Understanding Pile Foundations
Table of Contents
What are the types of pile foundations and why do we need pile foundations?
Pile foundations are often required for buildings that have large loads coming does their structural column that needs to be transferred to competent ground strata. For example, a 10-storey steel structure with concrete floors will have significant loads through the supporting columns. This will need to be transferred to a ground which has enough bearing pressure which may not always be close to the top layer of soil. These types of pile foundations are classed deep foundations.
If strong rock (i.e. sandstone) is located 10m deep below the ground level, it will not be economical to excavate 10m to place pad foundations and extend columns 10m below ground. It will be cheaper to install piles reaching the competent and using that as the support of the building.
Piles can be thought of as columns in the ground supporting the structure, as they undertake compression, tension and shear. It is best to avoid tension piles and shear as concrete piles can not always undertake theses forces and may require large bolt boxes in the pile caps and more piles in the pile group to distribute tension and shear forces. This increases the costs as complicated connections as bespoke designs are expensive.
In general, these types of pile foundations are installed and designed by specialists sub-contractors but it is always important for designers to understand the piling construction techniques and designs.
End-bearing Piles
End bearing piles are types of pile foundations, which supports that rely on the large bearing capacity of strong bed rock and involves installing piles and connecting/screwing them into bedrock, and its location in the ground depends on what the ground investigation indicates.
These piles are embedded into the bedrock (depth of embedment depends on SI report) but the embedment affects the bearing capacity (i.e. structure is supported on piles which act as columns).
Skin Friction Piles
Skin friction piles are types of pile foundations, which rely on shear stresses generated along the sides of the pile, which increases pile-bearing capacity. The load is transferred from the pile to the soil along the length of the pile through friction/adhesion. Therefore, the longer the pile, the greater the pile bearing capacity.
An easy way on thinking about it is placing a flake in a cornetto, the deeper the flake in the ice cream, the more secure it is, but if it is not deep enough, then the chocolate flake will fall off.
Skin frictions become an option when the depth of bedrock cannot be found or is too deep, where end bearing pile becomes un-economical.
CFA piles
Contiguous flight auger piles drill into the ground, removing the soil and pour concrete simultaneously leading to faster installation compared to bored piling. Reinforcement is then added into the wet concrete and depending of the mix and additives of the concrete, there is usually a 4-hour window of adding the reinforcement in before the pile fully sets. (After it sets, you can’t place the reinforcement in the pile).
The use of self-compacting concrete is common as it is not always possible to get a vibration rod at deep levels to compact the wet concrete.
Bored Piles
Bored piling is like CFA piles except the process of drilling the hole and pouring the concrete is not simultaneous.
The ground/soil location of the pile is penetrated using a temporary casing, and then the soil is drilled out and the concrete is then poured into the empty hole (once soil has been removed). The reinforcement can be added before the pouring of concrete or afterwards (depends on the contractor).
The reasons for using bored piles over CFA is to stabilise the slope stability in areas next to buildings or undermining the existing foundations of buildings.
Any existing foundations or basements need to be broken through with the temporary casing as the piling rig (drill components) will break/damage if it drills directly into any foundations. (It is common to find basements/pads/piles on a demolished site or in the city)
Driven piles
Driven piles can be either steel, concrete, pre-cast concrete or timber and are slender members which are installed on site using a pile hammer. The pile is compacting the ground along the side increasing the soil density, which also increases the bearing capacity.
This is useful for offsite manufacturing as it allows for a faster construction time of site (i.e. no need to wait for concrete setting or removing soil from ground). The installation of this type of soil removes the risk of subsidence in soil and there is no need for soil disposal.
However, these types of pile foundations can get damaged and must be designed to withstand the impact loads during installation. Vibration becomes an issue and can affect nearby building or construction sites.
Also, driven pile is noisy and will cause complaints in towns and cities, so it is unwise to use them in highly crowded areas.
Sheet piling
Sheet piles are generally steel but can be reinforced concrete or timber. They are generally installed with interlocking edges or grooves and used as a temporary measure to act retain the soil, which allows the excavation of the soil.
The type of sheet pile is dependent on the ground conditions, is it a temporary or permanent measure, the depths of the pile, type of protection required and the construction sequence, which can have a big impact on the design and depth of the sheet pile. Consideration should be taken for the piling rig, to see whether it can access the site or enough space for the rig (i.e. can this be done next to a busy highway or in a tightly restricted area).
Mini piles
Mini-pile or micro-piles are piles with a diameter ranging from 200-600mm using high strength, small diameter steel casing and is generally used for underpinning works or in areas where access is highly restricted. This could be in an existing building where additional piles are needed for a existing piled raft (i.e. retrofitting or changing the use of existing buildings changes the loads and load paths).