Soil Engineering

Bleeding is a form of segregation in which a layer of water migrates to the surface of the grout during the initial stage of cement hydration process. Later on, some of the floating water is re-absorbed into the grout due to further hydration reactions. Even without the problem of bleeding, there is a total reduction of volume of grout after hydration action when compared with the total initial individual volume of cement and reacted water. Bleeding tests should be carried out for grout because of the following reasons:

(i) During bleeding, the upflow of water from grout mixture leads to the formation of channel paths inside the grout mix. These channels act as potential paths for aggressive materials to pass through as these
channels would not be closed during further hydration of the grout.

(ii) The loss in volume by bleeding generates voids inside the grout mix which affects the properties and performance of the grout. Moreover, it increases the chance of corrosion of steel elements protected by the grout. (e.g. tendons)

(iii) In bleeding test, there is a usual requirement of total re-absorption of water after 24 hours of grout mixing because for some cold countries, this layer of water may cause severe freezing problem leading to frost damage.

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 use of horizontal drains to enhance slope stability in the following ways:

(i) It enhances an increase in soil strength by lowering the degree of saturation of soils.

(ii) It reduces the pore water pressure in the region of potential slip surface.

(iii) It hinders the development of seepage forces.

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.

Colluvium refers to common surface deposit rolling down hillsides under the action of gravity. It differs from residual soils which, upon decomposition the discontinuity, joints and textures are preserved. The rolling action of colluvium destroys these features.

Colluvium displays some important features which deserves attention:

(i) It appears to be more resistant than residual soil to erosion and tends to offer protection to the soils beneath.

(ii) Its relatively lower permeability when compared with underlying residual soils may form a perched water table at its base during heavy rainstorm.

(iii) For residual soils, when slope cutting is carried out the presence of relict joints may affect the slope’s stability. However, the base layer of colluvium is also a plane of potential failure location.

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.

Decomposed dolerite dykes contain high clay content display high plasticity. Moreover, the material is generally of low permeability than decomposed granite which is found in many slopes.

The presence of decomposed dolerite dykes in a direction parallel to slope surface may cause slope failure after heavy and prolonged rainfall. Owing to the low permeability of decomposed dolerite dykes, perched water table would be developed above the dykes and this essentially increase the positive pore water pressure which reduces the shear strength of soils.

The presence of decomposed dolerite dykes in a direction perpendicular to slope surface may also cause slope failure after severe rainstorm. The low permeability of the material tends to act as a dam building up the local groundwater level for water coming down from upper slopes. The increase of water level behind the “dam” causes an increase in pore water pressure so as to reduce the strength of soils in slope, leading to subsequent failure.

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 ground conditions in hillsides of Hong Kong normally consist of colluvium overlying weathered and fresh rock. The groundwater flow in sloping ground may involve both perched and main water flow. As permeability of soil tends to decrease with depth, groundwater flow may take place in the upper aquifer which contains a perched water table.
Read More

Geogrid allows the fill on one side of the grid can key with fill on the other side of the grid. Hence, it is different in its interlocking ability when compared with strip soil reinforcement. The keying-in of both sides of fill could be achieved by compaction and static load above the fill.

To trigger shearing across the plane of geogrid, work has to be done to dilate the soils and overcome the frictional forces.

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.

Reinforced fill consists of reinforcement embedded in fill with facing. With backfilling material of clayey nature, it may pose problems to potential corrosion of steel reinforcement. The low permeability of cohesive fill materials tends increase the duration of contact between reinforcement and water and may cause corrosion problems. Moreover, consideration should be given if cohesive materials could also achieve the required compaction.

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.

Rockfall barrier is intended to absorb the energy of rockfall safety without the need of future regular maintenance. The net of rockfall barrier is best to be arranged like Olympic Rings with interlacing (called ringnet) and it has proved to be very effective in its energy absorption capacity.

The main function of post of rockfall barrier is to provide support to the net. Impacts to posts should not result in the collapse of the structure. In the event of directly impact of rocks with the posts, there is a break devise at the post’s base plate which is designed to allow the post from separating the base plate. In this way, it serves to protect the anchor of base plate from being damaged.

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 rockfall barrier has to serve basically the following two purposes:

(i) It has to be sufficiently high so that it would not be jumped over by rock boulders.
(ii) It has to resist the impact of rockfall without structural failure.

For rockfall energy greater than 2,500kJ, it requires the installation of earth dams, underpinning (stabilized) or even complete removal. For rockfall energy less than 2,500kJ, it can be resisted by rigid structures made of concrete or steel. However, the recent trend is that most of these rigid structures are replaced by flexible structures like flexible wire rope nets. For total work on barrier system, it is defined as force times displacement. The merit of flexible system lies in the small forces generated with large displacement when compared with larges forces with little displacement in rigid structures. As a result, the flexible structures can be designed with low impact forces leading to better and economical design.

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.

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 becomes effective.

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.