Method of shear testing on cable bolt

Two test methods are available to test cable bolts in shearing: the single shear test and the double shear test. The only difference between these two methods is the number of joint faces in the shearing process. According to the conclusion of Li (2016), the factors, including joint friction, size of rock, pre-tension bolt de-bonding, boundary conditions, bolt installation angle ,contact condition and loading manner, would be different among different shear test method and should be considered in test apparatus design. The first trackable shearing test on encapsulated rock bolts was carried out by Dulacka (1972), as shown in Figure 2.31. Different factors were considered including installation angle, the strength of concrete and stirrup yield. The phenomenon of dowel action was examined in this test and a theoretical load-deformation relationship was proposed. However, only thin bolts can be tested due to small shear movement and limited size of concrete.

Figure 2.31 Single shear test apparatus (Dulacka 1972) e

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Since the study of Dulacka, the single shear test method started to motivate the interests of researcher and a number of test methods on rock bolts were developed by Bjurstrom(1974), Dight (1982), Ge and Liu (1988) and Spang and Egger (1990) in 1980s-1990s. Among these tests, different special designed compression machines were requested so the shear strength can be smoothly subjected to rock bolt. However, the most important common drawback of these test rigs is the limited size of encapsulation concrete or granite, resulting in just small rock bolt and cable bolts being tested. Except that, the pre-tension could not be added into cable bolts and the friction could also not be ignored in their test apparatus. It should be importantly noted that the embedment material would contact each other due to that embedment material with less constrain being rotated during test and stress concentration would occur in the joint, leading to larger shear load.

Standard cable bolts and modified cable bolts were tested by Bawden, et al., (1992), as shown in Figure 2.32. Different installation angles and modified cable bolts were treated as the main factors for research. The test sample was mounted into the compression machine and the form of test was like a pull-test, which actually applied combined axial and lateral loads on the cable bolt. Compared with other apparatus, the cable bolt was grouted in a small pipe (75 mm) and free constraint in one side. Hence, the apparatus could not reach the maximum capacity of cable bolts in most cases.

Figure 2.32 Single shear (Bawden, et al., 1992)

Direct shear test apparatus was studied in the tests carried out by Goris et al. (1996), as shown in Figure 2.33. Three factors, including friction in joint surfaces, grout condition and diameter of cable bolts was studied in this tests. The result showed that the shear resistance in rough joint surfaces is double the value of the test result in smooth surface

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and the shear resistance increases much more rapidly at shorter displacement in the block with grouted cable bolts. In this test, the pre-tension could not be applied on cable bolts and the friction could not be eliminated even within smooth surfaces.

Figure 2.33 Direct shear test apparatus (Goris, et al., 1996)

The granite and concrete was utilized in the test machine developed by Ferrero (1995), as shown in Figure 2.34. There is no contact between shear box and bolts during shearing by steel box confinement, which could partly reduce the influence of friction.

The shear box was designed to test in general compression machine. However, the sample tends to collapse during testing and no end constraint was applied to cable bolts.

Figure 2.34 Single shear apparatus without friction (Ferrero, 1995)

The British standard test method was commonly used in the laboratory to test the shear behaviour of rock bolts and cable bolts, as shown in Figure 2.35. The bolt was grouted into two separated steel pipes. After grouting, the shear pipe was placed into a single

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shear apparatus. This test method is easily conducted in the laboratory environment.

However, the stiffer contact between steel tubes and bolts gets bolts snapped early and the result of peak load would be smaller than that monitored in the field. Due to the limited length of steel pipes and no end constraint structure, pretension could not be applied on tested bolts and de-bonding may occurred during tests.

Figure 2.35 British Standard Single Shear (British Standard,2009)

A single shear test rigs for rock bolts and cable bolts developed by University of New South Wales was carried out by Mahony et al. (2006), as shown in Figure 2.36. Two concrete blocks with the length of 250 mm and 1000 mm with cross-sectional dimensions of 280 mm² were butted and a hole was then drilled through the two blocks.

Steel clamps were encapsulated on the concrete. A series of tests were conducted and factors, including strength of concrete, end- constrain, loading rates and pre-tension, were studies. However, the process of set-up and testing is complex, and friction would influence the accuracy of results.

Figure 2.36 Schematic of shear test laboratory facility (Mahony, et al., 2006)

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The friction could not be ignored in previous tests and this problem was solved in the test carried out by McKenzie and King (2015). There was no contact between steel tubes and bolts. Therefore, the friction in the joint was eliminated during the test. Peak load is similar to field results However, samples are too big and the whole length reaches to 1800 mm or 3600 mm, which is difficult to prepare and test. The phenomenon of de-bonding happened partly during testing. This Megabolt Single Shear Test (MSST) is considered as one of the accepted shear test methods. Hence, numerous cable bolt shear tests was conducted and analyzed in this thesis using this rig which is to be discussed in Chapter 4.

Figure 2.37 Megabolt Single Shear (McKenzie and King, 2015)

Aziz et al. (2003a) was the first to report on Double Shear Test. The 600 mm long shear box is known as MK-I Double Shear Box or simply MK-I DSB, as shown in Figure 2.38a. This box was relatively small in dimension and was found to be unsuitable for shear testing of large capacity rock bolts and cable bolts in low strength concrete.

Currently MK-I is used for shear testing of fiber glass rods and smaller diameter mild steel rib bolts as reported by Aziz, et al., (2015b). Pretension effect of bolts can be studied and both shear force and axial confining force are recorded during the test.

However, due to the limit size of this rig, the apparatus is used mainly for shear testing of fiber glass dowels, and small diameter rock bolts.

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Figure 2.38 Double shear test rigs (a) MK-I and (b) MK-II

To accommodate shear testing of larger capacity rock bolts and cable bolts, a larger size double shear testing rig was subsequently developed, as shown in Figure 2.38b. The rectangular MK-II DSB consisting of two 300 mm long outer cubic boxes and a 450 mm long middle central cuboid box with 300×300 mm² cross-sectional area. The overall length of 1050 mm has opposing concrete joint faces in contact with each other.

The maximum shear load of cable bolt was affected by a combination of friction effect and dowel effect. A new double shear test was developed to eliminate the friction at the joint, which would be described in the Chapter 5.

Figure 2.39 Double shear assembly without friction

However, it was found by Li (2014) that the rock sample cracked during testing. Once the concrete cracked, it can no longer provide a pure shear condition. Shear failure becomes bending failure due to the sample crushing around the intersection of joint and bolts. This phenomenon was also observed in the experiment carried out by Aziz, et al., (2016a, b). Excessive fractures in the rectangularly shaped concrete medium contribute

a

b

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to the inconsistency in test results. Axially cracked concrete contributed to increased displacement and increased tensile/shear failure of cable wires. The failure would normally increase with increased widening of the crack as well as internal crushing of the concrete.

2.6 Influence of factors in shearing property research of rock