Study On The Pore-water Pressure Change And Deformation Characteristics Of Frozen Soil Under Dynamic Load

Frozen soil foundation is prone to create large creep deformation and even structural damage under long-term dynamic load.Therefore,engineering structures built on frozen ground are observed to have some diseases including uneven settlements,cracks,etc.,which not only threaten the long-term stable operation of engineering buildings,but also cause huge economic losses and resources waste for later engineering maintenance.Warm frozen soil is one of the main reasons for the settlement of the foundation.Warm frozen soil refers to frozen soil with a temperature higher than-1.5°C.Because of relatively high negative temperature,there is a large amount of unfrozen water in warm frozen soil.Under the long-term action of dynamic load,the unfrozen pore water shows a significant increase and dissipation in pressure,causing a change in the stress field of frozen soil.Meanwhile,the processes like mutual compression and dislocation of soil particles,pore ice thawing and re-freezing,and development of micro-cracks occur.These changes will cause damage in the frozen soil and affect the macroscopic deformation characteristics.In addition,part of the mechanical energy imposed by dynamic load will converted into heat,leading to an increase in soil temperature and unfrozen water content,which eventually causes the deterioration of soil mechanical properties.Therefore,studying the pore-water pressure variation,soil deformation and temperature change of the warm frozen soil under dynamic load is of great significance for the stability of the frozen soil foundation.This paper carried out temperature variation tests,triaxial creep tests,and confined compression tests on warm frozen soil subjected to dynamic load.The temperature fluctuations,pore-water pressure variations and deformation behaviors of the frozen soil samples were observed through these tests.The influences of multiple factors,such as initial dry density,test temperature,dynamic stress amplitude and vibration frequency,were evaluated.The samples after triaxial creep tests were scanned by CT to observe the evolution process of the meso-structure of frozen soil.The main conclusions are as follows:(1)Under the action of dynamic load,the internal temperature of frozen soil increases.The greater the amplitude of dynamic stress,the higher the warming rate.The temperature rise amplitude of frozen soil is affected by whether the sample was damaged and the time length to damage.The sample with a continuous increase in temperature will eventually be damaged and deformed.The non-damaged samples show a temperature variation increasing to a specific value and then keeping stable or dropping.This is a result of the heat exchange with the ambient temperature.In the frequency range of 1-7Hz,the temperature growth rate relates positively with the vibration frequency.(2)It is found that the pore-water pressure changes with the development of sample strain,according to pore pressure curves and cumulative strain curves.In the initial stage of dynamic loading,the cumulative deformation as well as the pore-water pressure increases rapidly.When the accumulated strain reaches a certain value,it causes structural defects such as cracks in the soil,and the pore pressure dissipates subsequently.If the accumulated strain is small and there are no micro-defects in the soil,the pore pressure does not dissipate significantly.Under the same dynamic load amplitude,the increase in vibration frequency gives an increase in strain rate and pore-water pressure during the frequency range of 1-7Hz.When the frozen soil temperature is higher and the dry density is lower,the pore-water pressure has a quicker response and the cumulative strain rate is faster.(3)The lateral deformation of confined frozen soil is limited,and the soil structure under dynamic load is dominated by stre ngthening effect.The soil strain rate tends to be progressive flattening,the pore pressure maintains a stable or fluctuating growth trend for a long time,with no obvious dissipation phenomenon.As the test temperature rises,the ability to resist compre ssion deformation of frozen soil is weakened,accompanied by a steady growing strain and larger pore-water pressure.At-1℃,when the dry density is lower,the soil has a higher amount of unfrozen water content and a looser soil skeleton.Under the action of dynamic load,the pore-water pressure increases rapidly to a higher peak value,while the cumulative deformation develops fast to achieve bigger value.When the vibration frequency is constant,the larger amplitude of dynamic stress results in the inte nser compaction of the soil structure,the faster strain rate,the quicker growth of pore-water pressure and the higher peak pore pressure.(4)Micro-cracks generate and close in frozen soil at the early stage of the dynamic loading,causing an effect combination of strengthening and weakening inside the frozen soil.Afterwards,the sample damage is dominated by plastic slip,showing structural weakening.The micro-cracks in the lower part of the sample gradually widen and lengthen to form fissures,which eventually lead to excessive accumulated deformation and soil failure.