It is a common practice in the construction of bored piles by adding a cement plug before concreting of bored piles takes place. One of the possible explanations is that the cement grout serves as a barrier and protects fresh concrete from being washed away by water inside pile bore. The cement grout helps to set aside water when the first batch of concrete is poured down from the tremie pipe. As such, the quality of fresh concrete is anticipated not to be seriously affected by water and this helps to improve toe imperfection problem.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

For normal soils, water in bentonite slurry penetrates into the sandy walls and leaves behind a layer of bentonite particles on the surface of the soils. The bentonite particles form the filter cake of low permeability on the excavated faces. The filter would be formed only when slurry pressure is greater than the pore water pressure in excavated soils. The filter cake serves as impermeable layer and allows the application of full hydrostatic pressure of bentonite slurry on the excavated surface of soils.

For soils with low permeability such as clay, there is little water passage from slurry to excavated clay surface so that filter cake would hardly be formed. As a result, slurry pressure simply applies on clay surface.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

The use of high density bentonite slurry could improve trench stability. It helps to retain cuttings and particles in suspension and reduce the loss of bentonite slurry into soils of high permeability such as sand.

However, the use of low density bentonite slurry is desirable from operation point of view. Low density bentonite slurry tends to be more neatly displaced from soils and reinforcement. Moreover, the pumping of low density bentonite slurry is more easily to be carried out.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

Bentonite slurry is one of the most common excavation fluid used in constructing diaphragm wall. Bentonite clay (in powder form) and water are combined in a colloidal mixer and clay particles bond to each other and set to form a gel when left to stand for a period of time. When the bentonite is set in motion, it reverts back to the fluid state rapidly.

Bentonite slurry shores the trench to stabilize the excavation and forms a filter cake on the slurry trench walls that reduces the slurry wall’s final soil permeability and to reduce ground water flow. The gel strength and viscosity properties of the bentonite clay allow for cutting suspension and removal.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

Shallow foundation is commonly accepted as foundation with founding level less than 3m from ground surface. In case surface loads or surface conditions could still affect the bearing capacity, the foundation which sits on it is called shallow foundation.

Pad foundation refers to the foundation which is intended for sustaining concentrated loads from a single point load such as structural columns.

Strip foundation is used to support a line of loads such as load-bearing walls. For instance, closely-spaced columns render the use of pad foundation inappropriate and strip foundation may be a better alternative.

Raft foundation consists of a concrete slab which extends over the entire loaded area so that loads from entire structure are spread over a large area leading to a reduction of the stress of foundation soils is reduced. Moreover, raft foundation serves to avoid differential settlement which otherwise would occur if pad or strip foundation is adopted.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.