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How Does Basement Excavation Work?

Basement extensions are becoming a widespread and viable option to gain extra space for your home, especially in built up areas. For instance, most of London requires people to dig downwards if they are looking to extend.

Most existing basements are quite shallow and won’t be tall enough to accommodate your design plans. However, there’s a lot of potential when it comes to basement conversions. They can be used to create an office, a spare bedroom or another lounge area. Because of its presumed location in the house, there is more scope for what you can do with it as opposed to converting an attic or loft. Lofts lend themselves to being turned into bedrooms whereas a basement can be converted into almost anything. Some people estimate that a well-done basement conversion can bump up the price of your home by about 30%.

This article is going to explain the excavation process from start to finish to answer any questions or concerns you may have about it. At Berkeley Build, we can offer you our services from start to finish throughout this whole process, from planning to finishing.

Consultation and Planning

The process begins with consultation. Although you don’t have to call in the professionals for every step of this process, even if you’re a nifty handyman, you should consult and seek professional advice for this. We’re not just saying this because we’re professionals, excavating a basement involves working on the foundations of the whole house so it is very ill-advised to go into this without expert help as you could cause some serious damage to your home.

Consultation is where you can discuss what you want from your basement conversion and what your vision for it is. There’s loads you can do with a basement conversion and just by looking online you can glean a lot of inspiration and see what people have been able to do with their basements.

After the consultation, you will have a rough idea of the cost of what you’re undertaking and you have an opportunity to iron out the details with an expert. Your property will then need to be surveyed and any planning permissions that are needed will then be acquired.

Basement Underpinning

Once you have your planning permissions and a design you’re happy with, building can begin. The first stage of this process is called hoarding the construction site. This just involves organising the building site and making sure there are safe rights of way. A small structure may need to be installed at the opening of the excavation. Temporary waterproofing measures will be taken so as not to flood your foundations.

Your property then may be underpinned as the excavation process continues. Underpinning strengthens the foundations of a home. As you may have to dig beneath existing foundations, underpinning ensures that, chiefly, your house won’t collapse. There a few different methods for this.

Mass Concrete Underpinning

The most common way to underpin a house, the mass concrete method includes excavating below the foundations in stages. You will excavate an area below the foundations and then fill it with concrete. The building is transferred onto its new foundations through the use of dry sand cement but there are other factors such as groundwater level that may affect this. This method has been in use for centuries and is the generally the most cost-effective. Other methods of underpinning use this as a basic template but might transfer the load in different ways.

The type of basement conversion you choose will depend on your needs – what you plan on using your basement space for, and whether you currently have a basement/cellar space.

Expanding Resin Injection

A relatively new method of underpinning that has been around for about 30 years but is only more recently seeing more and more popularity. If your foundations need levelling out or certain weak points need fortifying for whatever reason, this is the least disruptive method. This involves injecting structural resins into the problem areas where through a chemical reaction causes them to expand, filling any cracks and strengthening the surround the soil by compacting it.

Mini-Piled Underpinning

This method is often turned to if the ground conditions vary a lot and you need to transfer the load at depth, normally over 5 metres.

Beam and Base Underpinning

This method involves using a beam to transfer the load to mass concrete bases. This is slightly more technically advanced and requires constructing a reinforced concrete beam.

If underpinning is required, that will be carried out and then excavation can begin. This involves digging into the existing foundations to increase the ceiling height and so on.

A structural engineer will advise on whether any steel, timber or reinforced concrete beams need to be installed during the excavation. This will support any loadbearing walls, the floors and the existing foundations.

Basement Tanking

After the basement has been cleared, you will see your new space taking form. The next step is called tanking. Put simply, this means waterproofing the basement. Using something called tanking slurry, which is a mixture of cements and chemical modifiers that combine to stop the passage of water, you apply it to the walls thoroughly.

Going underground involves careful consideration of the water table. If done incorrectly, hydrostatic pressure can build up and water will force its way into the basement. Waterproofing is definitely best left to the experts. If you aren’t totally sure what you’re doing, the risk of getting it wrong is far greater than what you would save by cutting corners.

On top of tanking, drainage systems are put in place. A good drainage system will ensure that your basement never gets flooded by ground water. One method of this is installing a French drain which will relieve hydrostatic pressure on your basement walls by redirecting surface water. Once the correct waterproofing measures have been undertaken, your basement has been excavated. At this point, you will be able to really see your basement conversion taking shape.

The excavation process isn’t too disruptive, and most people choose to stay in their homes whilst this is going on. The main sources of disruption will be the noise and mess. Excavating can be noisy business. In terms of how long this will take, a full basement conversion can take up to half a year but that depends on conditions and who you hire. Generally, you can expect it to take 12-24 weeks depending on the size of the project. The excavation is generally what takes up most of that time. Once the excavation has been done, its just a case of fitting carpets, electricity etc.

So, if you’re considering a basement conversion but are still unsure of what that entails hopefully this post has cleared up the excavation process for you. Our main piece of advice is to consult professional help if you are planning on having a basement conversion. We can help you through every step of the process so don’t hesitate to get in touch if you would like some more information or to get the ball rolling on your new basement.