深基坑双排桩结构支护效果有限差分数值模拟

[摘 要] 由两排平行的钢筋混凝土桩以及桩顶冠梁形成的双排桩支护结构可以有效控制基坑侧 向变形,被广泛应用于基坑支护设计中。本文以济南市西区文化中心项目大剧院为工程背景,对双排桩 支护结构下基坑坡面水平位移,坑底隆起,桩身弯矩、剪力,桩土接触面切向刚度和法向刚度等特征进行 研究,并通过“实体单元桩冶和“结构单元桩冶的计算结果进行对比分析。研究表明,桩与土体相互作用 力学模型能较好模拟双排桩支护施工过程,计算精度较高;桩顶的水平位移最大,基坑开挖底面的弯矩 和剪力最大;计算结果可为双排支护桩作用机理的研究提供参考。     

A Winkler model approach for vertically and laterally loaded piles in nonhomogeneous soil

An investigation is made to present analytical solutions provided by a Winkler model approach for the analysis of single piles and pile groups subjected to vertical and lateral loads in nonhomogeneous soils. The load transfer parameter of a single pile in nonhomogeneous soils is derived from the displacement influence factor obtained from Mindlin's solution for an elastic continuum analysis, without using the conventional form of the load transfer parameter adopting the maximum radius of the influence of the pile proposed by Randolph and Wroth. The modulus of the subgrade reaction along the pile in nonhomogeneous soils is expressed by using the displacement influence factor related to Mindlin's equation for an elastic continuum analysis to combine the elastic continuum approach with the subgrade reaction approach. The relationship between settlement and vertical load for a single pile in nonhomogeneous soils is obtained by using the recurrence equation for each layer. Using the modulus of the subgrade reaction represented by the displacement influence factor related to Mindlin's solution for the lateral load, the relationship between horizontal displacement, rotation, moment, and shear force for a single pile subjected to lateral loads in nonhomogeneous soils is available in the form of the recurrence equation. The comparison of the results calculated by the present method for single piles and pile groups in nonhomogeneous soils has shown good agreement with those obtained from the more rigorous finite element and boundary element methods. It is found that the present procedure gives a good prediction on the behavior of piles in nonhomogeneous soils. Copyright © 2011 John Wiley & Sons, Ltd.     

Thermotectonic evolution of Precambrian basement rocks of the Kuruktag uplift,NE Tarim craton,China: evidence from apatite fission-track data

The Kuruktag uplift is located directly northeast of the Tarim craton in northwestern China. Neoarchaean-to-Neoproterozoic metamorphic rocks and intrusive rocks crop out widely in the uplift; thus, it is especially suited for a more complete understanding of the thermal evolution of the Tarim craton. Apatite fission-track (AFT) methods were used to study the exhumation history and cooling of these Precambrian crystalline rocks. Nine apatite-bearing samples were collected from both sides of the Xingdi fault transecting the Kuruktag uplift. Pooled ages range from 146.0 ± 13.4 to 67.6 ± 6.7 Ma, with mean track lengths between 11.79 ± 0.14 and 12.48 ± 0.10 μm. These samples can be divided into three groups based on age and structural position. Group A consists of five samples with AFT apparent ages of about 100–110 Ma and is generally associated with undeformed areas. Group B comprises three specimens with AFT apparent ages lower than 80 Ma and is mostly associated with hanging wall environments close to faults. Group C is a single apatite sample with the oldest relative apparent age, 146.0 ± 13.4 Ma. The modelled thermal history indicates four periods of exhumation in the Kuruktag uplift: late-Early Jurassic (180 Ma); Late Jurassic–Early Cretaceous (144–118 Ma); early-Late Cretaceous (94–82 Ma); and late Cenozoic (about 10 Ma). These cooling events, identified by AFT data, are assumed to reflect far-field effects from multi-stage collisions and accretions of terranes along the south Asian continental margin.     

高临界循环荷载水平下钻孔桩的抗拔性状分析

为了研究抗拔桩在循环加载过程中桩身的性状,选取杭州萧山某工地两根抗拔桩作为试桩进行循环加载试验。在抗拔桩桩身埋设钢筋应力计,测量循环加载过程中桩身应力的变化情况;在桩身预埋钢管,测量桩顶及桩端上拔量。通过对桩身应力及桩顶、桩端上拔量的分析,研究循环加载对抗拔桩承载力及上拔量的影响。综合分析抗拔桩承受循环荷载时的桩顶、桩端上拔量、桩身轴力以及桩侧摩阻力分布可知,当抗拔桩承受循环荷载时,且临界循环荷载水平较大（接近或者达到1）,抗拔桩经过2～3次循环加载后就发生破坏,其研究结果可供其他类似工程参考。     

指数曲线模型预测基坑周边地面沉降

结合天津富裕广场三期工程基坑周围地面沉降观测的工作实践,利用指数曲线法对该工程的地面沉降过程进行了预测,得出实际观测值与预测值之间的拟合曲线,并通过比较得出该预测方法的优劣。     

Modeling of energy dissipation in structural devices and foundation soil during seismic loading

Though rocking shallow foundations could be designed to possess many desirable characteristics such as energy dissipation, isolation, and self-centering, current seismic design codes often avoid nonlinear behavior of soil and energy dissipation beneath foundations. This paper compares the effectiveness of energy dissipation in foundation soil (during rocking) with the effectiveness of structural energy dissipation devices during seismic loading. Numerical simulations were carried out to systematically study the seismic energy dissipation in structural elements and passive controlled energy dissipation devices inserted into the structure. The numerical model was validated using shaking table experimental results on model frame structures with and without energy dissipation devices. The energy dissipation in the structure, drift ratio, and the force and displacement demands on the structure are compared with energy dissipation characteristics of rocking shallow foundations as observed in centrifuge experiments, where shallow foundations were allowed to rock on dry sandy soil stratum during dynamic loading. For the structures with energy dissipating devices, about 70–90% of the seismic input energy is dissipated by energy dissipating devices, while foundation rocking dissipates about 30–90% of the total seismic input energy in foundation soil (depending on the static factor of safety). Results indicate that, if properly designed (with reliable capacity and tolerable settlements), adverse effects of foundation rocking can be minimized, while taking advantage of the favorable features of foundation rocking and hence they can be used as efficient and economical seismic energy dissipation mechanisms in buildings and bridges.     

Experimental investigation on seismic behavior of single piles in sandy soil

This paper describes a quasi-static test program featuring lateral cyclic loading on single piles in sandy soil. The tests were conducted on 18 aluminum model piles with different cross sections and lateral load eccentricity ratios, e/d, (e is the lateral load eccentricity and d is the diameter of pile) of 0, 4 and 8, embedded in sand with a relative density of 30% and 70%. The experimental results include lateral load-displacement hysteresis loops, skeleton curves and energy dissipation curves. Lateral capacity, ductility and energy dissipation capacity of single piles under seismic load were evaluated in detail. The lateral capacities and the energy dissipation capacity of piles in dense sand were much higher than in loose sand. When embedded in loose sand, the maximum lateral load and the maximum lateral displacement of piles increased as e/d increased. On the contrary, when embedded in dense sand, the maximum lateral load of piles decreased as e/d increased. Piles with a higher load eccentricity ratio experienced higher energy dissipation capacity than piles with e/d of 0 in both dense and loose sand. At a given level of displacement, piles with circular cross sections provided the best energy dissipation capacity in both loose and dense sand.     

Numerical simulations and parametric study of SDCM and DCM piles under full scale axial and lateral loads

The new kind of reinforced Deep Cement Mixing (DCM) pile namely, Stiffened Deep Cement Mixing (SDCM) pile is introduced to mitigate the problems due to the low flexural resistance, quality control problem and unexpected failure of DCM pile. The SDCM pile consists of DCM pile reinforced with concrete core pile. Previously, the full scale pile load test and the full scale embankment loading test were successfully conducted in the field. To continue the study on the behavior of SDCM and DCM piles, the 3D finite element simulations using PLAXIS 3D Foundation Software were conducted in this study. The simulations of full scale pile load test consisted of two categories of testing which are the axial compression and the lateral loading. For DCM C-1 and C-2 piles, the clay–cement cohesion, C_DCM, and clay–cement modulus, E_DCM, were obtained from simulations as 300 kPa and 200 kPa as well as 60,000 kPa and 40,000 kPa, respectively. For the SDCM piles, the simulation results show that increasing length ratio, L_core/L_DCM, increased the bearing capacity whereas the sectional area ratio, A_core/A_DCM, has only small effects on the bearing capacity for the axial compression loading. The verified parameters such as the clay–cement cohesion, C_DCM, and clay–cement modulus, E_DCM, from simulations of axial compression tests were 200 kPa and 30,000 kPa, respectively. On the other hand, increasing the sectional area ratio, A_core/A_DCM, significantly influenced the ultimate lateral resistance while the length ratio, L_core/L_DCM, is not significant in the ultimate lateral load capacity when the length of concrete core pile is longer than 3.5 m. In addition, the tensile strength of DCM, T_DCM, and concrete core pile, T_core, are very important to the lateral pile resistance. The back-calculation results from simulations of tensile strength were 5000 kPa and 50 kPa for the T_core and T_DCM, respectively.     

塔里木盆地卡塔克隆起卡1三维区块奥陶系鹰山组储层特征与影响因素

奥陶系鹰山组一直是塔里木盆地油气勘探的重点层位。综合运用岩心、薄片、成像测井、分析测试以及三维地震资料,对鹰山组储层特征以及影响因素进行了分析。结果表明:鹰山组整体岩性以粉-细晶白云岩,砂屑灰岩,灰质白云岩为主,储集空间以溶蚀孔洞、裂缝为主;上部为风化壳岩溶储层,溶孔-裂缝发育,中下部为裂缝性储层,裂缝发育,纵向上具有上孔下缝特征,储层非均质性强;加里东中期I幕运动形成的构造不整合面是该区岩溶储层发育的重要因素;加里东中期—早海西期形成的断裂和构造裂缝是该区储层发育的关键;早二叠世时期与火山作用伴生的岩溶烘烤以及埋藏溶蚀作用对储层发育具有积极作用。     

娄江特大桥深孔泵吸反循环工艺的改进

随着国家基础设施投入的加大,桩径1000－2000 mm、孔深100－130 m的钻孔灌注桩在各类工程中屡被采用。泵吸反循环传统上被用于孔深不超过80 m的深孔,在京沪高铁第六标段娄江特大桥钻孔灌注桩施工中,通过对现有的反循环钻机进行改进,很大程度上提高了泵吸反循环的工作效率,顺利地完成了孔深131 m、桩径1.8 m桩孔的施工,产生了很好的经济效益。