Study On The Damage Characteristics And Health Assessment Method Of Anchor Cable Under Seismic Effect

There are complicated geological structures and varieties of geological disastersin Longmen Mountain area. The prestressed anchor cable is an active preventionmeasure. They has many merits such as light self-weight, easy to be constructed, goodseismic performance, work together with the surrounding rock. So it is one of themost economical and efficient measures in prevention projects. But a large amount ofprevented engineering was badly damaged in Wenchuan earthquake. Although theprestressed anchor cable has better seismic performance than other preventedengineering, but it still damaged in different extent. Therefore, it is vitally importantthat we comprehensive evaluate the health conditions of prestressed anchor cable, andit is also useful to keep the engineering structure safety.This paper based on National Science-technology Support Plan Projects“preventing the geological disasters such as significant landslide and debris flow andecological restoration in the seismic disturbance area”. The damage characteristics ofprestressed anchor cable and its health diagnostic index system can be studied throughdata collection, site survey, physical simulation and nondestructive testing methods.And the health assessment model of prestressed anchor cable was set up using thefuzzy comprehensive evaluation. From them, a few conclusions can be made.(1)According to the investigation, the failure mode of prestressed anchor cableunder earthquake can be divided into four types:①the failure of anchoring section;②the failure of anchor cable;③the failure of anchor head;④the failure of rockmass. Comparing the failure mode of prestressed anchor cable under naturalconditions and earthquake conditions, we found three differences.①Under theearthquake conditions, the axial force of prestressed anchor cable was increasesharply because of the powerful impact within a short time, and tensile failure couldhappen more easily. But under the natural conditions, because of the slopedeformation the tensile failure of prestressed anchor cable were less likely.②Underthe earthquake conditions, the response of the anchor components to seismic wavesare slightly different. So the failure of anchor head was due to their relativedisplacement. But under the natural conditions, the components of anchor head could harmonize the stress. So the failure was mostly caused by the insufficient strength ofconcrete cover or material.③Under the earthquake conditions, the slope rock mayoccurred cracks and even whole sliding. There were probably no structural damagesin anchor cable. But under the natural conditions, the slope rock occurred wholesliding mostly because that the anchor cable engineering could not continue to keepthe slope safety.(2)According to the Low-Cyclic Loading Test, we studied the deformation andfailure laws of prestressed anchor cable under cyclic loading. The result shows thatthe failure modes of prestressed anchor cable are mostly the failure of anchor cableand anchor head. Under the cyclic loading, the force concentrated on the anchor cableand anchor head, and their damages produced a ton of energy. When the anchoringsection grouting had enough strength and density, and no quality defects, the responseof grouting to seismic waves were slow, so they could resistant to the cyclic loading.They were less likely to appear the phenomenon of cohesive force failure and bondforce failure. Ultimately, the deformation and failure of prestressed anchor cableunder cyclic loading mainly included four stages.①Safety Stage: The structuralsurface mainly provided sliding resistance in this stage. The change of prestressedanchor cable axial force was small. The displacement changes of whole slope were inless than1millimeter, and the displacement was increasing with the increase ofhorizontal seismic load. The anchoring section had obvious stress concentration, andthe free-segment deformation of anchor cable was correspondingly small.②StableStage: The prestressed anchor cable began to anchor the slope in this stage, and theycould resist this degree of horizontal cyclic load. The force concentrated on the anchorhead. The axial force change rates of prestressed anchor cable gradually improve. Theprestress and its compression will gradually lose effectiveness.③Linear DeformationStage: The prestress of anchor cable totally lose effectiveness in this stage. The slidingdisplacement and the change of axial force almost increased linearly. The slip massbegan to become unstable. The slope was more dangerous at this moment, if therewere not effective measure in time, it could happen partially or wholly damage.④Failure Stage: The sliding distance was more than143millimeters. All the anchorcables were failure. The test found that the upper anchor cable were more likely todamage than the lower anchor cable. And the failure limit value of anchor cable wasfar below their ultimate tensile strength under earthquake conditions.(3)In this paper, the anchor cable’s detection indicators of damage characteristicsand health assessments were picked up by the evaluation way of macro-micro-macro. At first, from the macroscopic evaluation, to determine the earthquake damage rangewas the primacy index (sensitive factor). And then, we evaluated the health conditionof a single anchor cable. With the help of corresponding nondestructive testingmethod, we could get the single anchor cable’s health diagnosis by detecting theinternal characteristics. In the end, we could analysis all the information of anchorcables’ damage level and distribution, and determined the health condition of wholeanchor cable engineering.