Settlement mechanism of piled-raft foundation due to cyclic train loads and its countermeasure

In this paper, numerical simulation with soil-water coupling finite element-finite difference（FE-FD） analysis is conducted to investigate the settlement and the excess pore water pressure（EPWP） of a piled-raft foundation due to cyclic high-speed（speed： 300km/h） train loading. To demonstrate the performance of this numerical simulation, the settlement and EPWP in the ground under the train loading within one month was calculated and confirmed by monitoring data, which shows that the change of the settlement and EPWP can be simulated well on the whole. In order to ensure the safety of train operation, countermeasure by the fracturing grouting is proposed. Two cases are analyzed, namely, grouting in No-4 softest layer and No-9 pile bearing layer respectively. It is found that fracturing grouting in the pile bearing layer（No-9 layer） has better effect on reducing the settlement.

Deep Foundation Pit Excavations Adjacent to Disconnected Piled Rafts: A Review on Risk Control Practice

Foundation pit excavation engineering is an old subject full of decision making. Yet, it still deserves further research due to the associated high failure cost and the complexity of the geological conditions and/or the surrounding existing infrastructure around it. This article overviews the risk control practice of foundation pit excavation projects in close proximity to existing disconnected piled raft. More focus is given to geotechnical aspects. The review begins with achievements to ensure excavation performance requirements, and follows to discuss the complex soil structure interaction involved among the fundamental components: the retaining wall, mat, piles, cushion, and the soil. After bringing consensus points to practicing engineers and decision makers, it then suggests possible future research directions.

Model test on vertical bearing capacity of X-section concrete pile raft foundation in silica sand

To reveal the bearing capacity of the X-section concrete piles pile raft foundation in silica sand,a series of vertical load tests are carried out.The X-section concrete piles are compared with circular section pile with the same section area.The load−settlement curves,axial force and skin friction,strain on concave and convex edge of the pile,pile-sand stress ratio,distributions of side and tip resistance are presented.The results show that bearing capacity of the X section concrete pile raft foundation is much larger than that of the circular pile raft foundation.Besides,compared with the circular pile,the peak axial force of X-section piles under raft is deeper and smaller while the neutral point of X-section concrete pile is deeper.Moreover,the strain on the concave edge is much larger than that on the convex edge of the pile,and the convex edge has more potential in bearing capacity as the vertical load increases.The X-section pile has higher pile-sand stress ratios and load-sharing between side resistance and tip resistance.Above all,the X-section concrete pile can significantly increase the bearing capacity of pile-raft foundations in silica sand.

秦望广场塔楼沉降后浇带提前封闭技术研究

研究目的:带超高层物业开发的大型地下综合体,为了不影响裙房和地下室提前投入使用,需要研究超高层建筑沉降后浇带提前封闭可行性,通过杭州富阳秦望广场通道工程实际项目,采用有限元整体建模分析,提出了残余沉降差计算方法,利用试桩静载试验P-S曲线确定了桩基线刚度参数,推导出沉降后浇带提前封闭楼层,通过实际监测数据,对理论计算结果进行对比验证。研究结论:(1)试桩线刚度与单桩承载力特征值比值在76~119之间,与传统经验值100倍单桩承载力特征值相比,试桩结果离散性较大;(2)塔楼总沉降量与楼层数关系不大,与桩基线刚度取值关系较大;(3)塔楼桩基采用相同线刚度,楼层数与总沉降量成正比;(4)塔楼沉降呈线性发展趋势,理论计算与监测数据基本一致,因成桩工艺控制不稳定,导致工程桩线刚度有离散性,理论计算沉降速率与监测相比均偏小,导致后浇带提前封闭楼层判断有误差;(5)本研究成果可为超高层建筑提前封闭沉降后浇带设计提供参考。

格栅式钢板桩与桩筏基础集成的基坑支护技术

针对软土地质基坑工程,提出格栅式钢板桩与桩筏基础集成的基坑支护技术,通过设置纵向双排钢板桩与横向钢板桩或钢板组成格栅式钢板桩,以提高支护结构强度与刚度,并与桩筏基础协同工作,以解决结构下部软土层承载力较差的问题,并提高支护结构稳定性。以某管廊工程为依托,通过计算分析与技术对比,论证集成支护技术的安全性与可行性。

大倾角钢箱提篮拱桥总体设计

考虑施工技术、经济性和景观效果等方面,陕西蔡家坡渭河特大桥主桥采用(35+210+35) m大倾角钢箱提篮拱桥。该桥采用摩擦摆和环向钢丝绳阻尼支座综合减隔震约束体系,以降低结构的地震响应。拱肋为单室等宽钢箱截面,采用主、副拱肋2条悬链线,主、副拱肋共面,设计为横向内倾20°的大倾角,在提高桥梁景观效果的同时,又有效提高了结构的整体稳定性;拱肋设钢-混结合段连接拱脚与混凝土拱座。桥面系采用自重较轻的正交异性钢桥面板结构。吊杆采用抗拉强度1 860 MPa的环氧柔性钢绞线。基础采用Φ2.5 m的钻孔灌注群桩,以抵抗软弱地质条件下荷载的竖向及水平作用力。该桥总体采用先梁后拱的施工方案。整体及局部有限元分析结果表明,桥梁结构力学性能均满足规范要求,设计安全可靠。

某高层剪力墙结构不均匀沉降分析及处理

无新增荷载情况下的持续不均匀沉降常常发生在地基有缺陷的高层建筑中,该类沉降产生的破坏是缓慢持续的。地基不均匀沉降会导致建筑物开裂、倾斜,进一步发生结构安全事故。依据某高层剪力墙结构以及不均匀沉降监测数据,采用ABAQUS建立该高层剪力墙结构三维有限元计算模型,分析不均匀沉降工况下上部结构的内力分布及整体变形发展规律。结果显示,有限元计算得到的模型内力增大位置与实际外墙主要开裂位置相吻合,整体沉降变形发展规律与现场实测数据基本一致,证明上部结构三维有限元模型的可行性。进一步建立剪力墙结构-桩筏基础-地基耦合计算模型,开展不同筏板厚度对整体结构不均匀沉降的影响分析。同时结合实际案例制定“增设筏板+补桩”处理方法,加固完成后监测数据显示房屋处于稳定状态,变形与裂缝均无发展。

高层建筑迫降法纠倾方案研究

建筑物因勘察、设计、施工或环境等不良因素影响,导致不均匀沉降,从而产生超过规范允许范围的倾斜。而高层建筑纠倾工程技术难度大、风险高。结合工程实例,通过对现有技术资料分析得出倾斜的原因,并根据工程实际情况,提出桩侧射水扰动迫降纠倾方案,对迫降法的原理、方案设计和施工工艺流程进行了介绍,可为类似工程提供参考。

某筏板基础居民楼顶升纠偏方案设计与实践

通过PLC顶升系统,对一倾斜量达16‰的6层砖混结构纠倾。该工程由于地基土层受到多次侵蚀,导致建筑基础发生不均匀沉降。根据现场实际情况,决定采用新增桩基作为反力系统对原建筑进行顶升,最终成功将建筑纠正。

双荷载箱自平衡法在超长桩基承载力测试中的应用

在黄河中游山西小北干流范围,地下以砂土、粉质黏土层等物理力学指标较低的地层为主。在这种复杂的地质条件下,为保证桥梁桩基承载力和上部结构的安全性要求,设计多采用大直径超长桩为主,施工难度较大,造价较高。双荷载箱自平衡法是指根据试验方案规定,在桩身特定位置依次嵌入两个荷载箱,然后按照加载试验方案对荷载箱进行逐级分段加载,分别得出3段桩身的承载力,可以解决超长桩基试桩试验中的很多问题,为桩长优化设计提供数据支撑,具有广泛的应用前景。

跨栈桥煤仓的基础设计及施工

为保证新建跨栈桥籽煤仓施工过程的安全,同时保证施工过程中选煤厂现有系统的正常生产,通过将新建籽煤仓范围内现有产品栈桥的部分土建结构拆除,并利用新建钢桁架将现有产品带式输送机托起,从而完成了跨栈桥煤仓的基础设计与施工。结果表明:该设计方法可在选煤厂不停产的情况下完成新建跨栈桥煤仓基础的安全施工,有效缩短了施工工期,使选煤厂如期达产,减少了企业的停产损失,同时也降低了建筑工程投资,增加了企业的经济效益;此外提出了部分无埋深筒仓的基础设计方法,可为选煤厂厂房或筒仓的设计提供一定借鉴。

剑潭东江特大桥3号墩钢围堰施工关键技术

新建赣深高铁剑潭东江特大桥部分斜拉桥为主跨260 m的预应力混凝土四线高速铁路桥,3号墩采用24根φ3.0 m钻孔灌注桩基础,桩基均位于弱风化花岗岩层。针对大型船机设备无法进场,研究采用了现场整体拼装钢套箱围堰、气囊法滚动下水浮运及锚定卷扬机缆绳定位安装技术。通过采取结构优化、增设隔舱等措施保证钢套箱围堰下沉稳定。针对承台基底为弱风化花岗岩,应用钻孔桩柔性技术实现封底混凝土与桩基之间的柔性连接,钢围堰封底混凝土通过分舱封底保证质量。

改进高应变法的桥梁基桩承载力检测研究

为实现对桥梁基桩承载力的检测,本研究引入高应变法,并对某桥梁建设项目中的基桩进行承载力检测。在明确了桥梁建设项目的基本概况后,选择重锤、荷载箱、基桩静载仪、基桩高应变承载力检测仪器等设备。利用高应变法,提出改进高应变法的桥梁基桩承载力检测方法。经过检测,发现该桥梁建设项目的承载力特征值均超过了设计承载力特征值。同时,通过检测得到的极限承载力数值结果将可作为该桥梁建设项目承载力验收的依据。

呈林状岩层起伏地基基础选型分析

文章对南宁某工程项目的基础选型进行分析,因该项目所处场地条件较为复杂,且基底下岩层起伏较大,呈林状,施工难度大,因此分别采用筏板基础、CFG地基加固+筏板及桩基础三种不同的基础类型,对该项目进行针对性的基础方案比选分析。综合考虑各方案的安全性、经济性、施工便利性等因素,从而得出最合理经济的基础方案。

成品储存库地基处理桩基布置方案对比

水泥、砂、骨料等建筑材料的生产常采用成品库进行储存,其荷载重、库容大;一般的天然地基难以满足其承载力要求,实践中多采用桩基础。案例中采用盈建科YJK软件对直径为Ф32 m成品库进行建模分析,计算不同桩基布置形式下的桩反力值。结果表明:环形布桩相比于梅花形布桩受力更均匀,相比于正方形布桩可以有效减少桩的根数,具有良好的经济效益。

某高层建筑基础控沉及纠偏加固工程实例

某高层建筑基础为平板式桩筏基础,该建筑出现不均匀沉降,导致筏板等构件出现开裂情况。文章通过对不均匀沉降原因的分析,提出针对性的基础控沉及纠偏微调加固方案。方案包含了采用钢管桩进行基础控沉托换、地基应力解除法实施迫降纠偏及对开裂构件进行加固。实际施工效果达到预期,可为其它类似工程提供借鉴。

土岩溶组合地区高层建筑地基处理方案优选——以龙岩铁发佳苑项目为例

土岩溶地区的地质条件较差,易造成高层建筑基础的承载力不足或不均匀沉降。以龙岩地区铁发佳苑项目基础工程为例,通过三种不同基础方案的优选,采用在桩顶设置刚度调节装置的桩筏基础设计,很好解决了土岩溶地区建筑地基承载力不足问题。经实测结果显示建筑地基基础的不均匀沉降和总体沉降量均较小,表明此基础设计方案能有效控制岩溶地区基础的差异沉降等问题,同时在工程造价和工期方面取得了较好的经济社会效益。

小三峡隧道D1K671+670处巨型溶洞处理措施研究

文章基于郑万高铁重庆段小三峡隧道D1K671+670处隧底大型溶洞发育特征和工程补勘资料,从施工难度、运营风险、工程投资等方面进行对比分析,最终确定箱梁桥方案跨越溶洞的处理方案。通过结构计算与工程类比进行隧道桩基托梁结构设计,隧道拱顶空腔采取护拱+缓冲层方案,地表河道采用混凝土进行铺砌。工程实践表明,该处大型溶洞的综合处理措施取得了很好的效果,可供类似工程借鉴参考。

Influence of dynamic soil-pile raft-structure interaction:an experimental approach

Traditionally seismic design of structures supported on piled raft foundation is performed by considering fixed base conditions, while the pile head is also considered to be fixed for the design of the pile foundation. Major drawback of this assumption is that it cannot capture soil-foundation-structure interaction due to flexibility of soil or the inertial interaction involving heavy foundation masses. Previous studies on this subject addressed mainly the intricacy in modelling of dynamic soil structure interaction (DSSI) but not the implication of such interaction on the distribution of forces at various elements of the pile foundation and supported structure. A recent numerical study by the authors showed significant change in response at different elements of the piled raft supported structure when DSSI effects are considered. The present study is a limited attempt in this direction, and it examines such observations through shake table tests. The effect of DSSI is examined by comparing dynamic responses from fixed base scaled down model structures and the overall systems. This study indicates the possibility of significant underestimation in design forces for both the column and pile if designed under fixed base assumption. Such underestimation in the design forces may have serious implication in the design of a foundation or structural element.

3D-Analysis of Soil-Foundation-Structure Interaction in Layered Soil

The analysis of building structure in contact with soil involves an interactive process of stresses and strains developed within the structure and the soil field. The response of Piled-Raft Foundation system to the structure is very challenging because there is an important interplay between the component of building structure and the soil field. Herein, soil-foundation-structure interaction of buildings founded on Piled-Raft Foundation is evaluated through 3D-Nonlinear Finite Element Analyses using PLAXIS3D FOUNDATION code. The soil settlements and forces demand of the high-rise building structures and foundation is computed. The parametric study affecting the soil-foundation-structure response has been carried out. The parameters such as construction phasing, sequential loading, building aspect ratios, soil failure models and thickness proportion of soil field stiff layer, are considered. It is concluded that the interaction of building foundation-soil field and super-structure has remarkable effect on the structure.