Important Point
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.
Also, read: What Is Superstructures | Difference Between Load-Bearing and Framed Structures
#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 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.
#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.
Also, read: Classification of Stone Work | What Is Stone Masonry
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).
Also, read: What Is Water Cement Ratio | Water-Cement Ratio and Concrete Strength | Role of Water in Concrete
Other Design Details of Pier Foundation:
The following are the steps involved in the design of drilled piers or drilled caissons:
- The loads coming on top of the foundation are calculated. The weight of the pier is not usually included.
- Establish the water level and soil profile at the location of the pier.
- Identify the bearing stratum, that is, depth up to which the pier has to be constructed. Calculate the allowable bearing capacity.
- Check for the safety of stresses in weak soil layers if present below the pier.
- Check for settlements.
- Design the shaft, bell (if required), and the cap.
- Check lateral load capacity, bending stresses, and eccentricity.
- 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.
Pier Foundation:
A pier foundation is a collection of large diameter cylindrical columns to support the superstructure and transfer large super-imposed loads to the firm strata below. It stood several feet above the ground. It is also known as “post foundation”.
Is a Pier Foundation Good?
Because they are elevated, homes built on pier and beam foundations are less prone to flooding than those built on slab foundations. Pier and beam construction also provides easier access to plumbing and electrical lines which are located in the crawlspace beneath the home.
Pier Footing:
A pier foundation is a collection of large diameter cylindrical columns to support the superstructure and transfer large super-imposed loads to the firm strata below. It stood several feet above the ground.
Is Pier a Deep Foundation?
A pier is generally considered the type of deep foundation which is constructed by placing concrete in a deep excavation large enough to permit manual inspection.
Are Piers Considered a Permanent Foundation?
Post and pier homes, manufactured homes, modular homes, and mobile homes are just a few homes supported by piers. Furthermore, piers can be made from stacking concrete blocks. So, often you will find piers considered a permanent foundation.
Masonry Pier:
Piers are essentially columns that might have wall segments-like appearance. It is quite common for some masonry walls in a building to have openings; the wall segments between the openings are commonly referred to as piers.
Can I Build a House on Piers?
Many old houses and small buildings use pier foundations. A pier foundation is an excellent way to reduce the overall cost of your home. If you don’t require the basement or crawlspace provided by a continuous foundation, a pier foundation can solidly support your house while drastically reducing your materials cost.
What Does It Mean to Pier a Foundation?
Piering is the technique of driving steel pipe pilings to remedy failing building foundations and to correct foundation settlement. Push piers consist of sections of galvanized or epoxy-coated steel pipe that are driven into the soil with a hydraulic ram.
Pier Foundation Construction:
Most of these homes have a pier, or pier-and-beam, foundation a construction style similar to a wood deck. … Basically, it is a series of vertical pillars or piles that transfer the building load to the soil. Beams are then built on top of these columns.
Caisson vs Pile:
Caissons are watertight structures made up of wood, steel or reinforced concrete built above the ground level and then sunken into the ground. Caisson is putting a box into underwater and pouring it with concrete. Pile is a column of material driven by a piledriver.
What Is a Pier Foundation?
Most of these homes have a pier, or pier-and-beam, foundation—a construction style similar to a wood deck. Basically, it is a series of vertical pillars or piles that transfer the building load to the soil. Beams are then built on top of these columns.
Pier Vs Pile:
Very similar to piles, piers are also deep foundation elements which are used to provide structural support. Piers, however, will invariably include the use of concrete/masonry and have a larger minimum-diameter than that of piles. There is no specific diameter at which a pile would become a pier.
Types of Piers:
- Solid Masonry piers.
- Solid RCC Piers.
- Cylindrical piers.
- Column piers or column bent.
- Trestle piers or Trestle bent.
- Pile piers or Pile bents.
- Typical Framed Piers.
- Hammerhead piers.
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