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.

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.

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.

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.

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.

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.

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

新型斜撑-排桩基坑支护体系工程应用分析

在软土较深基坑工程中,新型斜撑-排桩的基坑支护体系逐步得到推广与应用。以某基坑工程采用的斜撑-双排桩支护为例,采用有限元软件Z-soil对该支护体系进行数值模拟分析,通过对比现场监测数据,从支护体系内部的受力特性及围护结构变形对该体系的适用性进行评价。研究结果表明:该支护体系可用于软土基坑工程,斜撑轴力在基坑开挖阶段轴力稳步提升,基坑开挖到底后实测斜撑轴力可达800kN;该体系在控制围护结构水平变形及周边沉降方面效果明显,基坑开挖到底后,围护结构的水平位移最大约30mm。该新型基坑支护体系中的斜撑通过腰梁与围护桩形成较统一的整体结构,相互之间起到有效联合支撑作用,提供了有效的结构抗力,可广泛用于软土基坑工程。

双排桩-锚索支护体系在软土地区基坑中的应用

为研究双排桩-锚索结构在软土基坑中的内力变化规律及可行性,本文通过理论计算及数值模拟,以东莞某软土基坑工程为例,对结构水平位移、弯矩等参数的变化规律进行了研究,并将实测值与计算值进行了对比。结果表明,卸荷板可提供一个反弯矩,增加结构稳定性;支护结构水平位移理论及数值计算曲线与实测曲线变化趋势基本相似,变形值均在规范允许范围之内;双排桩-锚索结合卸荷板体系应用于场地受限的软土地区基坑支护效果较好。

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.

淤积-潮汐耦合作用下沉管隧道接头变形特征研究

重点研究了循环荷载耦合下沉管隧道接头的变形特征。通过建立管-土相互作用模型,将隧道简化为Timoshenko梁模型,将地基简化为Kerr地基模型,同时考虑基床系数随时间变化,建立管节-接头理论模型,分析接头处转角和挠度差。对比荷载叠加和荷载耦合两种不同加载方式下沉管隧道接头变形问题,讨论淤积荷载引发的结构变形在总变形中的贡献比例,以及适合分析运维期接头变形问题的理论解析方法。与ABAQUS数值计算结果进行比较,说明方法的适用性。以甬江沉管隧道实测数据对比分析表明:循环荷载耦合作用下对沉管隧道的动力响应影响显著,大于由单个荷载引起结构变形的叠加。淤积荷载引起的沉管结构变形占据耦合荷载导致变形的75%~125%,提出的理论模型可以很好地分析实测沉降结果,验证了提出理论模型的有效性。

大尺寸三维电动-水力渗透协同固结淤泥能耗特性

为解决目前电渗应用的高能耗困境及大尺寸模拟困难等问题,提出针对低渗透性、高含水率土体的电动-水力渗流协同作用的三维固结方法。自行研制了一套阴极-集水-排水协同作用的多功能集排水系统,采用间歇式抽水代替连续抽水。采用三维电动-水力渗流固结系统对取自贡湖湾湿地和白旄堆场的2种不同太湖底泥进行了电渗试验研究,同时对白旄堆场太湖底泥进行了传统一维电渗固结试验,并对2种试验条件下的单位体积排水能耗和单位体积单位排水量能耗等关键指标进行了对比。结果表明:三维集水井设计可大大降低土体阴极附近电阻,间断性提高系统电流,提高排水效率,降低电渗总能耗;间歇式抽水可间歇性降低系统内总电流,利用电动-水力协同作用,保持系统渗流的连续性;三维电动-水力渗流固结系统的电流呈周期性减小—增大模式,并且降低速率较慢,尤其对于有机质含量较高的土体,电渗过程电流始终保持在一个较高的水平,提高了排水固结效率。三维电动-水力渗流固结系统的单位体积排水能耗和单位体积单位排水量能耗分别约为一维电渗系统的2/3和1/30。在高含水量软土固结排水中具有显著的节能效果。三维电动-水力渗流固结系统可以提高排水固结效率、大幅度降低能耗,为实际工程应用提供了可靠的理论、设计依据和数据支持,具有很好的推广价值。

桩顶荷载对软土地基-群桩基础动力相互作用的影响与机理分析

设计并实施了软黏土地基-群桩基础体系的动态离心模型试验,通过设置两组不同桩顶荷载的模型预制桩,在50g重力加速度环境下,获取了天然地震动作用下软黏土层的加速度、承台横向位移及桩身弯矩,并据此推导计算了桩身竖向变形、桩土相对位移与桩周土抗力,分析研究了桩顶荷载对软黏土-群桩基础体系地震动响应的影响及作用机理。结果表明:①在地震作用下桩身弯矩的反应结果比较复杂,当桩身弯矩沿深度方向出现唯一零点时,桩身弯矩随埋深的增加呈现出先减小后增大又减小的趋势,且桩顶荷载增加时出现这一情况的概率提高了7.58%;②桩顶荷载对桩身弯矩有一定的影响,桩顶荷载增加时,桩身弯矩的最大值随之减小7%,桩周土抗力随之增大25.32%;③软黏土发生震陷时,土体相对位移在桩-土相互作用中起主导作用,桩周土抗力与桩身弯矩的最大值均出现在土体相对位移最大值附近。

竖直-水平向荷载作用下窄深型筒形基础承载特性研究

筒形基础最早源于深海吸力锚基础,被逐步拓展到防波堤、单点系泊基础、张力腿平台基础、海上导管架基础、海上风电等不同的海洋工程领域。采用PLAXIS岩土有限元软件,针对黏性土地基上高径比大于1的窄深型筒形基础在竖直向、水平向及V-H二维荷载作用下的地基承载特性进行分析,研究随着高径比的增长对筒形基础地基承载能力的变化规律,获得的筒形基础竖直向及水平向地基极限承载力与高径比的关系式,建立V-H复合加载模式下筒形基础破坏包络面方程,分析了不同受力条件下的地基失稳的破坏状态,可为工程应用提供参考。

软土-基岩条件下地铁车站施工间歇期的结构变形监测与分析

针对软土-基岩条件下地铁车站施工间歇期的结构变形监测与评价中存在的不足,将布拉格光纤光栅传感技术应用于车站施工间歇期的结构变形监测中。以深圳某地铁车站为例,将光纤传感器安装在车站主体结构上,远程自动化获取结构在施工间歇期的变形信息。监测结果表明:软土-基岩地基的主体结构在施工间歇期存在一定程度变形,监测期间车站底板的最大应变为62με,车站地下二层的墙柱结构最大变形为249με,位于软土-基岩分界面附近的墙柱结构同时受到剪切和压缩应力的影响,表明软土-基岩地基的车站设计应分别考虑底板和底层墙柱的抗弯和抗剪。

复合塑料排水板真空-堆载联合预压处理工艺创新设计研究

针对深厚软土地基的处理,真空-堆载联合预压法是在真空预压施工的基础上进行优化后的一种地基处理工艺。研究在传统真空-堆载联合预压法的基础上,提出一种复合塑料排水板为竖向排水通道,对深厚软土地基进行处理,利用有限元分析方法对软土地基复合塑料排水板真空-堆载联合预压效果进行模拟。研究结果表明,复合塑料排水板真空-堆载联合预压与一般的现场实际处理效果相比沉降值差异在2%左右,表明采用该方法开展深厚软土地基处理的堆载高度合理、可行。

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。