Experimental Investigation On The Freeze-thaw Deterioration Mechanism Of Rock Slope Anchorage System

Constructions in cold area mostly have day and night freeze-thaw or seasonal freeze-thaw phenomena,compared to those in normal temperature areas.Long-term freeze-thaw deterioration could cause the slope anchor system fail,even cause slope instability.As China’s new national projects shift to the southwest area,constructions in cold regions will face more severe problems of freeze-thaw degradation.In response to this scientific problem,this paper takes the Rumei Hydropower Station at lancing River as engineering background to deal with rock slope stability from three aspects:the freeze-thaw degradation mechanism of the anchoring system,the freeze-thaw durability of the anchoring system and the freeze-thaw durability of the prestressed bolt.The freeze-thaw stability of the anchorage system has been experimentally investigated,and the main works and conclusions are as follows:(1)The construction area of the Rumei Hydropower Station at Lancang River is affected by seasonal freeze-thaw and day-night freeze-thaw.The rocks in the left and right banks develop massive fissures.Based on the engineering geological conditions and meteorological data of the study area,this paper selects on-site rock samples as test materials and determines the test parameters of the freeze-thaw cycle.On this basis,four bolts commonly used in engineering are selected.Freeze-thawing degradation test of anchor,anchoring system freeze-thaw durability test and prestressed anchor freezethaw durability test are carried out.(2)Results of the freeze-thaw degradation mechanism test of the rock-anchoring system show that water is the key factor in freeze-thaw degradation of anchor systems.The three key factors of freeze-thaw degradation of rock-anchorage systems are the ice splitting,the uneven thermal expansion and the degradation of anchor resin protective layer.The ice in the rock mass and the grouting body fissures undergoes repeated freeze-thaw.Due to the different thermal expansibility of various materials in the anchorage system,the double interface of bolt-grouting and grouting-rock will debond under the long-term freeze-thaw conditions,which would affect the anchoring stability.In addition,the anchor will deteriorate in long-term freeze-thawcycle,especially when FRP(Fiber Reinforced Plastic)material is used as anchor,the resin protective layer in the FRP material will also be degraded by freezing and thawing,which will affect the long-term performance of the bolt.(3)The results of the freeze-thaw durability test of the anchoring system show that the pull-out failure modes of bolts can be roughly divided into four modes: bolt pulloff,cone failure,slurry debonding and pull-out,and bolt debonding and pull-out.When ordinary threaded steel bars are used as anchor rod materials,the anchoring force is higher and the anchor rod has better freeze-thaw durability than FRP anchor.The pullout failure modes are mainly slurry debonding and cone failure.When FRP bars are used as anchor rod materials,the bonding strength between the anchor rod and the grouting mass is low.Furthermore,the pull-out failure modes are mainly anchor rod breakage and anchor rod debonding unplug because the FRP material itself is deteriorated significantlt by freezeing and thawing.(4)The results of freeze-thaw durability test of rock-prestressed bolts show that the pre-stress of bolts will be slowly reduced by freeze-thaw action and higher prestress can alleviate more pre-stress freeze-thaw loss.It can also strengthen the rock mass and delay the rock mass degradation.Due to the difference in thermal expansion of the materials in the rock-anchoring system,rapid changes in temperature can significantly affect the prestress of the rock-anchoring system.The maximum loss of prestress can reach more than 10%,which is an important unstable factor in slope anchoring construction.The results of pull-out experiments on prestressed bolts show that the greater the prestress of the bolt,the smaller the loss of the anchoring force due to freeze-thaw degradation.Also,high prestress can enhance the long-term anchoring performance of the bolt.This paper includes 77 pictures,15 tables and 80 references.