pressure are the tunnel stiffness and dynamic soil-tunnel interaction. Moreover, the solid frame-pore water coupling has a prominent infl uence on the magnitude of the pore pressure. The fi ndings are benefi cial to obtain insight into the seismic soil pressure on underground tunnels, thus facilitating more accurate estimation of the seismic soil pressure.

Propagation of plane P-waves at interface between elastic solid and unsaturated poroelastic medium

A linear viscoporoelastic model is developed to describe the problem of reflection and transmission of an obliquely incident plane P-wave at the interface between an elastic solid and an unsaturated poroelastic medium, in which the solid matrix is filled with two weakly coupled fluids （liquid and gas）. The expressions for the amplitude reflection coefficients and the amplitude transmission coefficients are derived by using the potential method. The present derivation is subsequently applied to study the energy conversions among the incident, reflected, and transmitted wave modes. It is found that the reflection and transmission coefficients in the forms of amplitude ratios and energy ratios are functions of the incident angle, the liquid saturation, the frequency of the incident wave, and the elastic constants of the upper and lower media. Numerical results are presented graphically. The effects of the incident angle, the frequency, and the liquid saturation on the amplitude and the energy reflection and transmission coefficients are discussed. It is verified that in the transmission process, there is no energy dissipation at the interface.

Green's function of multi-layered poroelastic half-space for models of ground vibration due to railway traffic

This study proposes a Green＇s function, an essential representation of water-saturated ground under moving excitation, to simulate ground borne vibration from trains. First, general solutions to the governing equations of poroelastic medium are derived by means of integral transform. Secondly, the transmission and reflection matrix approach is used to formulate the relationship between displacement and stress of the stratified ground, which results in the matrix of the Green＇s function. Then the Green＇s function is combined into a train-track-ground model, and is verified by typical examples and a field test. Additional simulations show that the computed ground vibration attenuates faster in the immediate vicinity of the track than in the surrounding area. The wavelength of wheel-rail unevenness has a notable effect on computed displacement and pore pressure. The variation of vibration intensity with the depth of ground is significantly influenced by the layering of the strata soil. When the train speed is equal to the velocity of the Rayleigh wave, the Mach cone appears in the simulated wave field. The proposed Green＇s function is an appropriate representation for a layered ground with shallow ground water table, and will be helpful to understand the dynamic responses of the ground to complicated moving excitation.

Effect of Rotation on the Propagation of Waves in Hollow Poroelastic Circular Cylinder with Magnetic Field

Employing Biot’s theory of wave propagation in liquid saturated porous media,the effect of rotation and magnetic field on wave propagation in a hollow poroelastic circular of infinite extent is investigated.An exact closed form solution is presented.General frequency equations for propagation of poroelastic cylinder are obtained when the boundaries are stress free.The frequencies are calculated for poroelastic cylinder for different values of magnetic field and rotation.Numerical results are given and illustrated graphically.The results indicate that the effect of rotation,and magnetic field are very pronounced.Such a model would be useful in large-scale parametric studies of mechanical response.

The scattering of plane P1 wave by a 3-D sedimentary basin in a fluid-saturated poroelastic half-space using IBEM

The indirect boundary element method(IBEM)is applied to investigate the scattering of elastic waves around a 3-D sedimentary basin filled with fluid-saturated poroelastic medium.Based on this method,the free field and scattered field can be solved according to the boundary conditions.And the numerical accuracy has been verified.The effects of parameters on elastic wave scattering are studied,such as boundary condition,incident frequency,incident angle and porosity of medium.Numerical results illustrate that the amplification effect of surface displacement near poroelastic sedimentary basin is notable.In addition,for the case of large porosity the drainage condition has a significant impact on the response amplitude.Due to the fluid exchange at the interface under the drained condition,the displacement amplitude can be much larger than that under the undrained condition in present study.The study can provide a theoretical basis for the anti-seismic design of engineering structures located in sedimentary basin.

深水浅层钻井呼吸效应机制及影响因素分析

钻井呼吸效应常被误判为井涌而采取错误的措施,使呼吸效应强度增加。为明确深水浅层呼吸效应机制,基于深水浅层高孔高渗和裂缝不发育的特点,建立多孔弹性介质渗流模型,模拟深水浅层钻井呼吸效应过程,分析地层参数(弹性模量、泊松比、渗透率和孔隙度)、钻井液性能(塑性黏度和动切力)和井底压差对深水浅层钻井呼吸效应的影响。结果表明:深水浅层钻井呼吸效应属于渗透性诱导机制,使部分钻井液残留在地层中,具有瞬时性,可分为漏失、拟稳态和返排3个阶段。低弹性模量、低泊松比、高孔高渗的地层更容易发生渗透性呼吸效应。加入降滤失剂和膨润土、使用低密度钻井液、采用低排量的钻井方式和控压钻井技术等,可以降低渗透性呼吸效应强度。该结果为深水浅层安全高效钻井提供指导。

On the material constants measurement method of a fluid-saturated transversely isotropic poroelastic medium

The fluid-saturated transversely isotropic poroelastic medium could be widely found in nature, e.g., the sedimentary rocks underground. To determine the eight independent material constants for the transversely isotropic poroelastic medium, a series of tests are discussed. Two undrained tests and one drained test are suggested as a set of tests of the least amount. For the verification purpose, two additional drained tests are also introduced as an option. The atmospheric dried test is discussed as a replacement of the traditional infiltrated drained test to save the time waiting for an equilibrium state. Some microscopic material constants, i.e.,the unjacketed bulk coefficients, the porosity, and the compressibility of porous fluid, are measurable but unnecessary to determine the independent material constants of a poroelastic medium.

Uncoupled state space solution to layered poroelastic medium with anisotropic permeability and compressible pore fluid

This paper presents an uncoupled state space solution to three-dimensional consolidation of layered poroelastic medium with anisotropic permeability and compressible pore fluid.Starting from the basic equations of poroelastic medium,and introducing intermediate variables,the state space equation usually comprising eight coupled state vectors is uncoupled into two sets of equations of six and two state vectors in the Laplace-Fourier transform domain.Combined with the continuity conditions between adjacent layers and boundary conditions,the uncoupled state space solution of a layered poroelastic medium is obtained by using the transfer matrix method.Numerical results show that the anisotropy of permeability and the compressibility of pore fluid have remarkable influence on the consolidation behavior of poroelastic medium.

昆仑山Ms 8．1级地震震后变形场数值模拟与成因机理探讨

2001年11月14日,在青海和新疆交界处发生了昆仑山Ms8.1级强烈地震,GPS后观测显示,此次地震震后形变不仅在断裂南北两侧存在很大的差异,而且在短时间调整后断裂南北两侧表现为同向运动.本文以观测的地震形变为约束,通过有限元数值模拟分析昆仑山地震震后形变的物理机制.建立有关的有限元虚功方程,通过有限元数值方法模拟震后形变,从理论上分析介质的非均匀性、黏滞性松弛、流体调整对震后形变的影响.采用网格搜索确定昆仑断裂南北两侧下地壳的黏滞系数分别为5.0×1017Pa.s,9.0×1018Pa.s左右,正是这十余倍的差异引起了断裂两侧震后形变的非对称性和同向运动,这一差异既是长期地质作用的结果,又是现代地球动力学环境的决定因素之一.通过数值模拟定性讨论了断裂北侧地表形变在震后短期内的调整,对于靠近断裂附近的测点可能是黏弹性松弛和孔隙流体调整共同作用的结果,所以在分析短期震后形变时综合考虑黏弹松弛和孔隙流体调整是很有必要的.

井孔声场计算中的黎曼叶选择

针对耗散介质地层模型和双相介质地层模型，给出频率-波数域井孔声场函数Φ1(r,ω，k）在k实轴上满足辐射条件的单值定义，确定在积分围道上和围道内Φ1(r，ω,k)所在的黎曼叶．同时，澄清了首波分析中的一些概念．

平面P波在弹性介质和非饱和多孔弹性介质分界面上的传播

使用线性粘滞的多孔弹性介质模型,解决在弹性介质和非饱和多孔弹性介质分界面上平面P波的反射与透射问题,这里的非饱和多孔介质中固体骨架被两种相互耦合的流体(液体和气体)所充满.通过势函数的方法得到了振幅反射系数与振幅透射系数.然后推导得到入射波与反射波、透射波之间能量转换情况.研究发现:用振幅比和能量比所表示的反射系数与透射系数是与入射角度、饱和度、入射频率以及上下层介质的弹性常数有关的方程式.数值计算通过图形的形式表达出来,而且入射角度、频率及饱和度对振幅和能量的反射与透射系数的影响分别进行了讨论.证明了在整个波的传播过程中分界处并没有发生能量的耗散.

高速列车荷载作用下路轨系统及饱和地基的动力响应

将在高速移动列车荷载作用下的铁路系统分为上覆路轨系统和下卧土体两部分.对于下卧土体部分,通过Fourier变换求解Biot多孔饱和介质的动力方程.对于路轨系统,将钢轨简化为无限长弹性Euler梁,将枕木简化为连续质量块,同时考虑道渣层的影响.用一系列符合列车几何尺寸的点荷载来模拟列车荷载.在Fourier变换域内,联立路轨系统和下卧土体的动力方程,求解在列车荷载作用下的钢轨位移和加速度、土体位移和加速度及土体孔压的表达式.利用数值积分方法对表达式进行Fourier逆变换,得到钢轨位移和加速度、土体孔压在时域内的表达式.通过算例讨论了荷载移动速度和土体渗透系数对钢轨速度和加速度及土体孔压的影响.结果表明,水相介质的存在和荷载移动速度均对土体动力响应产生很大影响.

横观各向同性饱和土的基本方程组

两相饱和多孔介质的基本方程组和计算参数的选取在一定程度上是混乱的.本文利用连续介质力学理论,结合空间平均化方法,根据应力-应变关系、运动学关系、连续性方程及广义Darcy定律,建立了横观各向同性液体饱和多孔介质的基本方程组;通过将该基本方程组与Biot理论的基本方程组进行比较,确定了Biot方程组中的弹性常数Bi与弹性常数cij的关系,并得到了确定Biot参数mi和ri的计算公式——Biot参数与渗透率、孔隙率及黏性系数的关系;最后对几种特殊情形进行了讨论,给出了简化方程组.

层状孔隙介质孔隙度对地震属性影响分析

孔隙度是储层的重要参数之一,学者提出了多种根据地震属性分析孔隙度大小的方法.然而,层状孔隙介质中孔隙度对地震属性的影响至今没有系统的论述.鉴于此,本文建立了两种类型储层:其一为深度压实,固结程度好的储层;其二为欠压实,固结程度差的储层.对于上述两种类型储层,通过理论和数值模拟,分析了孔隙度对地震属性(与频率相关)的影响.研究表明,对于深度压实储层,反射系数随孔隙度的增加而减小;对于欠压实储层,反射系数随孔隙度的增加而增加.

轨道附近地面振动模型中的饱和地层动力格林函数

列车引起场地振动的建模需要能够表达地层的动力格林函数.本文兼顾饱和土的流固两相耦合性、场地土的分层性和波动的三维传播性,构建了半解析的场地动力格林函数.首先,基于Biot方程,在傅里叶变换域求解固体骨架和流体的位移和应力.然后采用传递矩阵方法建立地表位移和应力间的关系,得到格林函数矩阵.进而讨论矩阵的一些固有特征,提出改善竖向位移计算效率的措施.最后利用推导的格林函数计算了几个典型算例.数值结果与文献中其他方法得到的结果十分接近,与场地振动的现场观测试验基本符合.软土场地振动的计算结果高于饱和砂土场地,高速列车场地振动强度高于低速列车.当车速接近场地瑞利波速,模拟结果中显示出马赫锥.数值结果还显示,即使车速略低于瑞利波速,马赫锥也可能出现.本文推导的格林函数将有助于深入理解列车等移动激励作用下层状饱和土场地的振动特征.

不可压饱和多孔弹性梁的大挠度非线性数学模型

在孔隙流体仅存在沿梁轴线方向扩散的假定下,建立了微观不可压饱和多孔弹性梁大挠度问题的非线性数学模型.利用Galerkin截断法,研究了固定端不可渗透、自由端可渗透的饱和多孔弹性悬臂梁在自由端突加集中载荷作用下的非线性弯曲,得到了梁骨架的挠度、弯矩以及孔隙流体压力等效力偶等的时间响应和沿轴线的分布.比较了大挠度非线性和小挠度线性理论的结果,揭示了两者间的差异.研究发现大挠度理论的结果小于相应的小挠度理论结果,并且,大挠度理论的结果趋于其稳态值的时间小于相应的小挠度理论结果趋于其稳态值的时间.

非均布列车荷载作用下软土路基的振动分析

基于实测资料,假设列车轮载经钢轨传递下来的荷载由5根枕木承担。根据分担比,获得了道渣层表面非均布荷载的表达式。将道渣层视为单相弹性介质,软土地基被看成是考虑水土耦合作用的饱和多孔介质。借助势函数,利用Helmholtz矢量分解法及Fourier变换技术分别对弹性土体和饱和半空间土体进行求解,得到非均布移动荷载作用下软土地基位移、应力及孔压响应在变换域内的精确解答。利用FFT算法得到了数值结果,详细分析了荷载分布形式、观察点位置、道渣性质以及软土地基渗透系数对动力响应的影响。研究结果表明:高速情况下动力响应与低速情况有很大不同;对于软土路基,应特别注意列车高速运行时路基浅层范围内产生的孔压响应。

横观各向同性饱和地基的三维动力响应

首先引入位移函数,将直角坐标系下横观各向同性饱和土Biot波动方程转化为2个解耦的六阶和二阶控制方程;然后基于双重Fourier变换,求解了Biot波动方程,得到以土骨架位移和孔隙水压力为基本未知量的积分形式的一般解,并用一般解给出了饱和土总应力分量的表达式.在此基础上系统研究了横观各向同性饱和半空间体的稳态动力响应问题,考虑表面排水和不排水两种情况,得到了半空间体在任意分布的表面谐振荷载作用下,表面位移的稳态动力响应,文末给出了算例.

页岩气高效开采中钻井完井和水力压裂的关键力学问题

本文研究了在页岩气高效开采中钻井完井和水力压裂缝网改造的关键力学问题.提出了页岩多孔弹性介质的本构、强度和断裂韧性的各向异性模型,指出了微观均匀假设与微观各向同性假设在页岩多孔弹性本构中的简化应用,给出了横观各向同性多孔弹性岩石材料常数的简化测量方案,讨论了基于修正的能量最大释放率下裂纹扩展的弱面模型,提出了裂纹扩展禁止区现象.阐述了钻井完井过程中的多孔弹性介质井壁稳定性和剪切破坏的时间效应,提供了多种破坏模式下井壁许可压力范围的显式表达式,并与传统广义胡克定律得到结果对比.给出了水平井水力压裂缝网改造技术,包含水力压裂的大物模实验技术、水力压裂过程中基于扩展有限元与有限体积法的耦合流体/固体/裂缝扩展的数值模拟方法,并与黏性主导的水力压裂解析解结果对比,针对性分析了川渝地区的水力压裂施工现场实践应用问题.建立了基于数据驱动的页岩气采收率预测方法,指出了机器学习中极限梯度爬升法在小数据集情况下的有效性.

井壁稳定地质力学分析

系统分析了各种地质力学参数、井眼剖面参数对安全钻井液密度窗口的影响规律,这些参数有井斜角、方位角、原地应力状态、抗拉强度、抗剪强度、地层孔隙压力、有效应力系数和钻井液液柱压力对地层强度的影响等。同时研究了各参数定量变化对安全钻井液密度窗口影响的敏感性,为提高地层三个压力剖面预测精度进一步开展工作指明了方向。