Assessment and control of metro-construction induced settlement of a pile-supported urban overpass

A metro station in Beijing, consisting of two separate parallel platform tunnels, has been constructed in close range below a pile-supported overpass of a major municipal artery built more than a decade ago. In order to assure the normal operation and integrity of the overpass as well as smooth and safe tunneling, the condition and status quo settlement capacity of the overpass is assessed via structural measurements and analysis, and on the basis of which, criteria on allowable additional settlements of pile foundations are established. The probable additional settlements of the pile foundations caused by tunneling and dewatering are estimated by using empirical and theoretical analyses and numerical simulations of the construction process in combination with in situ monitoring data, and anti-settlement underpinning piles are installed to control the predicted excessive adverse effects of tunneling. The major conclusions include: (1) the surcharge effect from an existing structure in or on the ground may enhance the effect of tunneling on ground deformation, and thus should be taken into account when estimating the delimitations of zones of tunneling influence and the degree of closeness of an existing structure; (2) when tunneling in close range to an existing urban overpass supported on pile foundations, the general control procedure should consist of assessment of the contemporary capacity of the superstructure, establishment of criteria for and prediction of dewatering and tunneling-induced ground surface and pile foundation settlements, and, if necessary, execution of reinforcement measures; (3) in situ monitoring data close to areas sensitive to tunneling should be employed to adapt theoretical analysis and calibrate numerical models; (4) rational settlement criteria and mitigation measures should be worked out through consideration of the local composition of the superstructure of the overpass, as well as the type and depth of the pile foundations; and (5) a time period after installation may be needed for the underpinning piles to be effective in counteracting the settlement effect of tunneling.     

高铁不同刚度桩-网复合地基工作性状差异

结合沪宁城际铁路CFG桩网复合地基试验段和京沪高铁砂桩网复合地基昆山试验段,在现场试验断面埋置土压力盒、沉降计和孔隙水压计等监测仪器,获取地基沉降、桩和桩间土压力、孔隙水压等监测数据,对比分析CFG桩与砂桩网在高速铁路地基工作性状及工后沉降控制效果中的差异。分析结果表明:CFG桩网和砂桩网联合堆载预压均可满足高速铁路无砟轨道工后沉降控制要求,CFG桩网复合地基沉降总量与沉降速率均小于砂桩网复合地基,且收敛速度快;受桩刚度差异的影响,CFG桩网复合地基与砂桩网复合地基桩土应力规律存在较大差异,前者桩土应力比随着路堤填筑加载而增大,最终趋于稳定,后者桩土应力比随荷载增加先增大后减小,再增大,呈波浪形变化;CFG桩网地基超孔压消散速率远小于砂桩桩网地基的超孔压消散速率;在施工工期较短的情况下,与砂桩网复合地基相比,CFG桩网复合地基处理技术的工后沉降控制效果更优。     

Prediction of the settlement of shallow foundations using both limiting-state criteria

Equations are proposed for calculation of foundation settlements and the rates of their increase in the nonlinear stage of bed-soil deformation. The possibility of using bed reserves with respect to pressures is supported on the condition that the settlement is equal to the limiting allowable. The latter makes it possible to permit foundations with smaller dimensions; an example of the calculation is discussed. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 1, pp. 2–4, January–February, 1996.     

地铁停车场工程基槽灰土回填质量控制试验研究

基槽灰土回填是地铁停车场工程施工的一大难点.施工过程质量控制不严,可能造成基槽土方填土出现沉陷,甚至造成建筑物基础积水,引起建筑物结构不均匀下沉,出现裂缝.通过开展基槽灰土回填试验段试验研究,分析夯实过程中灰土含水率、石灰掺量、夯实时间以及松铺厚度对压实度结果影响程度的大小,研究其之间的关系,得出相关结论.在既定的施工...     

A Case History on Underpinning for a Distressed Building on Hard Residual Soil Underneath Non-Uniform Loose Sand

This paper presents a case history on the failure of Suranivet 9, a student dormitory in the campus of Suranaree University of Technology (SUT), Thailand. The dormitory encountered excessive differential settlement due to the variation in soil profile. Part of the building was underlain by very stiff to hard SUT silty clay and part by loose clayey sand. Underpinning to extend the foundations down to stable stratum was employed to strengthen bearing capacity and minimize settlement. The underpinning design and procedure were summarized. In practice, the static formula was used for the preliminary micro-pile design (selection of pile section and length for different loads and soil profiles). The undrained shear strength (S_u) of SUT silty clay was approximated using the Stress History and Normalized Soil Engineering Properties (SHANSEP) technique and standard penetration number (N). The finite element method was employed to predict the load-settlement curve of the micro-pile. Modified cam clay model was proved as a suitable model for this prediction. The measured settlements of the underpinned foundations after one year service were less than 0.5 mm. This small settlement guarantees the stability of the underpinned structure. It is also found that the settlement ratio (ratio of the measured settlement of underpinned foundations to the predicted settlement of single micro-pile) varied from 0.7 to 3.0.     

位于复杂地质条件下的某污水处理厂的地基处理

本文通过工程实例,介绍了强夯、强夯置换法及钢筋混凝土灌注桩在某污水处理厂地基及基础工程中的应用。主要探讨了在复杂地质条件下,结合建(构)筑物结构特性及其可能产生的不均匀沉降等问题,采用强夯与强夯置换法相结合,对不同构筑物地基及同一建(构)筑物基础位于不同地基土,采用不同强夯参数,来达到理想的处理效果。     

Settlement predictions of footings on sands using probabilistic analysis

The design of footings on sands is often controlled by settlement rather than bearing capacity.Therefore,settlement predictions are essential in the design of shallow foundations.However,predicted settlements of footings are highly dependent on the chosen elastic modulus and the used method.This paper presents the use of probabilistic analysis to evaluate the variability of predicted settlements of footings on sands,focusing on the load curve(predicted settlements)characterization.Three methodologies,the first-and second-order second-moment(FOSM and SOSM),and Monte Carlo simulation(MCS),for calculating the mean and variance of the estimated settlements through Schmertmann(1970)'s equation,are presented and discussed.The soil beneath the footing is treated as an uncorrelated layered material,so the total settlement and variance are found by adding up the increments of the layers.The deformability modulus(ESi)is considered as the only independent random variable.As an example of application,a hypothetical case of a typical subsoil in the state of Espirito Santo,southeast of Brazil,is evaluated.The results indicate that there is a significant similarity between the SOSM and MCS methods,while the FOSM method underestimates the results due to the non-consideration of the high-order terms in Taylor's series.The contribution of the knowledge of the uncertainties in settlement prediction can provide a safer design.     

Centrifuge study into the effect of liquefaction extent on permanent settlement and seismic response of shallow foundations

Centrifuge experiments were conducted to investigate how the liquefaction extent affects the seismic and post-seismic settlement of shallow foundations resting on saturated sand. Two rigid foundations with different bearing pressures were placed on the ground surface in a model container. Multiple input motions were applied to achieve different extents of soil liquefaction. The results indicate that foundation settlement can be divided into three distinct phases: (I) during shaking, (II) during the time period after shaking has ceased and before soil reconsolidation in the shallowest layers has taken place, and (III) during soil reconsolidation. Contrary to the free-field ground, most of the total settlement of the foundations occurred before soil reconsolidation, i.e., during Phases I and II. The volumetric strain during these phases was not significant as opposed to the shear strain produced by the foundation surcharge. It was demonstrated that foundation settlement is not necessarily proportional to the liquefied depth of the sand. The extent of the liquefaction in the sand medium mostly affected the post-seismic settlement of the foundations, while the co-seismic settlement was relatively the same for both foundations. The response of the foundations was significantly influenced by the liquefaction extent, whereas the foundations did not experience large accelerations when the soil profile was entirely liquefied. However, the foundations tolerated large settlement under severe liquefaction conditions. The results of this study highlight the role of the liquefaction extent on co-seismic and post-seismic settlement as well as the seismic response of shallow foundations.     

承载地层隧道开挖二次应力和塑性区的理论预测方法

地铁隧道往往不得不从既有结构基础附近穿过,隧道的施工会扰动周边地层,引起邻近基础及结构的沉降,甚至影响其安全或正常使用。为了预测并控制隧道开挖引起的结构沉降,就需要计算隧道开挖对地层的扰动程度。现有隧道开挖塑性区理论均以自由地层(地层中没有结构基础荷载作用)为前提假设,没有反映承载地层隧道开挖影响的特殊性。考虑均质各向同性理想弹塑性地层,利用自由地层隧道开挖影响的解析解和结构基础荷载影响的解析解,提出一个预测承载地层隧道开挖引起的地层二次应力及塑性区形状和范围的理论方法,并通过算例分析基础荷载形式、大小和位置对隧道开挖塑性区的影响。结果表明,达到一定量值的邻近基础荷载会使隧道开挖塑性区的形状和范围明显地不同于自由地层隧道开挖塑性区,具体与荷载的位置、大小及分布形式有关。     

物流仓库大面积堆载危害及规范方法预测沉降问题探讨

深厚软土地基上大面积堆载引起的沉降危害是建筑结构中出现的主要问题之一。然而,当前针对大面积堆载条件下地基沉降预测的规范方法仍为基于较小面积基础的沉降观测资料推算得到。由于大、小面积分别堆载时导致的下部地基压缩层深度的不同,在预测大面积堆载下地基基础沉降时,当前规范方法的适用性有待验证。为此,针对上海某物流仓库的沉降灾害开展实例分析,在此基础上,对当前我国GB 50007—2011《建筑地基基础设计规范》与上海规范DGJ 08-11—2010《地基基础设计规范》中地基沉降计算方法部分展开讨论。结果表明：地基过大沉降导致地坪板等平整度严重降低以及地坪板开裂等问题,极大危害了建筑结构的安全与正常使用;规范方法预测的沉降值远低于该工程的沉降监测值;不应取代表性分仓单元或局部面积进行沉降预测,而应取整体建模分析;上海规范的沉降计算经验系数取值不甚合理,导致随计算压缩层深度增加,最终沉降反而变小的不合理结果。     

带浅内坑基坑的动态施工数值模拟

结合上海市西藏南路越江隧道浦东接线段某坑中坑基坑工程,建立有限元数值模型对基坑动态施工进行数值模拟。内坑开挖造成两侧地表有不同程度的沉降,靠近内坑一侧地表沉降较大。当开挖至内坑底部后,右侧最大沉降为左侧的1.6倍。内坑开挖造成左墙底部往坑内发生较大侧向位移。中墙顶部发生坑外位移,中、底部发生坑内侧移。右墙中底部侧移较大,且最大侧移处位于地表以下1.2倍的开挖深度。     

An Experimental Study on the Additional Settlement of Footings Resting on Granular Soils by Water Table Rise

When the water table rises in a granular soil mass, a large additional settlement of footing resting on such a soil mass is commonly expected. Laboratory model tests show that when the water table rises to the footing level, there can be an additional 400 to 500% of the settlements compared to when the soil is dry. To understand the mechanics of the additional settlements of footings resting on the granular soil masses caused by water table rise, an investigation into the change in the Young's modulus of soil was therefore made using oedometer tests. A relation between the saturated and the dry Young's moduli of the granular soils is presented for use in the elastic analysis of footing settlements. The findings of the investigation were then used to predict the additional settlements of a model footing when subjected to a water table rise, using elastic analysis. The theoretical predictions of the additional settlements are lower than the actual additional settlements observed in the settlement tests conducted in the laboratory using model tests. The model tests show that the additional settlements are larger in loose sands than in dense sands.     

基于沉降控制的高路堤涵洞纵向调荷技术

为解决高路堤涵洞纵向不均匀沉降所带来的病害问题,基于纵向沉降控制的高路堤涵洞调荷机理,利用有限元软件研究涵洞填土与地基土特性以及不同EPS板参数对涵顶垂直土压力和涵底土体沉降的影响,通过离心模型试验探讨涵洞纵向铺设EPS板对高路堤及涵洞的沉降的影响。通过数值模拟计算,分析不同EPS板模量、铺设范围、厚度以及地基处理范围对高路堤涵洞纵向沉降差减少率的影响。研究结果表明:①填土模量与泊松比对涵顶垂直土压力及涵底土体沉降的影响不显著;②随着地基土模量与泊松比的增加,涵底土体纵向沉降趋于均匀分布;③数值仿真与离心模型试验成果得出,沿涵洞纵向分层铺设EPS板时,涵洞纵向调荷效果最佳;④通过数值模拟计算得到了基于纵向沉降控制的高路堤涵洞调荷设计计算方法。     

长江漫滩高承压水地基地连墙承载特性现场试验研究

 针对长江漫滩高承压水地基,以南京青奥轴线-梅子洲过江通道基坑为依托工程,开展格栅地连墙和普通地连墙承载特性的现场试验研究,分别测试研究其墙顶水平位移、墙体深层水平位移、地表沉降、支撑轴力等随基坑开挖及时间的变化规律。主要结论如下：(1) 墙顶水平位移在支撑设置后均有回弹变形趋势,变形受支撑架设、预加轴力及拆除影响较大;(2) 2种墙体深层水平位移随深度均呈“胀肚型”变化趋势,两者最大侧移均发生在埋深中上部区域;(3) 格栅地连墙在基坑开挖初期,地表先小幅隆起,普通地连墙无隆起现象,且沉降明显偏大,两者随距墙体距离增大沉降逐步变小,且不同距离处差异沉降在基坑开挖后期均有增大趋势。     

复杂环境下地铁深基坑变形行为的实测研究

本文通过对上海软土地区开挖深度为15.5m的多支撑基坑的现场实测,发现与世界范围其它工程实例相比,本基坑的墙体侧移和地表沉降是非常小,空间效应不明显,其原因是基于时空效应的分层分小块的快挖快撑、坑内注浆加固以及积极的支撑预加轴力等措施的应用。同时通过土体体积的变化量和平方根时间方法分析在中板养护期间的土体随时间变形,指出在开挖期间基坑变形不仅仅与土蠕变相关,土体的固结也对变形产生了相当的影响。     

Centrifuge modeling of hybrid foundation to mitigate liquefaction-induced effects on shallow foundation resting on liquefiable ground

A hybrid foundation is developed in this study to mitigate the liquefaction-induced effects on shallow foundations. The proposed hybrid foundation is a combination of a gravel drainage system and friction steel piles with spiral blades, framed under the footing. The motivation behind having a gravel drainage system, as an integral part of the hybrid foundation, is its ability to improve the liquefaction resistance of the ground in the most economical way. However, case histories and the development of recent research have highlighted that gravel drainage systems have exhibited poor performances and could not prevent ground liquefaction during strong ground motion. To counteract these shortcomings, friction steel piles are provided which are supposed to yield frictional resistance during earthquakes and are presumed to minimize the rocking/tilting behavior of the foundation-structure system even if the ground undergoes liquefaction. The evolution of excess pore water pressure, specifically in the vicinity of the foundation-structure system, dominatingly influences the settlement mechanism of shallow foundations. Centrifuge test results show that the presence of gravel drainage can minimize the post-liquefaction settlement of shallow foundations through the rapid dissipation of excess pore water pressure. Moreover, friction piles are able to minimize the tilting/differential settlement of shallow foundations. It is found that the proposed hybrid foundation provides the desired function of reducing the overall liquefaction-induced effects on shallow foundations resting on liquefiable grounds.     

大厚度自重湿陷性黄土地基处理深度和湿陷性评价试验研究

 为解决大厚度自重湿陷性黄土地区地基处理深度和湿陷性评价等难题,在湿陷性黄土厚度大于36.5 m的场地进行以下浸水试验：不同深度的挤密桩处理地基深层浸水载荷试验,不同深度的孔内深层强夯处理地基载荷浸水试验,不打注水孔、埋设TDR水分计的原位浸水试验。研究结果表明：(1) 大厚度自重湿陷性黄土地基处理6～12 m、深层浸水时,发生显著地基下沉;15～20 m时,地基沉降较小;处理深度大于20 m时,地基沉降基本可忽略。(2) 浸水试坑22.5～25.0 m以上土体含水率增加较快,甚至达到饱和,以下土体含水率增加缓慢,基本没有发生湿陷。建议22.5～25.0 m作为大厚度自重湿陷性黄土地基处理和湿陷性评价的临界深度。(3) 大厚度自重湿陷性黄土地基在采取有效的综合处理措施之后,甲类建筑可以不全部消除湿陷量,乙、丙类建筑可以根据控制建议适当放宽对剩余湿陷量的要求。(4) 不同地区、不同微结构类型土的湿陷性应当采用不同的湿陷系数 来判定,即“湿陷系数 = 0.015”在自基础底面至基底下15 m的范围内可继续使用;15 m以下适当放宽,按不同深度对 进行修正,可使大厚度自重湿陷性黄土湿陷性评价趋于合理,有效节约大量地基处理费用。     

强夯加固吹填砂地基试验研究

通过对某填海工程吹填砂地基进行强夯处理的现场试验,对夯坑沉降及夯点周围地表变形、场地平均沉降量、孔压的增长和消散、强夯振动等进行监测,然后采用载荷试验,重型动力触探(N63.5)、标准贯入、瑞利波波速测试等原位测试方法对地基的加固效果进行评价,在此基础上,给出了地基强夯参数,以供指导施工。通过试验研究可以了解强夯对提高吹填砂地基承载力的效果、孔隙水的响应规律、地基的振动影响等,有利于揭示强夯加固砂地基的特性,为我国沿海地区的"填海造地"工程提供了经济有效的地基处理方法和经验。     

Bewertung von Setzungsmessungen flachgegründeter Gebäude in weichen Böden

Evaluation of settlement measurements of buildings with shallow foundation on soft soils. Available settlement measurements of 10 buildings with shallow foundations on lacustrine clay in Southern Germany represent the basis of a statistical evaluation of the primary‐ as well as the creep‐settlement. Generally, the measurements show that the predicted settlements using the standard one‐dimensional consolidation tests are significantly larger than the actually occurred settlements. According to the evaluation a correction factor to the calculation of settlement in soft soils was derived. By means of back‐analysis of the measured settlements the coefficients of the consolidation and the secondary compression are determined so that the time‐settlement behavior could be also investigated.     

沉降控制复合桩基的工程应用

 以沉降控制复合桩基为对象,分析其受力机制、工作性态和变形特性,推导复合桩基桩数的计算公式;通过对沉降控制复合桩基沉降量的几种常规计算方法的讨论,建立计算复合桩基沉降量的实用新方法——修正实体深基础分层总和法;并用该方法计算2个沉降控制复合桩基工程的基础沉降量。结果表明：采用修正实体深基础分层总和法计算沉降控制复合桩基基础的沉降量要比天然地基浅基础的沉降量小;计算结果与现场跟踪检测沉降结果相吻合,比常规方法的计算结果更符合工程实际,满足现行地基基础设计规范要求;与常规桩基相比,更有利于充分发挥桩和桩间土的承载力,减少了用桩数量,在很大程度上降低了工程造价,并确保工程质量。