Soil Engineering

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.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Steep slopes are prone to intermittent high velocity flows during rainstorm and this causes erosion at slope surface which prevents the growth of vegetation.

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.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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^oto 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.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

From soil mechanics, it tells us that unsaturated soils get its strength from three main components, namely, friction, cohesion and suction. In building a sand castle in a beach, experience tells us that when sand is too dry or too wet, the castle can hardly be built. However, when the sand is partially saturated, the suction (negative pore water pressure) holds the sand together and provides the strength the build the castle.

In the event of intensive rainfall, the soils cannot get away the water at the rate it is penetrating into the slope and this results in wetting up of the subsurface soils. When the slopes gets too wet (but not yet saturated), it loose much strength in terms of suction (negative pore water pressure) and results in slope failure. This occurs despite the fact that the sliding mass is well above the ground water table.

In Hong Kong about 80% of landslides occur owing to erosion and loss in suction. Only less than 20% of landslides occur as a result of increase of pore water pressure, leading to the decrease in shear strength.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.