What Is Pier Foundation?
Drilled pier foundations, the subject matter of this article, belong to the same category as pile foundations. Because piers and piles serve the same purpose, no sharp deviations can be made between the two.
The distinctions are based on the method of installation. A pile is installed by driving, a pier by excavating. Thus, a foundation unit installed in a drill-hole may also be called a bored cast-in-situ concrete pile.
Here, a distinction is made between a small diameter pile and a large diameter pile. A pile, cast-in-situ, with a diameter less than 0.75 m (or 2.5 ft) is sometimes called a small diameter pile.
A pile greater than this size is called a large diameter bored-cast-in-situ pile. The latter definition is used in most non-American countries whereas in the USA, such large-diameter bored piles are called drilled piers, drilled shafts, and sometimes drilled caissons.
Types of Drilled Piers
Drilled piers may be described under four types. All four types are similar in construction technique but differ in their design assumptions and in the mechanism of load transfer to the surrounding earth mass.
• Straight-Shaft End-Bearing Pier
• Straight-Shaft Side wall Shear Pier
• Straight-Shaft Pier With Both Sidewall Shera and End Bearing
• Underreamed or Belled Pier
Straight-Shaft End-Bearing Pier
Straight-shaft end-bearing piers develop their support from end-bearing on strong soil, “hardpan” or rock.
The overlying soil is assumed to contribute nothing to the support of the load imposed on the pier.
Straight-Shaft Side wall Shear Pier
Straight-shaft sidewall friction piers pass through overburden soils that are assumed to carry none of the load and penetrate far enough into an assigned bearing stratum to develop design load capacity by side-wall friction between the pier and bearing stratum.
Straight-Shaft Pier With Both Sidewall Shera and End Bearing
A combination of straight shaft side-wall friction and end bearing piers are of the same construction as the two mentioned above, but with both side-wall friction and end bearing assigned a role in carrying the design load.
When carried into rock, this pier may be referred to as a socketed pier or a “drilled pier with rock socket“.
Underreamed or Belled Pier
Belled or under reamed piers are piers with a bottom bell or underream figure. A greater percentage of the imposed load on the pier top is assumed to be carried by the base.
Advantages and Disadvantages of Drilled Pier Foundations
Advantages Drilled Pier Foundations
• Pier of any length and size can be constructed at the site
• Construction equipment is normally mobile and construction can proceed rapidly
• Inspection of drilled holes is possible because of the larger diameter of the shafts
• Very large loads can be carried by a single drilled pier foundation thus eliminating the necessity of a pile cap
• The drilled pier is applicable to a wide variety of soil conditions
• Changes can be made in the design criteria during the progress of a job
• Ground vibration that is normally associated with driven piles is absent in drilled pier construction
• Bearing capacity can be increased by underreaming the bottom (in non-caving materials)
Disadvantages Drilled Pier Foundations
• Installation of drilled piers needs careful supervision and quality control of all the materials used in the construction
• The method is cumbersome. It needs sufficient storage space for all the materials used in the construction
• The advantage of increased bearing capacity due to compaction in granular soil that could be obtained in driven piles is not there in drilled pier construction
• Construction of drilled piers at places where there is a heavy current of ground water flow due to artesian pressure is very difficult
Construction of Drilled Piers
In the earlier methods like the Chicago method and Gow’s method, the drilling used to be done manually. The shaft excavations are presently done mechanically using augers.
When the excavation reaches the load-bearing stratum, the augers are replaced by underreaming tools to construct the bell, if required.
The casing is used to prevent caving in of soil as the bore-hole is advanced deeper into the soil. Sometimes drilling mud/bentonite slurry can be used in drilling through sandy and gravely soils instead of the casing.
The bottom of the hole must be inspected physically by descending to the bottom to make sure that the load-bearing stratum has been reached and also that the under reaming is properly done.
The construction details are very elaborate and these are carried out only by specialized construction agencies (Tomlinson, 1977, 2001).
Other Design Details of Pier Foundation
The following are the steps involved in the design of drilled piers or drilled caissons:
1. The loads coming on top of the foundation are calculated. The weight of the pier is not usually included.
2. Establish the water level and soil profile at the location of the pier.
3. Identify the bearing stratum, that is, depth up to which the pier has to be constructed. Calculate the allowable bearing capacity.
4. Check for the safety of stresses in weak soil layers if present below the pier.
5. Check for settlements.
6. Design the shaft, bell (if required), and the cap.
7. Check lateral load capacity, bending stresses, and eccentricity.
8. Check for the uplift force.
Most of the above steps are similar to those used in the pile foundations since drilled piers are essentially large diameter piles. However, a few additional approaches used in practice are given below