Development Of Hollow Cylinder Apparatus With Four-direction Dynamic Loads And Test On Sand Under Rotation Of Principal Stress Axes

The foundations of offshore engineering are faced with the dange of destrection caused by the wave load,so it is necessary to study the mechanical properties of soils under wave loads.In this paper,the existing torsional shear apparatus was modified to have the four-direction dynamic loads control capability including axial force,torque,internal and external pressure.Then the basic formulas for the monotonic shear test and the pure principal stress axes rotation test were deduced respectively.Moreover,the monotonic shear tests were carried out to discuss the influence of the main major stress angel on the stress-strain relationship,the pore water pressure accumulation and the non-coaxiality of the soil.Finally,the pure principal stress axes rotation tests under undrained and drained condition were carried out,respectively,to study the non-coaxial characteristics of the soil.The conclusions can be obtained as follows:(1)Only the torque or axial force can be dynamically controlled in the existing torsional shear apparatus,the hardware and software framework was designed to control the axial force,torque and internal and external pressure independently.Gaseous control method was used in the added air pressure automatic feedback control system,which has the advantages of easier to control stresses.Then,a series of tests were carried out to verify the loading capacity of apparatus under complex stress path,and the proposed hardware and software framework can provide the foundation for further function of instrument.(2)The monotonic shear tests on sand were conducted under undrained condition.It is considered that the shear direction can affect the dilatancy of sand and peak and inflection point of pore water pressure accumulation.The natural anisotropy exists in sand,which leads to the influence of shear direction on the strength of sand.The non-coaxial behavior can be observed in the monotonic shear tests.Specifically,the direction of strain increment is closed to the direction of the major principal stress angel of 45° when the principal stress angel is 30°.In contrast,the non-coaxiality weakens as the deviatoric stress increases,and direction of strain increment and stress increment tends to be same.(3)A series of the pure principal stress axes rotation tests under undrained condition were carried out.The results show that the deviatoric stress,relative density and starting point of rotation affect the mechanical properties of sand.Through the analysis of pore water pressure accumulation in the first cycle,more pore water pressure increases in the second and third quadrants,compared to the first and fourth quadrants.And these findings are understandable because of the natural anisotropy in sand.During the rotation of the tress axes,non-coaxiality between the strain increment and the principal stress axes can be significantly observed,and the strain increment is larger when the major principal stress angel in the range of 45°? 90°.(4)One group pure principal stress axes rotation test under drained condition was carried out.The test results show that hysteresis loop between stress and strain appears when the principal stress axes rotates,moreover the area of hysteresis loop will be reduced as the cycle number increases.Above all,non-coaxiality also can be significantly observed in the drained test,and the direction of strain increment tends to the stress increment direction when the cycle number increases.