| With the continuous development of our country’s economy and the continuous improvement of people’s living standards,the scale and quantity of geotechnical engineering construction show a growing trend.Non-penetrating fractured rock mass is a kind of complex engineering medium often encountered in engineering,and the existence of fractures plays a decisive role in the failure and mechanical properties of rock mass.The mechanical properties of the rock mass are affected by the degree of fissures.When calculating the degree of bidirectional fissures in rock mass engineering,usually only the area through which the fracture passes in a certain section of rock mass is counted,and the influence of the position and degree of fracture on the mechanical properties of rock mass is ignored.In this paper,cement,quartz sand,gypsum,barite powder and other materials are used to obtain a rock-like material ratio that simulates the mechanical properties of brittle red sandstone.Rock-like specimens with four kinds of crack openings(1mm,2mm,3mm and 4mm),the shear strength of the non-penetrating fracture surfaces of the rock specimens was tested by a direct shear tester,and the high-definition camera technology was used to monitor the real-time monitoring.During the test,the damage of the rock mass crack surface was studied,and the influence of different crack positions and opening degrees on the shear strength and failure mode of the rock mass specimen was studied.Through experimental research,the following conclusions are mainly obtained:(1)When the opening is constant,the shear strength of the rock mass with the crack located in the middle of the sample is about 65%～75% of that of the rock-like sample,which is obviously smaller than that of the rock mass where the crack is close to the shearing edge(about 73%～82% of rock-like samples)and the rock mass where the crack is far away from the shearing edge(about 73%～80% of rock-like samples);For each increase of 1MPa in normal stress(from 5MPa to 9MPa),the shear strength of specimens with openings of 1mm and 2mm increases by about 8% to 13%,and the shear strength of specimens with openings of 3mm and 4mm increases by about 5% to 10%.When the crack position is fixed,the shear strength of the rock mass with openings of 1mm and 2mm is generally greater than that of the rock mass with openings of 3mm and 4mm;when the normal stress is 5MPa and 6MPa,The rock mass with 2mm crack openings close to the shear edge has the greatest shear strength,which is about 4%～12% higher than that of the rock mass with the opening of 3mm and 4mm at the same position;when the normal stress is 8MPa and 9MPa,the shear strength of rock mass with cracks located on both sides of the sample with openings of 1mm and 2mm is the largest,which is about 3%-8% higher than that of rock mass with openings of 3mm and4 mm at the same position.(2)When the crack opening is 4mm,the cohesion c of the rock mass presents a "V" shaped distribution with the different positions of the cracks.The c value of the rock mass with cracks located near the shearing edge is about 9.29 MPa,the c value of the rock mass with cracks located far from the shearing edge is about 8.88 MPa,and the c value of the rock mass in the middle position is about 40%～50% lower than that on both sides;when the crack opening is 2mm,the c value of the rock mass with cracks on both sides of the sample is basically the same,about 6.5MPa,and the c value of the rock mass with cracks in the middle is slightly smaller,about 6.2MPa.When the two-way fissure degree is the same,the cohesion c of the rock mass is greatly affected by the opening and position of the fissure.Especially when the opening is 3 mm and 4 mm,the c value of the rock mass where the fracture is located in the middle of the sample is significantly smaller than that on the two sides.(3)When the crack opening is 4mm,the internal friction angle φ of the rock mass presents an inverted "V" shape distribution with the different crack positions.The φ value of the rock mass with cracks located near the shearing edge is about 41.66°,the φ value of the rock mass with cracks located far from the shearing edge is about 44.42°,and the φ value of the rock mass with cracks in the middle is about 10% ～20% higher than that on both sides;When the crack opening is 2mm,the internal friction angle φ of the rock mass presents an inverted "V" shape distribution with the different crack positions.The φ value of the rock mass with cracks located near the shearing edge is about 53.26°,the φ value of the rock mass with cracks located far from the shearing edge is about 54.46°,and the φ value of the rock mass with the cracks in the middle is about 5%～10% lower than that on both sides.When the two-way fissure degree is the same,the internal friction angle φ of the rock mass is greatly affected by the opening and position of the crack.When the opening is 4 mm,the φ value of the rock mass with the crack located in the middle of the sample is significantly larger than that on both sides;when the opening is 3 mm,the φ value of the rock mass with the crack located near the shearing edge is the largest;when the opening is 1 mm and 2 mm,the φ value of the rock mass with the crack located in the middle of the sample is significantly smaller than that on both sides.(4)For rock-like specimens with cracks located close to the shearing edge,no cracks will occur in the elastic stage when the specimen is sheared.After entering the plastic stage,cracks begin to occur.The initial cracks occur in the interior of the cracks and the contact part of the rock bridge.Then,cracks began to appear in the contact part between the sample and the baffle plate.With the further increase of the shear force,the cracks in the two parts gradually increased,and finally a through shear plane was formed;For rock-like specimens with cracks in the middle,no cracks will occur in the elastic stage when the specimen is sheared.After entering the plastic stage,cracks start to occur.The initial crack occurs in the rock bridge on the side close to the shearing edge,and then the crack occurs in the rock bridge on the side away from the shearing edge.With the increase of shear force,the cracks in the last two parts gradually expand and penetrate to form shear sliding surfaces;For rock-like specimens with cracks located far from the shearing edge,no cracks will occur in the elastic stage when the specimen is sheared.After entering the plastic stage,cracks began to occur.The initial cracks occurred on the side of the rock bridge close to the cutting edge,and then cracks began to occur at the end of the rock bridge away from the cutting edge.With the increase of shear force,the cracks in the last two parts gradually expanded and penetrated to form a shear sliding surface.(5)With the increase of the normal stress,the failure mode of the fractured rock mass changes from the combined tensile-shear failure to the shear failure.When the normal stress and opening are constant,and the fracture is located in the middle of the sample,the rock mass tends to undergo shear failure.When the cracks are located on both sides of the sample,the rock mass tends to have tensile-shear composite failure;When the normal stress and position are constant,the rock masses with 1mm and 2mm fissure openings tend to undergo shear failure,and the rock masses with 3mm and 4mm fissure openings tend to have combined tensile-shear failure. |
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