南海钙质砂宏细观破碎力学特性

Micro-and macro-mechanical behavior of crushable calcareous sand in South China Sea

针对取自我国南部某海域的钙质砂样本,做了以下两方面工作:一是通过电子显微镜获取了钙质砂颗粒的几何投影图像,利用图像处理技术对图形进行黑白二值化处理,获取单元颗粒形状轮廓边界,使用圆度和粗糙度2个参数对钙质砂的颗粒形状进行定义和量化。二是通过不同围压下的三轴固结排水剪切试验及试验前后的颗分测量对比,研究了颗粒破碎对钙质砂的变形、强度、能量耗散等特性的影响。研究表明,大粒径钙质砂(粒径大于2.0 mm)和小粒径钙质砂(粒径小于0.5 mm)形态比较接近圆形、颗粒表面相对光滑;相比而言,中间粒径(粒径介于0.5~2.0 mm之间)钙质砂形状较不规则,表面棱角较多。钙质砂在三轴排水剪切过程中发生颗粒破碎,试样向着级配均匀的方向发展。随着初始围压的增大,颗粒破碎程度加大,土样整体剪胀趋势减小,而破碎引起的能量耗散增加。而在高围压(初始围压为600 k Pa)剪切过程中,仅考虑摩擦耗散,以及同时考虑摩擦、体积耗散两种情况下,计算得到的最大颗粒破碎耗散分别可达土样总输入塑性功的25%和18%。

To develop offshore facilities in South China Sea, it is vital to understand the characteristics of particle shape and breakage properties of the calcareous sand. As a result, micro- and macro-mechanical behaviors of crushable calcareous sand sampled from South China Sea are addressed in this study. Firstly, calcareous sand particles are scanned by electron microscope and analyzed by an image processing software （i.e., ImageJ）. Based on the processed image, two shape parameters, namely circularity and solidity, are defined and quantified. Secondly, drained triaxial tests on calcareous sand at various confining stresses are carried out, so as to investigate the effects of particle breakage on deformation, shear strength and energy dissipation of the calcareous sand. Results show that the shape of the calcareous sand with relatively large particle size （diameter greater than 2 mm） and relatively small particle size （diameter less than 0.5 mm） tend to be circular and the particle surface is relatively smooth. Comparatively, shape of calcareous sand with moderate-size diameter （grain diameter ranged between 0.5 and 2 mm） is more irregular, and sand particles have more surface edges. Particle breakage is identified to occur in triaxial tests, leading to a better graded sand packing. With the increasing initial confining pressure, the degree of particle breakage and the energy dissipation due to the breakage both increase. In the meantime, dilation of the sand is partially suppressed by the breakage. Under two conditions only considering friction dissipation and considering both friction and volume dissipation, the dissipated energy resulting from breakage in the triaxial tests with relatively high （600 kPa） initial confining stresses could account for 25% and 18% of the total plastic energy input, respectively.