Tests on affecting factors for pretension loss of cables with small borehole

Abstract：In view of the features of the pretension cables anchored by resin capsules with small borehole in coal mines,and the research situation and existing questions in the filed of cable pretension loss,the mechanism and laws of pretension loss for cables with small borehole were studied by means of theoretical analysis,laboratory and underground tests.The pretension loss caused by anchor system and deformation of surrounding rock was emphatically analyzed.The pretension loss caused by anchor system includes friction loss and unloading loss;there is an optimal limit distance for one anchor system,11~13 mm is suitable for the cables with diameter of 22 mm.The cable length,tension oil pressure,tension machines,barrel and wedge anchors and their matching with tension machines have different influences on the pretension loss ratio：in general,the shorter the cables,the larger the pretension loss ratio,the cable tensioned by the jacks with smaller limit distance have the largest pretension unloading loss ratio under the pressure of 20~30 MPa,and the unloading pretension loss ratio of the cables tensioned by the jacks with appropriate limit distance is getting smaller with the higher tensioned oil pressure.Finally,several technical measures ensuring cable pretension were put forward,including tests before construction,determining the optimal limit distance,and modifying the jack;taking certain ultra tension into account in cable bolting design;applying the plates,combined beams and steel meshes with larger area,strength and stiffness in soft and broken surrounding rock.