Concrete Piles and drilled shafts are an important category of foundations. Despite their relatively high cost, they become necessary when we want to transfer the loads of a a heavy superstructure (bridge, high rise building, etc.) to the lower layers of soil. Another reason for choosing a pile foundation is the condition and quality of soil layers. Based on how they transfer the load into the subsoil, piles can be categorized as friction piles, and end-bearing piles. In friction pile, the load transfer is done through shear stress generated along the interface of pile and soil. In end-bearing pile, the load is transferred through the tip of the pile to a firm stratum. Drilled shaft, as its name implies, are drilled into the subsoil, and then filled with concrete. Generally, drilled shafts have larger cross sectional area (Barja M. Das, 2008)
Why and When to Use Concrete Piles?
Different types of concrete piles are used for different applications. Cast-in-place concrete piles, or driven shafts are two great examples of how the can be produced (made) and installed. When choosing a pile type, one should generally consider the following conditions:
1- Poor quality of upper soil layers
2 – When we have expansive soil in construction site
3- To resist uplift forces
4- To resist lateral loads (horizontal)
5- Bridge abutment and piers
Types of Concrete Piles
Concrete piles can be either pre-cast pile, or cast in-situ. Concrete piles are generally reinforced.
Pre-cast Concrete Piles
For pre-case piles, the reinforcement brings extra strength to resist bending moment during pile pick up, transportation, vertical loads, and bending moment as a result of lateral loads. They can be built in different sizes and shapes, as required for each specific use. Pre-case piles can be prestressed as well.
Cast in-situ piles are made by drilling a hole into the soil, and then filling out with concrete.
Cast-in-situ Concrete Piles
Cast in-situ piles can be divided into two main categories: cased, or uncased. Cased concrete piles are made by driving a steel casing into the soil. In this case, the mandrel is placed inside the casing. After reaching desired depth, the mandrel is withdrawn, and the casing is filled with concrete. In the case of the uncased piles, the casing will be gradually withdrawn.
Quality Control of Concrete Piles
Quality control of concrete piles is a challenging task. Engineers, and contractors rely on experience and well-established procedures and test standards to verify the strength, and consistency of pile materials. Non-destructive testing help reveal potential defects that might have happened during pile casting (in the case of cast-in-place piles), or transportation and installation (in the case of precast piles).
Different methods have been developed to evaluate the quality of concrete piles. Apart from general concrete tests (concrete cylinder samples, and slump test), different non-destructive testing (NDT) methods can be used to evaluate the quality and reliability of concrete piles. These test can help identify and quantify integrity and quality related issues. The following NDT methods are widely accepted and used for evaluating the integrity of piles:
+ Low Strain Impact Integrity Testing – Read More
+ Ultrasonic Cross Hole Testing for piles with accessible tip,
+ Parallel Seismic (ACI 228.2R) for piles covered up by pile-cap