Influence of the Elastic Modulus of the Soil and Concrete Foundation on the Displacements of a Mat Foundation

In this paper, we suggest to study the behavior of a mat foundation on subsoil from the plate theory taking into account the soil-structure interaction. The objective is to highlight the soil-structure interaction particularly the influence of the rigidities of the soil and the concrete on the subgrade reaction (k) and the displacements of the mat foundation subjected to vertical loads. From plate theory and the soil-structure interaction, the general equation is reached. This equation depends more on the subgrade properties than the concrete foundation properties. Consequently, the behavior of the mat foundation is more influenced by soil properties than the concrete.

Influence of the Soil-Structure Interaction in the Behavior of Mat Foundation

This article focuses on the study of the behavior of a soil mass under a plate subjected to a uniformly distributed load. The aim of this paper is to highlight the soil-structure interaction with a linear variation of the mechanical properties of the soil with the depth. The theory of plates and the soil-structure interaction has allowed reaching the general equation of the problem which depends on both the mechanical properties of the concrete and the subgrade. This study shows that the linearity of the elastic modulus of subgrade leads to larger displacements when this modulus is assumed to be constant in the soil mass. It also shows that the Poisson’s ratio of soil and mechanical properties of the concrete have an insignificant influence on the displacements. This analysis also shows that the points in the upper half-thickness of soil are the most sensitive to the parameters of the model.

Upper Bound Solution of the Resisting Moment Bearing Capacity of A Composite Bucket Shallow Foundation of An Offshore Wind Turbine in Sand

Bearing the large moment that is generated by the wind load that acts on the upper structure of offshore wind turbines is an important feature of their foundations that is different from other offshore structures.A composite bucket shallow foundation(CBSF)has been proposed by Tianjin University to address the soft geological conditions in the offshore regions of China for wind turbines.The CBSF is a new type of foundation and is effective against large moments.The soil deformation test of a CBSF and the numerical simulation study under the same working conditions are carried out to determine the failure mechanism of a CBSF under moment loading.The resisting soil compression rateηm is defined as a new empirical parameter that indicates the ability of the soil inside the bucket to resist moment loading.The upper limit of the resisting moment bearing capacity of the bucket foundation is derived through the upper bound theorem of classical plasticity theory based on the failure mechanism.The calculation method is validated by tests of bucket models with different height-diameter ratios in sand under moment loading.

Dynamic Analysis of Submarine Pipelines on an Elastic / Viscoelastic Foundation

This paper presents a new finite element method for solving static and dynamic problems in laying operation of pipelines. The effect of the viscoelastic soil behavior is considered by using the Pasternak foundation model. Some examples are also presented.

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.

Dynamic Response of Fluid-Single Leg Gravity Platform-Soil Interaction System

An approximate method is presented to investigate the earthquake response of the fluid-single leg (shortened for S. L.) gravity platform-soil interaction system. By assuming a suitable form of the velocity potential of the radiation waves and by using the motion equation and the boundary conditions, the unknown coefficients can be obtained. Thereafter the function of frequency for the interaction system may also be obtained. In this paper, the difference of the system dynamic response between rigid foundation is analyzed and the influences of the various foundation geometric dimension and the various water-depth on the hydrodynamic loading and dynamic response of the system is illustrated.

STUDY ON MESO-MECHANICAL SIMULATION OF GROUTING FOR UNFAVORABLE GEOLOGICAL CONDITIONS

The numerical simulation program of PFC2D(Particle Flow Code in 2 Dimension)particle flow based on the flow-solid coupling principle and,on its built-in FISHTANK function library and FISH language,defines the flow equation and pressure equation of fluid domain respectively,and carries out numerical simulation calculations on the diffusion process and,on the morphology and particle displacement of slurry during the slurry injection process.By adjusting the parameters of hist,n_bond,s_bond and measure in the PFC command flow,the tracking of granular body displacement changes is achieved,and the mesoscopic mechanism such as the diffusion law of soil slurry at different depths and the change of formation porosity is revealed.The numerical calculations show that:the grouting pressure has a significant effect on the alteration and destruction of the formation structure,and the fracturing effect becomes gradually worse with increasing adhesive strength,while the porosity increases significantly with increasing grouting pressure.Based on the elastic-plastic theory of the Mohr-Colomb criterion to theoretically derive the stress field of the soil around the borehole,it is pointed out that the mechanical mechanism of annular tension and radial compression is the fundamental reason for the appearance of fracturing grouting action mode.The increase of slurry viscosity is beneficial to improve the grouting effect of fracturing-compacting grouting,while the increase of friction coefficient has little effect on the grouting effect.The comparative analysis of the laboratory tests shows that the PFC2D simulation of the grouting process is feasible.

Reinforcement of Clay Soils through Fracture Grouting

Fracture grouting is widely used for building foundation reinforcement,however the underpinning mechanisms are still not clear.Using numerical results about a single-hole fracture grouting process as a basis,a model composed of soil and grouting veins has been created to analyze the reinforcement mechanism.The influence weights of the grouting vein skeleton and compaction effect have been studied,thereby obtaining relevant information on the compressive modulus of the considered composite soil.The research results show that the compaction effect plays a leading role in the soil fracture grouting reinforcement.The grouting pressure,the hardened grouting vein modulus,and the shape of the grouting veins all influence the compressive modulus of the composite soil.

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。

深基坑桩-撑-土组合支护结构的受力特性与优化设计

依托某高层建筑深基坑工程,通过现场试验获得不同施工阶段组合支护结构及基坑关键部位的变形规律。基于土体硬化模型建立深基坑组合支护体系的三维数值模型,对基坑开挖施工进行全过程模拟,分析支护结构体系的桩身弯矩、桩身位移和土压力分布规律。针对反压土的顶宽、高度、坡度以及斜支撑的间距、直径等影响因素设计正交试验,对围护桩受力变形的影响参数进行敏感性分析,并提出优化建议。研究结果表明:围护桩最大位移与最大正弯矩随反压土顶宽、高度、斜支撑直径的增大而减小,随着反压土坡度、斜支撑间距的增大而增大。桩身变形影响因素敏感性从大至小依次为反压土坡高、顶宽、反压土坡度、斜支撑间距和斜支撑直径;对组合支护体系进行设计时,建议在确定土坡自稳角度的前提下,尽量增大反压土土坡高度。

套筒强夯长短桩复合地基承载特性研究

【目的】随着大型海洋工程项目不断发展,对水下软土地基处理提出了新的挑战,水下挤密砂桩和水下水泥搅拌桩复合地基等传统方法具有定位难、制桩难、桩体质量差等问题,始终制约着其在海洋工程中的应用和发展。【方法】借鉴陆上孔内深层强夯施工方法,因地制宜地提出水下套筒强夯复合地基成桩技术,该技术使用套筒阻隔海水进入桩孔,具有定位准、加固深和强度高等优点,通过模型试验的方法对该技术进行研究。【结果】结果表明,在强夯碎石桩与传统碎石桩相结合的工况下,地基极限承载力提高50%以上,等长度群桩复合地基极限承载力比长短桩复合地基提升18%;在全部采用强夯碎石桩的工况下,地基极限承载力提高170%以上,等长度群桩复合地基极限承载力比长短桩复合地基提升3.7%。【结论】全部采用强夯碎石桩的复合地基比强夯碎石桩与传统碎石桩组合而成的复合地基承载力提高更多,在提高承载力方面,采用套筒强夯长短桩复合地基更加经济有效。

填方灰土桩复合地基大底盘建筑倾斜特性数值分析

目前建筑倾斜事故频繁发生,但对于大底盘建筑倾斜全过程的变形力学行为受到了较少的关注。因此,研究了填方灰土桩复合地基倾斜过程的沉降变形特性及受力特性。基于数值模拟,探讨了填方灰土桩复合地基大底盘建筑倾斜特性相应变化规律,分析填方灰土桩复合地基倾斜过程中沉降变形及大底盘基础变形受力特性,研究了填方区地基处理优化方案。研究结果表明:建筑倾斜过程中,基础边缘处剪切变形最大,剪切变形值与倾斜值呈正相关;大底盘建筑倾斜过程中基础顶板混凝土首先开裂,其弯曲点不断变化;基础底板尚未开裂但底板南半段出现隆起现象,在靠近倾斜建筑0.2 L(L为底板长度)位置处存在开裂风险,倾斜过程弯曲点基本不变;水泥粉煤灰碎石桩(cement fly-ash gravel,CFG)桩地基处理方案剪应变最大值比原有方案剪应变最大值降低了67.08%,沉降值和倾斜值分别降低了67.52%、73.83%,达到此类建筑倾斜率不应超过3‰的规范要求;研究结果为类似复合地基大底盘建筑纠偏加固提供了借鉴和参考。

On the instability of offshore foundations: theory and mechanism

As the offshore engineering moving from shallow to deep waters, the foundation types for fixed and floating platforms have been gradually evolving to minimize engineering costs and structural risks in the harsh offshore environments. Particular focus of this paper is on the foundation instability and its failure mechanisms as well as the relevant theory advances for the prevailing foundation types in both shallow and deep water depths. Piles, spudcans, gravity bases, suction caissons, and plate anchors are detailed in this paper. The failure phenomena and mechanisms for each type of foundations are identified and summarized, respectively. The theoretical approaches along with sophisticated empirical solutions for the bearing capacity problems are then presented. The major challenges are from flow-structure-soil coupling processes, rigorous constitutive modeling of cyclic behaviors of marine sediments, and the spatial variability of soil properties for large-spreading structures. Further researches are suggested to reveal the instability mechanisms for underpinning the evolution of offshore foundations.

回填土地基的强夯加固机理与承载力综合检测技术研究

针对回填地区土体的密实程度远远无法达到建筑建设需求的问题,开展回填土地基的强夯加固机理与承载力综合检测技术研究。明确强夯加固中的振动波理论,设计碎石土地基的强夯,实现对回填土地基的加固处理。再通过检测装置设计、加载与检测数据分析,提出一种新的检测技术。将强夯加固机理和综合检测技术应用于实际可以有效促进回填土地基承载力提升,进而增强地基整体稳定性。

土力学与地基基础课程思政教学设计与实践

为充分发挥课程的育人功能,课程思政以习近平新时代中国特色社会主义思想为指导,以“土力学与地基基础课程”为例,结合实际工程,根据学科培养目标,确定知识、能力、思想政治目标等进行教学课程设计,将课程思政有机融入课程教学中。教师坚持知识传授与价值引领相结合,挖掘理想信念、价值取向、政治信仰、社会责任等思政元素,培养大学生明辨是非、独立思考的能力,为社会培养德才兼备、全面发展的土木工程专业人才。

预留土和桩锚结构联合支护下基坑的力学性状

桩锚结构在基坑支护中应用广泛,但在其设计计算时往往以经验为准,存在一定的盲目性和不确定性。依托在建工程,通过数值模拟分析了预留土作用下桩锚结构设计参数的变化对基坑力学性状的影响。结果表明:基坑支护桩的最大水平位移发生在桩顶,桩底的位移非常小;基坑周边地表沉降呈槽形分布,最大沉降在距离基坑边缘5 m左右的位置;弯矩分布曲线沿桩身有2个峰值点,峰值点大概位于桩顶以下3 m和10 m处;当锚索锚固角为15°时,桩身水平位移和弯矩、基坑周边地表沉降都比较小;锚索锚固点距离桩顶越远,桩身水平位移、基坑周边地表沉降越大;当预应力大于200 kN后,桩身水平位移和弯矩、基坑周边地表沉降几乎不再减小。

不同含水率下膨胀土深基坑环形支护结构的力学响应机制研究

成都地区广泛分布膨胀土,其力学特性具有显著的水敏性,给基坑等建设工程带来了巨大的挑战,而基坑支护结构对不同含水率条件下膨胀土性质的力学响应是支护工程设计的关键。以成都膨胀土地区某环形支护体系深基坑为研究对象,开展了膨胀土在不同含水率影响下的三轴固结不排水剪切试验,建立了在不同含水率影响下深基坑环形支护结构力学响应的数值模型。研究结果表明:粉质黏土和硬塑黏土的抗剪强度参数均随着含水率增大而减小,且硬塑黏土的抗剪强度参数具有更高的水敏性;在环形支护体系中,支护桩桩顶和桩身的水平位移随着含水率的增大而增大,且桩身水平位移极大值在深度约13.0 m处;临近基坑建筑物的沉降变形随着开挖深度的增加和含水率的增大而增大,当含水率增大时,临近基坑建筑物易发生不均匀沉降,尤其是当含水率大幅增大时,临近基坑建筑物易发生整体沉降。

预留土堤对深基坑力学性状的影响分析

预留土堤在基坑支护中应用广泛,但在设计计算时往往以经验为准,存在一定的盲目性和不确定性。依托在建工程,采用FLAC3D建立了数值分析模型,通过改变预留土堤的高度和宽度,研究了基坑支护桩水平位移、基坑周边地表沉降、桩身弯矩、安全系数以及潜在滑动面的变化规律,最后提出了经济合理的预留土堤设计参数。结果表明:基坑支护桩的最大位移发生在桩顶,基坑周边地表沉降呈“凹槽形”分布,最大沉降量距离基坑边缘2~3 m;桩身弯矩沿桩长呈“鼓肚形”变化规律,峰值点在距离桩底8~12 m处;桩前预留土堤的设计宽度不应大于8 m,否则对于减小支护桩位移和弯矩、基坑周边地表沉降益处不大,而且会占用基坑有限的施工作业面;桩前预留土堤的高度不应大于5 m,否则其稳定性将难以保证。

地基处理中水泥强化回填土的力学性能及固化机理研究

为分析低碳水泥强化对地基回填土力学特性的影响效应,开展压实、抗压强度和微观结构扫描实验,全面分析水泥强化地基回填土的增强规律及其机理。结果表明:随着水泥掺量提高,回填土的最大干密度降低,最优含水率提高;当低碳水泥掺量由0增至6%时,抗压强度与弹性模量大幅上升;地基回填土的孔径分布会随着低碳水泥掺量的增加而逐渐变小,同时孔隙体积会先减少后增加。与此同时,孔隙率和强度值之间存在线性数学关系;低碳水泥的黏合作用使得地基回填土的微观结构发生重组,结构密实度提高,进而达到提高强度性能的效果。研究结果可为软弱地基处理的设计和施工提供科学参考。

基于有限元模拟的水下地基土体力学性质变化研究

针对目前水下地基施工现场土体强度精密测量的难度大、影响因素多等问题,分析水下地基成桩—固结过程,通过多阶段欧拉-拉格朗日技术构建有限元数值模型。结果表明,水下地基的成桩过程中对黏土体的挤压作用导致了大量的沉桩效果,地表最大抬升约为R。在固结90 d以后,黏土体的不排水性抗剪强度增加很快,可达原强度的1.3倍。