Pile Foundations and their Suitability within Construction
As housing developments upon the UK's brownfield sites continues apace the industry has had to adapt and change in order to consider the characteristics of such sites.
As housing developments upon the UK’s brownfield sites continues apace the industry has had to adapt and change in order to consider the characteristics of such sites. And whilst these developments have been gaining in commonality over the past twenty years the challenges that rising costs of excavated material disposal and the viability of piled foundations present continues to act as key concerns for many a house builder, contractor and architect. What’s more these challenges become ever more relevant for those who are constructing low rise housing. To this end this article looks at the landscape of pile foundations, explore their benefits and explain where they serve as a suitable option within construction projects.
Let’s Clear up Misconceptions Surrounding Pile Foundations
It’s an industry understanding that pile foundations generally tend to require less material to construct when compared to traditional trench fills, and as such they can provide cost reductions, as well as construction deadlines and sustainability. However despite these benefits the specific criteria for when piled foundations are to be used can seem somewhat unclear. This lack of understanding, and indeed the lack of resources and information to clear up such poor understanding, is a particular issue that the NHBC Foundation has focussed upon addressing, such as their recent publication upon Efficient Design of Piled Foundations for the Low-Rise has shown. So in this article, with the following sections, we aim to clear up the confusion that surrounds correct design approaches and selection of pile foundations, specifically in relation to the construction of low rising housing.
The Benefits of Pile Foundations
‘Direct savings’: Time and Money
Let’s begin with the many benefits of pile foundations. As a key finding within the report there were highlighted some benefits that went against previous perception, a key such example being that separate design criteria is not required when used for the construction of low rise housing.
Rather the most important factor that affects the foundation of a design is the possible damage that can occur upon the built structure. To this point it goes without saying that fissures and cracks are unacceptable within construction and the governing focus should always be upon the avoidance of potential differential movement (regardless of the foundation choice).
What’s more when avoiding differential movement, piled foundation presents another benefit in the form of less material being required for construction (which consequentially avoids the extensive excavation of ground material). This in itself then serves to speed up construction, all whilst avoiding the high associated costs with waste disposal. These benefits we shall refer to as ‘direct savings’ for the house builder, however there are additional benefits to be considered, with each falling under the umbrella of ‘indirect savings’.
‘Indirect Savings’: Avoiding Deep excavation, Environmental Agency concerns and sustainability
Let’s start with one of the most significant indirect savings that pile foundations can provide, which is the complete avoidance of deep excavation. This can serve to not merely avoid health and safety fears and concerns, but it can moreover address specific concerns of the Environmental Agency, such as the possibility of contaminated groundwater permeating underground water sources (which generally occurs as a direct result of the disruptions caused by foundation building).
This particular point is ever more relevant when constructing upon brownfields sites as contamination can present a real risk and should always be considered by the construction professional in question. To this end their choosing of the correct pile foundation can provide a tangible solution in minimising environmental risk.
What’s more when we consider the ever relevant issue of sustainability we find from the research within the report that pile foundations result in significantly lower embodied carbon than is found with alternative options. This means that the lower waste and reduced need for excavation is not only less costly, but also avoids the penalisation of traditional trench fill foundations by the 1996 Landfill Tax.
Going beyond the construction stage
When looking past the construction stage we find that pile foundations are helpful for ensuring that houses meet the demanding requirements of the Sustainable Homes scheme.
This scheme, which relies upon independent assessors, has been adopted UK wide by increasing number of local authorities in a bid to make housing within the UK greener. Specifications under the scheme encourage ‘Geothermal piles’, which utilise pile foundations along with a ground closed loop heat exchanger. This system then benefits from the subterranean thermal differences to heat the buildings that are based above them.
This is then another way in which pile foundations present an efficient, cost effective solution for the brownfield development, all whilst coming with impressive green credentials.
Where and When are Pile Foundations Used In Construction And Engineering
Construction and engineering are challenging fields at the best of times but often projects prove to be especially difficult and you are required to re-evaluate and come up with solutions which you may not have thought of before.
Screw pile foundations are often one such solution offered as an alternative to traditional pile foundations when the usually way of ensuring the stability of your building just isn’t feasible. There are a range of benefits of screw pile foundations and many reasons why this should in fact be any engineers preferred method of underpinning buildings and creating foundations for sky scrapers and other large buildings.
Here we look at some of the biggest advantages of using screw pile foundations; a technique which has been around for centuries but recently developed to become a cornerstone of the construction world.
Poor soil and Pile Foundations
One of the biggest advantages of using screw pile foundations is that they can be used when the ground and soil is poor quality and would usually be thought of as too weak to hold firm foundations. The helical fins which give the screw pile is screw-like appearance, can be adjusted and reconfigured depending on the type of ground and soil that they are being installed in. this means that they can be used in weaker soil as the screwing motion in which they are installed will compact the surrounding soil and a higher torque can be used to put them in the ground meaning the weaker soil isn’t an issue. While using grout isn’t usually necessary in the installation of screw pile foundations, this is an option if the soil is in really poor condition, to fill up any remaining gaps.
Negotiating restricted access
Another common scenario which would call for the use of screw pile foundations over traditional driven pile foundations is if there is restricted access to the site or not much room on the site itself. Driven pile foundations require a crane and a pile driver; both of which are large pieces of machinery. Screw pile foundations can be installed with simply a low speed motor with someone to operate it and so they are much easier to install on smaller sites where getting a crane in just isn’t possible. The fact that there is no need for the heavy machinery also helps to cut costs.
Money saving with piles
Using screw pile foundations is without doubt a much cheaper option than traditional driven pile foundations. As mentioned above, you don’t have to pay out for the large machinery and trained labourers to operate it, but you also save money in the costs of site clean-up once the foundations are installed. As screw pile foundations compress the soil they are installed in, you do not need to excavate or remove any soil before installation. This means there is less clean up and waste.
Use in an emergency
A big bonus of screw pile foundations is that they can bear weight immediately after installation. As there is no concrete or grout used there is not drying time to wait for. This means that the foundations can load bear immediately which makes these types of foundations perfect for use in an emergency. Where construction schedules are extremely short and the rest of the project relies on how quickly the pile foundations can be installed and ready for weight; screw pile foundations are the only option.
Environmental benefits of pile foundations
We are living in an age where being environmentally responsible is high on everyone’s list of priorities. Construction often has a large impact on the environment so any steps which can be taken to minimise that impact should be taken. Screw pile foundations have been developed so that they can be removed if needed and re-used elsewhere making them a much more environmentally friendly option than traditional driven pile foundations. They also create less noise and vibration during installation as well as creating less waste and not needing the heavy machinery that their alternative does. Contractors using screw pile foundations will have a much lower carbon footprint on that project than if they were to use standard pile foundations.
Any project manager will tell you how important finishing on schedule is as going past deadline on a construction schedule will end up costing much more money. Not only are screw pile foundations quicker to physically install but as they can bear weight immediately, do not need heavy machinery and most importantly, can be installed in any weather conditions, using this type of pile foundation is much more likely to see your build completed on time.
Pile foundations are being used increasingly in engineering - but what are they and why would you consider them
The foundations of a building are extremely important in construction for very obvious reasons. If your foundations are poor or not right, then you are risking the entire stability of your building and that simply isn’t acceptable.
Those in construction will know that the bigger or heavier the building, then the deeper the foundations need to be. There are different types of foundations which you may choose to use depending on the size of the building to be constructed above. A pile foundation is a very strong and reliable type of foundation which has been used in construction for quite some time due to its renowned reliability.
This article will take a look at what exactly is a pile foundation and why you may consider using them in your construction project.
Deep foundations and why a civil engineer would want to use them
Foundations fundamentally transfer the weight of the load above them to the earth below. They ensure that the building above remains stable and grounded. Deep foundations are capable of supporting a much greater weight than shallow foundations and are used for tall or large buildings such as sky scrapers. Engineers may also decide to use deep foundations if the soil they are building in on is not stable enough to hold the weight of a building. Firmer ground may be found at a deeper depth and so a deep foundation is dug to reach it. Another reason for deep foundations being used could be site restraints such as boundary lines which would restrict the width of the base of the building meaning deeper foundations are needed to spread the weight of the building evenly.
What are pile foundations?
Pile foundations are also referred to as driven foundations as they are driven into the ground. Pile foundations are poles which can be made from wood, concrete or steel which are driven with force into the soil in order to be able distribute the energy and force they bear from the building above, to the surrounding soil. Wooden piles are traditionally made from the trunk of a single tree which can restrict the depth at which they can be driven but historically they were spliced if a longer length was needed. The Italian city of Venice is famous for being built on wooden pile foundations driven into the sea bed.
Concrete piles are pre-cast as ground level and then hammered into the ground, as are steel and H shaped piles. These types of pile foundations are put into the ground using a pile driver which repeatedly hits the pile with a weight; forcing it into the ground. The surrounding soil can either be excavated by digging or displaced as the pile is driven into the ground.
Once placed in the ground, steel piles are topped with a concrete pile cap. This cap sits on the top of all of the piles used so that the weight of the building constructed on top of it will be evenly distributed across all of the piles in the ground. The thick concrete base serves as a strong, level and sturdy base for a column to be constructed upon.
Why use a pile foundation?
The process of drilling or driving a pile foundation into the ground displaces the soil into the surrounding area. This creates a tight space for the pile to sit and the friction created by the pile being in place helps the steel to transfer the weight energy it supports into the surrounding ground. This friction created between the pile and the soil actually increases the load-bearing capacity of the pile itself. Therefore, using pile foundations allows engineers to design taller and bigger buildings without the need of a large base area as the foundations will be capable of supporting the weight of the large column above. The way piles are often driven or screwed into the soil is also seen as a great test of their durability and strength due to the brutal way in which they are installed.
There are a range of types of pile foundations which have been developed to suit different types of installations. For example; large single piles, known as monopole foundations, use one large pile foundation for use in supporting wind turbines and screw pile foundations are shaped more like a screw than the smooth cylindrical steel pile and are screwed into the ground. These types of pile distribute the weight load and size to the soil accordingly.
Pile foundations are used the word over by engineers intent on creating buildings which will stand the test of time.
Pile Foundation Types
Dependent on how the piles are constructed, foundations are either known as drilled or driven foundations.
1. Driven Screw Pile Foundation Or Prefabricated Piles Foundation
The driven pile foundation are the type whereby prefabricated piles are delivered to a site and then driven into the ground to create a foundation. For this reason, driven piles come in a variety of suitable materials such as steel, reinforced concrete, timber or concrete.
2. Drilled Pile Foundation Or Cast-In-Situ Piling
When it comes to drilled pile foundations, the piles will be cast into the desired position. The pile casing is typically driven into earth or the ground and filled with concrete and reinforcement to arrive at the cast-in-situ pile. The other method involves an auger that is hollow stemmed that drills into the ground. This auger is then pumped with a material such as concrete in order to build up the pile.
Both drilled and driven techniques used for the pile constructions come with their own advantages and drawbacks. Here is a list of a few.
Benefits Of Prefabricated or Driven Piles Foundation
• The prefabricated piles are already ready-for-use. This can save on a lot of time on a construction site.
• The prefabricated pile is usually tested for its quality and strength. This means that any flaws found can be dealt with beforehand.
• The prefabricated pile can even be driven when placed under water which makes them suitable for much of the locations.
The Drawbacks Of Prefabricated Or Driven Piling
• The prefabricated piles are susceptible to damage while driving.
• Additional efforts for handling, transportation and costs are incurred when using the prefabricated piles.
• In most cases the prefabricated pile needs to be altered in its length when it arrives on site. This also requires additional costs and efforts.
The Benefits Of Cast-In-Situ Or Drilled Piles Foundation
• This type of pile does not require additional costs associated with transportation.
• The length of the piles is exact which means they are ready for casting on site.
• You can under ream drilled piles in order to maximize their bearing capacities. An under reamed pile always has a bigger base which means it can handle a larger load-bearing capacity.
The Drawbacks Of Cast-In-Situ Or Drilled Pile Construction
• The pile’s quality is not determined and the reinforcement inside the pile can displace during the process of casting.
• Fresh concrete is susceptible to damages from corrosive elements present inside the soil.
• Casting of piles takes a long time, which delays how quickly the construction proceeds.
• You cannot cast these piles under water.
• Adjoining walls of a pile can cave-in during casting which results in structurally weaker piles.
Friction And Bearing Piles
Piles can also be classified as friction or bearing in the way in which they are able to transfer loads. The bearing pile reaches the earth’s hard stratum and therefore acts as a load transmitter. The friction piles are the floating pile that are not able to reach the harder surfaces. These loads are endured by friction that occurs between the soil and the pile-surface.
What’s the Difference Between A Pile Foundation And A Screw Pile Foundation?
Pile foundations allow engineers to be able to design tall and heavy buildings without worrying about whether they will sway or topple as the foundations are strong enough to be able to take the weight.
There are a range of types of pile foundations, some with specific functions to enable constructors and engineers to be able to build varying projects in various locations. While screw pile foundations are quite similar to standard pile foundations, the differences offer an array of benefits to engineers and constructors alike.
The Shape of the Foundation
Screw pile foundations, often referred to as helical piles, have helical fins attached to a galvanised steel pipe which gives the pile the appearance of a traditional screw. Standard pile foundations are usually a cylindrical shape with a smooth surface to enable them to be driven into the ground easily. The physical nature of the screw or heliacal pile allows it to be screwed into the ground; minimising the impact of their installation. The shape of the screw pile compacts the surrounding soil as it is being installed, rather than displacing it. This means that there is less site clean up after installation as well as creating an increased force of friction which, in turn, increases the load baring capacity of the foundation.
The installation of the pile foundation
As mentioned above, the installation process of pile driving a standard pile foundation, and the installation of a screw pile foundation are vastly different. Pile driving creates a lot of noise and vibration which can be very disruptive, especially if you are working in an operational environment such as public building. Installing screw pile foundations also requires less heavy machinery as you do not need a crane or pile driver on site as all that is needed is a low speed, high torque motor with a single operator. Screw pile foundations are quicker to install, can also be installed at an angle of up to 45 degrees and can be weight loaded immediately as you are not waiting for any cement or grout to harden before capping.
Whereas standard pile foundations are designed to stay in place and support a building for the entire time that building is in use, screw pile foundations can be removed at any time. Due to their shape and the nature of their installation, screw pile foundations can be removed when required and then be used elsewhere. Screw pile foundations can also be adjusted to any depth so if you soil in which you are working has different qualities at different depths, you simply need to screw down as far you require and the cut pile off at the surface reducing waste. Screw pile foundation can be used in a variety of soil types, even in poor or weak soil. The helical plates can be adjusted and the configuration modified in order to increase the load capacity of the foundation or to gain extra traction in weaker soil types. Screw pile foundations can also be installed in any weather, rain or shine, due to the fact that the drill straight into the ground and there is no need for dry conditions for concrete or grout to set. These types of foundations can also be installed on sites where access may be limited and space at a premium. If you are working in tight space constraints, then the smaller machinery needed to install screw pile foundations will make the job much more straightforward.
The cost of the foundation
For any engineer, contractor or project manager, keeping on budget is essential. Screw pile foundations often work out as a much more financially viable option for construction companies due to a variety of reasons. As there is no excavation or site mess created then money is saved on the clean up after the installation. Money is also saved on heavy machinery and operators compared to driven pile foundations as cranes and rigs are expensive but low-speed motors are much cheaper to hire and run. The fact that screw pile foundations are quicker to install will also save you project money and that combined with not being delayed due to adverse weather means that screw pile foundations are much more likely to help get your project finished on time.
While there may be construction jobs where traditional pile foundations are the better solution; however, screw pile foundations are a cheaper and quicker solution for many smaller sites and businesses who are looking at the newest developments in foundation laying to help them keep on track and on budget too.
If you want to discuss pile foundations and how they can be of benefit to you, get in contact with us.
Copyright © Jan 2016