Research On Horizontal Load Characteristics Of Gravity Anchor Under The Condition Of Calcareous Rock And Soil Site

With the exploitation of oil and gas resources in the South China Sea,the number of various marine structures is increasing,anchoring foundation design plays a crucial role in maintaining the stability of marine structures.Considering the complexity of geological conditions in the South China Sea,it is mainly distributed in special rock and soil materials with easily broken calcareous sand,reef limestone and other carbonate materials which are different from the terrestrial sediments.Gravity anchor has the characteristics of simple structure,suitable for a wide range of soil conditions,and no need to survey geological data,especially in the hard rock seabed,it will play a unique advantage and be paid attention to.In view of the particularity of calcareous rock and soil,the problem of the horizontal bearing capacity of the gravity anchor foundation on this kind of ground is discussed.The influence of calcium carbonate content on the crushing and strength properties of carbonate sand soil is studied.The influence of the structure of the anchor bottom on the horizontal tension under the condition of the carbonate sand and the carbonate rock site is also studied by the centrifuge model test.With the help of the large deformation finite element technology,we simulate the complete process of gravity anchor horizontal movement under various site conditions,and analyze the influence of the location of the anchor point of the gravity anchor on horizontal pull.The main conclusions are as follows:1.The multiple regression analysis of particle breakage shows that the relative particle breakage increases with the increase of calcium carbonate content and the enlargement of particle size.With the increase of normal stress,the dependence of the particle breakage on the calcium carbonate content is enhanced,and the dependence on the particle size is weakened.The friction angles of mixed sand increased with the raise of calcium carbonate content,according to the distribution of the friction angle values in the grain group and the content of calcium carbonate space and the nomenclature of carbonate sand proposed by Clark,the classification of carbonate sand based on carbonate content index is suggested by dividing the similar strength of carbonate sand into one type.2.In centrifuge model test,under carbonate sand site condition,the displacement load response of gravity anchor with flat bottom,eight key and cross key is basically the same during horizontal movement.When the horizontal displacement reaches 0.1 times the anchor length,the horizontal tension reaches the "start peak".The measured friction coefficient is 20% larger than the theoretical friction coefficient.In the carbonate rock site condition,the load displacement response is an irregular fold line,and the error of the measured friction coefficient is relatively small with the theoretical value.Under two site conditions,the measured friction coefficient and anchorage efficiency of the eight anchors is the highest.Under the same base structural and the same horizontal displacement,the horizontal tension of the gravity anchor in the carbonate sand field is slightly higher than that of the standard sand.3.The large deformation finite element method can reflect the movement mechanism of the carbonate sand around the eight key anchor,and accurately simulate the displacement load response of the eight key anchor under horizontal motion in the carbonate sand site.During the horizontal movement of gravity anchor in the carbonate sand soil site,the high mooring point anchor is overturned and rotates around toes,low mooring point eight anchor performs horizontal motion.The initial peak of the low mooring point eight anchor is 37% higher than that of the high mooring point eight anchor.In the carbonate rock site,high mooring point anchor is easily overturned around the anchor toe.Based on the purpose of fast and without delay anchorage,and to ensure the anti-overturning stability of the gravity anchor structure,we suggest that the mooring point of the gravity anchor should set at the lower end.The structure of the eight-key anchor is optimized,then the cantilever anchor and four squaretable anchor are put forward,effectively improving the anchorage efficiency.