胶结钙质土的形成机制及其工程分类研究

The cemented calcareous soils are widely distributed in all islands and reefs in South China Sea. The cementations among the grains are carbonates either from the dissolving of the corals or from the growth of the carbonate crystals. Due to the randomness of this bounding among individual grains, the strength, density and the degree of cementations can have a signification variation even within a very short horizontal and vertical distance in the marine environment. This calcareous cementation can be summarized by its highly regional, randomness, discontinuities. These variations will unavoidably have an adverse impact on the axial performance of the piles in these soils. At current stage, a systematic investigation on the properties of these cemented calcareous formations has not been initiated yet in China. The ultimate objective of the current application is focused on serving the need of the defense as well as the resource development facilities in South China Sea. In order to achieve this goal, the high quality artificially cemented calcareous sand with a high void ratio that close to the naturally cemented coral soils is necessary. Therefore, the properties of the naturally cemented soils and the foundation performance in the cementetious coral formations could be simulated by the laboratory testing. Based on the microscopic investigations of the naturally cemented calcareous soil structures from different marine environments and regions, an engineering classification system of these cemented soils will be proposed. The proposed classification system will consider the contribution of the shape as well as the strength of the individual grains to the engineering properties. These chemically and artificially cemented calcareous samples should also have the characteristics of controllable and repetitive mechanical properties; more importantly, they could simulate the properties of a wide range of the naturally cemented soils with different classifications. The factors that affect the engineering performance of the calcareous soils could be revealed through extensive laboratory investigations.

南海诸岛礁普遍赋存胶结钙质土，其分布具有区域性、随机性及不连续性，这种钙质土颗粒之间的胶结是极其无序的，以至于在较小的范围内土的强度、密度和胶结度都会产生很大的变化，其独特的物理力学性质对桩基承载力产生显著的影响，迄今我国尚未对胶结钙质土的工程特性开展系统的研究工作。 本项目拟在对天然胶结钙质土的结构进行微观分析基础上，定量地考虑颗粒的形状和强度影响，采用可控的化学结晶胶结方法模拟天然胶结样的形成过程来制备人工胶结钙质土试样，开展人工胶结样的物理、水理及力学性状与天然胶结样的对比试验研究，逐步修正化学结晶胶结过程，进而探讨天然胶结的形成机制；研制出专门的、方便可控的、重复性好的高孔隙比人工胶结钙质土试样制备系统；通过对人工胶结钙质土试样开展系统的物理力学试验，确定影响其工程行为的关键参数；建立胶结钙质土工程分类方法。为钙质土的地基加固理论提供科学支持，服务于我国南海岛礁国防建设与资源开发。

项目以南海天然胶结钙质土为研究对象，通过采用试样切片分析、扫描电镜（SEM）、X射线衍射、飞秒激光切割、孔径分布测量及颗粒强度试验等细观测试手段，研究天然胶结钙质砂的结构特征；采用MICP及CIPS方法室内制备胶结钙质土试样，分析颗粒间产生钙质胶结的影响因素；开展胶结钙质土的物理、水理及力学性质研究，甄别影响其力学性状的关键参数，构建胶结钙质土的工程分类体系。研究表明，天然胶结钙质土多呈碎屑结构，颗粒粒径分布范围大，可从粉粒到砾粒，且分选差异性大。颗粒的矿物组成以文石和高镁方解石为主，且各试样的颗粒形状参数值十分接近，颗粒越小，形状参数值越接近于1。飞秒激光切割后的内孔隙分析表明，粒径大于2 mm 及1～2 mm 两个粒组颗粒的断面面孔隙度分别为1.07%、0.66%。天然胶结钙质土的粒间孔孔径为8×10－6～0.2 mm。对薄片的分析和扫描电镜图像的观察研究表明：天然胶结钙质土的胶结类型多为接触式胶结和镶嵌式胶结，胶结物主要为粒状方解石、针状文石以及生物微晶，基质主要是泥晶，少量亮晶；其胶结过程可以分为4 个期次，即犬齿状文石晶体在泥晶套上生长--粒状方解石晶体生长--溶蚀--针状文石在溶蚀孔孔壁上生长。采用MICP及CIPS方法均可在室内制备出不同强度及孔隙率的胶结钙质土试样。MICP方法中理想的钙源为氯化钙、最佳的营养液（尿素）浓度为1mol/L，且胶结效果小颗粒优于大颗粒，钙质砂优于石英砂，淡水环境优于海水。CIPS方法制备胶结钙质土试样中采用黄豆或黄豆种皮为脲酶的物质来源，用弱碱性碳酸氢钠和乙二胺四乙酸二钠混合溶液进行浸提，选用假酸浆籽胶质多糖（NPG）为水凝胶固定化载体，使用亚硫酸钠作为添加剂增加碳酸钙胶结“靶向性”。天然胶结钙质土的物理、力学性质及其对应关系研究结果表明，密度及胶结度（以抗拉强度表示）与单轴抗压强度间存在着良好的正相关性，可以作为胶结钙质土工程分类的主要控制参数。