How does the drilled-shaft foundation constructed?
How does the drilled-shaft foundation constructed?
Drilled shafts, also known as drilled piers, bored piles, cast-in-drilled-hole piles (CIDH), or caissons, are high-capacity deep foundation structures.
The construction of drilled shafts includes the following 3 phases:
- Excavation of piers
- Providing supports
- Concreting
- Excavation of piers: Usually, auger drill is used for the excavation of drilled pier. Other drilling equipment like hydraulic power,rotary systems, etc. are also used as per the requirement. An auger is attached to a shaft and rotated under pressure to dig into the soil. When it is filled with soil, it is lifted above the ground and emptied.
Belled piers are used where the soil stratum does not have the sufficient bearing capacity. For the forming of a bell, the auger is replaced by an under-reaming tool which comprises of a cylinder with cutting blades which are hinged at the top. The cutting blades are all in the folded position when the under-reamer is lowered into the cavity. On reaching the bottom of the cavity, the blades are extended outward by a mechanism.
When the under-reamer is rotated, a bell is created and the loose soil falls within the cylinder, which is lifted and emptied. This cycle is repeated until the bell is fully formed. The diameter of the bell is held two to three times the diameter of the shaft. The angle of the bell is 300 to 450 with the vertical.
The method stated above of drilling is useful for hard clays for which hole can be left open for a few hours without any support. For cohesionless soils underneath the water table, the hole is stopped from collapsing by having a drilling or by casing in a slurry. When rock is found during drilling, specific machines are needed.
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