Soil Engineering

It is not uncommon that granular fill layers and rockfill layers are placed beneath the bottom of concrete retaining walls. The purpose of such provision is to spread the loading in view of insufficient bearing capacity of foundation material to sustain the loads of retaining walls. Upon placing of granular fill layers and rockfill layers, the same imposed loads are supported by a larger area of founding material and hence the stress exerted by loads is reduced accordingly.

Layers of granular fill and rockfill materials are not standard details of concrete retaining wall. If we are fully satisfied that the founding material could support the loads arising from retaining walls, it is not necessary to provide these layers of granular fill and rockfill materials.

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.

Normally for reinforced concrete cantilever retaining walls, there is a 75mm kicker at the junction wall stem and base slab to facilitate the fixing of formwork for concreting of wall stems. If a higher kicker (i.e. more than 75mm height) is provided instead, during the concreting of base slab the hydraulic pressure built up at kicker of fresh concrete cause great problem in forming a uniform and level base slab.

Despite the fact that the position of kicker in a cantilever retaining wall is the place of largest flexure and shear, there is no option left but to provide the kicker at this 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.

The installation of shears keys helps to increase the sliding resistance of retaining walls without the necessity to widen the their base. The effect of shears keys enhances the deepening of the soil failure plane locally at the keys. The increased sliding resistance comes from the difference between the passive and active forces at the sides of the keys. In case weak soils are encountered at the base level of shear keys, the failure planes along the base of retaining walls due to sliding may be shifted downwards to the base level of the keys.

Shear keys are normally designed not to be placed at the front of the retaining wall footing base because of the possible removal of soils by excavation and consequently the lateral resistance of soils can hardly be mobilized for proper functioning of the shear keys. For shear keys located at the back of footings, it poses a potential advantage that higher passive pressures can be mobilized owing to the higher vertical pressure on top of the passive soils.

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 determining the external stability of retaining walls, failure modes like bearing failure, sliding and overturning are normally considered in design. In considering the criterion of sliding, the sliding resistance of retaining walls is derived from the base friction between the wall base and the foundation soils. To increase the sliding resistance of retaining walls, other than providing a large self-weight or a large retained soil mass, shear keys are to be installed at the wall base. The principle of shear keys is as follows:

The main purpose of installation of shear keys is to increase the extra passive resistance developed by the height of shear keys. However, active pressure developed by shear keys also increases simultaneously. The success of shear keys lies in the fact that the increase of passive pressure exceeds the increase in active pressure, resulting in a net improvement of sliding resistance.

On the other hand, friction between the wall base and the foundation soils is normally about a fraction of the angle of internal resistance (i.e. about 0.8p ) where p is the angle of internal friction of foundation soil. When a shear key is installed at the base of the retaining wall, the failure surface is changed from the wall base/soil horizontal plane to a plane within foundation soil. Therefore, the friction angle mobilized in this case is p instead of 0.8p in the previous case and the sliding resistance can be enhanced.

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 base slab thickness of a cantilever retaining wall is about 10% of the total height of retaining wall. The length of base slab is about 50-70% of the total height of retaining wall. Generally speaking, the thickness of wall stems may vary along the stem provided that its size should not be less than 300mm to facilitate concrete placement.

For retaining wall with a total height exceeding 8-12m, it is recommended to adopt counterfort retaining wall. The counterforts in counterfort retaining wall are normally spaced at about 30% to 70% of the total height. The length of base slab is about 40-70% of the total height of retaining wall.

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.

An excavator is defined as a power-operated digging machine and it includes different types like shovels, draglines, clamshells, backhoes, etc.

A dragline possesses a long jib for digging and dumping and it is used for digging from grade line to great depths below ground. Its characteristic is that it does not possess positive digging action and lateral control of normal excavators. A dragline is normally deployed for bulk excavation.

A backhoe is designed primarily for excavation below ground and it is especially employed for trench excavation works. It digs by forcing the bucket into soils and pulling it towards the machine and it possesses the positive digging action and accurate lateral control.

A shovel is a machine that acts like a man’s digging action with a hand shovel and hence it is called a shovel. It digs by putting the bucket at the toe of excavation and pulling it up. Though a shovel has limited ability to dig below ground level, it is very efficient in digging above ground like digging an embankment.

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.

Sand cone (replacement) test is normally carried out to determine the in-situ and compacted density of soils. This testing method is not suitable for granular soils with high void ratio because they contain large voids and openings which provide an access for sand to enter these holes during the test. Moreover, soils under testing should have sufficient cohesion so as to maintain the stability of the sides of excavation during the excavation step in sand cone (replacement) test. In addition, organic or highly plastic soils are also considered not suitable for this test because they tend to deform readily during the excavation of holes and they may be too soft to resist the stress arising from excavation and from placing the apparatus on the soils.

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 compaction curve of sandy materials is totally different from that of clayey materials. For sands or gravels, there are two situations of maximum density, namely the completely dry condition and the complete water saturation. For moisture content of sands and gravels between these two states, the dry density obtained is lower than that obtained in the above-mentioned states. The presence of capillary forces account for the difficulty of compaction sand at water contents between virtually dry and saturated state. They are formed in partially filled water void between soil particles and perform as elastic ties cementing soil particles together. Reference is made to Lars Forssblad (1981).

The compaction curve for clay is suitable for the majority of soil types except sands and gravels because a small amount of clay in soils is sufficient to make the soils impermeable.

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.

As suggested by Lars Forssblad (1981), the three main actions of compaction are static pressure, impact force and vibration. Different compactors contain one or more modes of these actions. For example, vibratory tampers perform mainly by the principle of impact while vibratory rollers work with principle of static pressure and vibration.

For sandy soils, vibration is adequate for normal compaction because the action of vibration sets the soil particles in motion and friction forces between soil particles are virtually demolished. During this vibration motion, the soil particles rearrange themselves to develop a dense state.

For normal soils, it is necessary to combine the action of vibration together with static pressure to breakdown the cohesion forces between soil particles in order to allow for better compaction. The static pressure of vibratory machines is adopted to exert a shearing force to eliminate the cohesion in clayey soils.

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.

Soil compaction is the process of increasing the soil density by reducing the volume of air within the soil mass.

Soil compaction depends mainly on the degree of compaction and the amount of water present for lubrication. Normally 2.5kg rammers and 4.5kg rammers are available for compaction in laboratories and the maximum dry densities produced by these rammers cover the range of dry density obtained by in-situ compaction plant.

Regarding the second factor of water content, it affects the compaction in the following ways. In low water content, the soils are difficult to be compacted. When water content is increased gradually, water will lubricate the soils and this facilitates the compaction operation. However, at high water content, as an increasing proportion of soils is occupied by water, the dry density decreases with an increase in water content.

For soil compaction tests, the dry density obtained from compaction carried out in-situ by vibrating roller/vibrating plate is compared with the maximum dry density conducted in laboratories using 2.5kg rammer of compaction with similar soils. In essence, the in-situ compaction is compared with the compacting effort of using 2.5kg (or 4.5kg) rammer in laboratories. In case the compaction test results indicate values exceeding 100%, it only means that the in-situ compaction is more than that being carried out in laboratories which is treated as the basic criterion for satisfactory degree of soil compaction. Therefore, the soil results are acceptable in case compaction test results are over 100%. However, excessive compaction poses a risk of fracturing granular soils resulting in the reduction of soil strength parameters.

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.