Initial excess pore water pressures induced by tunnelling in soft ground

Tunnelling-induced long-term consolidation settlement attracts a great interest of engineering practice. The distribution and magnitude of tunnelling-induced initial excess pore water pressure have significant effects on the long-term consolidation settlement. A simple and reliable method for predicting the tunnel-induced initial excess pore water pressure calculation in soft clay is proposed. This method is based on the theory of elasticity and SKEMPTON's excess pore water pressure theory. Compared with the previously published field measurements and the finite-element modelling results, it is found that the suggested initial excess pore water pressure theory is in a good agreement with the measurements and the FE results. A series of parametric analyses are also carried out to investigate the influences of different factors on the distribution and magnitude of the initial excess pore water pressure in soft ground.

Attenuation-type and failure-type curve models on accumulated pore water pressure in saturated normal consolidated clay

Based on dynamic triaxial test results of saturated soft clay,similarities of variations between accumulated pore water pressure and accumulated deformation were analyzed.The Parr's equation on accumulated deformation was modified to create an attenuation-type curve model on accumulated pore water pressure in saturated normal consolidation clay.In this model,dynamic strength was introduced and a new parameter called equivalent dynamic stress level was added.Besides,based on comparative analysis on variations between failure-type and attenuation-type curves,a failure-type curve model was created on accumulated pore water pressure in saturated normal consolidation clay.Two models can take cycle number,coupling of static and dynamic deviator stress,and consolidation way into consideration.The models are verified by test results.The correlation coefficients are more than 0.98 for optimization of test results based on the two models,and there is good agreement between the optimized and test curves,which shows that the two models are suitable to predict variations of accumulated pore water pressure under different loading cases and consolidation ways.In order to improve prediction accuracy,it is suggested that loading cases and consolidation ways should be consistent with in-situ conditions when dynamic triaxial tests are used to determine the constants in the models.

Experiment and analysis of the formation,expansion and dissipation of gasbag in fine sediments based on pore water pressure survey

Deep-seated gas in seabed sediments migrates upwards from effect of external factors,which easily accumulates to form gasbags at interface of shallow coarse-fine sediments.Real-time monitoring of this process is important to predict disaster.However,there is still a lack of effective monitoring methods,so we attempt to apply multi-points pore water pressure monitoring technology when simulating forming and dissipation of gasbags in sediments through laboratory experiment.This study focuses on discussion of sensitivity of pore water pressure monitoring data,as well as typical changing characteristics and mechanisms of excess pore water pressure corresponding to crack generation,gasbag formation and gas release.It was found that the value of excess pore water pressure in sediments is negatively correlated with vertical distance between sensors and gas source,and the evolution of gasbag forming and dissipation has a good corresponding relationship with the change of excess pore water pressure.Gasbag formation process is divided into three stages:transverse crack development,longitudinal cavity expansion,and oblique crack development.Formation of gasbag begins with the transverse crack at the interface of coarse-fine sediments while excess pore water pressure attenuates rapidly and then drops,pressure remains almost unchanged when cavity expanses longitudinally,oblique crack appeared in final stage of gasbag evolution while excess pore water pressure accumulated and dissipated again.The variation curve of excess pore water pressure in gas release stage has saw-tooth fluctuation characteristics,and the value and time of pressure accumulation are also fluctuating,indicating the uncertainty and non-uniqueness of gas migration channels in sediments.

The effect of pore fluid on seismicity: a computer model

The influence of fluid on seismicity of a computerized system is analyzed in this paper. The diffusion equation of fluid in a crustal fault area is developed and used in the calculation of a spring slide damper model. With mirror imagin boundary condition and three initial conditions, the equation is solved for a dynamic model that consists of six seismic belts and eight seismogenous sources in each belt with both explicit algorithm and implicit algorithm. The analysis of the model with water sources shows that the implicit algorithm is better to be used to calculate the model. Taking a constant proportion of the pore pressure of a broken element to that of its neighboring elements, the seismicity of the model is calculated with mirror boundary condition and no water source initial condition. The results shows that the frequency and magnitude of shocks are both higher than those in the model with no water pore pressure, which provides more complexity to earthquake prediction.

冲击荷载下含夹层饱和砂土孔压变化规律分析

砂土中夹层的性状会影响饱和砂土孔压发展,从而影响砂土层变形。为研究夹层位置、厚度和种类等不同夹层状态下砂土液化过程中的孔压变化规律,设计了冲击荷载作用下层状砂液化试验,建立了含夹层饱和砂土理论模型,并将试验结果与理论分析进行对比。结果表明:含夹层饱和砂土的孔压发展呈现3个阶段,即快速上升、快速消散、缓慢消散阶段。高渗透性夹层高度越高,其下方土层孔压快速消散时长越短,越快趋于稳定值,但消散总时长无明显影响;低渗透性夹层高度或厚度的增大,均会使夹层上方孔压快速消散阶段速率加快,孔压消散稳定阶段延长,孔压消散总时长随之线性增长;同时,孔隙水会在低渗透性夹层下方形成水膜,夹层高度或厚度的增加均会使水膜持续时间增长,但水膜形态主要受夹层厚度影响。试验结果与理论分析较为一致,说明了试验的可靠性。

静钻根植桩施工环境效应现场试验研究

静钻根植桩采用先钻孔注浆后植桩的施工技术,可以解决传统预制桩施工过程引起的挤土效应问题。然而,静钻根植桩施工过程中钻孔和注浆过程会对周围土体产生一定扰动,目前没有相关研究。文章通过一组现场试验对静钻根植桩施工过程中的钻孔、注浆搅拌和植桩阶段对周围土体的扰动规律进行研究。静钻根植桩施工前在钻孔周围土体中埋设土压力传感器,孔隙水压力传感器和测斜管,分别测试静钻根植桩施工过程中周围土体中水平土压力,超静孔隙水压力和深层土体水平位移的变化规律。研究结果表明:静钻根植桩施工过程会对周围土体造成一定扰动,而施工完成后周围土体中的水平土压力、超静孔隙水压力和深层土体水平位移迅速恢复;当水平距离达到4D(D为钻孔直径)时,静钻根植桩施工过程基本不会对土体产生影响;试验结果可以为静钻根植桩在地铁周围等施工位移控制敏感区域的应用提供理论依据。

饱和砂土液化特性的振动台试验研究

基于地震模拟振动台试验,配制3组不同平均粒径和3组不同细粒含量的6个砂土模型,通过埋置于砂土内部的传感器监测模型内部不同位置的超孔隙水压力等指标,分析砂土模型内部的超孔隙水压力时程曲线及孔压比时程曲线,归纳出地震波加载峰值、砂土平均粒径、细粒含量及埋置深度等因素对饱和砂土液化特性的影响规律。试验结果表明:随着地震波加载峰值的增大,砂土模型液化程度逐渐增大,液化势逐渐增大,抗液化强度逐渐减小;随着砂土埋置深度的增加,砂土细粒含量的增加,砂土平均粒径的增加,砂土模型液化程度逐渐减小,液化势逐渐减小,其抗液化强度逐渐增大。同时,试验结果还表明,砂土液化各影响因素对砂土液化的影响程度依次为地震波强度>砂土埋置深度>砂土平均粒径、细粒含量。试验结果可为后续数值模拟的参数选取提供支持,为研究其他因素对砂土液化的影响提供参考。

比奥固结理论下基于有效应力法的尾矿库稳定性分析

抗滑稳定计算是尾矿库安全评价的重要内容。由于无法有效估算因尾矿库上升而引起的超静孔隙水压力,目前常采用总应力法进行稳定性分析。然而,尾矿库的整个生命周期内,均可能存在超静孔隙压力,且该部分超静孔隙压力的影响不可忽视。以某平地型尾矿库为例,基于比奥固结理论,采用有限单元法定性分析尾矿库断面堆坝期和退役期超静孔隙压力的产生和消散,并以此进行有效应力法的稳定性计算。结果表明:1)超静孔隙压力在堆坝末期达到最大,约为130 kPa,全部消散需要约28 a;2)在堆坝末期,利用有效应力法计算的抗滑稳定安全系数为1.486,随着坝体有效应力的增长,超静孔压全部消散后的安全系数达到1.648,增长幅度约为11%;3)采用有效应力法可以较为细致地考虑尾矿库上升过程中超静孔隙水压力对坝体稳定性的影响,且结果更为科学可靠。

基于离心振动试验的深埋砂层液化研究

砂层地震液化的判别经验一般在10~20 m深度范围内,基于离心振动试验研究30~40 m埋深砂层是否地震液化的问题。用密度较大的钢球堆积体模拟上覆土层,进行埋深15 m和25 m这2种情况下砂层离心机振动台试验,研究深部12.8 m厚砂层在地震作用下孔隙水压力发展规律并进行液化判别。试验表明,2种埋深条件下砂土超静孔隙水压力比均<1,未发现砂土液化现象。通过对比,砂土中各测点初始孔隙水压力和试验过程中的超静孔隙水压力随着上覆厚度增加而逐渐变大,但超静孔压比随着上覆厚度的增加逐渐减小,表明砂层埋深对抗液化能力提升明显。

桩土界面超孔压和应力变化规律试验和数值模拟研究

为揭示黏土地区成桩后桩土界面超孔隙水压力和竖向有效应力的时空分布特征,本文依托岭南师范学院第五教学实训楼项目,进行了现场试验和数值模拟研究,并将数值计算结果与试验结果进行了对比。结果表明:竖向有效应力、孔隙水压力和超孔隙水压力的变化规律受传感器位置影响较为显著,传感器位置越靠近桩端数值越大。孔隙水压力变化特征受土层性质影响,当桩体位于粉质黏土层时,桩端孔压增幅更小。桩端位置处的超孔隙水压力峰值达到了25kPa,桩顶位置处超孔隙水压力始终保持在0附近。桩土界面的超孔隙水压力和竖向有效应力数值模拟结果与试验结果数值大小和随时间变化规律基本一致,验证了本文建立的桩基数值模型的有效性。

软基处理PST桩复合地基加固机理及CPTU验证

对于厂房、仓库、露天堆场等沿海软土地基加固处理工程,采用传统的软基处理方法如真空预压或堆载预压,其施工工期长、工后沉降大;采用水泥搅拌桩由于施工质量不易控制,桩体强度低、处理效果不佳;采用刚性桩复合地基存在地面堆载沉降不均匀等难题。“PST”复合地基处理方法采用预制桩与桩帽及厚褥垫层组合,将预制桩的高承载力转化为褥垫层均布承载力;地面均布荷载引起的变形应力传递到桩身而非其下的软土。以某大型地基处理工程为例,通过CPTU孔压静力触探技术评价PST桩地基处理加固效果与挤土效应,通过对打桩过程中,不同时期的超静孔隙水压力进行观测,掌握超静孔隙水压力消散和变化规律。通过打桩过程中的超孔压的变化情况,分析打桩顺序和速率对孔压的影响程度,了解PST桩施工对软土的扰动规律,以指导施工,降低PST桩施工对地基土的影响。

深厚淤泥地层超静水压力条件下超深灌注桩施工孔壁稳定性研究

依托某地铁车辆段管桩施工后超静水压力条件下的钻孔灌注桩工程,研究了超静水压力条件下钻孔孔壁的稳定条件。推导出了孔壁坍塌的临界水力坡度公式和保证孔壁稳定性的临界固结度公式,并分析了土体固结对孔壁稳定的影响;探讨了新奥法原理在淤泥质软土超静孔隙水压力下灌注桩成孔中的应用。研究结果表明:饱和淤泥质地层中管桩施工后超静孔隙水产生的渗透力是造成灌注桩钻孔过程中孔壁变形失稳的主要因素;土体的黏聚力是影响钻孔缩颈和塌孔的关键参数;超静水压力下软土中钻孔孔壁的稳定条件主要是圆拱支撑力和渗透力;钻孔期间孔壁土体的蠕变大、渗透路径短和土体能在较短时间内发生较大的固结可使孔壁达到稳定的平衡状态;超静孔隙水压力条件下的软土可以借鉴新奥法原理进行钻孔灌注桩成孔。

高频免共振钢管桩承载力恢复规律预测模型

高频免共振钢管桩因其施工效率高、泥浆污染小、对周边环境影响小等优点而被广泛应用。免共振沉桩原理是使桩周土体液化,产生的超孔隙水压力会导致桩承载力下降,且恢复缓慢。现依托上海杨高中路改建工程,通过现场孔压监测与高应变试验,分析了桩周孔压消散规律与桩承载力恢复规律,并提出了基于孔隙水压力的归一化桩承载力恢复预测模型,为相似工程应用提供技术支持和经验参考。

A Coupling Model of Nonlinear Wave and Sandy Seabed Dynamic Interaction

In the paper, a weak coupling numerical model is developed for the study of the nonlinear dynamic interaction between water waves and permeable sandy seabed. The wave field solveris based on the VOF （Volume of Fluid） method for continuity equation and the two-dimensional Reynolds Averaged Navier Stokes （RANS） equations with a k-ε closure. The free surface of cnoidal wave is traced through the PLIC-VOF （P/ecewise Linear/nterface Construction）. Blot＇s equations have been applied to solve the sandy seabed, and the u-p fmite dement formulations are derived by the application of the Galerkin weighted-residual procedure. The continuity of the pressure on the interface between fluid and porous medium domains is considered. Laboratory tests were performed to verify the proposed numerical model, and it is shown that the pore-water pressures and the wave heights computed by the VOF-FEM models are in good agreement with the experimental results. It is found that the proposed model is effective in predicting the seabed-nonlinear wave interaction and is able to handle the wave-breakwater-seabed interaction problem.

Centrifuge model test and numerical interpretation of seismic responses of a partially submerged deposit slope

Partially submerged deposit slopes are o ften encountered in practical engineering applications.Howeve r,studies on evaluating their stability under seismic loading are still rare.In order to understand the seismic behavior of partially submerged deposit slopes,centrifuge shaking table model tests(50 g) were employed.The responses of horizontal accelerations,accumulated excess pore pressures,deformation mode,and failure mode of the partially submerged deposit slope model were analyzed.In dynamic centrifuge model tests,EQ5 shaking event was applied numerically.The results indicated that in the saturated zone of the deposit slope,liquefaction did not occur,and the measured horizontal accelerations near the water table were amplified as a layer-magnification effect.It was also shown that the liquefaction-resistance of the deposit slope increased under multiple sequential ground motions,and the deformation depth of the deposit slope induced by earthquake increased gradually with increasing dynamic Ioad amplitude.Except for the excessive crest settlement generated by strong shaking,an additional vertical permanent displacement was initiated at the slope crest due to the dissipation of excess pore pressure under seismic loading.The result of particle image velocimetry(PIV) analysis showed that an obvious internal arc-slip was generated around the water table of the partially submerged deposit slope under seismic loading.

Simulation of Excess Pore Water Pressure During Deep Soil Mixing Columns Installing

Most of current studies of deep soil mixing (DSM) methods are focused on the soil strength improvement and soil treatment effectiveness. But the DSM installation leads to excess pore water pressure and soil disturbance, which will bring great harm to adjacent structures, such as shell tunnels and historic buildings. The procedure of excess pore water pressure buildup while large number DSM columns are installed is complicated. In order to find methods to predict and simulate the excess pore water pressure during DSM column installation, the complicated dissipation and buildup of excess pore water pressure through in-situ test are studied in this paper. In-situ test was conducted in soft clay near the Huangpu River in Shanghai. The pore water pressure was investigated by an automatic monitoring system. Test results indicate that the excess pore water pressure induced by one DSM column installation is composed of the compaction pressure and the reversing pressure. The empirical equations of excess pore water pressure dissipation and buildup were built by mathematical fitting methods. A compound method is proposed to simulate the excess pore water pressure due to DSM installation. Using this method to predict the excess pore water pressure in the situ test, results show a well agreement between the prediction and the measurements.

地震历史对各深度土体抗液化性影响的振动台试验研究

地震引起的液化会对岩土结构造成重大破坏,而震后伴随的余震可能导致砂土再次发生液化。为研究自由场地下地震历史对各深度饱和砂土的抗液化能力的影响,设计并开展了一系列振动台试验。试验中对砂土输入了4次不同加速度的震动事件,细分为7次小的事件。通过计算对比每次振动事件的超静孔压比、加速度响应以及土体沉降,以研究不同地震历史下各深度土体抗液化能力的变化规律。试验结果表明:输入的地震波加速度大小与加速度响应系数呈正相关关系;饱和砂土遭受轻微和中等地震时液化更容易发生在浅层而非表层;表层土体中,强烈地震后的中等余震使得土体对地震烈度的敏感性降低,孔隙水压力在峰值加速度后消散迅速,液化时间缩短;有强震历史的土体会降低浅层土体的抗液化性能,深层土体抗液化性反而会在经历强地震后增强,影响深度范围取决于地震的烈度;通过数据拟合得到了不同等级的循环地震在各深度土体的抗液化提升比的量化公式。试验结果可以反映在地震实际中各等级的地震历史对各深度土体的不同影响。

含细粒珊瑚土抗液化特性试验研究

珊瑚土作为新兴的热带地区岛礁与港口工程的首选土工材料,宽级配是其结构组成的主要特征。由于相关工程面临较高的地震风险,珊瑚土的抗液化能力正逐渐引起重视。为探究含细粒珊瑚土的抗液化能力,以东太平洋某热带港口工程的实际珊瑚土场地为背景,通过大粒径循环三轴液化试验测试了设计相对密实度为0.4~0.8的3组代表性级配试样及剔除细粒的两组试样的饱和不排水动强度。试验结果表明:幂函数可以模拟含细粒珊瑚土的循环应力比与液化所需振次关系;细粒的存在与相对密实度的提高不会显著提高珊瑚土抗液化能力;珊瑚土液化过程的超静孔压发展模式与砂土相近,两参数或三参数的反正弦模型可以较好地模拟含细粒珊瑚土的液化超孔压发展过程。研究表明,含细粒珊瑚土仍然属于可液化土类。以背景工程为例,同类型工程在设计施工及使用阶段都需要考虑对地震液化灾害的设防,该研究为珊瑚土液化防治工作提供了技术支持。

成桩后黏性土中超孔压和有效应力试验研究

针对预制桩沉桩挤土效应问题,采用现场原位试验和数值模拟方法,分析了桩土界面超孔压和竖向有效应力随时间的变化规律,并应用水力压裂理论、孔穴扩张理论和修正剑桥(modified Cam-clay,MCC)模型,揭示了成桩后桩周土体固结过程的超孔压沿桩长的分布形式。结果表明:桩周超孔压大小同孔压与传感器入土深度成正比;在外载荷作用下,黏性土中超孔压和竖向有效应力分别随时间呈“增-减-平”和“平-增-平”的三段式变化趋势;成桩后约25天,超孔压基本消散完毕,竖向有效应力趋于稳定最大值;基于修正理论公式的数值模型得出,不同载荷下,桩侧中下部出现负超孔压,距桩的水平距离和距桩端竖向距离越小,超孔压越大。可为同类静压沉桩挤土效应问题提供参考依据。

植入式排水桩固结理论

植入式排水桩可消除临近土层中的超静孔压、提升桩侧土体强度,是一种适用于滨海深厚软土地基的新型桩基技术。为研究桩侧临近土层中孔压消散和固结度发展规律,将水泥土视为涂抹区,并基于椭圆柱排水体假设和坐标变换方法,推导了真空负压作用下植入式排水桩固结解析解。在模型试验检验理论可靠性的基础之上开展算例分析,发现真空负压传递主要集中在塑料排水板附近,土层中超静孔压平面分布呈“8”字型;真空负压作用时间和水泥土渗透性对排水桩加固效果影响显著;水泥土渗透性在排水固结和硬化作用下快速下降,真空抽排水时间应控制在5 d以内。研究成果为植入式排水桩技术进一步研发及推广应用奠定了基础。