CIVIL 3811/9811/8811 Tutorial Solutions - Week 5 PDF

The University of Sydney School of Civil Engineering CIVIL 3811/9811/8811 – Tutorial Solutions – Week 5 Question 1-1 The connection between the surface elements and rock/cable bolts is often the weakest link in the system as it is near the surface of t...

The University of Sydney School of Civil Engineering CIVIL 3811/9811/8811 – Tutorial Solutions – Week 5 Question 1-1 The connection between the surface elements and rock/cable bolts is often the weakest link in the system as it is near the surface of the tunnel where many unsupported rocks lie, as the act of tunnelling has caused them to become unsupported. Therefore, the rock in the failure zone (shown below) is wanting to cave into the tunnel, whereas beyond the failure zone, the rock is relatively undisturbed from the tunnelling action, and is thus as stable as it has always been. Therefore, the connection of the bolts and cables need to ensure that they are driven deep enough to reach this stable rock outside of the failure zone in order to anchor the unsupported surface rock. Though, this is of course difficult to ensure as you cannot physically see where the joints are inside the rock, and therefore this makes the connection of the surface elements the weakest link of the support system. Furthermore, corrosion is also another factor that can make rock bolts the weakest link in a rock support system. Without the correct procedure, such as galvanising the rock bolts or applying corrosion reducing actions, rock bolts are directly exposed to the corrosive effects of ground water. In areas with highly aggressive corrosive environments, the rock bolt is likely to experience pitting, erosion, corrosion fatigue and fretting. Failure Zone Question 1-2 The shrinkage of shotcrete can vary, as it depends on the water content and the aggregate type/size/mix ratio. The high dry shrinkage which is experienced by the shotcrete will need closer control-joint spacings. Shrinkage is the contraction of hardened concrete, because of a loss of water, which creates an increase in the tensile stresses acting on the shotcrete. As concrete is weak in tension, tensile stresses cause cracking. This is most likely to occur at joints with internal warping and external deflection creating greater loads on the ground support system. To reduce this tensile stress, the addition of fibres in the shotcrete is favourable. Creep is defined as any time-dependent deformation of a material which experiences a load over time. When shotcrete is well designed with a low water-cement ratio, the magnitude of creep strain can be drastically lowered. On the other hand, when the water ratio is too high, creep will increase deflection over a period, creating deformation. Therefore, the force created by the deflection will act in the same direction as the pressure applied by the rock wall, which will create a collapse. Question 2 In this analogy bolts have 2 functions: 1.Increase the depth of the beam, and 2.Reinforce the lamination (reduce interlayer slip).