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.

After rainfall, groundwater pressure is built up and this elevates the ground water table. The water inside the pores of soil reduces the effective stress of soils. Since shear strength of soils is represented by the following relations:

Shear strength = cohesion + effective stress x tan0 where 0 is the friction angle of soils

Hence, the presence of water causes a reduction of shear strength of soils and this may lead to landslide. On the other hand, the rainfall creates immediate instability by causing erosion of slop surface and results in shallow slope failure by infiltration. In addition, the rain may penetrate slope surface openings and forms flow paths. As a result, this may weaken the ground.

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.

Slope cutting causes stress relief in slopes which may cause slope movement. For instance, for weathered rocks the horizontal stresses would be relatively low when compared with normally consolidated soils.

Consequently, a major cut on the slope formed by weather rock may result in the development of tensile stresses in the slope, leading to slope movement.

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.

Filled slopes constructed in many decades ago are mostly sub-standard. The relative density of filled slopes may be below 85% and is readily subjected to liquefaction. To rectify the situation, the sloped are reconstructed by excavation of 3m measured vertically from slope surface. Then, compaction should be carried out in thin layers to achieve in-situ density of not less than 95% of maximum dry density. After compaction, the compacted layer would not vulnerable to liquefaction failure. Moreover, it is less permeable than loose fill upon compaction and prevents water entry into underlying soils inside the slope.

For the case of Hong Kong, most fill slopes constructed before 1977 were formed by end-tipping so that they are in a loose state and poses hazard to developments nearby.

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.

When cohesive strength is zero (i.e. slopes of granular types), the slip surface is of shallow failure type and is parallel to the slope surface.

When friction angle is zero (i.e. slopes of clayey types), the slip surface is if deep seated failure. The factor of safety of slopes is nearly independent of the angle of slopes because the weight of deep seated failure regime is much greater than the slope.

Normally, non-circular failure surface is always more critical than circular one for two dimensional analysis.

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.

Other than static liquefaction, slow-moving slips driven by transient pore water pressure leading to high speed landslide are the other possible cause of failure of loose fill slopes.

For loose fill lying on low permeability soil layers, there is potential storage of infiltrating water when the slope of underlying low-permeability soil layer is mild. As such, there is a localized zone of high transient pore water pressure induced within the fill material. Flowslides normally start with a local slip caused by transient pore water pressure by soil layering or flow restriction. Then, the nature of slow-moving soil debris and the geometry of slip result in a fast landslide.

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 rainstorm, the runoff from rainfall infiltrate into the top layer of fill slopes. It may result in saturation of this layer of fills leading to the decrease in soil suction. Consequently shallow slope failure may occur.

If the fill slope is in a loose state, the soils would tend to decrease in volume during deformation. As a result this induces a rise in pore-water pressure which triggers slope failure in form of mud-avalanche.

If the fill slope is in a dense state, the soils would tend in increase in volume during deformation and it only fails like a mud slump.

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.

Janbu’s method and Morgenstern-Price method are non-circular analytical method and they are frequently used for soil slopes while Bishop’s method is circular analytical method. Bishop’s Simplified method and Janbu’s Simplified method assume that the inter-slice forces are horizontal and inter-slice shear forces are neglected.

Equilibrium Method	Moment Equilibrium	Force Equilibrium
		Horizontal	Vertical
Janbu’s Simplified	No	Yes	Yes
Bishop’s Simplified	Yes	No	Yes
Morgenstern-Price	Yes	Yes	Yes

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.

With the use of rockfill to build embankment, it is not a concern regarding the build-up of pore water pressure during construction so that the embankment can be filled at the faster rate. Moreover, the use of rockfill allows a steeper angle in forming road embankment when compared with earthfill so that it results in a small amount of fill. As such, it helps save the cost of construction.

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.

Hot applied thermoplastic road marking appears to be more commonly used than cold applied road paint as road marking materials. For hot applied thermoplastic road marking, it allows the addition of solid glass head which enhances reflectorisation effect. This essentially makes great improvement on the visibility of road markings at night time. Moreover, from past experience the durability of thermoplastic road marking is higher than that of road paint.

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.