中国科学院力学研究所机构知识库

我国南海海域广泛分布着钙质砂及天然气水合物地层。钙质砂是特殊的海源土，具有高孔隙、易破碎、易胶结的特性；天然气水合物分解将造成地基强度和其中结构物承载力的大幅降低。随着我国油气开采逐渐向南海深海扩展，这两种地基中结构物的稳定性是必须要解决的问题。本文分别针对这两种地层中的结构物承载力和稳定性进行了研究。首先对钙质砂基本力学性质、以及钙质砂地基中桶形基础在波浪荷载下的承载特性进行了实验研究；然后对水合物沉积物的力学性质进行了实验研究，并对天然气水合物分解引起的地层沉陷及海底管道的稳定性进行了数值方面的研究。

针对钙质砂力学性质实验，首先在实验室中分别采用一定比例的石膏粉、方解石粉以及硅酸盐水泥对钙质砂进行胶结，然后进行力学性质并与非胶结情况的结果进行对比。结果表明，石膏粉及方解石粉对于钙质砂的固结排水强度影响不大；硅酸盐水泥的含量提高对钙质砂的强度以及变形模量都有很大的提高。

钙质砂地基中桶形基础在循环荷载（模拟波浪载荷）下的试验结果表明：水平动循环荷载下，桶周围土体发生塑性变形，形成坡角约为 的沉陷区域，桶在没有压重的情况下向上移动，土体反力有较大幅度的降低。荷载幅值、频率、压重、桶长径比、地基材料参数等是桶形基础动态响应主要参数。

在实验室内利用四氢呋喃水合物沉积物研究了水合物分解前后沉积物的参数变化。结果表明：水合物的存在对土骨架具有粘聚及增强咬合的作用，它的分解将导致沉积物变形模量及强度的大幅降低。利用商用软件ABAQUS研究了输油管道穿过水合物层时，水合物分解引起的地层及管道位移变化。结果表明：分解半径的增加将使管道位移增加；但当分解半径超过水合物层深度时，分解半径的增加对管道及土体的变形不明显；分解后地层参数变化越大，引起的变形也越大；有覆盖层时地层沉陷更加明显。

今后在钙质砂研究方面，应找到更合适的能反应实际的胶结方法，并进行大尺度的实验；在水合物引起的结构物稳定性方面，应获得更准确的水合物分解前后的力学参数，然后考虑热－渗流－岩土骨架－结构物的耦合作用。

Calcareous sand and methane hydrate are widely distributed in South China Sea. Calcareous sand is a special kind of soil with high void, cementation and high grain breakage. Methane hydrate is easy to dissociate once the environmental temperature and pressure changes. With the oil and gas exploitation gradually extending into the deep water area in the South China Sea, the structure stability in these two types of stratums becomes an important problem. The experimental study has first been done on the mechanical properties of calcareous sand and the responses of bucket foundation in calcareous sand layer under static and cyclic loads; then the properties of hydrate deposit has been studied by tri-axial experiments, and numerical study on the stability of pipeline in ocean floor when gas hydrate dissociates has been carried out.

The mechanical properties of cemented calcareous sand with calcite and Portland cement as cement material have been obtained by tri-axial experiments. It is shown that the gypsum and calcite powder have little effect on the cementation of calcareous sand ,while Portland cement can improve Young's modulus and cohesion greatly if the proportion is suitable.

The dynamic responses of bucket foundation in calcareous sand layer under cyclic horizontal loading are experimentally investigated. It is shown that the sand layer around the bucket has plastic deformation, and the subsidence area with a slope angle about 20 degree is formed. Large vertical displacements of the bucket foundation occur and the stability of bucket foundation degrades. A series of experiments have been carried out to investigate the effects of the following factors:  loading magnitude, loading frequency, the soil properties, the weight, the bucket scale etc. It is shown that with the increase of the loading amplitude and loading frequency and the decrease of the bucket height (the same diameter), the dynamic responses becomes acute. The stability of bucket foundations is enhanced with the weight increase in the range of our experiments.

Pure tetrahydrofuran hydrate and tetrahydrofuran hydrate deposits with different materials as the skeleton are synthesized in laboratory. A series of experiments are carried out to study the mechanical properties. It is shown that the strength of pure gas hydrate is smaller than that of gas hydrate deposit. The mechanical properties of hydrate deposit with different skeletons are obviously different. The bigger the initial strength of the skeleton, the smaller the strength decreases of gas hydrate after dissociation.

Commercial software ABAQUS is used to study the stability of pipeline in ocean floor when gas hydrate dissociates. It is shown that with the increase of the dissociation radius and the over-layer’s depth, the stability of pipeline and soil layer becomes acute. Nevertheless, as the dissociation radius beyond the depth of hydrate layer, the increments of the deformation of pipeline and soil layer are not obvious with the increase of dissociation radius.