Engineering Geological Zoning of Soft-Soil Foundation Based on Combination Weighting and Extension Methods

Different criteria and factors are used in different methods of soft soil foundation settlement calculation and engineering geological zoning.The methods used are not universally suitable for complex geological environments.The post-construction settlement of soft soil foundations are especially large and difficult to calculate.In addition,there are many deficiencies in the current methods used for engineering geological zoning.Focusing on the need of establishing engineering geological zoning for areas with soft soil foundations in the Tianjin Marine Economic Area,combination weighting and extension methods were introduced.An evaluation model for the settlement of soft soil foundations was established using multiple factors and large amounts of data.This evaluation model is accurate and objective for delineating engineering geological zoning.These methods eliminate deficiencies by considering both objective and subjective factors,and help obtain an objective and accurate result.

Design parameter optimization of beam foundation on soft soil layer with nonlinear finite element

Finite element method was performed to investigate the influences of beam stiffness,foundation width and cushion thickness on the bearing capacity of beam foundation on underlying weak laminated clay.The comparison between numerical results and results from field test including plate-bearing test and foundation settlement observation shows reasonable agreement.According to the numerical results,the beam width,length,cross section and cushion thickness were optimized.The results show that the stresses in subgrade soil decrease greatly with increasing the cushion thickness and width of foundation.However,the foundation settlement and influencing depth of displacement also increase correspondingly under conditions of relatively thinner cushion thickness.For the foundations on underlying weak layer,increasing foundation width merely might be inadequate for improving the bearing capacity,and the appropriate width and cushion thickness depend on the response of subgrade.A comparison between rigid and flexible beams was also discussed.The influence of a flexible beam foundation on subgrade is relatively smaller under the same loading conditions,and the flexible beam foundation appears more adaptable to various subgrades.The proposed flexible beam foundation was adopted in engineering.According to the calculation results,beam width of 2.4 m and cushion thickness of 0.8 m are proposed,and a flexible beam foundation is applied in the optimized design,which is confirmed reasonable by the actual engineering.

Research of deformation prediction method of soft soil deep foundation pit

In view of the characteristics of soft soil deep foundation pit for the construction and geotechnical characteristics of the special medium,it is difficult to calculate theoreti- cally accurately structural deformation of the foundation pit,so in the course of excavation on the construction of the information is particularly important.The analysis and compari- son of several popular non-linear forecasting methods,combined with the actual projects, set up a grey theoretical prediction model,time series forecasting model,improved neural network model to predict deformation of the foundation pit.The results show that the use of neural network to predict with high accuracy solution,it is the foundation deformation prediction effective way in underground works with good prospects.

Highway Construction of Soft Soil Foundation Processing Analysis

Highway is an important channel to connect regional economic development,and is an indispensable part of modern transportation system.In view of the extensive nature of highway cover space and the existence of diversified construction environment,climate and geological influence in highway construction,soft soil foundation is one of the more typical geological forms.With wide distribution in our country,seen as a big difficulty,highway construction technology and directly affect the quality of highway construction,cost,if not properly handled,will cause the soft soil foundation highway engineering structure is not stable,prone to accidents in use.In this paper,we study the treatment of soft soil foundation in highway construction,and put forward some reasonable Suggestions.

Estimation of Undrained Bearing Capacity for Offshore Soft Foundations with Cyclic Load

The degradation strength of soils under cyclic loading is studied and a method for determining the cyclic degradation strength with cyclic triaxial tests is given in the paper. Furthermore, a dum my static method for estimating the undrained bearing capacity for offshore soft foundation under wave loads is developed. It can consider the effect of the difference of cyclic stress for different parts of the foundation on both the degradation strength of the foundation soil and the bearing capacity so that the estimated result can better reflect the real condition of foundation under cyclic loading. The method can be applied to plane and space problem.

Pseudo-Static Elasto-Plastic Cyclic Creep Model and Cyclic Stability of Offshore Soft Foundation

The accumulative shear deformation of soft clays under cyclic loads is considered as pseudo-static creep. A pseudo-static elasto-plastic cyclic creep model is developed based on the visco-elasto-plastic theory. The parameters in the model are determined by cyclic triaxial soil tests. A method for analyzing the stability of offshore soft foundation under wave loads is given by combining the model with pseudo-static creep analysis. An example is analyzed by the method. The results show that the horizontal and vertical stability of foundations under wave loads can be analyzed by it and the analytical results are qualitatively consistent with the observed failure modes of shallow foundations.

Review of research on high-speed railway subgrade settlement in soft soil area

Construction issues of high-speed rail infrastructures have been increasingly concerned worldwide,of which the subgrade settlement in soft soil area becomes a particularly critical problem.Due to the high compressibility and low permeability of soft soil,the post-construction settlement of the subgrade is extremely difficult to control in these regions,which seriously threatens the operation safety of high-speed trains.In this work,the significant issues of high-speed railway subgrades in soft soil regions are discussed.The theoretical and experimental studies on foundation treatment methods for ballasted and ballastless tracks are reviewed.The settlement evolution and the settlement control effect of different treatment methods are highlighted.Control technologies of subgrade differential settlement are subsequently briefly presented.Settlement calculation algorithms of foundations reinforced by different treatment methods are discussed in detail.The defects of existing prediction methods and the challenges faced in their practical applications are analyzed.Furthermore,the guidance on future improvement in control theories and technologies of subgrade settlement for high-speed railway lines and the corresponding challenges are provided.

New artificial boundary condition for saturated soil foundations

Anew artificial boundary model based on multi-directional transmitting and viscous-spring artificial boundary theories is proposed to absorb stress waves in a saturated soil foundation in dynamic analysis. Since shear waves （S-waves） are the same in a saturated soil foundation and a single-phase medium foundation, a tangential visco-elastic boundary condition for a single-phase medium foundation can also be used for saturated soil foundations. Thus, the purpose of the artificial boundary proposed in this paper is primarily to absorb two types of P-waves in a saturated soil foundation. The main idea is that the stress of the P-waves in the saturated soil foundation is decomposed into two types. The first type of stress, δra＇ is absorbed by the first artificial boundary. The second type of stress, δrb, is balanced by the stress generated by the second artificial boundary. Ultimately, both types of P-waves （fast-P-waves and slow-P-waves） are absorbed by the artificial boundary model proposed in this paper. In particular, note that the fast-P-waves and slow-P-waves are absorbed at the position of the first boundary. Thus, the artificial boundary model proposed herein can simultaneously absorb P-fast waves, P-slow waves and shear waves. Finally, a numerical example is given to examine the proposed artificial boundary model, and the results show that it is very accurate.

Time-dependent deviation of bridge pile foundations caused by adjacent large-area surcharge loads in soft soils and its preventive measures

Time-dependent characteristics(TDCs)have been neglected in most previous studies investigating the deviation mechanisms of bridge pile foundations and evaluating the effectiveness of preventive measures.In this study,the stress-strain-time characteristics of soft soils were illustrated by consolidation-creep tests based on a typical engineering case.An extended Koppejan model was developed and then embedded in a finite element(FE)model via a user-material subroutine(UMAT).Based on the validated FE model,the time-dependent deformation mechanism of the pile foundation was revealed,and the preventive effect of applying micropiles and stress-release holes to control the deviation was investigated.The results show that the calculated maximum lateral displacement of the cap differs from the measured one by 6.5%,indicating that the derived extended Koppejan model reproduced the deviation process of the bridge cap-pile foundation with time.The additional load acting on the pile side caused by soil lateral deformation was mainly concentrated within the soft soil layer and increased with the increase in load duration.Compared with t=3 d(where t is surcharge time),the maximum lateral additional pressure acting on Pile 2#increased by approximately 47.0%at t=224 d.For bridge pile foundation deviation in deep soft soils,stress-release holes can provide better prevention compared to micropiles and are therefore recommended.

A 3D Numerical Simulation of Sand Drain Element in the Soft Soil of Guangzhou-Zhujiang Highway,China

The behavior of sand drain was estimated so that the size of very large load-pressure could be eliminated by changing the configuration of the sand drain elements into sand wall.A 3D mathematical model was formulated to transform the configuration of a sand drain into a sand wall to minimize or eliminate the excessive stress and primary settlement on the road base.This was barely considered in the past. According to soil mechanics theory and seepage characteristics of sand drain in road base foundations, a 3D sand drain element in FEM format was generated,and a matrix expression was formulated which was introduced into 3D Biot Consolidation

基于ANSYS对软基预应力管桩在建筑施工中的应用

预应力管桩具有承载能力强、便于运输和吊装、接桩速度快等优势。在软弱地基工程的基础设计与建设中,预应力管桩对承载能力有较好的适应能力。在工程实践中,采用的方法有2种,一种是静力压桩,另一种是锤击桩。通过结合某软土地区房建小区建设中预应力管桩的工程实例,并依托ANSYS平台,对预应力管桩在软基房建施工中的应用效果进行了研究,并对其在实践中遇到的问题和解决办法进行了分析,同时介绍了需要注意的问题,其可为相似工程的建设提供借鉴。

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。

四参数Logistic模型在软土地基沉降推算中的应用

地基的沉降变形是反映工程(如建筑物、公路和铁路等)安全性的重要指标,也是岩土工程领域的热门研究课题。为进一步加深对软土地基沉降发展规律的认识和提高软土地基沉降推算的准确性,采用了四参数Logistic模型对软土地基沉降-时间曲线进行模拟推算,并基于实例计算结果将四参数Logistic模型与地基沉降推算方法中常用的三参数Logistic模型、Gompertz模型和双曲线模型进行了比较研究。计算结果表明,四参数Logistic模型作为软土地基沉降发展规律的模拟描述是适合的,比三参数Logistic模型推算沉降精度更高;相比于常用的三参数Logistic模型、Gompertz模型和双曲线模型,四参数Logistic模型推算后期沉降发展趋势效果更好;四参数Logistic模型在软土地基沉降推算应用方面具有更好的可靠性和实用性,值得推广应用。

考虑SSI效应的框架结构体系动力响应分析

为了研究软土地基上考虑土-结构相互作用(SSI)对高层结构动力响应的影响机理,文中设计了1:6比例的框架结构模型,分别进行了考虑结构-桩-土动力相互作用和刚性地基上框架结构的振动台试验。通过对比考虑SSI的框架模型和刚性地基框架模型的结构动力响应,总结出本试验中SSI效应对高层纯框架结构的影响规律:考虑SSI效应后,桩-土相互作用对地震波具有明显的过滤作用,大震时一般有削弱峰值的作用,且整体结构的振动频率降低,阻尼比提高。从结构内力来看,SSI效应减小了楼层剪力和弯矩,有效降低了地震动对结构的影响。从结构位移看,当震级较小时,SSI效应减小了结构的弹性位移和层间位移角,降低了地震对结构的作用;当震级较大时,上部楼层由于模型差异和损伤累积,部分工况下SSI效应增大了结构位移响应。从结构加速度看,SSI效应降低了楼层绝对加速度和相对加速度,同样减小了地震对结构的作用。这是由于SSI效应下桩-土相互作用产生辐射阻尼和滞回阻尼,消散了整个结构-桩-土系统的能量,同时软土地基传递给上部结构的辐射波改变了结构的动力特性。

深厚软土基坑放坡开挖结合SMW工法桩支护的数值分析

以广东中山某城市综合开发项目二层地下室基坑工程为例,开展了大面积放坡开挖结合悬臂式SMW工法桩支护形式在深厚软土基坑的应用研究。基于小应变土体硬化模型(HSS),利用三维有限元软件模拟和预测基坑的变形发展规律,研究放坡高度、被动区加固、支护桩型钢间距对基坑变形的影响,同时将现场监测结果与数值模拟结果进行对比分析。研究表明,本项目深厚软土基坑采用大面积放坡开挖结合悬臂式SMW工法桩支护的变形及内力均在合理控制范围内;基于小应变土体硬化模型(HSS)的数值模型可有效模拟和预测基坑开挖的变形,数值计算结果和现场实测结果具有较好的一致性;上部放坡开挖高度对基坑影响较大,被动区加固对基坑支护桩变形有一定影响,型钢间距从1.2 m至0.6 m对基坑变形影响较小。

Characteristic Test Study on Bearing Capacity of Suction Caisson Foundation Under Vertical Load

Suction caisson foundations are often subjected to vertical uplift loads,but there are still no wide and spread engineering specifications on design and calculation method for uplift bearing capacity of suction caisson foundation.So it is important to establish an uplift failure criterion.In order to study the uplift bearing mechanism and failure mode of suction caisson foundation,a series of model tests were carried out considering the effects of aspect ratio,soil permeability and loading mode.Test results indicate that the residual negative pressure at the top of caisson is beneficial to enhance uplift bearing capacity.The smaller the permeability coefficient is,the higher the residual negative pressure will be.And the residual negative pressure is approximately equal to the water head that causes seepage in the caisson.When the load reaches the ultimate bearing capacity,both the top and bottom negative pressures are smaller than Su and both the top and bottom reverse bearing capacity factors are smaller than 1.0 in soft clay.Combined the uplift bearing characteristics of caisson in sandy soil and soft clay,the bearing capacity composition and the calculation method are proposed.It can provide a reference for the engineering design of suction caisson foundation under vertical load.

Settlement Mode Analysis for An Immersed Tube Tunnel Considering A Nonuniform Foundation Under Tidal Load

Immersed tube tunnels are usually placed on soft soil layers in cross-sea tunnelling engineering.Owing to the influence of stratum conditions and slope design,the longitudinal distribution of substratum layers is generally uneven.Thus,the inhomogeneous deformation of the element-joint becomes the key factor in the failure of the immersed tube tun-nel.Therefore,a corresponding calculation method for joint deformation is needed to explore the deformation law of immersed tube tunnels.By constructing a three-section immersed tube tunnel analysis model(TTM),the relationship between the two types of deformation of the immersed tube tunnel structure in a longitudinal nonuniform soft soil foundation is described,and the deformation characteristics of the immersed structure under different boundaries are discussed.Based on the mechanical behaviour of the joint and foundation,according to the Timoshenko beam on the Vlasov two-parameter foundation(VTM),considering the tidal cyclic load during the operation and maintenance period,an example analysis is given.Moreover,the deformation characteristics and development trend of the immersed tube tunnel under the influence of different soil layers are discussed.The obtained results have a certain guiding significance for the deformation calculation of immersed tube tunnels.

软土地区深基坑中SMW工法桩+斜撑组合支护施工技术研究

文中以某高层建筑为例,对排桩+锚索支护、SMW工法桩+斜支撑两种方案进行比较分析,深入研究SMW工法桩+斜撑组合支护技术在软土地区深基坑中的应用,进一步提高SMW工法桩+斜撑组合支护施工质量及安全性,减少事故发生,以期为未来软土地区类似的基坑工程提供相关实践经验。

Bending the Foundation Beam on Elastic Base by Two Reaction Coefficient of Winkler’s Subgrade

A new method for analysis of counter beams is presented in the paper. The analysis has taken into account their stiffness EI, Winkler’s space with modulus of subgrade reaction k and equality deformities of the foundation beam with the ground. The solution is found by using the numerical analysis of the Winkler’s model, with variation of different moduli of the subgrade reaction k2 outside the force zone r, while under the force P exists the modulus of the subgrade reaction k, up to the definition of minimum bending moments. The exponential function k2(r), as the geometric position of the minimum moments is approximately assumed. From the potential energy conditions of the reciprocity of displacement and reaction, the width of the zone r and the modulus of the subgrade reaction k2 are explicitly determined, introducing in the calculation initial and calculation soil displacement wsi successively. At the end of the paper, it presented numerical example in which the influence of k and k2 values on bending moments of the counter beam is analyzed. The essential idea of this paper is to decrease the quantity of the reinforcement in the foundations, beams, i.e. to obtain a cost-efficient foundation construction.

软土地区基坑前排倾斜双排桩支护现场试验及工作机理

基坑倾斜桩支护大幅提高了无支撑支护结构的适用深度,具有造价低、施工便捷、绿色低碳、变形控制好等特点,然而目前研究多集中在单排倾斜桩,对前排倾斜双排桩支护的试验、理论与应用的研究均较少。针对前排倾斜双排桩支护基坑开展了现场试验,同时采用有限元方法分别从前排斜桩、桩顶连梁及桩间土体作用3个方面深入探究了前排倾斜双排桩支护的工作机理,进一步分析了直斜桩长度和排距对前排倾斜双排桩支护结构受力变形性能的影响。结果表明,相比传统竖直双排桩,前排倾斜双排桩支护结构的支护性能显著提升,前排斜桩主要发挥“斜撑”作用,连梁主要起到将直斜桩及冠梁连接成一个空间刚架的作用,桩间土体能够提高桩土摩阻力,进而有效减小前排倾斜双排桩变形。研究成果有助于前排倾斜双排桩的推广和应用。