Experimental Study On The Relationship Between Shear Characteristics And Saturation Of Rough Rock Discontinuities

Natural rock mass in the stratum is often composed of a number of rock blocks and discontinuities such as joints,fractures and faults.In recent years,with the increasing demand for the construction of water conservancy,transportation and the exploitation of various energy and resources,as well as the necessity for the geological deep buried storage of carbon dioxide and radioactive waste,increasingly complex rock stratum occurrence environments such as stress conditions,hydrological and permeability characteristics,and temperature field features will be involved in rock mass engineering.In view of the complexity of the occurrence environment of rock mass discontinuity and its main control effect on the stability,mechanical strength and hydraulic characteristics of most rock masses,in-depth research and discussion on the mechanical behavior characteristics of rock mass discontinuity under multi-field coupling is not only the practical demand of rock mass engineering practice,but also very important to evaluate the stability of rock mass engineering and serve the national economic and strategic construction.To explore the water rock coupling effect,especially the weakening effect of discontinuity saturation(S_r)on the mechanical properties of rock mass,and reveal the change law of shear behavior of rock mass discontinuity under unsaturated state.This paper uses the combination of three-dimensional laser scanning and three-dimensional printing technology,pouring cement mortar which mixed with high-standard cement,standard sand,water and fly ash at a certain mass ratio into the composite mold,to form multiple groups of rough rock discontinuities with the same surface morphology(i.e.,joint roughness coefficient JRC).The controlled discontinuity saturation measurement and method are studied by using the sample surface point cloud data,then the direct shear test of rock discontinuity is carried out under 3 roughness and 5saturations conditions.The main research results and conclusions are as follows:(1)The peak shear strength and residual shear strength of rock discontinuity decrease with increasing saturation.Under three roughness conditions,the average peak shear strength decreases by about 28.4%(S_r=100%)and the average residual shear strength decreases by about 11.4%(S_r=70%).In addition,the greater the surface roughness of the rock discontinuity,the more significant the weakening effect of the shear strength due to the increase of discontinuity saturation:when fully saturated,the peak shear strength of the rock discontinuity under all levels of roughness decreases by 16.1%(JRC=7.3),28.5%(JRC=10.1)and 35.7%(JRC=12.9),while the residual shear strength decreases by 7.6%(JRC=7.3),10.7%(JRC=10.1)and 11.3%(JRC=12.9),respectively.(2)The peak shear displacement of rock discontinuity is significantly reduced due to the existence of water.The average peak shear displacement under the three roughness is reduced by about 49.4%(S_r=100%),and the existence of only a small amount of water(S_r=10%)could reduce the peak shear displacement dramatically by about 45.5%.Although the peak shear displacement is not sensitive to the changing of discontinuity saturation,the results still remind that attention should be paid to the significant influence of water on the shear behavior of rock mass in engineering.(3)In this paper,the mechanical and lubricating effects of water on the discontinuity sample are the main factors that causing the weakening of the shear behavior.With the increase of discontinuity saturation,both the increase of fluid pressure in the cavity and the gradual decrease of capillary pressure could lead to the decrease of effective normal stress.In addition,it is estimated that the rougher surface of the discontinuity,the larger the actual surface area,and the larger the water-rock interaction area that can be provided,so the weakening degree of water on the rock discontinuity is enhanced.(4)According to the experimental results,the relationship between saturation and basic friction angle is established:as the saturation increases to 100%,the reduction ranges of the sample’s basic friction angle under all levels of roughness are8.60%(JRC=7.3),11.65%(JRC=10.1)and 16.90%(JRC=12.9),respectively,indicating that the influence of discontinuity saturation on the basic friction angle also increases with the increase of roughness.Then,an unsaturated shear strength criterion of rock discontinuity is proposed by introducing the established relationship between saturation and basic friction angle,the improved JRC-JCS shear strength formula may enable the prediction of the shear strength for unsaturated rock discontinuity.