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 9. What are the major causes of rockfall? (S6) The causes of rockfall can be broadly classified into the following two reasons: (i) Freezing and thawing and the subsequent development of vegetation root pressure in slopes is one of the major causes of rockfall in some countries like Europe. Moreover, rockfall can also be triggered by heavy rainstorms which bring about surface erosion and generate water pressure in rock joints. (ii) Rockfall can also be induced by poor joint patterns and low strength and water pressure in joints. 10. What are the functions of drainage system and protective covers of slopes? (SF1) In Hong Kong the angle of fill slope is about 30 o – 40 o to the horizontal while the angle of cut slope is about 50 o – 60 o to the horizontal. To protect the slope from surface erosion and water infiltration, it is covered with impermeable hard cover like chunam and shotcrete. Chunam is a mixture of soil, cement and lime and is usually applied in two layers with the thickness of each layer of 25mm. For shotcrete, mesh reinforcement may be provided inside it to enhance tensile strength of cover, thereby reducing the risk of tensile cracking of slope cover. Weepholes are normally provided in slopes to prevent building up of water pressure in the slope and subsequently this causes cracking and disintegration of slope cover. For gentle slope, hydroseeding may be used and geofabric may be introduced on slope surface to guard against possible surface erosion. U-channels are provided on the crest and toe of slope to divert and collect the rain falling on slope surface. Catchpits are provided at the intersection or junctions of drains to avoid possible splashing of water. 11. What is the purpose of installation of erosion control mat in slopes? (SF2) Steep slopes are prone to intermittent high velocity flows during rainstorm and this causes erosion at slope surface which prevents the growth of vegetation. 294
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU Erosion control mat is installed to control soil erosion and provide soil stability until vegetation can be established. The principal function of erosion control mat is to prevent pre-vegetated soil loss by stabilizing and protecting soils from rainfall and surface erosion. Moreover, it could provide a long-term artificial erosion control system which would increase the shear resistance of vegetation and provide long-term, tenacious reinforcement of the root system. 12. What is the reason of adding steel wire mesh when using erosion control mat in slopes? (SF2) It is not uncommon that the use of erosion control mat in slopes is accompanied by the addition of steel wire mesh. The system of erosion control mesh with a steel wire mesh proves to be a more effective method to control surface erosion. In case surface erosion occurs even in the presence of erosion control mat, the soil debris could be trapped between the steel wire mesh and the slope surface. As such, the steel wire mesh essentially serves as an additional protective layer to avoid further occurrence of erosion. 13. How can vegetation improve soil shear strength and slope stability? (SF3) Vegetation in slopes could modify the soil moisture condition in two ways: (i) Soil moisture is absorbed by roots of vegetation and the water is transpired to its root; (ii) It intercepts the rainfall by reducing the amount of rainwater penetrating into the slope. Both processes, i.e. evapotranspiration and interception effected by the presence of vegetation contribute to dry soil conditions. Moreover, evapotranspiration results in soil suction in unsaturated soil slopes, which further improves the shear strength of soil. As such, it tends to delay the time of saturation in slopes and this essentially improves the slope stability. 14. What is the difference between sprigging and turfing? (SF4) A sprig is a part of stem with crowns and roots which is cut from a rhizome or stolon. Sprigging is a type of vegetative planting by placing sprigs in spaced intervals in holes. With proper transplantation and maintenance, a 295
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