Figures 4-57 and 4-58 show the comparison between the result of plain 19 wire, 21.8 mm cable bolt with 0 and 10 t pretension load subjected to double shear tests with and without contact between concrete blocks surfaces.
The comparison between the performance of the plain 19 wire, 21.7 mm cable bolt without pretension load shows that the peak shear loads were 1291 kN and 761 kN, when the concrete blocks were with and without contacts. The shear load is the combination of the shear load provided by the breakage of the cable bolt and the sliding between the concrete blocks when the concrete blocks are in contact with each other. When concrete blocks are not in contact, the peak shear load decreases because it only includes the load due to the failure of the cable bolt. The performance of the plain 19 wire cable bolt with 10 t pretension load are 1369 and 738 kN when the concrete blocks are with and without contact respectively. When the concrete blocks are not in contact, the peak shear load is 54% of the peak shear load when the concrete blocks are in contact with each other. This shows a great contribution of concrete blocks in the total shear strength of system. Moreover, the comparison between the shear displacements at peak shear load shows that the shear displacement at peak shear load was higher when there is no contact between concrete blocks. This is due to higher freedom in movement of concrete blocks.
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Figure 4- 57 Comparison between performance of plain 19 wire, 21.7mm cable bolt
with 0 t pretension load when concrete blocks are in use and are not in contact with each other
Figure 4- 58 Comparison between performance of plain 19 wire, 21.7 mm cable bolts
with 10 t pretension load when concrete blocks are in use and are not in contact with each other
Chapter IV Shear behaviour of cable bolts subjected to single shearing
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The peak shear load is low in MKIII as opposite to what is happening with MKII. This is because the shear load in MKII is a combination of cable bolt shear strength and friction between concrete blocks however the shear load in MKIII only includes the cable bolt strength. It is clear from the results shown in Table 4-7 that because of the most wires of the tested cable are failing in tension, the final shear failure load appears to be closer to failure in tension as demonstrated by the axial load build up per side being nearly equal to shear failure load as shown in Figure 4-59.
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4.6 Summary
The performance of different types of cable bolts subjected to different pretension loads were studied using a double shear assembly with and without contact between concrete block surfaces. The following conclusions were drawn:
The shear behaviour of each cable bolt has three behaviour stages of elastic, strain softening and failure.
Cable type, pretension load, failure angle, lay length and the concrete block strength are important factors in the peak shear load of pre-tensioned fully grouted cable bolts.
On average, the contact between concrete block surfaces was 80% of the total surface area. However, the pure rubbing of the joint faces may be misleading as a significant damage can occur on joint surface during the snapping phase of the cable as one side of the central block leans against outer extremity block.
The plastic hinge location mostly occurs at 60 to 80 mm from the contact surfaces when the concrete blocks were in contact with each other; however the hinge point location was found to start at deeper location up to 110 mm when the shearing blocks are not in contact with each other.
The peak shear load occurs in smaller shear displacement by increasing the pretension load.
The peak shear load for the plain cable is higher than for the spiral/indented one at a similar amount of pretension load because the cross sectional area for plain cable is more than for the spiral cable bolts.
The ultimate tensile strength, lay length, number of wires, surface profile type (plain, spiral and indented) are important factors in the performance of cable bolts in
Chapter IV Shear behaviour of cable bolts subjected to single shearing
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shear. The Plain MW10 has the highest amount of peak shear load due to its highest UTS, 600 mm lay length and plain surface profile type.
No debonding occurred due to the use of barrel and wedge in the double shear assembly.
Increased pretension in double shearing without proper confinement leads to greater shear failure load and greater displacement. In this situation, the deflection is greater causing failure in tension instead of shear. In other words, the failure due to tensile is greater that in shear. Lack of concrete effective confinement is an influencing factor. Basically the cable behaves less stiff with increased pretension load. The shear load when testing in double shear without face contacts, the final shear failure load appeared to be closer to failure in tension rather than in shear as the axial load build up per side being nearly equal to shear failure load.
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Chapter V
SHEAR BEHAVIOUR OF PRE-TENSIONED FULLY GROUTED
CABLE BOLTS SUBJECTED TO SINGLE SHEARING
5.1 IntroductionLaboratory studies are essential to determine similarities and differences between the shear performance of pre-tensioned fully grouted cable bolts subjected to double and single shear testing. The main objective of this study is to investigate the shear behaviour of fully grouted cable bolts under newly developed single shear apparatus. Thus, a laboratory investigation was conducted based on a comprehensive experimental program using different types of cable bolts and various pretension loads.