Research On Model Test Of Bearing Characteristics Of Post Grouting Pile In Calcareous Sand Formation

Calcareous sand is a kind of special rock and soil medium rich in calcium carbonate.The shape of sand particles is irregular,with the special properties of multi porosity,easy to break and easy to cement.In recent years,the calcareous sand is often used as the filling material in the island and reef projects in the South China Sea.However,under the action of pile foundation and upper load,the calcareous sand is easy to break particles,resulting in creep or uneven settlement of the foundation,which seriously affects the use of buildings.The engineering practice shows that the post grouting pile can improve the bearing capacity of the foundation.It is of great significance to study the bearing characteristics of the non grouting pile and the post grouting pile in the calcareous sand formation.Scientific results show that the indoor model test can better reflect the actual bearing characteristics of the project site.In order to study the bearing characteristics of post grouting pile in calcareous sand formation,this project designed and improved the model pile bearing characteristics test device and indoor post grouting test device,and carried out indoor model test.Firstly,taking the river sand formation as the control group,the bearing capacity difference of single pile in the calcareous sand formation and river sand formation is explored,and then,the bearing characteristics of non grouting pile and post grouting pile in the calcareous sand formation are compared and analyzed.Based on the application and research status of pile foundation engineering in calcareous sand formation,a series of research work on bearing capacity of indoor model pile is carried out in this paper.(1)Because of its special material and sedimentary environment,the apparent and mechanical properties of calcareous sand are quite different from those of river sand.The bearing characteristics of model piles are tested in calcareous sand and river sand formation.In these two kinds of formation,the Q-s curve of single pile without grouting is of steep drop type;the N-h curve has a great slope,and the axial force decline rate is small;the qs-h curve is relatively uniform,and the ultimate side friction occurs.(2)The side friction and ultimate bearing capacity of single pile increase steadily with the increase of pile length and pile body roughness;the relative compactness of pile diameter and sand layer increases,and the ultimate bearing capacity of single pile increases obviously,but the side friction of pile body decreases slightly with the increase of pile diameter,and the relative compactness of sand layer is not sensitive to the exertion of side friction of pile foundation.Due to the different soil conditions at the pile end and pile side,the ultimate bearing capacity and ultimate side friction of single pile in river sand formation are always greater than that in calcareous sand formation.(3)In the calcareous sand formation,the post grouting technology is used to treat the single pile,and the bearing capacity model test of the post grouting pile is carried out.The relative displacement of post grouting pile is reduced to 2/3 of that of non grouting pile,and the ultimate bearing capacity of pile is increased by 1.6 times.The ultimate bearing capacity and side friction increase steadily with the increase of pile length and grouting pressure,but the softening effect will appear when the side friction increases with the increase of grouting pressure;the side friction of post grouting pile is more sensitive to the relative density of sand layer,and the ultimate bearing capacity and side friction of single pile will increase when the water cement ratio of slurry decreases.(4)On the basis of the above tests,combined with the test data and relevant theories,the influence of the properties of pile-soil interface,pile end conditions,pile material,cross-sectional area and depth effect of pile foundation on the side friction of single pile is analyzed comprehensively.Based on the above factors,the side friction function of post grouting pile foundation in calcareous sand formation is established,and it is found that the calculated value is generally consistent with the test value.