竖井地基固结解析理论与有限元分析

发布时间：2018-08-08 16:56

【摘要】：在含水量高、压缩性高、抗剪强度低的软土地基上设置竖井并结合堆载或真空负压,能加速地基排水固结、提高土体强度、减少构筑物工后沉降和差异沉降。该技术施工简便、效果可靠且造价低廉,已在公路、铁路、港口、机场等大型基础设施建设中广泛应用。为了合理预测竖井地基的固结发展,学者们在竖井固结解析解和数值解方面开展了大量研究且已取得大量成果。但同时应看到,目前各种计算方法得到的理论预测值与实测值还有一定差距,对可能的工后沉降和沉降差预测不准。另外,近些年来出现的竖井处理深度加大和工期要求加紧等情况,均给竖井地基的设计计算方法提出了挑战。为此,本文从解析理论和有限元法两方面开展了竖井地基固结问题的研究。 (1)首先对以往研究成果从理论方法和参数取值两方面进行了综述,结果发现：目前被国内外广泛接受的单井地基径向固结度简化计算公式均可表述为一个统一的公式,不同方法在涂抹效应和井阻效应的考虑上略有差异；单井固结理论近些年的新进展主要包括非线性理论、非达西渗流理论、考虑涂抹区渗透性渐变理论以及针对真空预压法的固结理论等；竖井地基固结分析参数取值方面已经开展了大量研究,但得到的参数取值范围较大,这将给计算结果的可靠性带来影响。 (2)通过对现有塑料排水板通水量特性研究进行综述发现,排水板通水量一方面随侧向压力增大而减小,另一方面在地基固结过程中会因为排水板的弯折和淤积堵塞等影响而逐渐减小。基于这一认识,假定通水量随地基深度线性减小、随时间呈指数函数减小,获得了描述井阻空间属性和时间属性的变井阻效应数学模型。然后,仿效Hansbo和谢康和竖井固结经典理论推导方法,建立了瞬时加荷条件下考虑变井阻效应的固结理论。计算分析表明：相对于采用短期通水量恒定值进行计算的竖井固结理论,变井阻固结理论得到的固结速率减缓；当考虑通水量随地基深度减小时,地基深部土层的超静孔压消散变得更加缓慢；当竖井渗透性减小到与井周土体相当,此时竖井不再起排水通道作用,径向固结将停止。另外,将变井阻固结理论应用于室内大型模型试验和工程实例分析,结果发现变井阻理论模拟的结果较传统理论预测结果与实际情况更为接近。 (3)通过引入Hansbo非达西渗流假定和变渗透系数假定,得到渗流双重非线性模型。然后,在Biot固结理论研究基础上,应用加权残数法推导了考虑该模型的有限元方程,并自编了相应的计算程序。通过开展参数影响分析得到：考虑渗透系数随时间减小后,地基固结速率减缓趋势明显；增大非达西渗流参数m或il,固结发展速率减缓,但该影响在10%以内；考虑非达西渗流后,地基固结速率随荷载减小而减缓。 (4)进一步引入土体分段线性e-lgp模型,得到了土体压缩非线性和渗流双重非线性模型。然后,结合宁波土样固结渗流联合测试试验,介绍了该模型的参数获取过程。在给出相应的控制方程和有限元方程基础之上,详细阐述了非线性有限元法编程实现过程。最后,通过算例验证了程序的可靠性,并对考虑土体多重非线性模型时竖井地基的固结性状进行了较为详细的分析,获得了一些有益认识。 (5)对目前广泛应用的竖井地基平面应变二维等效方法进行了评述。然后,以成层土竖井地基算例和未打穿竖井地基算例对六种二维等效方法进行了考量,结果发现：地基土成层性对二维等效方法计算结果的可靠性有较显著影响,而竖井未打穿情况的影响不明显；在本文算例参数取值情况下,各种方法的误差情况差别较大,Chai(2001)方法和Tran(2008)方法得到的结果相对可靠。此外,基于等效竖向渗透系数思想,建立了以单元体流量等效的一维变形、一维渗流简化分析方法。通过算例检验发现,该法计算效率大大提高,但相比于Chai法计算精度并未改善,且计算过程相对复杂。 (6)考虑到竖井地基三维有限元分析计算量大、前处理困难,提出了将井与涂抹区在地基横截面内划分为一个网格的复合单元法。复合单元的未知场由10结点单元和线性插值函数来刻画。复合单元内部全局坐标结点自由度和局部坐标结点自由度之间的联系则通过引入经典单井固结解析理论得到建立。之后,基于加权残数法详细推导了复合单元法有限元方程,并编制了相应的计算程序。通过对单井地基和群井地基算例进行验证分析,发现复合单元法计算精度较高、计算效率也有所改善。 (7)基于大型商业软件和自编程序开展了某电厂海堤下竖井地基固结发展过程的数值模拟。分析过程中,采用Chai(2001)二维等效方法和平面应变空间渗流分析方法分别进行了计算,结果发现：两种数值方法模拟得到的地基变形发展规律近似一致；后者考虑空间渗流得到的沉降略大、侧向位移略小；两种方法的孔压预测结果差异较为明显。数值模拟结果还表明：海堤下软土地基经过塑料排水板处理后,地基固结速率较快,地基中超静孔压在各级堆载间歇期均有明显回落；在给定的堆载方案下,软土地基变形发展速率均在控制范围内。
[Abstract]:Setting vertical wells on soft soil foundation with high water content, high compressibility and low shear strength, combined with loading or vacuum pressure, can accelerate the consolidation of the foundation drainage, improve the strength of soil and reduce the settlement and differential settlement of structures. The technology is simple, reliable and low cost, and has been used in large infrastructure such as highway, railway, port, airport and so on. In order to reasonably predict the consolidation and development of the shaft foundation, a lot of scholars have done a lot of research on the analytical solution and numerical solution of the vertical shaft consolidation, but there have been a lot of achievements. At the same time, it should be seen that there is still a certain gap between the theoretical prediction value and the measured value obtained by various calculation methods at present, and the possible settlement and settlement difference for the possible after work The prediction is not accurate. In addition, in recent years, the depth of shaft treatment and the demand for the construction period have been tightened, all of which have challenged the design and calculation method of the shaft foundation. Therefore, this paper has carried out the study of the consolidation of the shaft foundation from two aspects of the analytic theory and the finite element method.
(1) first of all, the previous research results are reviewed from two aspects of theoretical method and parameter value. It is found that the simplified formula for simplified calculation of radial consolidation of single well foundation, which is widely accepted at home and abroad, can be expressed as a unified formula. Different methods are slightly different in the consideration of the effect of smear effect and well resistance effect; the single well consolidation theory is different. The new progress in recent years mainly includes nonlinear theory, non Darcy percolation theory, the theory of permeation gradient in the smear area and the consolidation theory for vacuum preloading. A lot of research has been carried out on the parameter value of the consolidation analysis of vertical shaft foundation, but the range of parameters obtained is large, which will give the reliability of the calculation results. Impact.
(2) through the study of the current water flow characteristics of the existing plastic drain board, it is found that the water flow rate of the drainage plate decreases with the increase of lateral pressure, on the other hand, it will decrease gradually in the consolidation process of the foundation because of the influence of the bending of the drain plate and the clogging of the silt. The time exponential function decreases, and the mathematical model of the variable well resistance effect is obtained to describe the spatial and temporal properties of the well resistance. Then, the consolidation theory which considers the change of the well resistance effect under the instantaneous loading condition is modeled after the classical theoretical derivation of the Hansbo and Xie Kang and the vertical shaft consolidation. The consolidation theory obtained by the value is calculated, and the consolidation rate of the variable well resistance consolidation theory slows down. When the water flow is reduced with the depth of the foundation, the excess pore pressure dissipation of the deep soil layer becomes more slow; when the shaft permeability is reduced to the surrounding soil, the vertical well does not play the drainage channel, and the radial consolidation will stop. In addition, the theory of variable well resistance consolidation is applied to large-scale indoor model test and engineering case analysis. It is found that the result of the theory of variable well resistance is closer to the actual situation than that of the traditional theory.
(3) through the introduction of the Hansbo non Darcy percolation hypothesis and the assumption of variable permeability coefficient, the seepage double nonlinear model is obtained. Then, on the basis of the Biot consolidation theory, the finite element equation considering the model is derived by using the weighted residual method, and the corresponding calculation program is made up. By the analysis of the parameter influence, the permeability coefficient is considered. The consolidation rate slowed down with time, and the consolidation rate slowed down with the increase of non Darcy percolation parameters m or IL, but the effect was less than 10%. The consolidation rate of foundation slowed down with the decrease of load.
(4) further introducing the piecewise linear e-lgp model of soil, the nonlinear and percolation nonlinear model of soil compression is obtained. Then, combined with the combined test test of Ningbo soil sample consolidation seepage, the parameter acquisition process of the model is introduced. On the basis of the corresponding control equation and the finite Yuan Fangcheng Foundation, the nonlinear finite element is expounded in detail. In the end, the reliability of the program is verified by an example, and the consolidation behavior of the shaft foundation is analyzed in detail in the consideration of the multiple nonlinear soil model, and some useful understanding is obtained.
(5) the two-dimensional equivalent method of plane strain of shaft foundation, which is widely used at present, is reviewed. Then, six two dimensional equivalent methods are considered by taking the example of vertical well foundation and the example of not wearing the shaft foundation. The results show that the foundation soil stratification has a significant influence on the reliability of the calculation results of the two-dimensional equivalent square method. It is not obvious that the impact of the well is not worn; in this paper, the error conditions of various methods are different, the results obtained by the Chai (2001) method and the Tran (2008) method are relatively reliable. In addition, based on the idea of the equivalent vertical permeability coefficient, a one-dimensional deformation of one dimensional flow and a one-dimensional seepage analysis is established. The computational efficiency of this method is greatly improved, but the computational accuracy is not improved compared with Chai method, and the computational process is relatively complicated.
(6) considering that the three-dimensional finite element analysis of the shaft foundation is large and the preprocessing is difficult, a composite element method is proposed to divide the well and the smear area into a grid in the cross section of the foundation. The unknown field of the composite element is depicted by the 10 node element and the linear interpolation function. The degree of freedom and the local coordinate of the global coordinate node in the composite element are also described. The connection between point freedom is established by introducing the classical single well consolidation theory. Then, based on the weighted residual method, the finite element equation of the composite element method is derived in detail, and the corresponding calculation program is compiled. The efficiency has also improved.
(7) based on the large-scale commercial software and self compiled program, the numerical simulation of the consolidation and development process of the subgrade foundation under the sea embankment of a power plant is carried out. In the analysis process, the Chai (2001) two dimensional equivalent method and the plane strain spatial seepage analysis method are respectively calculated. The results are found: the development law of the foundation deformation simulated by the two numerical methods The settlement of the two methods is more obvious. The results also show that the consolidation rate of the foundation under the soft soil foundation under the sea embankment is faster after the plastic drainage plate is treated by the plastic drainage plate, and the hyper static pore pressure in the foundation is obvious in the batch period. Under the given surcharge plan, the rate of deformation development of soft soil foundation is within the control range.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2013
【分类号】：TU472

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