Seismic response of bridge pier on rigid caisson foundation in soil stratum

An analytical method to study the seismic response of a bridge pier supported on a rigid caisson foundation embedded in a deep soil stratum underlain by a homogeneous half space is developed. The method reproduces the kinematic and inertial responses, using translational and rotational distributed Winkler springs and dashpots to simulate the soil-caisson interaction. Closed-form solutions are given in the frequency domain for vertical harmonic S-wave excitation. Comparison with results from finite element （FE） analysis and other available solutions demonstrates the reliability of the model. Results from parametric studies are given for the kinematic and inertial responses. The modification of the fundamental period and damping ratio of the bridge due to soil-structure interaction is graphically illustrated.

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.

Upper Bound Solutions for Uplift Ultimate Bearing Capacity of Suction Caisson Foundation

Suction caisson foundation derives most of their uplift resistance from passive suction developed during the pullout movement. It was observed that the passive suction generated in soil at the bottom of the caisson and the failure mode of suction caisson foundation subjecting pullout loading behaves as a reverse compression failure mechanism.The upper bound theorems have been proved to be a powerful method to find the critical failure mechanism and critical load associated with foundations, buried caissons and other geotechnical structures. However, limited attempts have been reported to estimate the uplift bearing capacity of the suction caisson foundation using the upper bound solution. In this paper, both reverse failure mechanisms from Prandtl and Hill were adopted as the failure mechanisms for the computation of the uplift bearing capacity of the suction caisson. New equations were proposed based on both failure mechanisms to estimate the pullout capacity of the suction caisson. The proposed equations were verified by the test results and experimental data from published literature. And the two solutions agree reasonably well with the other test results. It can be proved that both failure mechanisms are reasonably and more consistent with the actual force condition.

Numerical analysis on bearing capacity of middle pylon caisson foundation of Taizhou Bridge

Because of the computation difficulty of the bearing capacity of large underwater caisson foundation on thick overburden layer ground, the geotechnical software FLAC3D was utilized in the 3D numerical analysis on the bearing capacity of middle pylon foundation. From the computational results, it is concluded that the caisson foundation has a good bearing capacity on thick overburden layer ground and the bearing capacity can be improved assuming that the soil near the area of basal corner is reinforced.

Prediction Method of Seismic Residual Deformation of Caisson Quay Wall in Liquefied Foundation

The multi-spring shear mechanism plastic model in this paper is defined in strain space to simulate pore pressure generation and development in sands under cyclic loading and undrained conditions, and the rotation of principal stresses can also be simulated by the model with cyclic behavior of anisotropic consolidated sands. Seismic residual deformations of typical caisson quay walls under different engineering situations are analyzed in detail by the plastic model, and then an index of liquefaction extent is applied to describe the regularity of seismic residual deformation of caisson quay wall top under different engineering situations. Some correlated prediction formulas are derived from the results of regression analysis between seismic residual deformation of quay wall top and extent of liquefaction in the relative safety backfill sand site. Finally, the rationality and the reliability of the prediction methods are validated by test results of a 120 g-centrifuge shaking table, and the comparisons show that some reliable seismic residual deformation of caisson quay can be predicted by appropriate prediction formulas and appropriate index of liquefaction extent.

Study on Unloading Creep Characteristics of the Soil and Application of the Stress-Dependent Creep Model in Suction Caisson Foundation

As the anchoring foundation of the tension leg platform(TLP),suction caisson foundation is subjected to the long-term vertical pullout loads.But there are few studies on the mechanism of the unloading creep of soft clay and long-term uplift bearing capacity of suction caisson foundations.To address this problem,unloading creep tests of soft clay were carried out to analyze the strain development with time under different confining pressures.The test results show that the creep curve rapidly develops in the early stage and tends to stabilize in the later stage.The unloading deviator stress is higher,the unloading creep deformation is greater and the soft clay has typical nonlinear creep characteristics.Therefore,by introducing the creep model and considering the influence of the deviator stress,the stress-dependent Merchant model is proposed to describe the unloading creep of soft clay.Then,the stress-dependent Merchant model is extended to a three-dimension constitutive model,and a finite element subroutine is developed to establish a finite element analysis method for analyzing the long-term uplift capacity of suction caisson foundations and validated with the long-term uplift bearing capacity results of caisson model.

Technical research on control of caisson construction for the middle tower foundation of Taizhou Bridge

The real-time informational monitoring system is adopted in the construction of middle tower foundation of Taizhou Bridge for the first time. The geometric state of the caisson, the stress of upstream and downstream anchorage cables, underwater topography, the drag forces of the caisson cutting edge and frictional resistances of the sidewall and etc. are monitored in real time. According to the synthesized data analysis and decision-making system, the spatial states of the caisson are adjusted in time to locate and embed the deep water caisson precisely. The offset error of the caisson is less than 30 cm and the vertical error is 1/363 at the final stage. The control technology for construction of large caisson under deep water is concluded and would be helpful for the construction of bridge foundation in the future.

Application of bi-directional static loading test to deep foundations

Bi-directional static loading test adopting load cells is widely used around the world at present, with increase in diameter and length of deep foundations. In this paper, a new simple conversion method to predict the equivalent pile head load-settlement curve considering elastic shortening of deep foundation was put forward according to the load transfer mechanism. The proposed conversion method was applied to root caisson foundation in a bridge and to large diameter pipe piles in a sea wind power plant. Some new load cells, test procedure, and construction technology were adopted based on the applications to different deep foundations, which could enlarge the application scopes of bi-directional loading test. A new type of bi-directional loading test for pipe pile was conducted, in which the load cell was installed and loaded after the pipe pile with special connector has been set up. Unlike the conventional bi-directional loading test, the load cell can be reused and shows an evident economic benefit.

Experimental Studies on Modified Suction Caissons in Fine Sand Subject to Uplift Loading

A modified suction caisson(MSC), which was reported by the authors of this paper previously, comprises an external short-skirted structure that is added to a regular suction caisson(RSC). It has been proved that MSCs can improve the lateral bearing capacity and limit the deflection of the caisson compared with RSCs. A series of model tests were conducted to investigate responses of MSCs subject to uplift loading in saturated sand. The effects of external skirt dimensions on the uplift bearing capacity of MSCs were considered. In addition, the influences of the sealed top lid of the skirted structure on the uplift bearing capacity and the resulting passive suction of MSCs were also studied. It was found that the uplift bearing capacities of MSCs are 1.4-1.7 times that of RSCs. Moreover, test results in serviceable conditions show that the sealed external skirted structure of perspex-made suction caissons significantly contributed to the uplift bearing capacity as a result of passive suction.

An Experimental Investigation on the Stability of Foundation of Composite Vertical Breakwaters

一系列 2D 模型测试被进行估计合成垂直防波提的基础稳定性。在这篇论文，从试验性的学习的结果与一个公式相连地被介绍估计基础的稳定性数字，它是为如此的结构的基础稳定性的评估的最重要的参数。有不同盔甲石头尺寸的合成防波提的稳定性上的波浪高度，波浪时期和城墙与外濠间之狭道宽度的影响被调查。45 测试被执行盖住这些参数的影响。根据现在的研究，城墙与外濠间之狭道宽度是有关盔甲基础的侵蚀的一个重要参数。作为城墙与外濠间之狭道宽度增加，城墙与外濠间之狭道侵蚀的数量减少。比较在现在的学习和 Kimura 等建议的估计的稳定性数字的结果之间被做。(1994 ) ，它是 Tanimoto 的扩展公式。结果证明以后的公式低估稳定性数字。然而，由使用一个改进因素大约 1.7 米到 Kimura 等。公式，结果与现在的试验性的结果相关……

大型沉井接高过程中地基抗剪强度的有限元计算

为探讨大型沉井在强度较低且具有时变效应的软土地基上产生的突沉、偏沉等问题,运用ABAQUS有限元软件对南京市龙潭大桥南锚碇沉井前3节的接高过程进行模拟,观察沉井地基中超静孔压的消散情况,并通过规范法和有效应力法计算并比较其抗剪强度的变化。结果表明,在沉井混凝土养护期间,超静孔压迅速消散,地基抗剪强度得到提升,且浅部的强度提升较大,向深部则存在衰减;沉井能否进行下一节的浇筑接高不受超静孔压消散时间的控制,而主要受控于混凝土的养护情况;规范法的抗剪强度计算结果相较于有效应力法大,在工程实践中为保守取值,可选取有效应力法计算强度增长。

基于离心模型试验的水下保护沉箱沉降规律研究

水下保护沉箱可以有效防护水下生产系统免受船锚和渔网拖曳、海底滑坡冲击以及冰山撞击等风险,但沉箱一旦发生超出允许变形量的沉降,将会引起沉箱内采油树失稳,管汇及沉箱跨接管、海管、脐带缆等发生断裂,进而酿成油气泄露等重大工程事故。通过开展离心模型试验研究了水下保护沉箱基础在基坑开挖安装就位和自重固结过程中的沉降规律,并采用Weibull曲线模型拟合出最终沉降量的预测公式。离心模型试验预测的最终沉降量与数值模拟、规范理论计算结果的对比,发现三种方法的预测沉降量较为接近。通过分析离心模型试验结果可知:基坑开挖完成后,基坑底部出现较为明显的回弹变形;在沉箱基础安装后的固结过程中,基础底部土体的回弹量有所减小,但回弹变形依旧存在。

硬质岩基条件下沉箱码头的抛石棱体优化设计

对于硬质基岩条件下的重力式沉箱码头,通常在沉箱后方设置抛石棱体,用以减小沉箱宽度,从而缩减基槽开挖范围内的炸礁量。依托某海外工程,对棱体高度和工程造价的关系进行分析,探究抛石棱体的优化设计规律。结果表明,当抛石棱体高度约为沉箱高度的60%~80%时,工程造价最低;在硬质岩面低于沉箱高度的1/2时,抛石棱体的最优高度为沉箱高度的60%左右;在硬质岩面高出沉箱高度的1/2时,抛石棱体的最优高度为沉箱高度的80%左右。揭示的规律可供类似项目参考,实际工程中应综合考虑工程地质条件、地材价格、施工能力等多方面因素,分析确定经济合理的棱体高度。

桩基与空箱结构一体化围堰施工技术及应用

在水利工程施工中,围堰施工时常会遇到场地狭小且水位波动频繁的工况,传统的重力式围堰和双排桩围堰很难适用。淮河航道临淮岗复线船闸工程,利用双排灌注桩结合旋喷桩与地下连续墙组成围堰下部防渗与承载体系,上接钢筋混凝土空箱结构,将常规基础防渗处理方法与空箱结构进行有效结合,形成一体化挡水围堰,有效地解决了有限空间和水位大幅频繁变化情况下的围堰施工难题。这种一体化挡水围堰防渗效果明显,结构稳定性高,工期短,工程造价低,对复杂作业面区水利工程围堰施工具有较高的借鉴价值。

沉井法在变电站事故集油池工程中的应用研究

在变电站改扩建工程中,事故集油池等地下水池结构的施工场地条件受限,基坑开挖极易对周边的已有建、构筑物造成影响。因此,提出了一种事故集油池沉井施工工艺,可用于各电压等级的新建或改、扩建变电站工程中。该施工工艺将事故集油池设计为井筒状结构,通过沉井法施工池壁开挖土体,由井壁的自重作用克服土壤摩阻力下沉,最后在池壁设计标高位置封底,并施工事故集油池顶板及其他附属设施。事故集油池沉井法具备施工安全风险较低、施工操作面较小、无需额外基坑支护、对邻近建构筑物影响小等优点。在新疆某变电站扩建工程中成功应用事故集油池沉井施工工艺,缩短了施工工期并产生了较高的经济效益。

沉井结构在海边桩基础泵房设计中的应用

国外某大型燃煤电站取水泵房位于海边红树林滩涂的软弱土层上,泵房下部采用预制沉井结构,软基处理采用混凝土灌注桩方案,结合施工方案,对泵房施工期和运行期的沉井结构分别进行设计优化,控制下沉方式和下沉系数,并对沉井泵房桩基与沉井施工次序进行优选,针对工程特点,结合工程实际,简述带桩基的沉井结构泵房设计要点,为同类工程设计提供参考。

Model testing of tripod caisson foundations in silty clay subjected to eccentric lateral loads

In this study,model tests were conducted to investigate the bearing capacities of tripod caisson foundations subjected to eccentric lateral loads in silty clay.Lateral load–rotation curves of five eccentric-shaped tripod suction foundations were plotted to analyze the bearing capacities at different loading angles.It was observed that the loading angle significantly influenced the bearing capacity of the foundations,particularly for eccentric tripod caisson foundations.Compared with eccentric tripod caisson foundations,the traditional tripod foundation has a relatively high ultimate lateral capacity at the omnidirectional loading angle.By analyzing the displacement of the caissons,a formula for the rotational center of the tripod caisson foundation subjected to an eccentric lateral load was derived.The depth of the rotation center was 0.68–0.92 times the height of the caisson when the bearing capacity reached the limit.Under the undrained condition,suction was generated under the lid of the“up-lift”caisson,which helps resist lateral forces from the wind and waves.

某超深雨水泵站结构方案比选

杏花路下穿通道雨水泵站项目位于武汉市新洲区,泵站深度较深,为选取合适的建造方案,在设计前期针对雨水泵站常见的两种施工工艺——明挖现浇法和沉井法,做了详细的方案设计,并在施工难度、施工工期、对周边环境的影响和工程造价等方面进行了对比分析。结果显示:沉井法比明挖现浇法的优势在于综合造价低、风险小,但工期较长,在工期不是控制因素的前提下可以优先采用;在软土地区施工沉井时,需要在设计阶段充分考虑沉井下沉过程中可能出现的各种问题,采取充分的措施降低风险,确保沉井顺利实施。

沉井与基坑支护结合法在某增压泵站中的应用

与常规泵站不同,增压泵站具有单体较多、开挖深度不一及平面形状不规则等特点,采用沉井法或基坑支护法单独实施时,会产生施工方式不合理、工程成本较高或工期较长等问题。本文结合某增压泵站工程案例,从安全、经济、便捷等角度对沉井法、基坑支护法及沉井+基坑支护法等方案进行对比分析,提出了一种沉井与基坑支护相结合的结构形式。该结构形式不仅具有沉井结构工程成本低、施工便捷的优势,还具有基坑支护结构形式灵活、便于保护周边环境的优点;同时也为类似工程项目的结构选型提供了一种新的思路。

桥梁大型沉井基础建造技术发展与展望

沉井基础在大型桥梁主墩、锚碇基础中得到广泛应用,并在沉井工程勘察、工程设计与施工技术方面取得了一定的进展。在工程勘察技术发展方面,地质参数获取方法在现有理论分析法、室内试验法、现场试验法的基础上进一步发展了现场载荷板试验法,研制了侧摩阻力监测装置,对地基承载力、侧摩阻力等地质参数认识不断加深。在工程设计技术发展方面,通过对平面形式与尺寸、结构安全、软弱地基砂桩加固等方面不断进行优化设计,形成了适用于大型沉井的结构与地基处理的设计方法。在沉井施工技术发展方面,针对沉井浮运定位与着床,提出了井孔封闭助浮、多阶段多方式长距离浮运技术,以及液压千斤顶多向快速定位着床技术,研发了锚系定位系统;针对锅底开挖下沉的不足,提出了全节点支撑、中心块状支撑等新型开挖下沉工艺;针对高压射水结合泥浆泵设备取土的不足,研制了四绞刀快速破取土设备、可自移动式快速取土设备、机械臂水下定点取土机器人等新型设备;针对人工监测的不足,采用信息化监测系统进行沉井施工监测,形成了自动监测-风险预警-辅助决策控制-设备自动化执行的智能化监测控制技术;在沉井工业化建造技术方面进行了有益探索,将取土平台与供气管、供水管、排泥管、施工电缆桥架等进行了集成化设计,推动了大型沉井的应用与发展。但在大型沉井理论、智能化装备、智能化建造方面还存在不足,随着工程建设的不断发展,地质参数高精度获取、理论分析方法、大型沉井设计与施工规范、特殊地层建造技术、工业化与智能化施工是大型沉井建造技术发展的方向。