基于改进荷载传递法计算降水引起的基桩沉降

现有研究采用荷载传递法时均未计算桩体自重,若直接应用于国内软土地区超长桩基的沉降计算,其精度难以满足高速铁路线下工程严格的沉降控制要求。基于佐藤?悟双折线模型提出侧阻荷载传递函数的假定模式,引入桩体自重并对荷载传递法的基本微分方程进行修正和求解,结合降水引起的桩周土体沉降计算和基于端阻弹性模型的桩端土体沉降计算,获得地下水位变化诱发的基桩沉降计算方法;采用理论推导的基桩沉降解析解,并借助嵌入荷载传递函数的有限元方法,分别对单纯桩顶荷载作用和桩顶荷载与降水共同作用两种工况下的桩侧摩阻力、桩体轴力和基桩沉降进行算例对比分析;有限元计算因考虑了由桩体沉降产生的桩周土体附加竖向位移而与理论计算略有偏差,但两种方法计算结果的变化规律基本一致,验证了降水引起的基桩沉降理论计算公式的合理性及正确性。     

Stone column effectiveness in soils with creep: a numerical study

While it is well established that vibro stone columns reduce primary settlement and improve bearing capacity, their effect on creep compression has largely been overlooked to date. However, with increasing pressure to develop marginal sites underlain by soft organic soils, the effect of ground treatment on creep is an important emerging issue in geotechnical engineering. In this paper, a series of axisymmetric unit cell analyses have been carried out using the PLAXIS 2D finite element program in conjunction with the Soft Soil Creep (SSC) model. Examination of the evolution of settlement improvement factor with time has indicated that the presence of creep leads to a lower ‘total’ improvement factor than would be obtained for primary consolidation settlement alone. Separate ‘primary’ and ‘creep’ improvement factors have also been derived; the latter are much lower than the former, but are nevertheless greater than unity. Creep results in a stress transfer process; as the soil creeps, vertical stress is transferred from the soil to the stone column. The additional load carried by the column induces additional yielding and shear-plane formation in closely-spaced columns. The additional increment of stress transferred to the already yielded column reduces its efficacy.     

Analysis of piled raft with nodular pile subjected to vertical load using a Winkler model approach

An investigation is made to present analytical solutions provided by a Winkler model approach for analysis of piled rafts with nodular pile subjected to vertical loads in nonhomogeneous soils. The vertical stiffness coefficient along a piled raft with the nodular pile in nonhomogeneous soils is derived from the displacement given by the Mindlin solution for elastic continuum analysis. The vertical stiffness coefficients for the bases of the raft and the nodular part in the nodular pile in a soil are expressed by the Muki solution for the 3‐D elastic analysis. The relationship between settlement and vertical load on the pile base is presented considering the Mindlin solution and the equivalent thickness in the equivalent elastic method. The interaction factor between the shaft of the nodular pile and the soil is expressed taking into account the Mindlin solution and the equivalent elastic modulus. The relationship between settlement and vertical load for a piled raft with the nodular pile in nonhomogeneous soils is obtained by using the recurrence equation of influence factors of the pile for each layer. The percentage of each load carried by both nodular pile and raft subjected to vertical load is represented through the vertical influence factors proposed here. Comparison of the results calculated by the present method for piled rafts with nodular piles in nonhomogeneous soils has shown good agreement with those obtained from the finite element method and a field test. Copyright © 2016 John Wiley & Sons, Ltd.     

Pile group settlement: A probabilistic approach

The uncertain settlement response of pile groups is determined using a ‘hybrid’ formulation and a first-order perturbation technique. The spatially varying soil modulus, which gives rise to the uncertainties in the pile group settlement, is modeled as a homogeneous random field. The random field is assumed to be one-dimensional since the ‘hybrid’ formulation does not account for horizontal variation in the soil properties. Using the proposed method, the coefficient of variation of the pile group settlement is computed. The single-pile solutions obtained compare favorably with the solutions from a conventional stochastic finite element analysis. Pile groups of sizes ranging from two to twenty-five piles are studied. It is observed that the coefficient of variation is not significantly affected by the pile spacing as well as the group size. By defining an appropriate performance function, the reliability index of a pile group system is also found to be approximately the same as that of a single-pile system. These observations suggest that the solutions for a single pile may be used to estimate the uncertainties in the settlement response of pile groups.     

基于一维和三维固结挤土桩沉降时效计算分析

饱和软黏土地基中的挤土桩,施工引起的桩侧土体超孔隙水压力消散和竖向荷载引起的桩底土体固结是导致其沉降的主要因素。基于三维固结理论,研究了桩侧地基土再固结沉降的变化规律,并从桩-土相互作用的原理出发,结合边界条件,推导出了在桩侧地基土再固结沉降的作用下挤土桩沉降的计算公式。运用太沙基一维固结理论并采用分层总和法计算竖向荷载作用下桩底土体的沉降。提出了饱和软土中挤土桩沉降时效的计算方法。并对工程实例进行了计算分析,其结果符合工程实际沉降规律。     

群桩沉降简化计算方法

将均质土和成层土中的单桩桩顶沉降分成桩端力引起的沉降、桩身压缩和桩侧阻力引起的沉降3部分分别计算,获得单桩沉降后,运用等代墩法可获得群桩的平均沉降。单桩沉降计算方法可考虑桩端力与桩端位移的非线性关系和桩侧阻力引起沉降的非线性特性,同时计算方法可考虑桩身压缩对桩顶沉降的贡献。算例分析表明,计算值与实测值和其他方法的计算值有较好的一致性,验证了该方法的合理性。     

考虑二期荷载的高填明洞加筋减载土压力计算

随着城市建设的迅速发展,既有高填明洞填土顶面将不可避免地出现新增结构物等二期荷载的情况。为了明确明洞回填完成后新增二期荷载对其顶部垂直土压力的影响,在已有明洞减载结构土压力计算公式的基础上,利用土体沉降弹性理论计算公式建立了土工格栅变形与土体沉降变形的关系,并考虑二期荷载作用,进一步完善了高填明洞减载前后的洞顶垂直土压力计算公式。建立数值分析模型,将数值分析结果与文中公式计算结果进行了对比。结果表明：公式计算与数值分析结果较为接近,表现出相同的规律性;有无减载措施对明洞回填土压力影响较大,填土越高,减载越显著,实际作用在明洞顶土压力越小。新增二期荷载作用下的明洞顶垂直土压力计算,应考虑其对土体沉降变形的影响,而非简单叠加。     

Load transfer analysis of rock-socketed drilled shafts by coupled soil resistance

The load distribution and deformation of rock-socketed drilled shafts subjected to axial loads are evaluated by a load transfer method. The emphasis is on quantifying the effect of coupled soil resistance in rock-socketed drilled shafts using 2D elasto-plastic finite element analysis. Slippage and shear-load transfer behavior at the pile–soil interface are investigated by using a user-subroutine interface model (FRIC). It is shown that the coupled soil resistance acts as pile-toe settlement as the shaft resistance is increased to its ultimate limit state. Based on the results obtained, the coupling effect is closely related to the ratio of the pile diameter to soil modulus (D/E_s) and the ratio of total shaft resistance against total applied load (R_s/Q). Through comparison with field case studies, the 2D numerical analysis reasonably estimated load transfer of pile and coupling effect, and thus represents a significant improvement in the prediction of load deflections of drilled shafts.     

深置矩形基础与地基共同作用解析分析

基于集中力作用下的Mindlin解位移公式,推导得到均布荷载作用下的地基柔度系数,对地基上的四周自由板与地基共同作用进行简化计算,得出了考虑埋深时地基基础共同作用的解析公式。在基础板为刚性和弹性、荷载为集中荷载及均布荷载的情况下,对考虑埋深时的Mindlin解与不考虑埋深时的Boussinesq解的计算结果进行比较,表明,埋深比对于基础沉降、地基反力的分布、基础板的弯矩等都有一定的影响。给出了考虑埋深时的沉降修正系数公式,以供工程应用参考。     

层状地基群桩沉降研究

考虑了桩的加筋与遮帘效应,通过剪切位移法依据力与位移的协调条件得出桩的柔度系数。在此基础上建立土的成层性模型,根据桩顶与桩底的物理关系推导了桩顶沉降与荷载关系的解析解。通过工程算例进行比较,结果表明, 以土层状模型建立的群桩沉降计算方法是可行的。分析表明, 加固区土层不同分布对桩沉降有所影响,浅部土模量的提高有利于减小沉降。     

地基固结沉降随机有限元计算和可靠度分析

基于比奥（Biot）固结理论，采用邓肯-张本构模型，考虑随机场的影响，首次给出了地基固结沉降随机有限元分析方法，并基于随机有限元对点固结沉降和差异固结沉降的可靠度进行了分析。     

工作荷载下温度循环对桩基变形与应力的影响分析

目前针对工作荷载下温度循环对桩基承载力特性的研究相对较少,基于室内模型试验方法,对饱和砂土中工作荷载下桩体在多次冷热循环作用时的承载特性和传热特性进行研究,测得温度循环作用下桩体和桩周土体温度、桩周水平土压力、桩体应变以及桩顶位移随时间的变化规律。试验结果表明,施加温度循环作用后桩体及桩周土体温度变化不大,桩周水平土压力也能基本恢复到初值,但在桩体内部则会产生较少残余应变,桩顶下沉并随着循环次数的增加不断累积,从而影响结构的整体承载能力。     

考虑地下水浸没作用的固结沉降计算方法

研究了地下水浸没对附加应力减小的作用,同时分析了附加应力变化对最终沉降量的影响,给出了考虑地下水浸没作用的大、小应变条件下的固结沉降计算公式,并对几种工况下的计算结果进行了对比分析。结果表明,考虑地下水浸没作用对地基固结沉降的影响随荷载和土压缩性以及土层厚度的增加而增大,采用大变形固结理论计算固结沉降较符合实际情况。     

粉喷桩多桩复合地基沉降因素的数值分析

利用有限单元法对粉喷桩多桩复合地基进行数值模拟,通过对荷载大小、垫层厚度、垫层模量、桩长径比以及桩距等影响因素的分析,得出在整个加荷期间粉喷桩多桩复合地基沉降随着这些因素变化而变化的规律：随着荷载的增加,粉喷桩复合地基沉降呈现了非线性的增长势态;褥垫层越厚,桩间土的沉降越大,厚度应控制在300mm左右且其模量不应过大;随着桩长的逐渐增加,桩侧摩阻力发挥得就越明显,桩土有共同的沉降趋势;随着桩间距的增加,导致桩间土沉降的非线性趋势更加明显。     

考虑加筋与遮帘效应的层状地基群桩沉降计算

桩群在土中的加筋与遮帘效应是客观存在的,但目前的理论与实践均未能或有效地考虑该效应.基于剪切变形法原理,在计算某一根桩沉降时,考虑了其他各相邻基桩的存在对该桩沉降的折减,即加筋与遮帘效应,得到了桩侧桩-土接触等效剪切弹簧刚度,建立了桩身位移微分方程,分别求得桩顶沉降-桩端沉降、桩顶荷载-桩端压力的递推关系,从而得到了各桩在自身荷载作用下引起自身沉降的柔度系数; 同理,也求得了各邻桩在其桩顶荷载下引起它桩沉降的柔度系数,最终建立了群桩沉降计算的柔度矩阵方程.推导过程中,考虑了地基土的成层性及桩端沉降的相互影响,并提出了基于一定深度内的Mindlin位移解且考虑桩径影响的桩端压力-桩端位移关系新模式.算例结果表明,本文方法与实测值较为接近,且按本文方法求得的群桩中基桩相互作用系数明显小于弹性理论计算结果,且与实测值吻合较好.     

Geogrid-reinforced railroad bed settlement due to cyclic load

Results of large-scale laboratory model tests conducted to determine the permanent settlement due to cyclic load of the railroad bed for a proposed high-speed train route extending from Seoul to Pusan in South Korea are reported. The possibility of using geogrid layers as reinforcement to reduce settlement of the subbase layer was investigated. Based on the present model test results, it appears that practically all permanent settlement due to cyclic load is completed after application of 10⁵ cycles of load. The most beneficial effect of reinforcement is derived when one layer of geotextile and one layer of geogrid are placed at the interface of the subgrade soil and subbase course.     

厚层沉渣嵌岩桩承载性状研究

首先对嵌岩桩进行了现场钻孔抽芯试验和静力载荷试验,试验结果表明：当采用冲击成孔工法施工且桩侧存在弱胶结土层时,桩底沉渣厚度普遍较大,远厚于规范允许值;当荷载达一定值时,呈现突发性下沉且下沉量较大,主要与厚层沉渣导致端阻力未发挥作用有关。试验结果说明：当采用冲击成孔工法施工且桩侧存在弱胶结土层时,现有嵌岩桩设计方法同时考虑端阻和侧阻作用是不妥的,嵌岩桩单桩承载力宜仅按土层和嵌岩段侧阻力确定。进一步基于数值分析对嵌岩段摩阻力分布及变形等承载性状进行研究。在施加荷载一定条件下,嵌岩深度与基岩强度越大,下沉量越小,且厚层沉渣嵌岩桩沉降存在嵌岩深度效应。随着桩顶荷载增大,嵌岩深度与基岩强度差异导致的下沉量的差值增大。嵌岩段桩侧摩阻力沿深度呈马鞍型分布,嵌岩深度与基岩强度较小时,嵌岩段下部峰值比上部峰值大;嵌岩深度与基岩强度较大时,嵌岩段下部峰值则比上部峰值小。在桩顶荷载一定条件下,嵌岩深度越小,下沉量越大,越有利于侧摩阻力发挥,嵌岩段桩侧摩阻力越大。由于厚层沉渣存在,嵌岩段桩身轴力沿深度方向从桩顶荷载逐渐衰减为0,嵌岩深度越小,衰减速度越快。     

Grey correlation-hierarchical analysis for metro-caused settlement

Monitoring Shanghai metro tunnels and their surroundings indicates that the maximum settlement caused by metro operation for years has been more than 25 cm in soft soil of tunnel bottom, which has seriously affected the normal operation of subway, and even caused some old house cracking around subway lines. To investigate main influence factors on metro-caused settlement so as to provide the basis for the control of metro-caused settlement, the analytical hierarchy process was adopted to make their weight arrangement. With the experience analogy and grey correlation theory, the grey correlation-hierarchical analysis method was proposed to evaluate the possibility of metro-caused settlement in soft soil of tunnel bottom. The correlation of metro-caused settlement and its influence factors (such as dynamic load conditions of metro, soil conditions, burial conditions of soil prone to dynamic settlement, and hydrogeology conditions) was probed. Taking Shanghai Yangtze River Tunnel project in China (it is now the largest shield tunnel in the world) as an example, the division estimation and the comprehensive evaluation of possibility and severity for metro-caused settlement were also done. The results were observed as follows: (1) the severity of settlements caused by dynamic load of metro in soft soil of tunnel bottom at the same position increases as the speed of subway trains increases; (2) On comparison between the foundations of silty clay and clayey silt, the former is generally more prone to the metro-caused settlement under the same conditions as dynamic load conditions of metro, void ratio of soil, natural water content of soil, burial conditions, and hydrogeology conditions; (3) the grey correlation-hierarchical analysis method is more applicable than grey correlation method for the possibility estimation of metro-caused settlement in the soft soil of tunnel bottom; (4) it may provide the reference for the possibility evaluation of metro-caused settlement in other similar projects.     

带帽PTC单桩和复合地基承载特性试验研究

结合苏-沪高速公路疏桩复合地基处理工程,进行了自由带帽PTC管桩和双带帽PTC管桩复合地基的现场足尺试验研究。根据试验结果,研究了桩身-桩帽-桩间土的相互作用机理和承载特性,分析了桩帽、桩间土和桩端承载的滞后效应,以及桩帽、桩间土对桩身的消减作用。认为管桩复合地基的第1阶段桩身控制沉降,第2阶段土体控制沉降,探讨了桩帽-桩间土荷载分担比及应力比在不同沉降阶段的变化规律。最后,还提出了桩帽尺寸的设计方法以及取消碎石垫层的建议。     

横-竖立体加筋地基中附加应力的分析计算

建立了横-竖立体加筋（H-V筋）地基的有限元模型,通过分析地基中的竖向应力分布、水平向位移分布以及筋-土界面相互作用,发现横-竖立体加筋地基中的竖向应力在筋材下方出现扩散和重分布,并逐渐向土体下部传递,使得土体中整体的应力分布更加均匀;同时,横-竖筋材中的竖筋类似于一个侧壁,其提供的垂直侧向力约束了介于竖筋间的土体,限制了土体的侧向水平位移,使得地基中筋材上部土体的侧向水平位移变小。基于有限元模拟对横-竖立体加筋地基加固机制的认识,将横-竖立体筋视为作用在地基上的一维弹性地基梁,通过弹性地基梁理论,根据弗拉曼解推导求解了横-竖立体加筋地基中任意一点竖向附加应力的计算表达式。将模型计算结果与有限元模拟所得结果进行对比发现两者吻合良好。