细粉砂土含水率对混凝土扩盘桩抗压破坏状态影响研究

发布时间：2019-01-14 12:48

【摘要】：混凝土扩盘桩是由普通混凝土灌注桩衍生而来,在普通混凝土灌注桩桩身增加一个或多个承力扩大盘,根据地质情况选择适合的土层设置承力扩大盘来提高桩的抗压或抗拔承载力,实践表明它具有承载力高、沉降变形小、施工简便快捷等优良特性。但是,实际工程中的地质条件千变万化,十分复杂,当采用混凝土扩盘桩时,各盘就可能设置在不同土层中,目前对扩盘桩的理论研究大多是将承力盘置于粘性土中,而关于砂土土层性状对其承载力影响的研究相对较少。已有研究表明细粉砂土可以设置承力盘,土层的含水率、密实度对混凝土扩盘桩的承载力产生很大的影响。为完善混凝土扩盘桩在竖向压力作用下的理论基础,本文在前期混凝土扩盘桩研究成果的基础上,进一步研究细粉砂土的含水率对承力盘周围土体破坏状态以及扩盘桩承载力的影响。研究方法主要采用室内半截面桩小模型试验以及ANSYS有限元模拟,并对两类分析结果进行对比,进一步验证破坏机理和承载理论的可靠性。通过试验的研究发现:加载过程中盘尖端水平裂缝逐渐向桩身收敛成“心形”形状,并最终沿裂缝发生滑移破坏,说明承力盘周围土体破坏状态符合滑移破坏理论,可以按此理论进行承载力计算。同时,对于含水率对研究结果的影响,试验研究得出,当细粉砂土含水率在10%—17.5%范围内时,含水率越高扩盘桩的抗压承载力相对越小,当含水率高于14%左右时抗压承载力下降较明显。从有限元模拟分析的结果显示:细粉砂土的含水率在10%—18%范围内,其含水率低于14%左右时对扩盘桩的承载力及最大位移值影响较小,其含水率高于14%左右时对扩盘桩的承载力及最大位移值影响较大。结合试验过程中记录的破坏状态和位移-荷载曲线与有限元模拟分析的结果进行对比,其含水率相同的情况下,二者桩周土的影响范围以及破坏状态基本一致,位移-荷载曲线的变化趋势相同,至于曲线变化率的不同主要是由于室内小模型试验条件的限制以及模拟分析的结果是属于纯理想状态所致,总体来看试验结果与有限元模拟分析的结果是贴合的,说明分析研究的结论是可靠的。所以在工程实践中尽量将承力盘置于含水率低于14%的细粉砂土中。本论文通过室内小模型半面桩试验和有限元模拟分析两种方法进行研究,探讨了含水率对混凝土扩盘桩处于细粉砂土中破坏状态及承载力的影响,弥补了实际工程中忽略土体含水率对扩盘桩竖向承载力影响的欠缺,为混凝土扩盘桩在细粉砂土土层的应用提供理论基础。
[Abstract]:The concrete expansion pile is derived from the ordinary concrete cast-in-place pile, and one or more load-bearing expanded plates are added to the ordinary concrete cast-in-place pile body. According to the geological conditions, a suitable soil layer is selected to increase the bearing capacity of the pile under compression or drawing. The practice shows that it has the advantages of high bearing capacity, small settlement and deformation, simple and quick construction, and so on. However, the geological conditions in practical engineering are varied and complex. When concrete pile is used, each plate may be placed in different soil layers. At present, most of the theoretical researches on the pile are based on putting the load plate in the clay. However, there are few studies on the effect of soil properties on the bearing capacity of sandy soil. It has been shown that the load bearing plate can be set in fine silty sand soil, the moisture content of soil layer and the compactness of soil layer have great influence on the bearing capacity of concrete pile with expanded plate. In order to improve the theoretical foundation of concrete expanded disc pile under vertical pressure, this paper is based on the previous research results of concrete expanded disc pile. The influence of the moisture content of fine silt on the failure state of soil around the bearing plate and the bearing capacity of the pile is further studied. The small model test and ANSYS finite element simulation are used to test the failure mechanism and the reliability of the bearing theory. Through the experimental study, it is found that the horizontal crack at the tip of the plate converges gradually to the "heart" shape of the pile body during loading, and finally slips along the crack, which indicates that the failure state of soil around the bearing plate accords with the theory of slip failure. The bearing capacity can be calculated according to this theory. At the same time, for the effect of moisture content on the research results, the test results show that when the moisture content of fine silty sand is in the range of 10%-17. 5%, the higher the moisture content is, the smaller the compressive capacity of the pile is. When the moisture content is higher than 14%, the compressive capacity decreases obviously. The results of finite element simulation analysis show that the moisture content of fine silty sand is in the range of 10- 18%, and when the moisture content is less than 14%, it has little effect on the bearing capacity and maximum displacement of the pile. When the moisture content is higher than 14%, the bearing capacity and the maximum displacement of the pile are greatly affected. Comparing the failure state and displacement-load curve recorded during the test with the result of finite element simulation analysis, the influence range and failure state of the soil around the pile are basically the same when the moisture content is the same. The variation trend of displacement-load curve is the same, and the variation rate of the curve is mainly due to the limitation of indoor small model test conditions and the result of simulation analysis is due to the pure ideal state. As a whole, the experimental results are consistent with the results of finite element simulation analysis, which shows that the conclusion of the analysis is reliable. Therefore, in engineering practice, the bearing plate is placed in fine silt with moisture content less than 14%. In this paper, the influence of moisture content on the failure state and bearing capacity of concrete pile in fine silt soil is studied by two methods: small model semi-plane pile test and finite element simulation analysis, and the influence of moisture content on the failure state and bearing capacity of concrete expanded sheet pile in fine silt is discussed. It makes up for the deficiency of neglecting the influence of soil moisture content on the vertical bearing capacity of the pile, and provides a theoretical basis for the application of concrete pile in fine silt soil.
【学位授予单位】：吉林建筑大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TU473.1

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