上拔荷载作用下的土体细观结构分析

ANALYSIS OF SOIL MESO-STRUCTURE UNDER UPLIFT LOAD

对上拔荷载作用下桩桶基础周围土体细观结构特征进行试验研究。通过对桩桶基础的全模和半模模型试验,研究土中破坏面,对土中破坏面和桩侧不同位置的测点进行图像拍摄。桩桶基础的上拔破坏为:桶上部土体的压缩变形塑性区开展整体剪切破坏的渐进性破坏;桶桩周围的土体由于桶和桩侧的摩擦力,致使土中的塑性区开展整体剪切破坏的渐进性破坏。土表裂缝是以基础中心为圆心的同心圆环状裂缝,并伴有放射状裂缝。分析土体结构细观结构特征变化,随荷载增加,土的大孔隙增多,土结构变化从土孔隙体积变化为主导作用逐渐变为土颗粒的定向为主导作用,土结构松弛,颗粒接触点减少,颗粒的咬合接触减少,在破坏面上颗粒有显著的定向性。研究破坏面的细观结构参数与抗拔强度的关系,上拔荷载作用下破坏面的孔隙率、孔隙率增量、颗粒总数、颗粒面积和颗粒偏心度均有明显的突变,其变化率均为最大值。随荷载增加,孔隙率增加,而颗粒总数、颗粒面积和颗粒偏心度减少。沿桩侧的土结构孔隙性受上拔荷载影响显著,不仅受桩的滑移影响,还受土体的整体剪切的影响。分析结果对上拔地基破坏的细观力学研究具有重要价值。

Meso-structure characterization of sand was studied on pile-bucket foundation bearing uplift load. Based on a series of uplift loading model tests,analysis of the failure modes and the development of slide rupture body in soil of pile-bucket foundation and the deformation process of soil is performed. Then soil meso-structure on plane of rupture and pile side is recorded with photos. Uplift failure of the pile-bucket foundation can be analyzed as compressive deformation of soil on the top part of the bucket-developed plastic zones—complete shear fracture of progressive failure. The frictional force of the bucket and pile will cause an increase in the shear stress of the soil surrounding the bucket and pile,and it will also cause developed plastic zones—complete shear fracture and progressive failure in the soil. The cracks on the surface of the soil are based on the centre of the concentric circle ring cracks in the centre of the foundation. The radius of the outer flank of the circle ring cracks has a good correlative relationship with the width of the foundation,about approximate 5.6 times against that of the bucket radius. Change of soil meso-structure characterization was researched. Load increasing,quantities of large pores increasing,and pore volumes will play leading roles in the change of soil structure;and gradually the direction of sand grains plays a leading role. Therefore,soil structure is loose,and contact points of grains are decreased;interlocking contact of grains are decreased. The grains have been marked directionality on plane of rupture. The relation of meso-structure parameters and pull strength on plane of rupture,porosity,incremental porosity,total grains,grains area and eccentricity of grains of rupture of plane have sudden change on uplift load;and their rates of change all are maximum. When load increasing and porosity are increased,the total grains,grains area and eccentricity are decreased. The porosity of soil structure of pile side is influenced by the uplift load,it will be influenced by pile sliding,and be influenced by general-shear failure. The results have important values on meso-mechanics study of subgrade bearing uplift load.