开孔管桩动孔压消散特性的理论研究

可液化地基在遇到地震作用或任何其他突变应力条件下,土体会迅速丧失强度和刚度,发生液化,从而失去承载能力。传统的抗液化措施主要集中在地基处理层面,而开孔管桩则兼具抗液化性能和承载能力。在假设开孔管桩孔洞内液体流动满足Poiseuille方程且等应变假设成立的前提下,结合开孔管桩复合地基的应力集中现象,推导出地基在时变荷载作用下的径竖向组合孔压解析解。考虑到循环荷载作用下的孔压积累现象,进一步利用孔压发展的经验模型改进该解析解。在此解析解的基础上,分析、对比开孔管桩、碎石桩、不排水桩以及天然地基的孔压发展情况,得出开孔管桩具有最好的孔压消散能力。桩-土模量比、井径比等因素对孔压的发展都有重要的影响,桩-土模量比越大、井径比越小,孔压消散速率越快。桩本身开孔系数对孔压发展的影响不明显。荷载频率只影响初始孔压峰值,并不影响孔压振荡衰减的速率。     

软土固结中孔压消散的有限元方法

在有凶中实现土体固结时孔压消散有两种方法,并对一实例作了分析,比较了它们的异同。     

软土路基固结中孔压消散的有限元法分析

在有限元中实现土体固结时孔压消散有两种方法,并对一实例作了分析,比较了它们的异同.     

现场软土欠固结程度孔压消散试验评价方法

原位测试技术分析软土的应力历史可以避免取样及室内土工试验对土样的扰动,其结果能真实地反映现场土体的工程特性。以往基于孔压静力触探（CPTU）测试技术的超固结比（OCR）计算方法主要是针对超固结土取得的,缺乏对现场处于欠固结状态土体的考虑,具有一定的局限性。在已有研究成果的基础上,提出采用不完全孔压消散曲线的末段及时间平方根倒数外推法计算原位初始孔压,如果初始孔压大于静水压力,表明原位土层中存在固有孔压,为欠固结土。在此基础上,通过计算固结状态参数,可进一步对软土层的欠固结程度进行定量评价。工程应用表明,该方法不受软土性质和地域限制,具有普遍适用性,是合理可行的。在缺乏室内固结试验成果的情况下,可高效快捷地判别软土层的固结状态。研究成果对合理评价欠固结软土的工程特性具有一定的指导意义。     

软黏土层一维有限应变固结的超静孔压消散研究

根据土力学固结理论计算分析软黏土层固结过程的超静孔隙水压力值,确定软黏土体固结过程的强度增长,对排水固结法处理软土地基至关重要。软黏土层固结过程中土体变形较大时,有限应变固结理论和小应变固结理论计算分析软黏土固结所得结果差异较大。利用非线性有限元法及程序,通过对软黏土层固结工程算例的计算结果分析,研究了有限应变固结理论和小应变固结理论计算分析软黏土层一维固结超静孔压值消散的差异;探讨了软黏土体一维固结过程中,几何非线性、土体渗透性变化和压缩性变化对超静孔隙水压力消散的影响。研究结果表明,当土体的变形较大时,有限应变固结理论计算出的超静孔压要比小应变固结理论得到的值消散的更快。考虑土体固结过程中渗透性的变化时,超静孔压消散变慢;可用软黏土渗透性变化指数ck 反映渗透性变化对超静孔压消散的影响,渗透性变化指数ck值越小、超静孔压消散越慢。固结过程中软黏土压缩性的大小及变化也影响超静孔压的消散,可用软黏土的压缩指数cc反映固结过程中压缩性的大小及变化对超静孔压消散的影响,软黏土的压缩指数cc越小,固结过程软黏土层中的超静孔压消散越快。     

冲击荷载作用下饱和软粘土孔压增长与消散规律

通过大量室内试验,研究饱和软粘土在冲击荷载作用下的变形和孔压增长规律,讨论孔压增长的内在机理和不同围压、不同冲击能作用下孔压量的变化。讨论土体的再团结变形规律,提出再固结体积压缩系数的确定方法。最后分析冲击荷载作用后的强度变化。这些讨论为动静结合法处理软基提供了依据。     

循环交通荷载下软土路基长期沉降理论解

基于循环交通荷载下软黏土累积塑性变形、累积孔压经验公式与分层总和法结合的方法,通过计算不排水循环累积塑性变形引起的沉降和不排水循环累积孔压消散引起的固结沉降叠加,建立了一个软土路基长期沉降拟静力计算模型。对交通荷载应力路径下的软黏土空心圆柱扭剪试验数据分析,获得了不排水累积塑性应变模型和不排水累积孔压公式的参数。基于弹性理论解积分计算交通荷载下路基中的动应力,再结合分层总和法计算了路基沉降与循环周次的关系。通过工程实例分析,对比了计算沉降结果与实测结果并与以往理论分析结果,验证了所建立模型的合理性。     

循环荷载作用下超固结软黏土软化-孔压模型研究

随着循环次数的增加,循环荷载作用下正常固结软黏土孔隙水压力逐渐增加,而孔压的增加将导致土体发生软化现象。对于超固结软黏土,循环荷载作用下在循环初期将产生负孔压,而此时也发生循环软化现象,这显然与有效应力原理是相矛盾的。以往的研究往往通过建立软化指数与循环次数的关系来描述土体的循环软化特性,从而不能反映土体的循环过程中残余孔压变化对土体循环软化的影响。通过对杭州饱和软黏土进行应力控制的循环三轴试验,对循环荷载作用下超固结软黏土的软化特性及孔压发展规律及两者关系进行了研究。在试验的基础上建立了超固结软黏土循环软化-孔压模型,该模型反映了残余孔压增长对超固结软黏土循环软化特性的影响规律。     

地铁荷载下不同固结度软黏土的孔压试验模型

地铁运营前下卧层土体存在不同程度固结,这会影响到运营期隧道周围土体中孔压的变化,进而产生不均匀沉降。通过GDS循环三轴试验系统对杭州饱和软黏土进行动力测试,研究了固结度、固结应力、循环应力比对孔压发展的影响。在现有研究成果的基础上建立了考虑初始固结程度的孔压模型。研究表明：循环荷载下饱和软黏土的孔压发展形态比应变发展形态更具规律性,固结程度愈高的土体孔压发展愈缓慢,并且随振动次数增加,在较低的孔压水平就可达到稳定。预测地铁长期荷载下土体的孔压和沉降时考虑孔压测试时的滞后现象会达到更好的效果,所得的孔压模型能够较好地模拟地铁运营荷载下孔压的发展。     

剪切-体积应变耦合的孔压增量模型试验

以南京细砂为对象,通过共振柱试验及应变控制的排水/不排水分级和单级加载循环三轴试验的系统性研究,建立了能够描述饱和砂土孔压增长规律的新模型。该模型遵循Martin和Byrne提出的基本理论框架,孔压的增长是由循环剪切作用下体积改变所引起的。提出的孔压模型属于应变控制的孔压增量模型。基于排水循环单级加载试验,通过引入体积门槛剪应变γ_(tv)的概念,以具有代表性的15周体应变(ξ_(vd))15为基准点,归准化体应变发展与循环剪应变之间的关系,建立了三参数的体应变增量模型及其参数标定方法。基于排水和不排水循环单级加载试验结果,揭示了体应变与孔压比之间的内在联系,进而导出回弹模量表达式。通过耦合新的体应变增量模型及回弹模量公式,建立了新的剪应变-体应变耦合的孔压增量模型。验证性试验表明,新的孔压增量模型的预测值与试验结果的吻合度较高。     

Coupling of solid deformation and pore pressure for undrained deformation—a discrete element method approach

This paper presents a numerical scheme for fluid‐particle coupling that uses the discrete element method by taking into consideration solid deformation and pore pressure generation. A new water particle element is introduced to calculate pore water pressure due to porosity changes. The water particle element has the same size and shape as the solid element and experiences the same amount of deformation. On the basis of the effective stress principle at the element contact, the total force is equal to the sum of the force transmitted through the solid element contact and the water particle force due to pore water pressure. Analytical solutions of traditional soil mechanics problems, such as isotropic compression and consolidated triaxial undrained test, are used to quantitatively validate the proposed model. The numerical results show good agreement between the model and the analytical solutions. The model therefore provides an effective method to calculate pore pressure in a porous medium in discrete modeling.     

考虑孔隙水压力作用的岩土体弹塑性模型及其有限元实现

岩土工程中孔隙水的作用一直是工程安全稳定分析中不容忽视的问题。基于Viesca提出的考虑饱和孔隙水压力作用的孔隙弹塑性模型,推导其在满足D-P准则的理想弹塑性情况下的具体形式。对于不排水条件,饱水状态下材料强度参数被等效化,使得屈服面的形状发生改变,同时根据最近点投影法确定其在弹塑性计算中的本构积分策略。不排水孔隙水压力对岩土体的塑性响应的影响得以表达,而对结构的稳定性的影响通过基于过应力概念的不平衡力反映出来。基于该模型开发非线性有限元程序PoreTfine,选取非均质边坡降雨入渗下的稳定问题作为算例验证其实用性,边坡随降雨作用的非平衡演化过程得到研究。Skempton系数的增大对连续降雨条件下边坡有弱化作用,对其稳定性不利。     

基于衬砌长期渗漏水影响的隧道施工扰动诱发超孔隙水压消散及地层固结沉降解

基于Terzaghi-Rendulic固结理论,采用保角映射和分离变量法,计算得到隧道衬砌半渗透漏水边界条件下,盾构施工扰动引起的隧道周围土体超孔隙水压力消散解和地表土体固结沉降,通过工程实例进行验证,发现理论方法与实测数据趋势一致;此外,利用参数分析获得了土体固结沉降和超孔隙水压力的分布影响规律。结果表明:衬砌与土体的相对渗透比越大,固结沉降的初始速率越大,但不影响最终收敛值;土体弹性模量越小,最终固结沉降量越大;土体中超孔隙水压力随着时间增加,在隧道开挖后较短时间里以较大幅度逐渐消散,消散到约为初始值的1/10时减幅放缓;距离衬砌外壁越远,土体初始超孔隙水压力越小,其消散速度也越慢。     

基于扩展有限元法的岩体水力劈裂耦合模型

1. 浙江水利水电学院 水利与环境工程学院,浙江 杭州 310018;2. 上海交通大学 船舶海洋与建筑工程学院,上海 200240; 3. 浙江省钱塘江管理局勘测设计院,浙江 杭州 310016     

Cone penetration-induced pore pressure distribution and dissipation

The excess pore water pressure distribution (u) induced by the penetration of a piezocone into clay and its dissipation behaviour have been investigated by laboratory model tests, theoretical analysis and numerical simulation. Based on the results of the tests and the analysis, a semi-theoretical method has been proposed to predict the piezocone penetration-induced pore pressure distribution in the radial direction from the shoulder of the cone. The method can consider the effect of the undrained shear strength (s_u), over-consolidation ratio (OCR) and rigidity index (I_r) of the soil. With a reliably predicted initial distribution of u and the measured curve of dissipation of pore water pressure at the shoulder of the cone (u₂), the coefficient of consolidation of the soil in the horizontal direction (c_h) can be back-fitted by analysis of the pore pressure dissipation. Comparing the back-fitted values of c_h with the values directly estimated by a previously proposed method indicates that the previously proposed method can be used reliably to estimate c_h values from non-standard dissipation curves (where u₂ increases initially and then dissipates with time).     

Coupled meshfree and fractal finite element method for unbounded problems

This paper presents a coupling technique for integrating the element-free Galerkin method (EFGM) with the fractal finite element method (FFEM) to analyze unbounded problems in the half-space. FFEM is adopted to model the far field of an unbounded domain and EFGM is used in the near field. In the transition region interface elements are employed. The shape functions of interface elements which comprise both the element-free Galerkin and the finite element shape functions, satisfy the consistency condition thus ensuring convergence of the proposed coupled EFGM–FFEM. The proposed method combines the best features of EFGM and FFEM, in the sense that no structured mesh or special enriched basis functions are necessary. The numerical results show that the proposed method performs extremely well converging rapidly to the analytical solution. Also a parametric study is carried out to examine the effects of the integration order, the similarity ratio, the weight function, the scaling parameter and the number of transformation terms, on the quality of the numerical solutions.     

A new approximation for pore pressure accumulation in marine sediment due to water waves

The residual mechanism of wave‐induced pore water pressure accumulation in marine sediments is re‐examined. An analytical approximation is derived using a linear relation for pore pressure generation in cyclic loading, and mistakes in previous solutions (Int. J. Numer. Anal. Methods Geomech. 2001; 25 :885–907; J. Offshore Mech. Arctic Eng. (ASME) 1989; 111 (1):1–11) are corrected. A numerical scheme is then employed to solve the case with a non‐linear relation for pore pressure generation. Both analytical and numerical solutions are verified with experimental data (Laboratory and field investigation of wave– sediment interaction. Joseph H. Defrees Hydraulics Laboratory, School of Civil and Environmental Engineering, Cornell University, Ithaca, NY, 1983), and provide a better prediction of pore pressure accumulation than the previous solution (J. Offshore Mech. Arctic Eng. (ASME) 1989; 111 (1):1–11). The parametric study concludes that the pore pressure accumulation and use of full non‐linear relation of pore pressure become more important under the following conditions: (1) large wave amplitude, (2) longer wave period, (3) shallow water, (4) shallow soil and (5) softer soils with a low consolidation coefficient. Copyright © 2006 John Wiley & Sons, Ltd.