Experimental Investigation On Uplift Behaviors Of Shallow Foundation In Clay

Shallow foundations whose embedment depth is less than the width or diameter have been widely used in the offshore oil and gas industry.For example,mudmats and skirted foundations are usually used to support gravity-based structures or subsea infrastructure,and skirted foundations are even used to anchor floating facilities as their excellent uplift resistance.Depending on their application,shallow foundations may be subjected to uplift loadings due buoyancy,overturning moments or recovery operations.When shallow foundation,located on the soft clayey seabed,is subjected to uplift loading,negative excess pore pressure may be generated in the soil at the base of the foundation.The suction is beneficial to improving the uplift bearing capacity of foundation and increase the stability of foundation.However,the suction may increase the difficulty when recovering the mudmat.Therefore,it is necessary to investigate the generation,dissipation and distribution of invert suction to give full play to the advantage of invert suction to increase the uplift resistance of foundation and reduce the adverse effect of invert suction on recovery process.In the first part of the thesis,the generation,development,dissipation and distribution of invert suction and the uplift capacity of the mudmat under the monotone uplift loading were studied.A series of model tests were carried out to simulate the uplift process of mudmat.The factors influencing the uplift capacity and invert suction were investigated,which include uplift velocity,soil strength,soil heterogeneity and degree of consolidation.The tests results show that the uplift capacity of the mudmat is provided by the invert suction and the uplift capacity of mudmat and invert suction increase with increasing uplift velocity,which is related to the drainage conditions in the process of uplift.With the increase of the soil heterogeneity and degree of consolidation,the uplift capacity of mudmat increases linearly.In addition,it is found the distribution of invert suction is related to the distance from the center of the mudmat invert.The closer to the center of the mudmat invert,the greater the invert suction is and later the invert suction disappears during uplift.Therefore,the invert suction can be reduced more effectively by perforating in and around the center of mudmat.In the second part of the thesis,the uplift capacity of skirted foundation under sustained loading or cyclic loading were studied.A series model tests were carried out to study the long-term stability of skirted foundation under sustained loading or cyclic loading.The effects of the length of skirt,loading types and loading magnitude were investigated.The tests results show that the invert suction of skirted foundation accounts for more than 80% of the uplift bearing capacity.When the magnitude of the loading is relatively small,the foundation can remain stable under sustained loading or cyclic loading.On the contrary,the foundation may move upward at a certain rate.Under sustained or cyclic uplift loading,the instability loading and failure loading of foundation with different embedment ratios are different.The increase of skirt length can significantly improve the long-term uplift stability of foundation.In addition,by monitoring of soil surface cracks around the foundation,it is found that the failure of foundation under sustained or cyclic uplift loading is related to the development of circumferential cracks around the foundation.