超大跨度悬索桥隧道锚承载特性的岩石力学综合研究

 针对四渡河特大桥宜昌岸隧道锚承载特性问题,采用基于岩石力学的综合研究方法,从围岩地质与力学特性、隧道锚1∶12实体模型试验及隧道锚承载特性数值分析等方面,对隧道锚与围岩岩体变形机制、时效特征及超载安全性等方面开展系统研究。结果表明,通过岩石力学试验及基于实体模型试验获得的隧道锚围岩弹塑性及流变参数符合实际;在设计水平下,隧道锚锭围岩变形在mm级水平;隧道锚极限抗拉拔力≥7.6倍设计载荷,满足锚固安全系数＞4.0的设计要求;实桥隧道锚碇的长期安全系数≥2.6。通过工程实际施工过程中的监测实施,对研究成果和结论的合理性进行验证。研究技术路线及成果可供山区类似桥梁建设借鉴。     

岩石地基承载力的几个认识问题

我国工程界对岩石地基承载力问题有不同认识,作者就其中的一些重要问题提出了自己的看法。包括Mohr-Coulomb准则是否适用于岩石地基?可否用抗剪强度指标计算岩石地基承载力?用单轴抗压强度指标确定岩石地基承载力是否合理?岩石地基承载力是否可做深宽修正?怎样看待岩石地基承载力的理论计算和经验方法估计?并对用剪切波速进行岩土分级和承载力估计、地基承载力的综合判断问题提出了看法。     

输电线路岩石锚杆基础试验研究

岩石锚杆基础已逐渐在输电线路工程中获得广泛应用,针对节理裂隙发育的灰质板岩岩体,进行了现场拉拔真型试验。试验结果表明:在满足锚筋自身强度的情况下,应以锚孔表面混凝土的位移值来确定基础的极限荷载;当锚杆达到一定长度后,再单纯依靠增加锚杆长度提高极限抗拔承载力效果甚微;适当改善锚筋外表面的粗糙度可提高锚筋与混凝土间的界面粘结强度;在岩石锚杆基础设计中要注意群锚效应的削弱作用。     

岩层设承力盘的嵌岩桩抗拔静载试验及计算方法研究

为提高东南沿海桩基础抗拔承载性能,开发应用了一种新型的、在桩身嵌岩段的上侧岩层中设置双向旋扩承力盘的抗拔嵌岩桩。结合广东佛山市某工程,进行单桩竖向抗拔承载特性静载试验。试验表明,该桩抗拔性能优越,抗拔承载力大大优于抗拔直孔桩及扩底桩。基于实测数据,建立适用于该桩的极限承载力双曲线理论模型。基于Hoek-Brown岩体破坏准则及嵌岩桩单桩抗拔承载机理,提出该桩型的抗拔破坏模式,并建立相应的单桩抗拔极限承载力计算方法。该计算方法可有效反映盘下嵌岩段的岩体性质、承力盘所处岩层的岩体性质、盘角及上覆土层厚度等因素对基桩抗拔极限承载力的影响。与规范方法、现场实测的抗拔承载力计算结果对比表明,该计算方法与现场测试结果吻合度高,具有很好的应用前景。     

对软质岩石地基承载力的一点新认识

 岩石地基承载力的确定不能简单的运用查表法或经验类比法,而应通过多种测试手段取得评价依据,充分挖掘岩石地基的承载力。     

Pile end bearing capacity in rock mass using cavity expansion theory

Much empiricism is involved in design of rock-socketed piles in rock masses. In light of this, an analytical solution based on the cavity expansion theory is proposed for calculating the ultimate bearing capacity at the tip of a pile embedded in rock masses obeying the Hoek-Brown failure criterion. The ultimate end bearing capacity is evaluated by assuming that the pressure exerted at the boundaries of a pressure bulb immediately beneath the pile tip is equal to the limit pressure required to expand a spherical cavity. In addition, a relationship is derived to predict the pile load-settlement response. To demonstrate the applicability of the presented solution, the results of this study were compared to those of 91 field tests from technical literature. Despite the limitations, it is found that the end bearing resistance computed by the present work is in good agreement with the field test results.     

Axial bearing capacity of socketed single cast-in-place piles

In this paper, a comparison is made of the axial bearing capacities estimated with pile load tests and empirical methods for seven rock-socketed single cast-in-place piles constructed in Turkey. The unconfined compressive strength of rock, obtained from pressuremeter tests, is used in the empirical correlations. It is commonly assumed that axial loads applied at the top of a socketed pile are transferred to the sides of the socket until a certain displacement is reached and that the end bearing capacity contributes only after this threshold displacement is exceeded. In practice, however, due to typically small displacements occurring in rock sockets, most, if not all, of the axial capacity is estimated to derive from the side shear. The limit displacement up to which the side frictional capacity of a socketed pile governs and the end bearing capacity starts mobilizing is examined, and no such threshold value is observed based on the findings of this study. Nevertheless, the bearing capacities obtained from the empirical correlations agree reasonably well with those calculated from pile load tests, when a systematic approach for estimating the threshold value from pile load tests is utilized and the unconfined compressive strength of socketed rocks can be estimated within reasonable accuracy applying actual field conditions.     

Empirical methods for determining shaft bearing capacity of semi-deep foundations socketed in rocks

Semi-deep foundations socketed in rocks are considered to be a viable option for the foundations in the presence of heavy load imposed by high-rise structures, due to the low settlement and high bearing capacity. In the optimum design of semi-deep foundations, prediction of the shaft bearing capacity, rs, of foundations socketed in rocks is thus critically important. In this study, the unconfined compressive strength(UCS), qu, has been applied in order to investigate the shaft bearing capacity. For this, a database of 106 full-scale load tests is compiled with UCS values of surrounding rocks, in which 34 tests with rock quality designation(RQD), and 5 tests with rock mass rating(RMR). The bearing rocks for semi-deep foundations include limestone, mudstone, siltstone, shale, granite, tuff, granodiorite, claystone, sandstone, phyllite, schist, and greywacke. Using the database, the applicability and accuracy of the existing empirical methods are evaluated and new relations are derived between the shaft bearing capacity and UCS based on the types of rocks. Moreover, a general equation in case of unknown rock types is proposed and it is verified by another set of data. Since rock-socketed shafts are supported by rock mass(not intact rock), a reduction factor for the compressive strength is suggested and verified in which the effect of discontinuities is considered using the modified UCS, qu(modified), based upon RMR and RQD in order to take into account the effect of the rock mass properties.     

长沙国际金融中心软岩载荷试验及其成果分析

长沙国际金融中心是目前华中在建第一高楼,建筑高度452 m,地上93层,地下5层,基础埋深约40 m。场地第四系覆盖层厚度约20 m,基岩为白垩系泥质粉砂岩,属极软岩。按现行(GB50007—2011)《建筑地基基础设计规范》确定的地基承载力远低于原位试验。文章采用承压板直径分别为φ800 mm、φ1130 mm的静力载荷试验与室内单轴抗压试验进行对比分析,提出了软岩承载力的试验取值原则,对同类地区类似岩基承载力的取值具有参考价值。     

孔壁粗糙度对嵌岩桩承载力的影响

总结了孔壁粗糙度对嵌岩桩承载力影响的规律 ,探讨了孔壁粗糙度影响嵌岩桩承载性状的机理 ,分析了影响嵌岩桩孔壁粗糙度的因素 ,指出改善孔壁粗糙度是提高钻孔灌注桩承载力的有效途径。     

输电线路岩石锚杆基础试验研究

首次针对新疆地区220kV输电线路工程典型的强风化岩体地质条件,进行了六根单锚基础的抗拔承载力真型试验,得到了其破坏性态与极限承载力,给出了相关设计计算参数的取值,为其在输电线路工程中的应用提供参考。     

人工挖孔桩软岩桩端承载力的研究

结合软岩工程实例,对如何确定软岩桩端承载力特征值进行了研究,并将试验结果与单轴抗压强度试验结果进行比较,发现软岩地基的承载力大于根据单轴抗压强度试验确定出的承载力,指出深层平板载荷试验更符合地基的受力实际情况,对合理确定软岩地基的承载力取值有一定的参考价值。     

Effects of inelastic volume increase on fractured rock behaviour

This study investigated the correlation between inelastic volumetric deformation and the bearing capacity of fractured rock. Triaxial compression tests on marble specimens have been performed under constant and controlled deformation rates using a servocontrolled loading machine supported by an electro-hydraulic volume-measuring unit connected to a data acquisition system. The triaxial compression tests were carried out on two different marble samples under constant confining pressures ranging from 1–12.5 MPa. The results indicate that stress–inelastic volumetric strain curves are generally linear through the initial part of the post-failure curve where fractured rock behaviour is dominant. It was found that brittle rock loses its strength in the post-failure region depending on volumetric deformation; the slopes of post-failure curves change with an increase in confining pressure. As a result it is concluded that there is a relationship between strength and volumetric strain of fissured rock. Electronic Publication     

武汉绿地中心大厦大直径嵌岩桩现场试验研究

武汉绿地中心大厦高636 m,基底平均压力达1500 k Pa,开展了桩径为1200 mm,有效桩长25.9~33.6 m,以较硬岩(微风化砂岩)和软岩(中—微风化泥岩)为桩端持力层共4组嵌岩桩承载力静载荷试验,并对桩身轴力与变形进行了量测。试验表明:4根试桩Q–s曲线皆为缓变型,极限承载力不小于45000 k N,对应工程桩桩顶标高沉降为9.7~10.8 mm,桩端沉降为2.4~2.8 mm,桩顶沉降的90%为桩身压缩量。软岩嵌岩桩与较硬岩嵌岩桩的侧摩阻力分布曲线及承载特性存在显著差异。四根试桩的端阻比介于45.3%~58.7%。软岩嵌岩桩的实测桩端阻力大于基岩单轴抗压强度,采用桩基规范方法计算将低估其实际承载力。本工程4根试桩均表现出较好的承载与变形控制能力,静载试验结果为本工程嵌岩桩设计提供了依据,同时为武汉地区大承载力嵌岩桩实践与理论研究提供了有益参考。     

扩底桩的抗拔承载力试验及计算

通过对干旱地区黄土中扩底桩的抗拔试验 ,测试了扩底桩在上拔荷载、水平荷载作用下的上拔位移和水平位移以及位移与荷载的关系。研究了极限上拔承载力和抗拔桩的破坏机理。在相同条件下 ,增加扩大端的高度对提高桩的极限上拔承载力是有效的 ,破坏机理为土的减压软化和损伤软化的渐进性破坏。提出了极限上拔承载力的理论计算模式 ,并与实测资料进行了对比 ,理论计算结果与实测值是吻合的     

软土中预制桩承载力时间效应三参数模型

预制桩承载力的时间效应是岩土工程中一个非常重要且尚未很好解决的问题。在承载力初始增长速率和承载力最大增量的基础上,引入承载力增长半衰期的概念,揭示双曲线模型的数学缺陷。根据机理分析和能量耗散假定,在建立承载力时间效应控制微分方程的基础上,推导出预制桩承载力时间效应的三参数模型。分析表明:该模型可克服双曲线模型的缺陷,双曲线模型及其承载力增长半衰期均是三参数模型对应的特例。最后,通过对两个工程实例的拟合与实测对比进一步验证新模型的正确性。     

砂砾软岩的极限承载力分析

利用Hoek-Brown强度准则建立砂砾软岩极限承载力分析方法,提出岩基剪切破坏模型,推求潜在破坏面上正应力计算公式。结合某工程现场载荷试验探讨砂砾软岩的极限承载能力及自重应力对极限承载能力的影响。研究结果表明:极限承载力的计算结果大于预估设计荷载,砂砾软岩在强度方面尚有很大潜力可挖;同时,极限承载力的计算结果弥补了现场载荷试验中由于试验条件所限造成的荷载–变形曲线上峰值荷载不能给出的缺陷,为建立完整的变形曲线模型奠定了理论基础。研究成果为研究砂砾岩一类软岩的承载力提供了新的途径。     

厦门海底隧道强风化花岗岩力学特性研究

 厦门海底隧道海域隧道地段存在多处风化深槽,岩体主要为全、强风化花岗岩。由于该类岩石强度低,压缩性高,自稳和自承能力差,在隧道衬砌结构的设计和施工工艺的选择方面会遇到一系列特殊的问题。主要通过对天然和重塑强风化花岗岩岩样进行一系列的室内试验,在掌握其基本物理力学特性的基础上,重点对其流固耦合作用下的力学特性进行研究,并建立该类强风化花岗岩的力学模型,通过反演分析对力学模型进行验证。研究成果对风化花岗岩类工程的力学参数取值有重要借鉴意义,对该类岩体中隧道的设计施工具有指导作用。     

基于广义Hoek-Brown非线性破坏准则的节理岩体地基承载力研究

 针对节理岩体非线性破坏特征,发展基于Hoek-Brown破坏准则的临界滑动场理论进行节理岩体地基承载力的计算。首先将Hoek-Brown准则的剪切强度逐点等效到Mohr-Coulomb强度线上,求得每点的瞬时内摩擦角和瞬时黏聚力;在此基础上,改进基于Mohr-Coulomb准则的临界滑动场理论,建立新的迭代算法,将Hoek-Brown强度准则与临界滑动场理论结合起来,求解被动土压力和地基承载力。同时,综合分析GSI和mi值对地基承载力的影响。算例对比分析结果表明,该方法能迅速准确地确定节理岩体地基最危险破坏滑面并得到相应的地基极限承载力值。     

下卧软弱夹层的软岩隧道式锚碇承载特性研究

为了研究下卧有软弱夹层的软岩隧道锚的承载特性,以拟建的某大桥隧道锚工程为依托,开展缩尺比例为1∶10的隧道锚原位模型试验。通过对模型锚开展荷载试验、蠕变试验和破坏试验等,隧道锚在下卧有软弱夹层的软弱围岩(泥岩)中,是具有一定的承载能力和长期稳定性的。该类隧道锚的拉力向和锚塞体前部的铅直向的地表围岩变形曲线以隧道锚中心轴线为对称轴分别近似呈现出"M"形和"倒V"形,铅直向地表围岩变形从锚塞体前端至后端逐渐减小。对于深部围岩变形,该类隧道锚在拉力向的变形控制应以锚间深部岩体变形为主,在铅直向的变形控制应以锚塞体前部深部围岩变形为主。此外,研究还得出下卧有软弱夹层的软岩隧道锚的破坏模式。研究成果可为类似的隧道锚工程的设计、施工等提供参考和借鉴。