A Self-Learning Manual - Institution of Engineers Mauritius

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A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU sprig is capable of growing as a plant. The cost of sprigging is moderate and vegetation is expected to grow for around six months. However, this method requires high manual input and is labour intensive. Turfing is the direct application of vegetation with developed roots. The cost of turfing is high when compared with sprigging. However, it is the quickest way to get vegetation. Within a month mature-looking vegetation is ready for use. 15. What is the difference between hydroseeding and turfing on slopes? (SF4) Hydroseeding is the provision of grass seed mixed with fertilizer and nutrient by spraying. The grass seed shall grow and the root of grass serves as reinforcing fiber to hold tightly the surface soils. Turfing is the direct application of grass with developed roots on slope surface. The already-developed grass is expected to grow easier when compared with hydroseeding. The roots of grass shall extend onto soils to strength the overall surface. 16. How can concrete buttress stabilize rock fall?(SF5) Sometimes there is a cavity in rock slopes which is caused by previous rock falls. Concrete buttress, together with the use of rock dowels, could help to stabilize the rock slope. The buttress serves to avoid local toppling of rock face. Moreover, it keeps protects the unstable rock and keep it in place. The presence of buttress also helps hold up the overhang. The size of buttress should be large enough to support the weak rock. In case the surface on which the buttress is founded is not normal to the resultant forces exerting on the buttress, buttress is tied to rock surface by using rock dowels to avoid the occurrence of sliding. 17. How does rock reinforcement function? (SF6) For rocks with widespread fractures, individual blocks resulting from these fractures may fall out and as a result slope failure may occur. Rock reinforcement (e.g. rock dowels, bolts or anchors) is installed to bolt through the discontinuities in rock to enhance the rock to behave as a single unit. With the bolting across block interfaces, the stresses would be 296
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU altered within the rock mass. For untensioned rock dowels, they may be subjected to tensile forces arising from rock movement. Other than the provision of rock reinforcement, shotcreting is another method to reinforce the rock. It functions by griping the rock together and maintaining the small blocks which hold the large blocks in position [37]. 18. Do inclined rock bolts perform better than rock bolts normal to joints? (SF7) The use of rock bolts can be dated back to Roman Empire and it was commonly believed that rock bolts pin surface rock (bedded rock strata or individual rocks) to more stable rock. With the insertion of rock bolt into rock slope, it increases the rock mass’s stiffness and strength with respect to shear and tensile loads. Axial force in rock bolts consist of a force component normal to joint plane which contributes frictional resistance, and a force component parallel to joint plane which contributes to dowel action. Inclined rock bolts helps to stiffen shear and results in an increase in shear strength at smaller displacements. Rock bolts perpendicular to shear planes provide the lowest shear resistance. The optimal inclination of rock bolt is 30 o to 60 o based on past experimental works. 19. What are the differences between rock dowels and rock bolts? (SF7) Rock dowels are passive reinforcing elements which need some ground displacement for activation. In installation of rock dowels, a hole is drilled and untensioned steel bars are inserted into the hole. When displacements along joints occur, rock dowels are subject to both shear and tensile stresses. The level and ratio of shear and tensile stress depends on the properties of the surrounding ground, the grout material filling the annular gap between the dowel and the ground and the strength and ductility parameters of the rock dowel itself. Moreover, the degree of dilation during shear displacement affects the level of stress acting within the dowel. Rock bolts are tensioned once the anchorage is attained to actively set up a compressive force into the surrounding rock. This axial force increases the shear capacity and is generated by pre-tensioning of the bolt. The system requires a bond length to enable the bolt to be tensioned. In essence, rock bolts start to support the rock as soon as they are tensioned and the rock does not have time to start to move before the rock bolt 297
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A Self-Learning Manual - Institution of Engineers Mauritius

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