既有建筑物地下室增层开挖桩基屈曲稳定研究

既有建筑物地下室增层开挖可有效利用地下空间,是目前改善停车难问题的主要手段之一。既有建筑地下室增层开挖将改变原有桩基础的受力性状,从而可能引起原有桩基础屈曲失稳。提出一种预测地下室增层开挖既有桩基失稳的临界荷载及稳定计算长度的理论解法。首先,结合浙江饭店地下室增层开挖工程,简单介绍增层开挖方案。之后,通过Winkler弹性地基梁理论建立增层开挖工况下单桩基础桩-土体系总势能方程。最后,利用最小势能原理导得增层开挖工况下单桩基础屈曲失稳的临界荷载及稳定计算长度的表达式。在此基础上,研究了桩身临界荷载的影响因素。计算结果表明：随着半波数n的增加,既有桩基的临界荷载逐渐收敛;既有桩基的临界荷载随开挖深度的增大而急剧减小。分析结果可为类似地下室增层开挖工程的设计提供依据,减少开挖引起既有桩基础失稳隐患。     

紧邻在建主楼桩基的地下室基坑围护设计实例

该文介绍了一个紧邻在建主桩基的地下室基坑围护设计的实例。通过对工期、施工空间、安全性以及造价的综合分析,确定了以土钉墙为主的围护方式。通过有限元计算对开挖所造成的主楼桩基与地基土的相互作用机理的分析, 表明桩对土的约束作用限制在一定的范围内。根据分析结果,在设计中采用比较大的安全系数以及开挖前的超前支护结构以控制开挖过程中主楼基础地板下地基土的侧向位移,最大限度地减少由于地下室开挖卸载而造成的主动土压力对已有主楼桩基的影响,保证在地下室开挖过程中主楼的施工安全,同时也保证了地下室已有桩基的安全。实践证明,这种方法是有效的,对同类工程的围护设计具有一定的借鉴价值。     

A study into the behaviour of the formation level of an excavation under different unloading patterns in soft deposits

The construction of basements in urban areas is often associated with the possible damage to existing structures and services. The varying construction processes inevitably lead to different stress unloading patterns and therefore the dissipation of these excess pore-water pressures may lead to non-standard deformation profiles. The three main types of basement construction processes are layered excavation (LE), basin excavation (BE) and island excavation (IE). The effect of the various unloading patterns has been investigated by a three dimensional (3D) effective stress analysis method using the developed computer program 3DBCPE4.0. An excavation of length 50 m, width 50 m and depth 9 m in a certain homogenous and isotropic saturated soft soil was modelled. This included a diaphragm wall of 800-mm thickness embedded 18 m deep into the soft soil. The different excavation deformation profiles under different excavation patterns were related to the different unloading process, the exposure time of excavation face and the dissipation of negative excess pore-water pressures. The most favourable process for controlling the horizontal deformation of a retaining wall or the heave deformation of the formation level is suggested. The ground water potentials within the formation level are also presented.     

超长基坑开挖的空间效应

为了研究超长基坑开挖的空间效应,对盘锦某地下商业街基坑工程开挖过程进行了数值模拟,主要对基坑工程中分段、分块开挖等问题进行了计算分析。结果表明:随着基坑开挖分段长度的增加,支护桩水平位移的最大值呈先减小后增大的趋势,基底隆起量最大值呈增大的趋势;随着基坑开挖分块长度的增加,受保护侧支护桩的水平位移呈先减小后增大的趋势。超长基坑开挖时考虑空间效应可有效地减小支护结构、周围地层的变形;在基坑分段、分块开挖过程中,要考虑未开挖土体是否具有足够的强度限制支护结构及地层的变形,使得空间效应得以发挥。     

Cantilever Contiguous Pile Wall for Supporting Excavation in Clay

In urban areas, there are many situations where basement of new constructions or underground utilities are proposed to be constructed adjacent to old buildings. Of greatest concern are buildings with shallow foundations that do not extend below the zone of influence of the adjacent excavation. For deep excavation, the presence of a cantilever stage at the beginning of a construction sequence can often lead to excessive movements. Therefore over-excavation can also be a primary cause of excessive movements. In the present study, shallow excavation was considered. Three Dimensional Finite Element (3D FE) study was carried out in the present study. A cantilever contiguous pile wall was selected because it is common and relatively economic to be used in cohesive soil. Parametric study were performed considering excavation depth, pile embedded depth, and wall stiffness. Some design recommendations were given to provide a safe supporting system in clay.     

某工程坑中坑塌滑原因分析及加固设计

基坑支护结构设计中,坑中坑的设计直接影响到基坑支护结构的安全和经济,况且不少基坑支护结构的工程事故与坑中坑的不合理设计密切有关。结合某土钉墙基坑支护结构失稳的工程实例,分析了基坑支护结构失事的根本原因,提出了基坑支护结构的加固设计方法,同时也提出了基坑支护结构设计中坑中坑的设计方法,研究了基坑支护结构与坑中坑之间的相互制约和相互影响。研究表明,坑中坑对地下室基坑支护结构的整体稳定性影响较大,而基坑支护结构对坑中坑中的支护桩的内力影响相对较小。     

开挖卸荷对既有群桩竖向承载性状的影响分析

既有建筑下增层开挖对已有桩基础的影响不同于基坑开挖对坑内桩基的影响。基于工程实例验证的有限元参数,用硬化土弹塑性模型模拟土体,用接触面单元模拟桩土相互作用,建立了桩筏基础-地基-增层开挖三维有限元模型,对增层开挖后群桩基础的竖向承载性状进行研究。分析了桩顶刚度、桩身轴力、桩侧摩阻力、桩端阻比以及土体回弹的变化规律,并研究了不同桩端土体刚度和增层开挖深度对这些参数的影响。结果表明,增层开挖后群桩中不同基桩表现出不同的承载性状,增大桩端土体刚度可明显提高单桩承载力和端阻比临界值;随着增层开挖深度的增加,侧阻和端阻的发挥程度也随之提高。研究结果有望为地下室增层开挖施工中的结构托换变形控制和补桩设计提供依据。     

某软土深基坑险情分析与处理

介绍一软土地区深基坑由于超挖引起基坑险情的工程实例。该基坑土层分布以淤泥质粉质黏土为主,为典型的长江漫滩地质条件,具有一定的代表性。基坑土方开挖期间,由于地下室方案调整,基坑深度加深,而未按设计要求对支护进行事先加固,导致基坑出现险情。通过对比前后不同挖深条件下支护结构的内力及变形,分析了险情产生的原因,并依据实测支护结构和周边环境的变形,针对基坑出现的问题提出有效的处理措施,从而保证了基坑的安全。     

钻孔引发基坑管涌事故的分析与处理对策

地下室开挖与基础施工过程中,当坑底存在承压水,如设计考虑不周或施工方法不当,因钻孔穿透承压含水层等原因,将导致管涌发生。结合浙江宁波某基坑管涌处理工程实例,提出采用钢套管、高压旋喷桩、双液注浆的综合处理技术,实践证明,提出的技术可以有效处理类似事故。     

高填方涵洞减载机制与数值分析

高填方引起的涵顶土压力集中往往造成涵顶纵向开裂,通过涵洞-填土-地基共同工作机制的研究,分析了涵洞减载的原理。通过数值模拟分析了中松侧实、柔性填料、先填后挖法以及地基处理措施对涵顶的土压力集中系数的影响。结果表明：上述方法和措施可以有效减小涵顶土压力,先填后挖法宜采用竖直边坡,开挖沟槽的宽度宜与涵洞宽度相当;涵洞地基应采用柔性处理措施,并延伸至基底以外一定的范围。研究结果可为高填方涵洞减载措施的选取以及涵洞地基设计提供参考。     

Hypervelocity collisions into continental crust composed of sediments and an underlying crystalline basement: comparing the Ries (∼24 km) and Chicxulub (∼180 km) impact craters

The Chicxulub and Ries impact craters were excavated from layered continental terrains that were composed of carbonate-bearing sedimentary sequences and underlying crystalline silicate basement materials. The Chicxulub and Ries impact events were sufficiently large to produce complex peak-ring impact craters. The walls of transient craters and excavation cavities, with diameters of 12-16 km for the Ries and 90-100 km for Chicxulub, collapsed to form final crater diameters of ∼24 and ∼180 km, respectively. Debris from both the sedimentary and crystalline layers was ejected during crater formation, but the bulk of the melting occurred at depth, in the silicate basement. The volume of melt and proportion of melt among shock-metamorphosed debris was far larger at Chicxulub, producing a central melt sheet ∼3 km in depth. The central melt sheet was covered with melt-bearing polymict breccias and, at the Ries, similar breccias (crater suevites) filled the central cavity. Also at the Ries (and presumably at Chicxulub), large hill-size megablocks of crystalline basement material were deposited near the transient crater rim. Blocks and megablocks of sedimentary lithologies were ejected into the modification zone between the peak ring and final crater rim, while additional material was slumping inward during crater growth, and buried beneath a fallout deposit of melt-bearing polymict breccias. The melt and surviving clasts in the breccias are dominantly derived from the deeper, basement lithologies. At greater distances, however, the ejecta is dominated by near-surface sedimentary lithologies, large blocks of which landed with such high energy that they scoured and eroded the pre-existing surface. The excavation and ejecta pattern produced lithological and chemical variations with radial distance from the crater centers that evolve from basement components near the crater centers to sedimentary components far from the crater centers. In addition, carbonate (and anhydrite in the case of Chicxulub) was vaporized, producing environmentally active gases. The vaporized volume produced by the Ries impact event was too small to dramatically alter the evolution of life, but the vaporized volume produced by the Chicxulub impact event is probably a key factor in the Cretaceous-Tertiary boundary mass extinction event.     

大断面开挖成型控制技术在三峡工程中的应用

开挖成型控制是在大断面地下洞室开挖施工中的一项关键技术,开挖成型控制得好,既能降低后续施工的成本,又能提高工程的安全。在三峡工程某大型地下厂房开挖施工中,通过对造孔设备选型、精度控制及精细化装药等方面的研究和实践,取得了顶拱平均超挖8.5 cm、边墙平均超挖5.3 cm、岩锚梁平均超挖控制在2 cm以内的结果,远低于规范所要求的小于20 cm的标准,并且开挖平整度小于10 cm,爆破半孔率达到了95 %以上,在成型质量控制方面较以往工程有了突破性的进展,具有较高的应用及参考价值。对具体控制过程予以分析总结,供类似工程借鉴工程实例。     

A 2D coupled finite element and boundary element scheme to simulate the elastic behaviour of jointed rocks

In this paper a coupled finite and boundary element formulation is developed for the analysis of excavation in jointed rock. The presence of joints in the rock mass has been included implicitly by treating it as an appropriate anisotropic elastic continuum. The boundary element formulation for an anisotropic medium is briefly discussed. Good agreement has been found between numerical and analytical solutions for several example problems, demonstrating the accuracy of the present formulation. Numerical solutions are also presented for the problems of a deep circular tunnel and a basement excavated in a variety of jointed rock masses.     

杭州黄龙饭店支腿式地下连续墙施工技术探讨

支腿式地下连续墙在杭州地区工程中首次运用。结合实际工程实践,就支腿式地下连续墙的技术要点进行介绍,通过跟踪分析实际施工过程,简单介绍整个工艺流程和确保支腿式地下连续墙的施工质量、保证地下室开挖深度及基坑安全性、稳定性的技术措施。展望支腿式地下连续墙推广应用前景。     

桩基础托换开发地下空间不均匀沉降的数值分析

桩基础托换法开发既有建筑物地下空间的施工过程可分为桩基托换前、托换后和土方开挖至设计标高3个工况。施工过程会使既有建筑物地基产生二次沉降变形,由此产生的附加内力会影响到上部结构的安全。为了研究一个3层框架结构建筑物增加一层地下室桩基础托换过程的地基沉降规律,建立了3个工况的ANSYS三维有限元模型,并进行数值分析。通过定义路径的方法,可以得到3个工况的沉降曲线和柱脚处地基的沉降差。从沉降分析的结果可以看出：桩基托换前,地基总的沉降趋势是中间部位柱脚大于周边部位柱脚的“盆式沉降”;桩基础托换后,柱脚处的沉降差进一步增大;土方开挖后,由于下层土被“卸载”,中间部位柱脚处的地基明显回弹,使得中间部位柱脚与周边部位柱脚的沉降差出现了减少的趋势。最大的沉降差出现在桩基托换后土方开挖前,此时由沉降差引起的上部结构的附加弯距可以通过ANSYS有限元分析求出。     

基坑开挖与支撑拆除对周围环境影响的研究

地下连续墙和内支撑是基坑工程常用的支护形式,一般认为该支护形式下的变形主要发生在基坑开挖阶段,忽略了支撑拆除对基坑变形以及周围环境的影响。本文以深圳市万科滨海置地基坑工程为依托,通过有限元数值模拟分析,研究基坑开挖和支撑拆除对基坑周围环境的影响,最终得到以下结论:支撑拆除阶段与基坑开挖阶段的变形规律一致,受时空作用影响明显,“中间大、两边小”;两阶段变形量大小相差不大,变形量之和占总变形量的85%以上,发展迅速的变形分别发生在底层土开挖以及底层支撑拆除阶段,其原因主要是三道支撑不等距,底层开挖土厚度大、花岗岩残积土厚度差别大以及仅在地下室底板位置处设有换撑块。模拟结果显示,基坑顶部水平位移、地面沉降以及建筑物沉降与监测数据基本一致,其结果可以为同类工程提供借鉴。     

隧道围岩稳定性的有限元分析

根据地下结构设计理论和岩石屈服的Drucker-Prager准则，考虑到了围岩的自身承载能力，采用有限元的分析方法对广西柳州一处公路隧道围岩的稳定性进行了分析。分析了开挖方式、开挖顺序对围岩稳定性的影响，对围岩的开挖过程进行模拟。确定不同开挖方式下隧道围岩的位移、应力状态，以及位移、应力状态随时间的变化规律，为隧道施工过程中开挖方案的制定、支护时间的选取提供依据。     

斜支撑支护基坑与相邻地下室有限土体土压力反演分析

为探讨斜支撑支护基坑与相邻地下空间有限土体土压力分布规律,对某基坑现场监测数据进行反演分析。研究基于试算法改进后的三次样条法,运用Matlab软件进行反演计算得到支护结构弯矩值和桩后土压力值,结果表明:受有限土体位移模式、非极限状态、边界条件的影响,有限土体主动土压力在开挖面以上,呈现明显的"R"字形分布,比经典土压力计算值小约16.3%;被动土压力与主动土压力差值在开挖面以下,呈现近似矩形分布,比经典土压力计算值小约65%,分析结果可为该类工程支护设计及计算提供依据和参考。      

上下交叉隧道爆破振动控制技术研究

钻爆法是隧道开挖中一种主要的施工方法,爆破开挖不可避免会对围岩产生扰动。以新建走马岗隧道上穿东深供水走马岗引水隧洞工程为背景,开展上下交叉隧道爆破振动控制技术研究。选取与交叉段岩性一致的爆破施工开挖区域进行爆破振动监测,得到了现场爆破施工方案条件下走马岗隧道爆破振动规律。对实测数据进行回归分析,计算得出走马岗地区爆破质点峰值振动速度（PPV）的Sadovsk公式,反演得到控制爆破振动的最大掏槽药量及安全距离,制定出交叉段施工的安全控制范围以及相应的爆破方案。通过数值模拟进行了验证并付于交叉段现场施工。现场监测数据表明,提出的振速控制标准及爆破方案符合安全要求,保证了在建隧道的顺利开挖以及既有隧道的安全运行。研究成果可为类似工程爆破开挖及振动控制提供参考。     

不良地质环境中基坑边坡加固技术分析及其应用

结合现有边坡加固理论和加固经验,对一紧临道路和8层老楼房且下部有厚度达14 m淤泥软弱土层的基坑边坡进行了复合加固设计。通过对边坡稳定性计算、分析,加固后的边坡满足最小安全系数的要求。按照本设计进行施工后,基坑边坡的变形和周围建筑物的沉降均在安全允许的范围之内,保证了地下室结构部分的顺利施工。所提供的加固技术对同类深基坑支护工程具有重要的参考价值。