基于自钻式旁压仪的残积土原位力学特性试验研究

残积土属区域性特殊土,准确获得其力学性质指标是各类工程场地地基处治及承载力评价的必要条件。针对典型花岗岩残积土层,在20 m深度范围内开展自钻式旁压试验,获得表征土体原始水平应力、强度与刚度的相关指标,类比分析深度与水平方向土体力学性质的差异性,并论证了土体刚度在不同应变水平下的衰减规律。结果表明,该场地浅层土体刚强度指标差异性较大,存在明显的风化壳,导致剪切模量及变形参数沿深度呈先减小后增大的趋势;土体割线剪切模量Gs,切线剪切模量Gt随剪应变变化关系具有明显的非线性特性,在小应变范围(γc≤0.3%)内Gs,Gt快速衰减,在γc=2.0%~3.0%时,趋于平稳,采用非线性分析方法得到的模量值较为合理,认为在该类场地工程设计中,剪切模量的选取宜基于自钻式旁压试验结果,根据不同应变水平确定,采用线性割线法得到的剪切模量Gur偏于冒险。同时发现,小应变时剪切模量值相对于土体差异较为敏感,为准确获得土性参数,可适当增加浅层土体测试数量。     

饱和度及收缩变形对土体小应变刚度特性的影响

饱和度影响下的浅表层土体小应变刚度与泊松比是预测土与结构物相互作用引起变形的关键参数。基于自主设计的试验装置,针对细粒土(黏土和粉土)通过蒸汽平衡法控制环境湿度,开展了无外荷载作用下土体饱和度变化对小应变刚度及泊松比影响的试验研究。试验结果表明:细粒土弹性波速及小应变刚度随饱和度演化均可分为3个阶段,即边界效应阶段、过渡阶段及残余阶段。压缩波速及小应变体积模量随饱和度的降低先减小后增加,剪切波速及小应变剪切模量随饱和度的降低逐渐增加,黏土的泊松比随饱和度的降低呈线性减小,粉土的泊松比在饱和度低于85%后无明显变化,仅在0.37附近波动,与粉土相比,黏土的小应变刚度特性及泊松比受饱和度影响更为显著。     

膨胀土原位剪切模量–剪应变衰减曲线与特征分析

为了评价原位应力状态膨胀土在小应变作用下的刚度衰减特性,综合原位地震扁铲侧胀和室内共振柱试验,分析应力历史、应力状态和试样扰动对膨胀土剪切模量衰减规律的影响。结果表明,应力状态和应力历史对膨胀土剪切模量影响较大,相同应力历史下,剪切模量随围压增加而增大,随着应变的发展,衰减较快;相同应力状态下,经历加载–卸载过程的剪切模量–剪应变(G-γ)衰减曲线位于仅经历加载过程的曲线上方,但经历加载–卸载和加载–卸载–再加载过程后土的G-γ衰减曲线基本重合;另外,相同应力历史条件下,土的原位剪切模量较室内共振柱试验所得剪切模量值大,且衰减更快,说明取样卸荷、运输及制样等扰动因素对土体结构造成了不可逆的损伤,通过室内加卸载过程无法还原其原位力学性状;参考剪应变不同时,推求得到的原位G-γ衰减曲线的中间部分差异较大,参考剪应变越小,剪切模量衰减越快,说明合理选取参考曲线和小应变剪切模量是推求原位G-γ衰减曲线的关键。该研究可为类似场地地震分析和沉降分析提供理论基础和技术支持。     

共振柱和弯曲元研究石灰固化粉土的小应变刚度特性

为研究石灰固化粉土的小应变刚度特性,利用共振柱和弯曲元对固化土和素土开展了不同围压下的小应变刚度测试。试验结果表明,石灰处理后粉土的小应变刚度显著提高,相同围压下固化土和素土的最大剪切模量比值为1.69～2.59。素粉土和石灰固化粉土的小应变刚度具表现出非线性特征,不同围压下的G-γ衰减曲线符合Hardin-Drnevich模型。固化土和素土的最大剪切模量和围压具有较强的线性相关性,并且固化土的刚度对围压的敏感性高于素土。研究还发现,采用共振柱和弯曲元得到的最大剪切模量非常接近。建议工程实践中采用弯曲元快速准确测定固化粉土的小应变刚度特征。本研究有助于对深化加固土的小应变刚度特性的理解,并为有关粉土的加固工程提供施工设计参数和理论指导。     

基坑开挖对坑内土体刚度特性影响室内试验研究与本构模型应用分析

考虑深基坑分层开挖与降水过程,进行了具有加卸荷的快速固结试验、三轴固结排水剪切试验和三轴加卸荷循环荷载试验,分析不同围压条件下不同土体的变形及刚度特性,并根据试验结果利用Plaxis软件中摩尔库伦模型、土体硬化模型、小应变硬化模型进行对比分析,得到结论:粉质黏土试样的试验关系曲线呈硬化型,粉土试样的试验关系曲线呈应力软化型。加载模量值随加载次数增长而升高,卸载模量值则相反;MC模型、HS模型和HSS模型,均能在整体上较好的模拟出曲线应变硬化发展趋势,但不包括软化行为。HSS模型比HS模型更加适用于工程实际。     

考虑土体真三维波动效应时桩的振动理论及对近似理论的校核

从三维轴对称土体模型出发,同时考虑土体径向和竖向位移,对完整端承桩在垂直谐和激振力作用下与土的耦合振动特性进行了分析。假定桩为竖直弹性等截面体,土为线性粘弹性体,其材料阻尼为滞回阻尼。首先通过引入势函数对土体位移进行分解,将土体动力平衡方程解耦并求解,得到土层的振动模态形式和阻抗因子;然后利用该解以小应变条件下桩土接触面上力平衡和位移连续条件来考虑桩土耦合作用,求解桩的动力平衡方程,得到桩的频域响应解析解,进而得到土层局部复刚度和桩顶复刚度。从土层阻抗因子、土层局部复刚度和桩顶复刚度3方面,将所得解与平面应变解和其他忽略土体径向位移的简化解在主要影响参数变化情况下进行比较,研究表明,不同解在硬土、细长桩并处于低频时有较大差别,随激振频率的提高,它们将趋于一致。     

一种新型光纤光栅局部位移计在小应变测量中的应用

小应变下的土体模量的变化对基坑周围既有建筑物的变形和路基沉降分析具有重要意义。土体模量会随土体的应变呈非线性变化,特别是应变范围为10~(-5)~10~(-3),土体模量随应变的增加显著降低。由于土体应变的变化范围广,传统的单一测量仪器无法连续量测土体从微小应变到小应变的模量变化情况。通常测量土体小应变模量的方法有弯曲元和局部位移计方法。弯曲元方法能有效测量土体微小应变的模量,但不能得到土体模量在小应变情况下变化规律,因为弯曲元方法无法得到和控制土体的应变。设计了基于布拉格光纤光栅传感技术的局部位移计。通过设计和标定试验,成功将布拉格光纤光栅局部位移计应用于改装的三轴试验仪器中。为进一步的对比分析,在改进的三轴试验装置中安装了弯曲元试验系统。基于改装的三轴试验仪器,进行了不同围压下的固结不排水试验,得到了土体从微小应变到小应变下模量的变化曲线。试验结果验证了光纤光栅局部位移计能有效的测量土体在小应变情况下的模量非线性变化特征。     

非线性条件下砂土地基中水平受荷长桩的性状分析

提出了水平荷载作用下,考虑土体非线性性质的砂土地基中长桩性状的分析方法。依据前人的研究成果,认为任一深度处水平土反力模量与地面处桩的位移存在一个衰减的指数函数关系。假定水平土反力模量和极限土反力均沿深度线性增长,认为随着桩位移增大,桩前土体由弹性逐渐达到塑性屈服,对于2种状态,分别建立桩的基本微分方程。分别采用Gleser有限差分结合迭代法和Reese-Matlock法给出了2组四阶变系数非线性微分方程的近似解答。现场试验和数值算例证明,该法较传统的m法计算结果更接近实测值,较p-y法方便可行,既适用于小位移的情况,也适用于桩顶出现较大位移的情况,因此,在缺乏试验参数的情况下,可以代替p-y曲线法用于工程计算。     

黏性土三轴排水蠕变试验及经验模型研究

人工岛地基通常以深厚的黏性土为主,过大的工后沉降会对人工岛的使用产生不利影响。针对大连人工岛地基黏性土原状样,采用三轴排水蠕变试验对黏性土的蠕变特性进行了研究,分析了轴向应变、体应变、轴向应变速率与蠕变时间及偏应力间的关系。基于蠕变试验结果,耦合Mesri模型与线性方程,建立了可描述土体衰减蠕变与加速蠕变过程的经验模型。研究结果表明,该地区土体具有典型的非线性蠕变特性,随偏应力的增加,非线性蠕变特性愈加明显。在低偏应力作用下,蠕变过程表现出衰减蠕变特性;在破坏偏应力下表现出加速蠕变特性。轴向应变速率随着蠕变时间的增加而减小,随着偏应力的增加而增大,而偏应力对m值(曲线斜率)的影响较小。试验结果与模型计算结果吻合较好,表明新建模型适用于描述该地区黏性土的蠕变特性。     

应变相关修正剑桥模型在基坑开挖中的应用

提出考虑上海软土在小应变情况下刚度随应变非线性衰减的应变相关修正剑桥模型(SDMCC),将其应用于基坑分步开挖和支护的数值分析中。采用三维快速拉格朗日方法(FLAC3D),建立了考虑支护结构与土体的接触作用的基坑有限差分数值模型,土体采用应变相关修正剑桥模型,分析了基坑开挖过程中围护墙侧移、地表沉降、坑底隆起等变形性状。     

沉积软岩强度和变形特征的研究综述

The strength and deformation characteristics of sedimentary soft rock evaluated in relation to several large-scale researc and construction projects in Japan are reviewed.The elastic stiffiness at small non-linearity due to strain and pressure level,and viscous properties are described.The elastic stiffness from triaxial tests using high-quality core samples while measuring stresses and strains accurately is basically the same as the corresponding value from field shear wave velocity.It is necessary to take into account the dependency or stiffness on shear and pressure level,which could be evaluated by relevant laboratory stress-strain tests while referring to results trom relevant field loading tests.Loading rate effects due to the material viscous properties could be simulated by a non-linear three-component model.     

Stiffness of Frozen Soils Subjected to K0 Consolidation Before Freezing

The stiffness characteristic of artificially frozen soils subjected to K₀ consolidation at small strains are investigated at different initial confining pressure and frozen temperature by secant Young's modulus representing the soil stiffness. The influence of freezing on stiffness is investigated by comparing the behavior of frozen and unfrozen soils with similar conditions. It is observed that the frozen soils secant Young's modulus may decay with increasing strain after keeping a stage of constant value, the threshold yield strain was found to be approximately 0.05% for all tested conditions. On the other hand, the stiffness of studied frozen soils and its degradation was heavily response to the initial confining pressure amplitude and temperature fluctuation.     

Small Strain Stiffness and Non-Linear Stress-Strain Behaviour of Cement-Mixed Gravelly Soil

The stiffness at small strains and non-linear stress-strain relation of compacted cement-mixed well-graded gravelly soil as well as the ageing effects were evaluated by drained triaxial compression tests on compacted moist specimens cured for different periods at isotropic and different anisotropic stress states. In all the tests, the initial stress-strain relation at small strains less than about 0.001% was essentially elastic and the initial Young's modulus, E₀, was essentially the same as the E_eq value evaluated by applying unload/reload cycles under otherwise the same conditions. The E_eq values were rather independent of strain rate. The E_eq value from the first unload/reload cycle applied during otherwise continuous ML became noticeably lower than the elastic modulus evaluated at the same stress state, more as approaching the peak stress state. After a number of small unload/reload cycles and long sustained loading, the E_eq value became closer to the elastic modulus due to a decrease in the viscous effects. The ratio of E₀ to the compressive strength (q_max) was similar to that of concrete but noticeably larger than those of uncompacted cement-mixed soil, sedimentary softrock and unbound gravelly soil. Both E₀ and q_max increased with time by ageing, while the E₀/q_max ratio decreased with time. When ML was restarted at a constant strain rate after ageing with a shear stress, the tangent stiffness became very high for a large stress range with a substantial change in the non-linearity of stress-strain relation.     

Field and Laboratory Measurements of Small Strain Stiffness of Decomposed Granites

Non-linear stress-strain characteristics and stiffness-strain relationships of sedimentary soils and sands at small strains have been reported by many researchers. Research work on the behaviour of weathered or decomposed granites at small strains, however, has rarely been reported. This paper compares some stiffness measurements of decomposed granites from field investigations involving crosshole seismic, self-boring pressuremeter (SBPM), and high pressure dilatometer as well as results from laboratory tests using bender element and internal transducers. The in-situ crosshole measurements show that the elastic stiffness of Moderately Decomposed Granite (MDG, approximately 7000 MPa) is about 25% greater than that (about 5500 MPa) of Highly Decomposed Granite (HDG), which is in turn approximately 18 times higher than that (about 300 MPa) of Completely Decomposed Granite (CDG). This is likely attributable to the materials' different bond strengths and structures. A new method has been adopted to interpret the SBPM data. Measured data from crosshole seismic and self-boring pressure meter tests for CDG are found to be consistent. Bender element laboratory tests on CDG indicate that the measured A-coefficient in the expression of G₀/p_r = A(p/p_r)ⁿ lies between the results from clay and sand as reported in the literature. However, the measured n-value for CDG is generally larger for clays and sands. The measured bender element results are consistent with data from internal transducers. Highly non-linear characteristics of CDG were observed in both the laboratory and field tests. Generally the elastic stiffness of CDG as determined by laboratory tests is about 50-80% of that from field tests. Some possible reasons are discussed.     

不同土性的地基-结构动力相互作用振动台模型试验对比研究

通过结构-地基动力相互作用(SSI)体系的振动台模型对比试验,研究了不同土性的地基条件对动力相互作用效果和规律的影响。在试验现象方面发现,地基土越软弱,结构沉降和倾斜越大;不同土性条件下结构裂缝形态相似,但土越硬,裂缝出现越早,发展也越严重。在体系动力特性方面发现,SSI体系频率均小于不考虑SSI的结构自振频率,阻尼比大于结构材料阻尼比,但不同土性条件时由于土、基础、结构三者刚度比不同,SSI对动力特性的影响程度和机理存在差异。在地震反应方面发现,土越硬,加速度反应的峰值放大系数越大;加速度反应的主要组成成分和频谱特征是,土较软时以基础转动引起的摆动分量和平动分量为主;土较硬时以结构弹塑性变形分量为主。结果表明在不同地基土性条件下,结构-地基动力相互作用具有相似的规律,但其效果和机理存在差异。     

土体的小应变特性(英文)

岩土工程的设计与计算，常常需要研究土的力学性质，并以数学模型描述其基本特性。随着现场测试技术和土工试验的进步，对土在小应变下应力－应变性质，以及新近应力历史影响有了新的认识。表征这些特性的数学模型，如非线性的块串模型等，已经得到广泛应用。本文首先讨论土在小应变下的非线性特征，接着介绍表征小应变特性，特别是新近应力历史影响的块串模型，最后以一实例说明模型之应用。     

土体小应变剪切模量的现场和室内试验对比及工程应用

土体小应变剪切模量G_0在场地地震响应和基坑变形等计算分析中有着重要作用。统计了上海典型土层的室内共振柱试验和现场原位波速测试测得的小应变剪切模量,给出了两种试验方法下的G_0的经验公式,并对比分析了室内试验和现场原位测试结果的差异。研究结果表明土体的G_0值主要由土体类别、孔隙比和平均有效应力决定,且现场试验的结果远高于室内试验的结果。进一步采用HS-Small本构模型对上海新金桥广场基坑工程的开挖变形进行数值模拟,分析了小应变剪切模量对于基坑工程变形的影响。分析结果表明,采用现场原位测试测得的小应变剪切模量进行基坑变形计算的结果与现场变形监测数据更为接近。