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砂土中刺桩抗压承载性状模型试验研究
Model Tests on Compressive Bearing Properties of Thorn Piles in Sand
在软土区或岩溶区,存在桩端持力层性质不良或难以入岩等问题,利用端承桩转化为中承桩的思想,提出一种新型非连续变截面的刺桩。通过自行研制的加载装置,开展了不同刺长、刺层数的室内模型试验,探究了刺桩在砂土条件下的抗压承载性状。试验表明:刺桩相比光面桩具有更优异的抗压和抗沉降性能;刺部结构的存在增加了刺土的嵌固作用和桩周土的有效应力,延缓了桩侧阻力软化,扩大了桩周土的影响范围。刺长和刺层数与桩基承载力均呈正相关,相比于刺长,刺层数对单桩极限承载力和桩侧阻力的影响更为敏感。当刺间距为桩长的1/8,刺长为1倍的桩径时,刺桩抗压承载性能最经济。创新性地引入了中承度的概念,量化了刺桩桩刺的中承程度,并提出了刺桩工程设计方法,为后续开展刺桩和其他类似变截面桩的试验和理论研究提供有效方法。
[Objective] Traditional piles with uniform cross-sections have inadequate properties of the bearing stratum at pile tip or difficulties in penetrating into rock when used in soft soil areas or karst regions. Based on the concept of transforming end-bearing piles into mid-bearing piles, this study proposes a novel discontinuous variable cross-section thorn pile, aiming to systematically reveal the mechanisms by which thorn length and the number of thorn layers affect the compressive bearing properties of thorn piles through indoor model tests, thereby providing a theoretical basis for establishing a performance-based optimized design method. [Methods] Using a self-developed loading device, a geometric similarity ratio of 1∶75 was adopted, and PVC hollow circular pipes and 304 stainless steel cylindrical pins were used as the model piles and pile thorns, respectively. Using the controlled variable method, a total of 21 test groups were designed, and indoor model tests with different thorn lengths and number of thorn layers were conducted to investigate the compressive bearing properties of thorn piles in sand. [Results] (1) compared with smooth piles, thorn piles exhibited superior compressive bearing and settlement resistance. Under sand conditions, the ultimate bearing capacity of the 17-layer Type-38 thorn pile increased by 278% compared with that of the smooth pile. (2) Both thorn length and the number of thorn layers showed positive correlations with the bearing capacity of the pile foundation; compared with thorn length, the number of thorn layers was more sensitive to the ultimate bearing capacity of a single pile and the pile shaft resistance. (3) From an engineering economic perspective, when the thorn spacing was approximately one-eighth of the pile length and the thorn length was about equal to the pile diameter, the thorn pile exhibited optimal compressive bearing properties. [Conclusion] (1) The presence of thorn structures increases the interlocking effect between thorns and soil as well as the effective stress of the surrounding soil, delays the softening of pile shaft resistance, expands the influence range of the surrounding soil, and effectively redistributes the applied load, thereby significantly enhancing the overall bearing capacity of the pile foundation. (2) The concept of mid-bearing degree is introduced to quantify the mid-bearing contribution of pile thorns, and a calculation method for the ultimate bearing capacity of a single thorn pile is proposed, laying a foundation for subsequent theoretical studies. (3) Future research may consider the effects of layered foundations, soil squeezing during thorn formation, and thorn geometry on the compressive bearing properties of thorn piles, so as to further improve the comprehensiveness of bearing mechanisms and the applicability of theoretical models. The findings of this study have important theoretical and practical value in promoting the transformation of modern pile foundation engineering toward green and intelligent development, and their application will profoundly influence future modes of underground space development.
桩基础 / 刺桩 / 抗压特性 / 模型试验 / 刺层数 / 刺长
pile foundation / thorn pile / compressive property / model test / number of thorn layers / thorn length
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为对比研究等截面面积与等截面周长Y形桩与圆形桩的竖向承载性能,基于自主研发的大型桩基模型试验加载系统,通过在桩体内埋设钢筋计,桩端土体内布设土压力盒和砂雨法施工埋设桩体,进行了Y形桩与圆形桩承载机理对比模型试验研究。结果表明:与等截面圆形桩相比,Y形桩极限承载力是圆形桩的1.60倍,桩侧表面积是圆形桩的1.56倍,极限承载力的提高与桩侧表面积增大效果基本一致,在各级荷载作用下圆形桩的端阻力均小于Y形桩的端阻力,异形桩端截面增大了其端阻力;与等周长圆形桩相比,Y形桩极限承载力是圆形桩的84%,但混凝土用量仅为圆形桩的41%,单位体积混凝土发挥的极限承载力为圆形桩的2.04倍,在各级荷载作用下Y形桩总桩侧阻力均大于圆形桩。
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In order to investigate and compare the vertical bearing behaviors of Y-section pile and circular pile with the constant section area and the same section perimeter, by means of burying steel bar gauge in piles, laying the earth pressure cell under the pile tip, and installing the piles using sand pour method, the comparative model test research on the bearing mechanism between Y-section piles and circular piles was carried out based on the self-developed loading system of large-scale pile foundation model test. The results show that the ultimate bearing capacity and the pile side surface of Y-section pile are 1.60 times and 1.56 times those of circular pile with the constant section, respectively; the effect of improving ultimate bearing capacity is consistent with that of the increase of pile side surface; and the tip resistance of the circular pile is always lower than that of the Y-section pile under different load levels due to the fact that the special Y-section pile tip section enhances the tip resistance. The ultimate bearing capacity of Y-section pile is 84% that of the circular pile with the same perimeter, however, the amount of concrete in Y-section pile is only 41% of that in circular pile. The ultimate bearing capacity of per unit volume of concrete for Y-section pile is 2.04 times that for circular pile; the total side resistance of the Y-section is larger than that of the circular pile under each load level.
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整体桥因其全周期寿命长、整体性好和养护费用低等特点，得到了广泛应用，但对其在地震荷载作用下的受力特点和变形规律还缺乏深入研究。基于此，以某整体桥为背景，制作桥台-H形钢桩试验模型，开展整体式桥台-H形钢桩-土体系抗震性能拟静力试验研究，分析桥台-H形钢桩的破坏模式、滞回性能、骨架曲线、水平变形和桥台转角等变化规律。试验结果表明：H形钢桩出现较大的负向残余变形，但负向加载下H形钢桩未出现破坏；台后、台底及桩顶土体均出现大范围脱空；试件的等效黏滞阻尼比约为0.35，具有良好的耗能能力；正向加载下试件的弹性抗弯刚度是负向的12.6倍，最大承载力是负向的3.85倍，台后土对试件的刚度和承载力影响显著；破坏时试件刚度减小至初始刚度的33%，退化不显著；相比位移延性和割线刚度，采用环线刚度分析其抗震性能更为合适，改进后的割线刚度能更准确地反映试件的刚度退化；考虑整体和局部累积变形的影响，大加载位移作用下，桩身出现较大的负向整体累积变形，且桩身沿深度方向多处出现局部累积变形；加载过程桥台仅发生刚体位移，正向转角逐渐增大，负向转角先增大后减小再转为正向倾斜。研究发现整体式桥台-H形钢桩-土体系拥有优越的抗震性能。
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The integral abutment jointless bridge (IAJB) has been widely applied in bridge engineering for its advantages such as long service life, good integrity, and low maintenance cost. However, its mechanical characteristics and deformation law under earthquake loads lack thorough investigations. In this study, a practical engineering of IAJB was adopted as a sample, and an abutment-steel H-pile structural model was manufactured to conduct on the seismic performance of integral abutment-steel H-pile-soil system under cycle quasi-static test. Some significant results were obtained, including the failure mode, hysteretic behavior, skeleton curve, horizontal deformation law, and abutment rotation angle law. The results indicate that steel H-pile carries a large negative residual deformation but does not damage under negative loading. Behind and at the bottom of the abutment and at the top of the pile, some large-scale voids are observed. The equivalent damping ratio of the abutment and pile is about 0.35, which indicates a good energy dissipation capacity. Furthermore, the elastic bending stiffness of the specimen under positive loading is 12.6 times that under negative loading, and the maximum bearing capacity is 3.85 times that under negative loading. Additionally, the backfill has a significant influence on the stiffness and bearing capacity of the specimen. After the test, the stiffness of the specimen reduced by 33%, which is relatively unremarkable. Compared to the displacement ductility and secant stiffness, the loop stiffness is more appropriate for analyzing the seismic performance of the specimen, and the improved secant stiffness can more accurately reflect the stiffness degradation of the specimen. Considering the effect of full and local accumulative deformation, under the large displacement load, the steel H-pile appears to have a larger negative, full accumulative deformation, and there are many local accumulative deformations along the depth direction. During the loading process, the abutment only exhibits rigid body displacement, and the positive angle gradually increases. Moreover, the negative angle of abutment increases and decreases before finally turning to the positive angle. These results show that the integral abutment-steel H-pile-soil system exhibits excellent seismic performance.
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To obtain the mechanics behavior at an enlarged-part and predict the pile settlement when a squeezed and branch pile was vertical loaded, the relationship between the force and displacement at the enlarged-part was solved by using the cavity expansion theory, and the load transfer method was applied to the squeezed and branch pile. When a pile is vertically compressed, the enlarged-part will squeeze the soil and move downward, which can be regarded as a cavity expansion problem. Based on a rational assumption, the relationship between the enlarged-part horizontal internal pressure and the vertical displacement was deduced. From the mechanical analysis on the subsurface, a varying pattern of the pile-soil contact surface, <i>A'C</i>, and relationship between the enlarged-part resistance and vertical displacement were confirmed, and some influential parameters were also researched. Based on the above, by applying a hyperbolic model and linear elastic model in the pile shaft and pile tip, respectively, the load transfer method was applied to the squeezed and branch pile to obtain the pile-head settlement curve and pile inner force. The research result shows that by using the cavity expansion theory, the approach for obtaining the relationship between the enlarged-part resistance and vertical displacement completely considers the soil squeezing effect and geometric construction features of the enlarged-part. The enlarged-part resistance is more sensitive to the squeezed angle, which is in more accordance with engineering practices. The enlarged-part horizontal internal pressure has a nonlinear distribution on the reversed-truncated cone-shaped subsurface and decreases with the increase in the initial radius, and the nonlinearity becomes more obvious as the vertical displacement increases. The value of the enlarged-part horizontal internal pressure increases as the vertical displacement increases, but decreases with the increase in the squeezed angle. The load transfer method for squeezed and branch piles considering the cavity expansion theory can determine the settlement and mechanics behavior effectively. Regarding the squeezed and branch piles, the variation in the enlarged-part resistance caused by the squeezed angle has a stronger effect on the final bearing capacity than the simple side-resistance variation. The relevant method and conclusions can provide references for engineering.
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针对桩在近海滩涂中下沉的问题，提出了一种根系桩，给出了其施工工艺。本文以根系桩为研究对象，采用了室内模型试验的方法，研究了根系桩的竖向承载力，对比分析了根系桩与圆桩、锚刺桩的承载力差异，给出了根系桩在江苏盐城市的斗龙港、川东闸的海平面高程观测中的应用实例。结果表明：根系桩比圆桩和锚刺桩的竖向承载力高，施工可行、简单、实用。本文研究成果可以为软基处理、河道防冲刷、边坡加固等类似工程提供技术参考。
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Aiming at solving the problem of pile sinking in tidal areas，this paper proposes a new pile type-root pile． The root pile is taken asthe research object． The indoor model test method is used to study the vertical bearing capacity of the root pile． The bearing capacitydifferences between the root pile and the circular pile and the anchor pile are compared and analyzed． The application examples of the rootpile in the sea elevation observation of the Longgang and East Sichuan sluice in Yancheng City，Jiangsu are given． The results show that thevertical bearing capacity of root pile is higher than that of round pile and anchor pile． Meanwhile，the construction of root piles is alsofeasible，simple and practical． The research results in this paper ca
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