| In the past 20 years,infrastructure construction in permafrost regions of our country is in the ascendant,and many theoretical and technical problems have been encountered in foundation engineering of buildings and structures.During the pile foundation construction of permafrost foundation,the excavation of holes,concrete pouring,hydration and heat release of cement concrete will bring heat into the frozen soil layer,generate thermal disturbance,destroy the frozen state of the soil layer,form frozen soil melting circle,reduce the strength and greatly reduce the bearing capacity of the pile foundation.With the passage of time,under the influence of the initial ground temperature and atmospheric environment,the pile and soil around pile gradually refreeze,the soil strength increases,and the bearing capacity of the pile foundation gradually increases.The temperature sensitivity and unique engineering properties of permafrost bring many difficulties to the design and construction of pile foundation engineering.In order to ensure the normal use of pile foundation for various buildings,the study on the pile-soil interface in permafrost foundation and the related long-term bearing capacity and deformation of pile foundation are the key to affect the safety and long-term stability of pile foundation.In this paper,the permafrost pile foundation engineering in Qinghai-Tibet Plateau is taken as the research object.Considering the thermal disturbance effect of soil around the pile during pile foundation construction,thermal disturbance effect on frozen soil around piles after rotary drilling and concrete pouring,and re-freezing process of pile-soil system are studied.Based on the analysis of disturbance effect of soil around pile,the formation mechanism of pile frozen soil-ice film interface in permafrost was explored through model tests of water migration and ice film formation.Through a series of low-temperature shear tests and creep tests,the mechanical properties of the interface between pile and soil in permafrost are studied.Considering the characteristics of interface between pile and soil,static load model tests of piles with different thickness of ice film and different pile forming methods are carried out to calculate the bearing capacity of permafrost pile foundation and study the bearing mechanism and load transfer characteristics of permafrost pile foundation.Taking the sinking disease of pile foundation in a site on permafrost foundation of Qinghai-Tibet railway as the engineering background,this paper proposes auxiliary pile reinforcement measures,considers the influence of temperature rise on the bearing capacity of pile foundation,and studies the variation law of pile foundation bearing capacity under different temperature rise conditions.The main research contents and innovations are as follows:(1)According to the rotary drilling and drilling concrete pouring pile for high temperature frozen soil and low temperature frozen soil,the test pile foundation is selected,thermocouples are embedded,and the ground temperature observation is started immediately after the process that pile foundation concrete is poured to obtain the ground temperature test site data.Considering different ground temperature and hydration heat conditions,through model tests,the variation law of temperature of soil around pile at different depths with time is analyzed.Through numerical calculation of thermal disturbance,the variation curves of ground temperature with time at pile center,pile wall and different distances from pile wall are obtained after concrete pouring.Based on the results of field tests,model tests and numerical analysis,the thermal disturbance effect of rotary drilling and concrete pouring on frozen soil around pile is studied,and the change of ground temperature and the thawing process of pile-soil system in permafrost foundation under different hydration heat conditions are analyzed.The result shows that,for high-temperature permafrost area,test site because of the influence of the initial ground temperature of permafrost and temperature of casting concrete,at the pile bottom section and the temperature gradient of upper limit of natural permafrost were 0.122 ? per day,0.12 ? per day,the rate of re-freezing in high-temperature permafrost area is slower,for cold permafrost foundation,50 days later,at various points along the pile ground temperature drop to minus temperature,the ground temperature gradient in pile bottom section and the upper limit of natural permafrost were 0.33 ? per day and 0.28 ? per day.(2)From the point of view of water migration,the water migration test in the freezing process of silty clay is carried out.The effects of initial water content,dry density,temperature gradient and freezing time on the temperature change and water migration rule in the permafrost silty clay under the condition of unidirectional freezing are analyzed,and the formation mechanism of ice film on the pile surface is studied.Taking the pile-soil interface as an object,low temperature shear tests are carried out under different ice film thickness,pile material,temperature and normal pressure conditions to obtain stress-strain curves,analyze the change rule of interface shear strength parameters with temperature,and study the influence of ice film thickness and pile material on the shear characteristics of pile-soil interface.Taking the silt-concrete interface as an object,creep shear tests under different temperatures and normal stress conditions are carried out under graded loading.The change curve of shear displacement of pile-soil interface with time is obtained,the creep deformation curve is analyzed,the failure mode of interface creep deformation is determined,and the creep characteristics of contact interface are studied.The results show that the creep failure mode of the frozen silt-concrete interface is characterized by brittleness,and the shear creep curve can be divided into two types: attenuated creep and non-attenuated creep,and the non-attenuated creep can be divided into four stages: instantaneous creep stage,unstable creep stage,stable creep stage and accelerated creep stage.The creep behavior of frozen silt-concrete interface is affected by shear stress level,test temperature and normal stress.The shear stress affects the creep deformation rate and creep time of the interface exponentially,the long-term strength and normal stress of the interface comply with the molar coulomb criterion.(3)For bored piles in high-temperature permafrost and low-temperature permafrost regions in the hinterland of Qinghai-Tibet Plateau,a steel strain gauge is embedded in the foundation piles.After 60 days of foundation pile perfusion,the static loading test of pile foundation is carried out by using the fast load method in permafrost regions to analyze the ultimate bearing capacity of foundation piles.According to the readings of the steel bar meter,the distribution curves of axial force of the pile body and freezing force on the pile side are obtained,and the distribution law and the exertion mechanism of freezing force on the pile side and the exertion characteristics of pile end resistance are analyzed.The bearing capacity of single pile is calculated,and the variation law of resistance characteristic value at pile end,basic bearing capacity of permafrost foundation and freezing strength characteristic value with temperature is analyzed.The calculation results of standard method are verified with the measured bearing capacity of single pile,and the bearing mechanism and load transfer characteristics of permafrost pile foundation are studied.The research results show that: when the test of bored pile in the high-temperature permafrost area is loaded to 4800 k N,the average freezing force is 64.8kpa,and the pile tip resistance is 735.7kpa,producing a large plastic settlement.When bored piles in low-temperature permafrost areas are loaded to 7600 k N,the displacement of foundation pile top is 4.93 mm,the residual settlement after unloading is 1.01 mm,and the rebound rate is 79.51%,mainly due to elastic deformation.(4)Considering the characteristics of pile-soil-ice film interface,a model test of pile bearing behavior under different ice film thicknesses is carried out in an indoor filling model,and distribution curves of pile axial force and pile side freezing force along the pile body under different ice film thicknesses are obtained.Four pile-forming methods of bored pile,pre-drilled pile,pre-drilled inserted pile and pre-drilled driven pile are selected respectively to carry out model tests on bearing behavior of foundation piles in permafrost foundation under different pile-forming methods.Distribution curves of pile top displacement,pile axial force,pile side freezing force and pile end resistance under different pile-forming methods are obtained to study the influence of pile-forming methods on bearing behavior of foundation piles.The results show that the pile tip resistance of pre-bored pile is more effective than that of bored pile,which reflects the compacting effect of driven pile and difference in load transfer characteristics of bored pile caused by two different pile formation modes of bored pile.The long-term bearing capacity of bored pile is similar to that of pre-bored pile.(5)According to the decline of pile bearing capacity in permafrost regions,the prediction formula of pile bearing capacity decline in permafrost regions under climate warming is obtained.According to the reinforcement measures of auxiliary piles,by means of finite element method,the ground temperature distribution laws of auxiliary cast-in-place piles and inserted piles at different times are obtained,the bearing capacity of pile foundation is calculated,and the variation laws of pile foundation bearing capacity under different temperature rise conditions are analyzed.The research results show that: as bored perfusion auxiliary pile brings a lot of hydration heat to the pile soil system,making the temperature of soil around the original pile grow which causes the land bearing capacity is reduced,at the beginning of filling auxiliary pile,there is an obvious strength degradation of the pile foundation,the bearing capacity cannot back to before until 2 months after the auxiliary pile construction.The overall bearing capacity improved gradually with the auxiliary pile back to frozen,it reached a stable state 6 months after the auxiliary pile construction. |
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