Foundations and footings, what lies beneath
Updated: Aug 11, 2020
Foundations and footings, what lies beneath
One of the most important parts of your build is constructing your new home on well-designed foundations. Considering 30% of your build-cost will be underground there is a lot to think about with the majority needing to be considered well before any building works can commence.
In this blog, we share our knowledge to help troubleshoot this area of the building process, whether you are considering a new build or conversion.
A suitably qualified building professional such as an architect, engineer or surveyor would normally design the foundations (don’t just rely on your builder). Investment in both time and money at this stage is crucial as there are a number of factors you need to consider to enable the design to be completed.
A site investigation needs to be carried out to confirm what soil type your site has. More often than not, this is either forgotten about or not done due to financial pressures - as it is seen as an unnecessary expense. Based on many years of experience this is not only false economy but it could present possible issues later on. There is a commonly used phrase in the construction industry “…money (your budget) is either made or lost in the ground…”.
Whilst guided by your appointed professional, the investigation could either be a simple desktop study, prior knowledge of the local area, or specific site information. If none of these can be achieved or there are other influencing factors, a formal site investigation should be carried out to confirm the subsoils below. Depending on the locality, further investigations such as Radon, Contamination evidence or a mining report may also be needed. In some instances, you may need to seek arboricultural advice especially if there are Tree Preservation Orders or trees on land owned by other parties.
Ground water is something that also needs to be considered at the design stage as this may have a bearing on the type of foundation used, especially if you are considering a basement where additional requirements will need to be met under the Building Regulations or Building Standards. It may be possible to ‘dewater’ locally by pumping or, on larger projects, freezing the water may be an option.
Remember at this point other legislation that may need to be taken into account such as the Party Wall Act and the Construction Design Management Regulations - where failure to comply won’t just delay your build but it could lead to financial penalties/fines or even a custodial sentence!
Once all the information is to hand, your building professional can move forward in the design of your foundations. These could vary considerably depending on the soil type and may range from a simple strip or trench fill foundation to a complex piled foundation.
Once the design has been completed this is approved by the relevant Building Control Body in the Plan Check. Whilst historically this is signed off by them as part of a site visit, we would recommend that the foundations are also reviewed and signed off by your contracted building professional (hopefully also the designer) to provide you with additional protection.
With the design agreed, works can commence. Always use a competent groundworker, who is adequately Insured with an A-rated insurer because there could be potential dangers underground such as utility cables, pipes and sewers that may need redirecting leading to multi-million pound claims. Insist on seeing proof of their insurances or a letter from their Broker or Insurer.
As with all the planning, it is still possible to find the unexpected and particular attention should be made where you are building close to other buildings or on brownfield sites. Traditionally with greenfield sites, i.e. no previous development, it may be possible that a small trial hole could be dug to confirm the soil type and this would also be combined with a percolation test to enable any soak away design to be carried out. This would normally be done with your Building Control Body and Designer in attendance in order that a way forward can be agreed.
In choosing the foundation a “traditional” cut off point of a 3m depth is often used before a change in the type of foundation is considered. This is due to the financial costs in the use of concrete but also the health and safety of personnel working at depth. At this point piled foundations or reinforced concrete slabs could be considered as a viable option.
There are many types of piling but in the main these are limited to augured and displacement piles and these would be designed by your nominated piling contractor.
In considering trench fill foundations the Building Control Body and Warranty provider would normally approve the trenches shortly prior to concreting. The trenches should be cleaned out with the sides true and square and any water which has seeped into the trench removed. Be prepared to do some last minute work, and sometimes you may be asked to dig a little deeper!
Another aspect to consider is a sloping site where there may be a stepped foundation or other external factors such as trees. In a cohesive (clay) soil you may need to go deeper because of the possibility of the presence of tree roots and you will need to comply with Table 4.2 “building near trees” of the NHBC Guide. Removing trees in close proximity of your proposed dwelling shortly before building may not be an option as you could cause long term damage to any nearby house (as well as to yours) in the form of heave.
Questions? get in touch and we will be happy to help with any queries.