Experimental study on the settlement of marine deposits of Anzali under cyclic loading by laminar box apparatus

The central Alborz mountain range, located in northern Iran, neighboring the Caspian Sea and where the two Persia and Eurasia plates meet, is known as a seismologically active area. In this regard, investigation of the behavior of saturated sand deposits located in this area may be of particular interest. Saturated sand deposits are subjected to instabilities owing to liquefaction or volume change due to earthquake shakings. A particular type of saturated sand deposits is Anzali sand which is abundant in Anzali port and other cities located in this area in northern Iran. This type of sand is a representative for most sands found in this area, i.e., the southern coastal line of Caspian Sea. This research is solely focused on the volume change behavior of marine deposits of Anzali area, often characterized as Anzali sand, in terms of the settlement of a model footing located on the surface of the sand by the aid of a transparent laminar shear box apparatus. Effects of a number of factors such as the frequency of the cyclic loading, the initial density of the sand, and the sample preparation method have been investigated. Observations indicated that the density index and the frequency of loading which are proportional to the energy of an earthquake have direct effects on the accumulation and amount of the final settlement of Anzali sand.     

Centrifuge modeling of single pile response due to lateral cyclic loading in kaolin clay

In offshore engineering, pile foundations are commonly constructed in marine deposits to support various structures such as offshore platforms. These piles are subjected to lateral cyclic loading due to wind, wave action, and drag load from ships. In this paper, centrifuge model tests are conducted to investigate the response of the existing single piles due to lateral cyclic loading. The cyclic loading was simulated by a hydraulic actuator. It is found that the residual lateral movement and bending strain are induced in the existing pile after each loading–unloading cycle. This is because plastic deformation is induced in the soil surrounding the existing pile during each loading–unloading cycle. By increasing the applied loads during cyclic loading–unloading process, the lateral movements and bending strains induced in the pile head increase simultaneously. As the cyclic loading varies from 10 to 50 kN, the residual pile head movement increases from 40 to 154?mm, and the residual bending strain of the existing pile varies from 100 to 260 με. The ratio of residual to the maximum pile head movements varies from 0.17 to 0.22, while the ratio of residual to the maximum bending strains is in a range of 0.12–0.55.     

沿深度强度增加软土中吸力锚循环承载力模型试验

采用分层底部真空预压技术,在模型试验箱内制备强度沿深度增加的模型试验土层;进而利用自行开发的电动伺服加载装置,在荷载控制方式下,针对水平破坏模式,进行两种直径的吸力锚在最佳系泊点受倾斜方向静荷载与循环荷载共同作用下的承载力模型试验。试验结果表明,循环荷载作用过程中锚的循环变形没有明显增加,导致锚破坏的是循环累积变形。强度沿深度增加软土和均一强度软土中吸力锚循环承载力的相对变化规律一致,当循环破坏次数为100时,循环承载力是静承载力的90%左右;当循环破坏次数增加至2 000时,循环承载力减小至静承载力的70%左右。锚径的变化只影响循环承载力的大小,不影响其相对变化。强度沿深度增加土层中的吸力锚,其循环荷载下竖向阻力弱化程度大于水平阻力弱化程度,这与均一强度土层中吸力锚受循环荷载作用时的性态一致。     

袋内材料对土工袋动力特性参数影响的试验研究

土工袋加固地基技术已普遍应用于房屋地基、边坡加固等工程中。大量研究表明,土工袋具有显著的减振消能效果。为了进一步探讨不同的袋内充填土体材料对土工袋减振消能效果的影响,通过对不同粒径、级配、黏粒含量的填充土体材料及装有相应填充土体材料的土工袋进行水平循环剪切试验,研究袋内填充土体材料与动剪切模量及等效阻尼比2个参数的关系。试验结果表明,土体装入土工袋后,动剪切模量降低,等效阻尼比增大;当填充土体材料及土工袋内土体材料粒径相对较小、级配均匀、黏粒含量高时,等效阻尼比和动剪切模量较大。总体而言,粒径和级配对土工袋减振消能效果影响不大。     

土工袋减振隔振机制分析及试验研究

设置于地基中的土工袋不仅可以提高地基承载力,而且具有减振隔振效果。通过水平循环剪切试验研究了土工袋的动力特性,验证了土工袋具有可变的水平刚度和较大的阻尼比,表明土工袋是一种良好的基础减振隔振材料。土工袋的等效阻尼比随着竖向压力的增大而减小,随着最大剪应变的增大而增大。同时,采用离散单元法进行了土工袋在动力荷载作用下的数值模拟计算,土颗粒间采用弹簧-阻尼器接触模型;而土工袋被看成具有张力的小颗粒薄层,这些小颗粒间的接触与袋内土颗粒间的接触模型相同,但仅设置法向的接触而无切向接触,且只受拉不受压。结果表明,土工袋的减振消能效果主要来自于袋内材料的摩擦耗能、黏滞耗能以及袋子张力引起的耗能。另外,还进行了土工袋沟槽的原位振动测试,验证了土工袋的减振隔振效果。     

循环温度场作用下PCC能量桩热力学特性模型试验研究

PCC能量桩是河海大学岩土所开发的一种新型能量桩技术。在常规桩基静载荷模型试验基础上,将PCC能量桩放置在南京典型砂土中,并通过导热管内水体的循环对模型桩体施加温度场,以模拟PCC能量桩在实际运行过程中的承载力特性与受力机制,PCC能量桩先加载至工作荷载（极限荷载的一半）,再施加热-冷循环一次,最后加载至极限荷载,测得不同温度下PCC能量桩的荷载-位移关系曲线、桩身应力-应变关系曲线等变化规律。试验结果表明,能量桩换热过程中,热量更容易从桩体传向土体（即夏季模式的热循环）;热循环及制冷循环都明显改变了桩顶位移值,且往复循环作用下产生的塑性变形不能完全恢复,其积累变形可能危害上部结构安全;桩身受温度场作用产生的热应力相对较大,且不同约束条件下其变化值有所差异;在制冷循环下,桩底部甚至可能产生较大拉应力。     

循环荷载下考虑主应力旋转的软土力学响应研究

目前用于轨道路堤分析计算的参数多是通过三轴试验获得,而列车荷载经过时土单元应力路径与循环三轴试验加载路径的显著差异可能导致预测失真而引发工程问题。针对某城市轨道进行了三维计算,并分析了移动列车荷载下土单元应力幅值、循环周数、主应力轴旋转的分布规律。在该基础上,采用空心圆柱循环扭剪试验对该复杂应力路径进行了模拟,研究了饱和软黏土的孔压及变形累积特性。结果表明,列车荷载经过时地基土单元大主应力将在(－90°, 90°)内发生旋转;该主应力轴旋转将显著促进软黏土孔压和应变的累积,竖向应力幅值为15 kPa时,循环扭剪试验产生的孔压值比循环三轴试验高77%,累积应变增大了近50%;随着循环应力水平提高,二者累积孔压及累积应变的差值还会进一步增大,甚至出现循环三轴下仅产生较小应变而循环扭剪下已破坏的本质性差异。     

上海软土动剪切模量衰减规律的试验研究

为了获得上海软土在循环动力荷载作用下的动力特性,特别是动剪切模量的衰减规律,采用原状第④层上海软土进行等向固结不排水动三轴试验,分析了加载频率和动应力比 对动剪切模量的影响。试验结果表明,对于上海第④层软土,动应力比达到0.2时,土样才发生破坏;土样的剪切模量-累积轴应变曲线不受加载频率和动应力比的影响;剪切模量的衰减下降主要发生在轴应变0～1%区间。最后提出了一个描述剪切模量衰减规律的数学模型。     

循环加-卸载岩石本构模型研究

循环加-卸载岩石本构模型是预测压气储能洞室长期稳定性的关键,但目前还没有适用的本构模型,因此,提出了一种能够描述岩石循环加载和卸载的本构模型。鉴于岩石在循环作用下损伤不断累积,将基于Weibull分布的岩石损伤软化模型进行拓展,并用内变量疲劳本构模型描述每个循环的初始模量和卸载模量的变化,进而得到循环加-卸载作用下的岩石本构模型,然后将该模型与现有的试验结果进行对比。该模型物理意义明确,涉及的参数较少,且便于拟合。提出的循环加-卸载下岩石本构模型对试验数据拟合效果较好,能较准确地反映循环荷载上、下限值对应的轴向应变,也能反映出循环内部变形模量衰减的趋势。该模型的成功建立为循环加-卸载下岩石本构模型的研究提供了新思路。     

Behavior of a helical anchor under vertical repetitive loading

Abstract

In the field of ocean engineering, anchors are used for several purposes. This article studies the behavior of a helical anchor embedded in soft marine clay under vertical repetitive loading. Helical anchors are simple steel shafts to which one or more helical plates are attached at regular intervals. The tests are conducted on a model helical anchor installed in a soft marine clay bed prepared in a test tank. Repetitive loading is applied using a pneumatic loading arrangement. Different cyclic load ratios and time periods are adopted. In each test, after the application of repetitive loading, poststatic‐pullout tests are conducted to observe the effect of repetitive loading on anchor behavior. From the test results, it is found that, up to a cyclic load ratio of 55%, there is no reduction in capacity. Instead, there seems to be a marginal increase in capacity and reduction in displacement. The reasons for this behavior are explained in terms of induced changes in strength and deformation behavior of marine clay under repetitive load. However, at higher cyclic load ratios, there seems to be reduction in pullout capacity of the anchor, and the reason for this is explained in terms of strain criteria. From this investigation, it can be concluded that the deep anchor is more suitable to a marine environment than a shallow anchor.