Soil Engineering

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.

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.

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 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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.