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1.
Application of an optimum design technique for determining the coefficient of consolidation by using piezocone test data 总被引：4，自引：0，他引：4
For normally consolidated clay, several researchers have developed a number of theoretical time factors to determine the coefficient of consolidation from piezocone test results. However, depending on assumptions and analytical techniques, it could vary considerably, even for a specific degree of consolidation. In this paper a method is proposed to determine a consistent coefficient of consolidation by applying the concept of an optimum design technique over all ranges of the degree of consolidation. Initial excess pore pressure distribution is assumed to be capable of being obtained by the successive spherical cavity expansion theory. The dissipation of pore pressure is simulated by means of a two-dimensional linear-uncoupled axi-symmetric consolidation analysis. The minimization of differences between measured and predicted excess pore pressure was carried out by the BFGS unconstrained optimum design algorithm with a one-dimensional golden section search technique. By analyzing numerical examples and in-situ test results, it was found that the adopted optimum design technique gives consistent and convergent results. 相似文献
2.
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 (su), over-consolidation ratio (OCR) and rigidity index (Ir) 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 (ch) can be back-fitted by analysis of the pore pressure dissipation. Comparing the back-fitted values of ch with the values directly estimated by a previously proposed method indicates that the previously proposed method can be used reliably to estimate ch values from non-standard dissipation curves (where u2 increases initially and then dissipates with time). 相似文献
3.
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。 相似文献
4.
软黏土层一维有限应变固结的超静孔压消散研究 总被引：1，自引：0，他引：1
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。 相似文献
5.
The dissipation test evaluation method presented here employs: (i) a point-symmetric, linear, coupled consolidation model with a new boundary condition and a new time factor, (ii) an automatic and mathematically precise, non-linear, inverse problem solver which includes some reliability testing methods and a kind of regularization technique, and (iii) a method to identify the initial condition.The evaluation method is equally applicable to pore water pressure data displaying monotonic or non-monotonic time variation, with the only difference that the initial condition is identified differently during the inverse problem solution.The necessary testing time is very short provided that the pore water pressure is measured well above the tip. This is attributed to the ability of the one-dimensional, linear consolidation model to account for the geometry and the unloading effects that occur when steady penetration of the static cone penetrometer ceases. 相似文献
6.
在砂井地基固结度计算中,土的固结系数是一个十分重要的参数,土的固结系数通常通过室内试验求得,但室内取得水平向固结系数制样比较困难。利用软基处理过程中孔隙水压力监测资料来推算软土的固结系数,并和室内试验加以对比分析,总结出二者之间的相互关系及变化规律,可供同类工程设计借鉴。 相似文献
7.
Soft clay exhibits creep behavior, but simple methods of surcharge preload assessment generally do not take into account creep during primary consolidation. Because Yin–Graham’s model can predict both primary and secondary settlement, it is employed in this paper to obtain the formulae for critical settlement at the unloading time during surcharge preload and for final settlement at the end of the service life. Because “aged” soft natural clay exhibits obvious apparent preconsolidation pressure during a long-term sedimentary history as a result of creep effects and because the field permeability coefficient is considerably larger than the laboratory permeability coefficient, most field measurements indicate that the theoretical excess pore pressure based on Terzaghi’s theory is greater than the measured excess pore pressure even in soft natural clay with obvious viscous behavior. Because of the widespread application of the degree of consolidation in terms of effective stress based on Terzaghi’s theory in real preload projects, the analytical solution for the surcharge preload period subject to creep is derived through the combination of Yin–Graham’s model and Terzaghi’s theory for consolidation. Compared with the existing solution considering secondary settlement, the formula for the preload period presented in this paper is easily applied to assess the preload period using a chart. The case study described indicates that when the consolidation parameters of Terzaghi’s theory are calculated from field-measured excess pore pressure in preload tests, the surcharge preload period determined as described in this paper is suitable for preload design and performance. 相似文献
8.
沉积形成的水底黏性泥砂自重固结过程表现出显著非线性大变形固结特征,应采用大变形固结理论进行泥砂沉积固结计算。基于软黏土一维非线性大应变固结理论,应用有效应力、渗透系数与孔隙比间扩展幂次函数固结本构关系,由达西定律、有效应力原理、连续介质方程等建立大变形固结控制方程,根据固结单元孔隙水渗流、单元变形与泥砂沉积层固结沉降耦合关系形成黏性泥砂大变形自重固结数值模型。泥砂自重作为固结荷载,数值模型假定沉积泥砂各向同性且固结沉降应变、孔隙水渗流仅发生于竖直方向,为一维单向沉积固结过程;采用泥砂沉降柱试验确定泥砂非线性扩展幂次函数关系参数。模型应用中,划分竖向固结单元,由沉积泥砂固结本构关系确定各固结单元有效应力及超孔隙水应力,通过超孔隙水应力时间维度上的消散过程及各固结参数间的耦合关系计算泥砂固结沉降。数值模型计算结果表明,沉积黏性泥砂自重固结初期表现为有效应力调整过程,初始有效应力与孔隙比根据固结本构关系匹配调整为扩展幂次函数关系;沉积泥砂应变与应力固结度存在20%左右误差,泥砂固结沉降发展快于超孔隙水应力消散过程,证明沉积泥砂固结沉降变形的发展与超孔隙水应力消散并非同步耦合。计算模型应用于室内沉降柱试验模拟淤积黏性泥砂自重固结沉降预测中,模型输出与试验结果符合良好。 相似文献
9.
将隧道周围土体视为均质连续各向同性的饱和弹性介质,采用保角变换的方法将含有隧道的半无限平面映射为同心圆环计算域。根据Terzaghi-Rendulic二维固结理论,建立隧道在不透水的情况下周围土体超孔隙水压力分布的控制方程。然后,采用分离变量法计算得到土体超孔隙水压力分布的解析解,最后,根据弹性理论计算得出隧道中线上方地表固结沉降的计算公式。结合算例,分析盾构施工扰动程度、土体渗透系数、土体弹性模量及隧道埋深等因素对隧道中心上方地表处固结沉降的影响。研究结果表明,地表固结沉降的增加值与隧道外侧初始超孔隙水压力值C0的变化量成正比例关系,施工扰动程度越大所引起的固结沉降越大;土体的渗透系数越大固结沉降速度越快,但土体的渗透系数与最终的地表固结沉降量无关;土体的弹性模量越大,最终的地表固结沉降量越小;隧道埋深越深,地表固结沉降所需时间越长,最终的地表固结沉降量也越大。 相似文献
10.
11.
An analytical solution of one-dimensional consolidation for soft sensitive soil ground is presented. The moving boundary is introduced to indicate the notable change of consolidation behaviour of sensitive soil with the increase of stress level. It is assumed that the soil structure of the upper subsoil gradually destroys downwards with the dissipation of pore pressure, and the coefficient of consolidation as well as the coefficient of permeability of the upper subsoil become small, which hinders the dissipation of pore pressure of the lower subsoil. The consolidation degree curve obtained from the present solution is found to lie between the two curves obtained from Terzaghi one-dimensional consolidation solution with the parameters of the undisturbed soil and the remolded soil. The calculated results provide a new explanation for a general phenomenon in the consolidation of soft sensitive soil ground, as that for high loadings the consolidation is longer than for small ones. It should be pointed out from this study that both the deep mixing method and the long vertical drains methods are effective techniques for improving deep sensitive soil ground. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
12.
The piezocone penetration test (CPTU) is commonly used as a fast and economical tool to identify soil profile and to estimate relevant material properties in soils ranging from fine to coarse-grained. Moreover, in the case of fine-grained soils (clays and silts), the consolidation coefficient and the permeability can be estimated through the dissipation test. Undrained conditions are commonly assumed for the interpretation of CPTU in fine-grained soils, but in soils such as silts, penetration may occur in partially drained conditions. This aspect is often neglected in data interpretation thus leading to an inaccurate estimate of soil properties. This paper investigates numerically the effect of partial drainage during penetration on the measured tip resistance and the subsequent pore pressure dissipation response contributing to a more accurate interpretation of field data. A realistic simulation of the cone penetration is achieved with the two-phase Material Point Method, modelling the soil response with the modified Cam-Clay model. The approach takes into account large soil deformations induced by the advancing cone, soil–water, and soil–structure interactions, as well as nonlinear soil behavior. 相似文献
13.
14.
The coefficient of consolidation is one of the most important parameters that control the rate of consolidation. Conventional consolidation theories assume that the coefficient of consolidation is constant during the whole consolidation process. In the case of sensitive clay, the coefficient of consolidation is strongly dependent on the level of effective stress of clay. With the dissipation of pore water pressure and the increase of effective stress, the soil structure of the upper subsoil is gradually destroyed downwards and its coefficient of consolidation becomes smaller. At the same time, the coefficient of permeability of the vertical drains drops down because of the kinking or bending effect. The destructured upper subsoil and the kinking of the vertical drain hinder the dissipation of the pore pressure in the lower subsoil. This paper presents a model to describe the above important phenomena during the consolidation of sensitive clay with vertical drain. The solution for proposed model can be obtained by extending the closed‐form solution of the consolidation of double‐layered ground with vertical drain by the interactive method introducing the concept of the moving boundary and the reduction of discharge capacity of vertical drain. The numerical results obtained from the finite element method package PLAXIS (Ver. 7.2) are adopted to compare those obtained from the present algorithm. Moreover, the rationality of the moving boundary is explained by the distributions of the excess pore water pressure in natural soil zone along the radial direction. Wenzhou airport project is taken as a case study in this paper. The results for the sensitive soil with decaying sand drain agree very well with the in situ measured data. The calculated results can properly explain two general phenomena observed in the consolidation of soft sensitive soil ground with vertical drains: one is that the time to achieve the same consolidation degree is much longer under heavy loading than that under light loading; the other is that the consolidation speed is much slower in the lower subsoil than in the upper subsoil. Finally, it is indicated that the vertical drains can decrease the hindrance effect of the destructured subsoil. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
15.
Binh-Le Ly 《国际地质力学数值与分析法杂志》1978,2(4):405-409
A method is derived to estimate the degree of consolidation of a saturated, non-homogeneous clay layer. In this method, the diffusion equation govering the dissipation of pore water pressure is transformed so that the pore water pressure can be expressed by a Fourier series. The Fourier coefficients are then evaluated along the trajectory of the diffusion process. The Fourier series does not converge point-wise to the pore water pressure; instead, it converges to the pore water pressure in the mean., The Fourier series can be used to estimate the degree of consolidation, because, by Schwartz's inequality, convergence in the mean implies convergence in the degree of consolidation. 相似文献
16.
软土固结系数刍议 总被引：1，自引：1，他引：0
在土的固结分析中,要正确估算土体中超静孔隙水压力的消散与沉降过程,重要的是确定可靠的固结系数Cv值.通过在天仙一级公路软弱地基路基工程沿线选择的二个试验场地上开展原位试验及室内试验,并结合珠江口某海区软土的固结试验数据,研究软土固结系数的规律,探讨其确定方法.研究表明：（1）由原位孔隙水压力消散试验得到的固结系数Cv值比室内高压固结试验时间平方根法所得到的相应值大一个数量级;（2）由室内高压固结试验时间平方根法所得到的固结系数Cv值大于时间对数法、反弯点法、三点法及司各脱法所得到的Cv值.（3）由于反弯点法、三点法及司各脱法确定固结系数Cv值有其各自的优点,在实际工程中均值得去尝试和推广. 相似文献
17.
Bryan A. McCabe Brian G. Sexton Barry M. Lehane 《Geotechnical and Geological Engineering》2013,31(1):183-197
Although finite element packages facilitating coupled consolidation analyses are increasingly in use, many practitioners still favour linear uncoupled analysis out of familiarity with the use of coefficients of consolidation. However, coefficients of consolidation measured by any single means tend to exhibit significant variation, with mean results from different laboratory and field tests also varying widely, leaving uncertainty over the correct values to apply to field problems. In this paper, a finite difference approach is used to back-calculate operational coefficients of consolidation from pore pressure measurements pertinent to a pile group driven in clay–silt. The research shows that this method is capable of successfully capturing the process of pore pressure dissipation, and that the operational coefficient of consolidation around the pile group is higher than that derived from piezocone dissipation tests in the same material. 相似文献
18.
Finite element cavity expansion analysis investigating the effect of penetration rate on piezocone tests in clay is presented. A coupled analysis was performed, in which the rate of cavity expansion was linked to the penetration rate of the cone and the cone angle, using the assumption that the deformation was wholly radial, and took place only between the cone tip and the cone shoulder. The soil was modelled using modified cam clay with two sets of parameters and varying values of overconsolidation ratio (OCR). The influence of penetration rate on the stress and pore pressure distributions was examined. For slower penetration rates, the excess pore pressure at the cone shoulder is lower since consolidation is permitted coincident with penetration. The radial profiles of post‐penetration voids ratio demonstrate that partially drained penetration is permitted by volume change in the near field, in addition to radial movement in the far field. The radial distribution of excess pore pressure after slow penetration differs from the undrained case, with a relatively low radial gradient existing at the cone face. As a result, the dissipation curves after slow penetration lag behind those following fast penetration. The cone velocity is made dimensionless by normalizing with the coefficient of consolidation and the cone diameter. ‘Backbone’ curves of normalized velocity against normalized tip resistance and excess pore pressure capturing the transition from undrained to drained penetration are derived. The normalized pore pressure backbone curve is unique, whilst the normalized tip resistance shows a small dependency on OCR. These backbone penetration curves are compared with centrifuge model piezocone tests conducted at varying rates, and subsequent dissipation tests. The numerical and experimental results suggest that the value of consolidation coefficient operative during the dissipation phase is 2–4 times higher than the virgin compression value due to changes in the operative soil stiffness, as demonstrated from the stress paths of individual soil elements. The use of multi‐rate penetration tests to deduce values of consolidation coefficient is discussed, in light of these differences. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
19.
盾构施工会对周围土体产生扰动,形成超孔隙水压力,引起工后固结沉降。运用应力释放理论推导与衬砌相邻的土体初始超孔隙水压力计算公式。假定扰动范围边界呈圆弧状,确定初始超孔隙水压力的分布范围;同时运用应力传递理论,推导分布范围内任一点土体的初始超孔隙水压力计算公式。通过对实测资料的分析可知,计算值与实测值吻合较好。算例分析表明,与衬砌相邻的土体初始超孔隙水压力呈近似圆形（顶部小、底部大）;随着到衬砌的径向距离增加,土体初始超孔隙水压力呈凹曲线形状;隧道底部的等值线最密,即变化最快;隧道顶部上方土体、不同深度处土体初始超孔隙水压力,以隧道轴线处为最大,呈现类似Peck曲线形状。 相似文献
20.
假设桩周土体为饱和黏弹性介质,采用Burgers流变模型进行描述,同时考虑竖向和径向固结,建立了固结控制方程。根据不排水和自由排水情况,将边界条件分为3类并分别得到超孔隙水压力消散的级数解答,该解答能够为孔压静力触探反求固结系数提供一定的理论依据。在此基础上编制了应用程序,对Burgers流变模型中主要参数进行了分析。结果表明,地基表面自由排水、桩端地基不排水条件下,在一定深度以内的桩周土体的固结速度随深度降低,但超过某一范围后固结速度趋于稳定;上、下边界均自由排水条件下,固结速度随深度增加呈现下降、稳定、升高;上、下边界均不排水条件时,孔压消散速度不随深度变化,可简化为本解答仅考虑径向固结的特例。同时土体的流变特性对超孔隙水压力消散的影响比较显著,流变参数G1/1的变化使超孔隙水压力趋于某不为0的定值,且该值随G1/1比值的增大而增加;其他参数不变时,土体剪切刚度比G1/G2的增大会引起孔压消散速度的下降。 相似文献