Pier Foundation and Drilled Pier Foundations: Types, Construction, and Design

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, sometimes known as drilled concrete piers or drilled shaft foundation, 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, which are central to pier foundation design, may be described under four types. A pier foundation diagram would typically showcase these 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.

1. Straight-Shaft End-Bearing Pier.
2. Straight-Shaft Side wall Shear Pier.
3. Straight-Shaft Pier With Both Sidewall Shera and End Bearing.
4. Underreamed or Belled Pier.

1. 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.

2. Straight-shaft side wall Shear Pier

Straight-shaft sidewall friction piers pass through overburdened 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.

3. 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“.

4. 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 of drilled pier foundation design and construction:

1. Pier of any length and size can be constructed at the site
2. Construction equipment is normally mobile and construction can proceed rapidly
3. Inspection of drilled holes is possible because of the larger diameter of the shafts
4. Very large loads can be carried by a single drilled pier foundation thus eliminating the necessity of a pile cap
5. The drilled pier is applicable to a wide variety of soil conditions
6. Changes can be made in the design criteria during the progress of a job
7. Ground vibration that is normally associated with driven piles is absent in drilled pier construction
8. Bearing capacity can be increased by underreaming the bottom (in non-caving materials)

Disadvantages Drilled Pier Foundations:

1. Installation of drilled piers needs careful supervision and quality control of all the materials used in the construction
2. The method is cumbersome. It needs sufficient storage space for all the materials used in the construction
3. 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
4. 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 piers for house foundation 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 pier foundation design example and 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.

Frequently Asked Questions (FAQ) About Pier Foundations

What is a pier foundation?

A pier foundation is a type of foundation that uses large diameter cylindrical columns, known as piers, to support and transfer heavy loads from a structure to the firm strata below the ground.

How does a pier foundation differ from a pile foundation?

The primary difference lies in the installation method: a pile is driven into the ground, while a pier is installed by excavating a hole and then filling it with concrete.

What are the main types of drilled piers?

Drilled piers can be categorized into four types:

1. Straight-Shaft End-Bearing Pier
2. Straight-Shaft Sidewall Shear Pier
3. Straight-Shaft Pier with Both Sidewall Shear and End Bearing
4. Underreamed or Belled Pier

What is a straight-shaft end-bearing pier?

A straight-shaft end-bearing pier derives its support from end-bearing on strong soil or rock, with the overlying soil assumed to contribute nothing to load support.

What is a straight-shaft sidewall shear pier?

This type of pier relies on sidewall friction between the pier and the bearing stratum for support, penetrating deep enough to achieve the design load capacity.

What is a straight-shaft pier with both sidewall shear and end bearing?

This pier combines sidewall friction and end bearing to support the load. When carried into rock, it is also known as a socketed pier or a “drilled pier with rock socket.”

What is an underreamed or belled pier?

An underreamed or belled pier has an expanded base, called a bell, which increases the bearing area and helps support a larger portion of the load.

What are the advantages of drilled pier foundations?

1. Can be constructed in various lengths and sizes
2. Rapid construction with mobile equipment
3. Allows for inspection of drilled holes
4. Supports very large loads, potentially eliminating the need for a pile cap
5. Suitable for a wide range of soil conditions
6. Design changes can be made during construction
7. Minimal ground vibration
8. Increased bearing capacity with underreaming

What are the disadvantages of drilled pier foundations?

1. Requires careful supervision and quality control
2. Cumbersome construction process needing ample storage space
3. Lacks the compaction benefit of driven piles in granular soils
4. Difficult to construct in areas with heavy groundwater flow

What materials are used in the construction of drilled piers?

Drilled piers are typically constructed using concrete and reinforcing steel. Drilling mud or bentonite slurry may be used to stabilize the borehole in sandy or gravelly soils.

How is the construction of drilled piers carried out?

Construction involves excavating the shaft using augers, underreaming the bottom if needed, using casing or drilling mud to prevent soil caving, and inspecting the hole to ensure proper depth and underreaming before pouring concrete.

What are the key steps in the design of drilled piers?

1. Calculate the loads on the foundation
2. Establish water level and soil profile
3. Identify the bearing stratum and calculate its bearing capacity
4. Check stress safety in weak soil layers
5. Assess settlement potential
6. Design the shaft, bell (if needed), and cap
7. Evaluate lateral load capacity, bending stresses, and eccentricity
8. Verify resistance to uplift forces

Are drilled piers suitable for all soil conditions?

Drilled piers are versatile and suitable for a wide variety of soil conditions, although they can be challenging to construct in areas with significant groundwater flow or highly unstable soils.