Drilled Pier Foundation Design
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A drilled pier stands for a deep foundation system under which a large diameter concrete cylinder is used. The cylinder is built up with the arrangement of fresh concrete and reinforcing steel into a drilled shaft.
It is also known as a caisson, drilled shaft, Cast-in-drilled-hole piles (CIDH piles) or Cast-in-Situ piles.
To develop a drilled pier, a hole with large diameter is drilled in the ground and stuffed with concrete later.
Normally, bored piles contain diameters under or equal to 0.6m. The shafts with sizes greater than 0.6m are normally designated as drilled piers. A drilled pier belongs to a type of deep foundation that is built up to deliver greater axial or lateral loads to a deep stratum underneath the ground surface.
Construction of Drilled Piers:
The drilled piers are constructed in the following three phases: Excavation of piers, Providing supports, Concreting
Excavation of Piers
1. Normally, the drilled pier is excavated with auger drill. Besides, different types of drilling equipment like rotary systems, hydraulic power etc. are also utilized as per the requirements.
2. An auger is connected with a shaft and rotated under pressure to dig into the soil. Once it is stuffed with soil, it is uplifted above the ground and vacated.
Also Read: Benefits of Pier and Beam Foundation
3. If the soil stratum does not contain the sufficient bearing strength, belled piers are normally applied. To build up a bell, the auger is substituted with an under-reaming tool comprising of a cylinder with cutting blades which are pivoted at the top.
4. The cutting blades remain in the folded position when the under-reamer is lowered into the hole. After attaining the bottom of the hole, the blades are expanded externally through a mechanism.
5. When the under-reamer is rotated, a bell is created and the loose soil drops inside the cylinder, that is lifted & vacated. The method should be reiterated unless the bell is totally developed. The diameter of the bell remains two to three times the diameter of the shaft. The angle of the bell should be 30o to 45o with the vertical.
6. This process is suitable for hard clays where the hole are provided open for a few hours devoid of any support. In cohesionless soils underneath the water table, a casing is arranged or a drill is made in a slurry so that the hole is not collapsed.
Providing Supports to Piers
The supports are provided with the following two methods: Chicago method, Gow method
Chicago Method: Under this system, a circular hole is created by excavating up to the depth at which the soil will locate unsupported (about 0.5m for soft clay and 2m for stiff clay). Vertical boards alias laggings, are then placed in position around the excavated face and retained firmly against the soil with steel rings.
The shaft is then excavated again for 1 to 2m and further setting of boards and rings is done. Reiterate the method unless the required level is attained. The base of the shaft is then belled out.
Gow Method: Under this system, excavation of the hole is made physically. By using the telescopic steel shells, the soil is supported and expanded since the hole gets deeper. The shells are eliminated when the concreting starts. One section of the shell is detached at one time. The least diameter of the hole in this method remains about 1.25m.
Concreting of Piers
1. Once the hole is drilled to the desired depth, the shaft is dewatered and the bottom is cleansed. The casing(if applied) is detached.
2. The reinforcement cage is then provided and concreting is completed. So far as possible, concreting should be accomplished in a dry shaft hole.
3. Concreting is accomplished in a dry hole by gravity pouring, given that the concrete does not hit the sides. However, when dewatering can't be done or slurry is applied to support the shaft sides, concrete is arranged with a tremie.
Benefits of Drilled Piers:
a. A single drilled pier sustains the load of a group of piles.
b. They contain greater resistance strength against lateral loads.
c. Lighter equipment is required for drilled pier and it does not create any noise throughout construction.
d. Ground heaving is not necessary in the case of drilled piers.
e. Inspection of the base and sides of the pier is simple.
f. Base of drilled pier comes with higher bearing strength and superior resistance to uplift.
g. They are utilized even when the soil includes boulders.
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