桩–筏–土体系的地震软化行为及ABAQUS模拟研究

通过开展离心机动力试验,充分揭示在地震荷载的持续作用下,土体将发生一定程度的软化,表现为周期的持续增大;然而基础(筏板)周期和桩身弯矩包络图几乎不发生改变,说明整个桩–筏结构在地震作用下的行为不会类似于土体发生软化(恶化的现象),而是能持续保持稳定,其动力行为主要取决于自身特性如桩身刚度和上部结构传递给筏板的荷载水平(惯性)等,同时土体的动力软化效应对桩–筏结构的影响很小。为了验证试验结果的正确性和合理性,采用ABAQUS 6.9程序对试验结果进行计算分析,土体模型包括软件内嵌的Hypoelastic非线性模型和所开发的考虑动力降强效应的HyperMas用户子程序模型(UMAT)。计算结果符合试验实测结果,尤其能准确地捕捉桩身最大弯矩值和周期等重要的工程设计参数。且有侧重于基础单元的地震响应分析,非降强模型和降强模型的计算差异很小,再次说明土体软化对既有桩–筏结构影响很小,但采用非降强模型计算更为快捷、高效

软粘土的地震行为研究

通过开展离心机动力试验,揭示了从基岩输出的地震波经过一定厚度的软粘土层传播到地表时,地震波高频成分将被过滤,地表运动的周期将变长,反映出土体在地震荷载的持续作用下将发生一定程度的软化,同时在一定的周期范围内,加速度反应谱幅值将得到不同程度的放大;最后开展了ABAQUS计算和理论分析以验证试验结论的合理性。该研究为自由场土体地震反应的基础性研究,成果对于结构抗震设计中场地选择以及土结动力相互作用研究具有一定的参考价值

桩筏基础–土动力相互作用的离心机模型试验研究

通过开展离心机动力模型试验,深刻地揭示桩筏基础与土之间的动力相互作用将造成基础和土体运动的非一致性,土体的运动不能代表基础的运动,两者在加速度时程、反应谱特征、共振周期等方面均存在显著的差异,传统结构抗震设计中采用自由场土体的动力参数直接作为基础的动力参数是极不严格的;基础与土之间的动力相互作用将导致周围土体发生重塑软化,且基础附近的土体(近场)软化程度明显高于远场土体(自由场土体),加之土体自身的应变软化和强度降低的内在机制将共同导致土体的周期明显延长;然而,在地震作用下,桩筏基础几乎不发生软化,这与土体有着本质的区别;在相同条件下,基础的共振周期小于土体周期(近场和远场),远场土体(自由场土体)周期小于基础附近的土体周期(近场土体);土体的动力软化对结构的影响甚微

桩–土动力相互作用问题中桩身刚度的影响效应研究

通过开展2类刚度差异较大的桩基础的离心机动力模型试验,系统地讨论柔性桩和刚性桩的地震反应差异,以及周围土体对桩体的不同作用效应。对于柔性桩,周围土体的作用主要体现在对桩的约束效应上;然而对于刚性桩,土体更大程度上对桩体施加附加惯性力的作用,以致桩筏基础的周期明显高于无土时的周期;针对柔性桩和刚性桩显著的地震反应差异,巧妙地提出用桩筏结构自振频率wp r界定两者的行为,可为今后类似研究提供参考

VARIATION of THE SHEAR STRENGTH of XIAMEN UNSATURATED RESIDUAL SOILS WITH WATER CONTENT

采用普通应变控制式直剪仪对非饱和残积砂质黏性土和黏土进行慢剪强度试验,试验结果表明,砂质黏性土的黏聚力和内摩擦角均对含水量变化敏感,黏聚力随含水量的变化呈二次抛物线关系,内摩擦角随含水量增大呈幂函数减小;含水量对黏土强度的影响主要是降低了黏聚力,对内摩擦角影响很小。黏聚力随含水量的增加呈指数减小。针对上述变化规律,分别建立实用的强度经验公式,避免了吸力量测和计算困难。对提出的强度公式进行试验验证,计算预测强度并与实测强度相比较,结果表明不同竖向应力条件下,高竖向应力下强度的预测精度更高。2类残积土的强度公式预测精度均满足工程要求,可为工程实际应用提供参考。Shear strength of unsaturated sandy and clay residual soils were conducted by consolidated-drained shearing method using the conventional strain controlled direct shear apparatus.Results show that the shear strength of two kinds of residual soils decreases with increase of water content.The initial water content has effect on both cohesion and frictional angle of sandy clayey soils.The relationships between cohesion,frictional angle and water content of sandy clayey soils are quadratic and power respectively.While the initial water content has negligible effect on friction angle of clay.The considerable influence of increasing water content on shear strength of residual clay is decreasing the cohesion.The variation of cohesion with water content can be described by an exponential relationship.On the basis of variation of shear strength indexes with water content,two new strength formulas of unsaturated sandy clayey and clay were established without suction component and were demonstrated by further tests.Comparisons of the calculated and measured shear strength show that the accuracy of the prediction is higher under high vertical stress.Two strength formulas can provide the good estimation of shear strength of unsaturated residual soils in Xiamen for engineering application.福建省自然科学基金资助项目(2014J01208

Calculation of passive earth pressures on retaining wall considering soil arching effects of backfill clayey soil

为解释挡土墙后填土被动土压力的非线性分布现象,在考虑土拱形状为圆弧,滑裂面采用朗肯滑裂面的基础上,给出考虑土拱效应的被动土压力系数kAWn,进而基于应力状态法及土楔形体静力平衡两种思想求解了竖向平均应力V？公式,在该基础上,给出黏性土填料下的挡土墙被动土压力分布公式、合力公式及作用点高度计算公式。通过与试验与其他方法对比,文中提出的方法得到验证。最后,研究了黏性土填料下的挡土墙被动土压力变化规律,即考虑土拱效应求得的黏性土填料的被动土压力分布呈现上小下大的指数型分布。此外,随着δ/φ(δ为墙土摩擦角,φ为内摩擦角)的增大,土拱效应逐渐增强,土压力合力点逐渐降低。The nonlinear phenomenon which the passive earth pressure behind the retaining wall of both sand and cohesive soil is nonlinear distribution could be explained by the soil arching effect theory.The formula which calculated passive earth pressure coefficient of which assumed the stress state method and the soil wedge static equilibrium method were derived considering the soil arching effect, assumed arch as circle and the angle of slip surface as Rankine's theory.Then, the distribution of lateral earth pressure considering the soil arching was derived by the above formulations; also the formulas that calculating the magnitude and the point of application of lateral passive earth pressure were derived.The accuracy of proposed method is confirmed by comparing with the experimental tests and values from existing equations.Finally, the trends of distribution and the height of total passive earth pressure of clayey soil are studied; distribution of passive earth pressures considering the soil arching is likely an exponential curve; and the height of resultant earth pressure point is gradually reducing with increasing the ratio of soil-wall friction angle to internal angle δ/φ.国家自然科学基金资助(No.51209180); 浙江省自然科学基金资助(No.Y1091175

DIFFERENCE of SEISMIC BEHAVIOR of HIGH AND LOW CAPS of PILE FOUNDATIONS

对于采用桩基的各类结构,按承台埋没于地基土体与否,可区分之高、低承台桩基形式。在许多实际工程中,因回填土或欠固结土地基的继续沉降,或因土体流变或场地震陷等后沉降效应,均可致使承台与土体接触解除,即先前按设计采用的低承台桩基可能转为高承台形式。在静力条件下,这将导致桩身产生负摩阻力而降低其部分竖向承载能力;然而,在地震作用下,通过开展离心机地震模型试验和AbAQuS计算分析发现:当承台与地基土体脱离时,桩基最大弯矩设计值将会增大,且弯矩有效深度也将变深;并且,基础(承台)周期明显延长,充分表明高、低承台桩基形式地震行为差异迥然,高承台桩基形式较之相应低承台情况更为不利,在抗震设计时应充分予以认识、考虑。The pile foundation could be classified into the high or low types according to the position of cap relative to the surface of grounds.In many engineering practice, the settlement of unconsolidated clay or backfilled soil after construction,the soil rheology or the seismic field subsidence may lead to the separation of the cap from the clay surface, i.e., the cap was embedded originally into clay to a new state of high cap pile foundation.For the static condition,this kind of separation may result in the negative skin friction force along the piles and reduce the vertical bearing capacity of piles.However, under the seismic shaking condition, the centrifuge shaking table experiments and simulation with ABAQUS on both the cases indicated that piles underwent a higher maximum bending moment and a much larger active depth under high cap case than those at low cap case.This suggested that the embedding condition played an important role in the seismic response of pile-cap foundation.It becomes more disadvantageous when the embedding condition of the cap changes from the low cap case to the high cap case.国家自然科学基金资助项目(51209180); 成都理工大学地质灾害防治与地质环境保护国家重点实验室开放基金(SKLGP2012K014); 厦门大学大学生创新创业训练计划项

Teaching reform of tunnel engineering course

隧道工程是土木工程学科岩土及地下方向的核心专业课程,理论性、实践性及应用性强,涵盖的知识面广而深。为了满足新时期创新型工程技术人才培养的迫切需要,结合笔者近年来在教学一线所积累的经验及深切体会,以工程科学角度,从教学方式、实践性教学理念、科研促教等方面对隧道工程课程的教学改革问题进行探讨,以期进一步提高教学质量与效果。Tunnel engineering,as a main fundamental course of geo-& underground engineering direction in civil engineering,features strongly in theory,practice as well as application,and takes a wide scope of professional knowledge.To meet the requirement of training excellent engineers in the new situation,based on my teaching experiences in recent years,I discussed the existing problems in teaching and then presented some reform approaches in teaching methods,practical teaching idea,and research for teaching to improve the teaching quality

Research on Several Partial Excavation Methods of Underground Engineering

介绍了几种分部开挖方法的主要特点,对其在软弱破碎围岩特殊地质条件下的实用性进行了对比,重点分析了地表沉降要求对软弱破碎围岩隧道施工工法选择的影响,同时通过计算给出了各种分部开挖法施工受力解析参数,从而结合现场施工经验提出一些有利的建议。Several key characteristics of partial excavation methods are introduced,then the practicality of the excavation methods in weak and fractured rock are compared.The influence of the surface subsidence on tunnel construction methods is emphatically analyzed,meanwhile the construction parameters of several partial excavation methods are given.Thus some favorable recommendations are made combining on-site construction experience.国家自然科学基金青年项目(51209180); 中国博士后科学基金(2012M510137

Field experimental study of transmitted characteristics of pile foundation under vertical load in loess slope

结合某黄土斜坡桥梁桩基工程,对同一坡度相近位置的两根桩基开展现场试验,研究其在不同竖向荷载工况下的桩身上、下坡面两侧的轴力、桩侧摩阻力及桩侧土压力的传递规律与分布特征。试验表明:桩基荷载传递具有明显的区域性,第1区域是从桩顶至约3倍桩径深度处,此区域上坡面桩侧轴力较下坡面相应位置小,且其轴力随深度递减幅度及侧摩阻力发挥程度均较下坡面桩侧大;第2区域是从约3倍桩径至10倍桩径深度处,桩身上坡面一侧竖向受力减小幅度小于桩基下坡面一侧,但此时下坡面桩侧摩阻力发挥幅度大于桩基上坡面一侧;第3区域是从约10倍桩径直至桩端,该区桩身两侧轴力差异不大,摩阻力发挥幅度相近,荷载平衡协调且向下传递稳定。针对斜坡桥梁桩基上、下坡面两侧所受土压力的不均匀性,对设计桩基竖向承载力进行了一定程度地折减,并对计算公式作了相应修正,研究结论可供类似工程参考、借鉴。Combined with the pile foundation engineering constructed in loess slope for supporting super bridge, field experiments on two neighboring piles at the same slope region are conducted to investigate transmitted characteristics as well as distribution feature of axial force, skin friction and lateral earth pressure of both upslope side and downslope side under different vertical load cases.The test results show that load transmitting of pile foundation can be obviously classified into 3 regions.The first region covers from pile head to about the depth of three times pile diameter; within this region, the axial force developed in pile upslope side is less than corresponding position at downslope side; moreover, its decrease extent with pile depth and work efficiency of skin friction are greater than pile downslope side.The second region is within about pile depth of three times to ten times pile diameter, which shows a less axial force decrease extent with pile depth in upslope side and higher work efficiency of skin friction along downslope side at the same elevation.The third region starts from the pile depth of about ten times pile diameter and ends with pile tip.For pile body in this region, the difference of axial force and skin friction for two sides at the same elevation are insignificant; and the load from super region could be even and well-distributed transmitted downwards.For the special issue of uneven earth pressure of pile foundation installed in loess slope, certain reduction in some degree for vertical bearing capacity of pile design is conducted; and some necessary revision of the calculation formula is made.The acquired study results can provide a reference for other similar pile foundation.交通部西部交通建设科技项目(No.2009318812004); 国家自然科学基金项目(No.51209180;No.40872179); 中国博士后科学基金(No.2012M510137