Study On Thaw Settlement And Mechanical Stability Of Foundation Soils Along A Slope-skirting Crude Oil Pineline Buried In Ice-rich Permafrost

Frozen soil is a four-phase mixture of soil aggregates,ice,unfrozen water and vapor.The contents of pore ice and unfrozen water strongly affect the hydrophysical and mechanical properties of frozen soil.The higher total water content of ice-rich frozen soil may result in large thaw-settlement deformation upon thawing,adversely impacting cold regions engineered infrastructures.For the design,construction and operation of oil pipeline projects,two types of problems are often encountered: thaw settlement and subsidence problems of the pipeline foundation soils and the mechanical stability of the slope-skirting crude oil pipeline buried in ice-rich permafrost,which pose great challenges for the design,construction and operation of pipeline projects.On the basis of the China-Russia Crude Oil Pipeline Project(CRCOP,Mo'he-Daqing Segment)and taking advantage of numerical models,in this paper we studied these two types of engineering problems and analyzed the processes of and mechanisms for ground thawing and slope destabilization in order to provide references and theoretical guidance for future similar pipeline engineering in mountainous permafrost regions.We carried out the following research and obtained relevant results as follows:Firstly,after collating and reviewing the relevant literature,we identified the inadequacies of the existing research.Based on these studies,a multi-physics coupling model was established and the strength reduction method was selected for studying the mechanical stability of the foundation soils of buried crude oil pipeline skirting a permafrost slope;then,we did the parameterization and provided the corresponding determination conditions after taking into account of actual situation of the two types of pipeline problems related to the CRCOP.In the finite-element software environment of COMSOL Multiphysics,we built the geometry domain and governing equations for the model and followed steps for conceptual and computational analyses.Secondly,we calculated the two-dimensional problems of thawing settlement of pipeline foundation soils and found out that there was a certain lag in changes of ground temperatures after the air,ground-surface and pipeline oil-flow temperatures.The thawing under the pipe bottom would deepen progressively with elapsing time.The thawing would be large during the first five years of pipeline operation.The thaw would increase by 0.92 m in the first year of pipeline operation,and would increase by 1.7 m during the first five years.The increase during the next 25 years(6th to 30 th years)is only equivalent to that occurred in the first year of pipeline operation due the slowed thawing and settling of foundation soils.In the parameter analysis,it was found that,in contrast,changes in pipeline oil-flow temperature had the greatest impact on the thawing settlement of pipeline foundation soils.If the pipeline temperature lowered by 2?,at the 30 th year of pipeline operation,the maximum thaw depth would be reduced by about 2 m and the amount of thaw settlement would be reduced by about 8 cm.Third,we calculated and evaluated the mechanical stability of the pipeline-skirted slope and found that the depth of the soil affected by the buried pipeline was mainly in the range of 0.5 ? 4.0 m.During the freezing period of the first year in pipeline operation,the safety factor Fr would increase from 1.86 in December to 2.95 in May,and would decrease from the value of 2.95 in May to 1.5 in October during the ground-thawing period,showing a cyclic change.When the slope became unstable,the plastic zone first would appear near the pipeline and would then extend to both sides of the pipe,forming a sliding surface.The thawing interface would be weakened;the position of the sliding surface would be in line with the thaw bulb formed by the warmer pipeline.Finally,we studied the influences of geo-environmental parameters on thaw settlement of pipeline foundation soils and the stability of the slope with buried pipeline.It is found that the thaw depth and thaw settlement deformation would increase with rising ground and oil-flow temperatures and declining water content,while the slope stability would decrease with higher ground temperature,richer water content and the increasing number of freeze-thaw cycles.