球型静力触探仪贯入双层土的困土效应

Effect of trapped soil on penetration of ball penetrometers in two-layered clay

为研究各种几何类型的球型静力触探仪在双层黏土地基中的贯入特性,采用改进的RITSS(remeshing and interpolation technique with small strain)大变形有限元方法对贯入过程进行数值模拟,研究土体流动特性和贯入阻力变化规律。利用理论解和试验结果验证模型的有效性,通过参数分析探讨上层土厚度、土体强度、球土间摩擦因数和杆轴投影面与球截面面积比等因素对贯入特性的影响。结果表明,球型静力触探仪贯入上硬下软双层黏土地基时存在困土现象(即上层硬土陷于探头底端并被带入下层土)。困土尺寸是影响其困土效应的关键因素。研究发现,困土尺寸随下层土与上层土不排水抗剪强度比的减小而增大,随球土间摩擦因数的增大而增大,杆轴面积比对贯入特性有一定的影响,但是相对摩擦因数和土体强度比影响较小,与上层土厚度和下层土不排水抗剪强度无关。困土厚度变化为0~0.20 D、困土宽度变化为0~0.50 D。困土效应增大下层软土贯入阻力,导致不排水抗剪强度计算值偏大。基于影响因素量化分析,提出考虑土体强度特性、杆轴比和摩擦因数因素的困土效应修正公式,得到更为准确的下层软土不排水抗剪强度值。

To investigate the penetration characteristics of ball penetrometers with various geometry characteristics in two-layered clay,large deformation finite element(LDFE)analyses incorprating modified remeshing and interpolation technique with small strain(RITSS)were conducted to simulate the ball penetration process.Soil flow mechanism and variation law of penetration resistance were analyzed.The numerical model was verified by theoretical solutions and experimental results.Through parametric analyses,the effects of top layer thickness ratio,soil strength,friction coefficient between soil and ball penetrometer,and shaft-ball area ratio on penetration characteristics were discussed.Results show that in stiff-over-soft clay,the trapped soil formed underneath the ball penetrometer was carried into the bottom layer.The size of the trapped soil was the key factor affecting the trapped soil effect.The size of the trapped soil increased with the decrease of the undrained shear strength ratio of bottom clay to top clay,and the increase of the friction coefficient between soil and ball penetrometer.The shaft-ball area ratio had certain effect on the penetration resistance,but the influence was less than those of friction coefficient and soil strength.The size was not affected by the thickness of the top layer or the undrained shear strength of the bottom clay.The thickness of the trapped soil varied from 0 to 0.20 D,and the width ranged from 0 to 0.50 D.The effect of trapped soil increased the penetration resistance in soft bottom clay and thus led to a larger value of the undrained shear strength.Therefore,considering the effects of soil strength,shaft-ball area ratio,and friction coefficient,a modified calculation formula was proposed to obtain more accurate results of undrained shear strength of soft bottom clay.