Research On The Instability Mechanism And Protective Measures Of Dolomite Slope Under Freeze-Thaw Cycle

The cold area of China is widely distributed,accounting for about three-fourths of the country’s land area.In recent years,with the construction and development of highways and railways and other infrastructure facilities,the problem of freezing damage to rock slopes in cold areas has gradually became prominent and attracted attention.Therefore,it is of great significance to research the instability mechanism and protective measures of rock slope under freeze-thaw cycle to guarantee the operation and construction of infrastructure in cold regions.Based on the high dolomite slope of a highway in the northern mountainous area of Hebei province,this paper designs and conducts 50 indoor rapid freeze-thaw cycle tests,acoustic wave test and uniaxial compression test of the dolomite rock samples under different cycles.Then combine experimental research,theoretical research and FLAC3D numerical simulation,This article were systematically studied the damage and degradation laws of various physical and mechanical properties of the complete dolomite rock block under the freeze-thaw cycle,discussed the degradation effects of the strength parameters of fractured rock masses under the freeze-thaw condition,and reveals the instability mechanism of rock slopes in seasonally frozen areas.Finally,according to the freeze-thaw damage characteristics of the slope,design and proposed the corresponding protective measures of rock slope,verified the safety of the main protection scheme by numerical simulation.The main conclusions of this article are as follows:（1）Freeze-thaw cycles have limited macro damage to low-porosity dolomite samples,most rock samples have good apparent integrity during freeze-thaw cycles,and only closed micro-cracks appear on the end faces of very few samples.The loss and increase of rock sample quality during freeze-thaw process are mainly caused by the supplement or absence of water in the internal pores of rock sample.The variation amplitude of rock sample quality is small and has no obvious regularity,which is not suitable for the indicator of rock freeze-thaw damage.During 0 to 50 cycles of freeze-thaw,the uniaxial compressive strength and elastic modulus of the rock both show a rapid decline in the early stage and a gentle decline in the middle and late stages.The number of micro-cracks and the intergranular space in the rock increase obviously due to freeze-thaw cycles,the compaction stage in the uniaxial stress-strain curve increases obviously with the increase of cycle times,and the rock brittleness decreases and ductility increases.（2）The global structure damage caused by freeze-thaw to the dolomite is obvious,and the longitudinal wave velocity decreases obviously during freeze-thaw cycles and has a good correlation with the number of cycles.As an economical and convenient non-destructive testing indicator,longitudinal wave velocity can well characterize the global structural damage caused by freeze-thaw in rocks.The crack water will interfere with the characterization of longitudinal wave velocity.Therefore,it is more accurate and reasonable to use the longitudinal wave velocity to characterize the structure damage under the dry state of the rock after freez-thaw cycles.（3）Based on the generalized Hoek-Brown rock mass strength criterion,used the longitudinal wave velocity as a reference quantity to characterize and quantify the rock mass geological strength index（GSIn）and the freeze-thaw disturbance factor（Dn）under freeze-thaw cycles.The estimation formulas of various strength indicators of fractured rock mass under freeze-thaw cycles were deduced,and the accuracy and validity of the evaluation formulas were preliminarily verified.The transformation of freeze-thaw rock sample parameters to freeze-thaw rock mass parameters were realized,which provides a basis for quantitative parameter selection for numerical simulation.The freeze-thaw degradation effects of various strength indicators of fractured rock masses were ranked in descending order:rock mass fragmentation（s）,rock mass uniaxial compressive strength(σ_cm),rock mass elastic modulus（E_m）,and rock mass hardness（m_b）.（4）In the freezing and thawing environment,the plastic zone mainly appears and expands in the freeze-thaw layer of the slope.The plastic failure is most serious at the position near slope toe,and the local rock mass collapse may occur at the position near the slope shoulder.As the freeze-thaw continues,eventually a through plastic zone will be formed in the freeze-thaw layer of the slope,which will cause the overall slope to collapse.The displacement and the displacement increment after several cycles at the slope toe are the largest,followed by the middle in the slope and slope shoulder.Under the same number of cycles,there is a obvious displacement difference between the inside and outside of the freeze-thaw layer at the same elevation position,and the displacement difference in slope toe is the largest,followed by the middle in the slope and slope shoulder.The stability of slope is significantly reduced by freeze-thaw action,after 50 cycles,the safety factor was reduced from 2.34 to 1.08,which did not meet the safety factor requirements of the highway cutting slope under normal working conditions.The critical sliding surface of the slope basically coincided with the freeze-thaw layer.（5）Aiming at the failure characteristics of slopes that have undergone 50freeze-thaw cycles,proposed a comprehensive protection scheme which is using spray-anchor grouting and freeze protection and structural drainage facilities.The numerical simulation results of the slope after grouting and anchoring indicate that the full-length bonded steel anchor pipe and grouting technology can effectively improve the stability of such rock slopes prone to shallow slip in the seasonal frozen region.For the protection design of rock slopes in seasonal frozen regions,it is advisable to focus on improving the integrity of the rock mass,and choose light support structures with less engineering disturbance.