Bored Pile Foundation Techniques and Benefits
Bored pile, also called drilled shaft, is a type of reinforced-concrete foundation that supports structures with heavy vertical loads. A bored pile is a cast-in-place concrete pile, meaning the pile is cast on the construction site. This differs from other concrete pile foundations, like spun pile and reinforced concrete square pile foundations, which use precast concrete piles. Bored piling is commonly used for bridge work, tall buildings, and massive industrial complexes, all of which require deep foundations.
Bored Piling Process
Installing a bored pile starts with drilling a vertical hole into the soil, using a bored piling machine. The machine can be outfitted with specially designed drilling tools, buckets, and grabs to remove the soil and rock. Piles can be drilled to a depth of up to 60 meters and a diameter of up to 2.4 meters. The drilling process may include driving a temporary steel cylinder, or sleeve, into the soil. This remains in place in the upper portion of the hole until the pile is poured.
Once the hole is drilled, a structure of reinforcing steel rebar is built and lowered into the hole, then the hole is filled with concrete. The top of the pile may be capped with a footing or pier near ground level to support the structure above.
Bored piling work must be done by bored piling contractor, It is a highly specialized operation requiring extensive knowledge and experience of bored pile construction and engineering as well as soil and site conditions.
Bored Pile Applications
When used for foundations, bored piles are specified for buildings and other structures that impose loads in the thousands of tons. They are also particularly well suited to unstable or difficult soil conditions. In addition to foundation systems, bored piles are used to create structural underground walls for earth retention. Piles can be placed close together, with 75 to 150 mm between piles (called a contiguous pile wall), or they can be placed so they overlap, to create a secant pile wall, which is often used to control migration of groundwater.
Challenges to Bored Pile Construction
Like any other deep foundation system, bored piles present challenges for the contractor. Because the drilling method used depends on the soil type, the contractor must complete a thorough soil investigation and report. The piling contractor relies on the soil report and past experience to choose the best drilling method that will minimize the disturbance of the surrounding soil.
When the soil is cohesion-less, such as with sand, gravel, and silt, or the hole extends below the water table, the hole must be supported using steel cages or stabilizing mud, such as bentonite suspension. This solution can be a very messy process and add considerable complexity to the project.
Bored Pile Advantages
The main advantages of bored piles or drilled shafts over conventional footings or other types of piles include:
- Piles of variable lengths can be extended through soft, compressible, or swelling soils into suitable bearing material.
- Piles can be extended to depths below frost penetration and seasonal moisture variation.
- Large excavations and subsequent backfill are minimized.
- Less disruption to adjacent soil occurs.
- Vibration is relatively low, reducing disturbance of adjacent piles or structures.
- High-capacity caissons can be constructed by expanding the base of the pile shaft up to three times the shaft diameter, thus eliminating the need for caps over groups of multiple piles.
- For many design situations, bored piles offer higher capacities with potentially better economics than driven piles.