CAST-IN-SITU BORED PILES
This type of pile installation is used to replace soil removed by drilling rather than occupying the space of displaced soil as in driven piles and thus, it mostly relies on end-bearing capacity of the earth layer at the drilled depth. Bored piles provide the largest pile dimensions and thus the highest load bearing capacities.
One method of installation is to drill a shaft using a continuous auger with a hollow stem at its center, which is later used to grout the drilled shaft. This method is known as Continuous Flight Auger (see following section).
Another technique is to drill the shaft, insert the reinforcement steel cage (if required) and then pour concrete under pressure to fill the entire shaft space. The shaft is drilled using an auger in dry land conditions and a drilling bucket in wet land conditions.
When shaft wall support is required, the drilling process takes place inside a steel casing or with the aid of drilling slurry such as Bentonite.
STEEL CASING SUPPORT
Steel drill casing is advanced into the ground using a vibratory hammer leaving a 1-meter protrusion. The casing is driven about 50 cm into hard stratum to provide shaft wall support during the drilling process.
After drilling to the specified depth, the bottom of the shaft is cleaned using a drilling bucket, the shaft depth is measured and caliper logging is performed to ensure that no major collapses have occurred in the borehole walls under the casing.
Steel cage reinforcement fitted with spacers (to provide uniform concrete cover) is placed within the borehole using a crane and concrete is poured into the shaft under high pressure using a tremie pipe and a suitable concrete pump.
Drilling mud or slurry such as bentonite is mostly used for drilling support in situations where the bore hole is unstable. A positive head of bentonite suspension above the water table is maintained during the drilling process, thus providing continual support to the borehole walls. Fluid concrete displaces the clean bentonite which is collected, cleaned and reused several times before it is discarded.