Basement Construction Guide

Here’s our guide to what’s involved in a basement construction project.

Simpile has been creating basements for over ten years to the highest construction standards and in many different types of building and site locations.

It’s important to realise that just because a builder is proficient at work above ground, it doesn’t mean they have the specialist skills and experience to build below ground. Basement projects can go horribly wrong.

We pride ourselves on being basement specialists, and we have a long track record in rock-solid construction projects built to stand the test of time.

Why build a basement?

Basement projects create more space in a home or commercial building, and add value to a property. Basements are particularly appealing for owners in urban areas where space is limited, and we have worked on a growing number of projects in London, Brighton and as well as basements in Surrey, Kent and Hampshire.

The reasons for building a basement are similar to those for investing in a normal extension, but instead of building on the same level, our expertise is in building below the surface. There is long list of possible uses for a basement including:

  • Gym
  • Games room
  • Indoor cinema
  • Playroom
  • Swimming pool
  • Sauna
  • Utility room
  • Library
  • Wine cellar
  • Music room
  • Secure room
  • Garaging

At Simpile we have successfully completed projects on basements for existing buildings and on basements for new builds. We have worked on one storey and two storey basements, as well as basements under gardens. 

Why do basements need a team of experts?

Basement projects involve a range of specialist skills. Construction projects are invariably a team effort, and we pride ourselves on working successfully in partnership with building professionals, including main contractors appointed by the client to take overall responsibility for the project.

  • We work with architects who provide the design and architectural drawings for the project and an estimate of costs. Where planning permission is required it is always advisable to use an architect with knowledge of local planning constraints.
  • We work with structural engineers who take the plans and undertake the load-bearing structural design after ground surveys and site investigations. The design needs to meet the strict building control standards required by your local authority. 
  • Once approved, Simpile will take the final plans, provide detailed costs, and undertake the below ground piling and construction work. 

Once the piling, foundations and other below ground work is completed and signed off, we will hand over to the above ground contractor.

How many people are needed for a basement project?

The reality of most domestic basement projects is that there is only adequate space for three or four people on site at any one time. Our groundworking team will start the project with excavation. They then hand over to the piling team which includes our grout operators responsible for managing cement and grout pumping and filling equipment.

All members of our team are trained and certified in site safety. They always work to Health and Safety Guidelines and we undertake full site risk assessments. We operate with full insurance cover for public and professional liability.

What are the steps in a basement project?

Creating a new basement begins with digging – a lot of digging! Our groundwork team will get to work with mechanical diggers (aided if necessary by physical manual digging). We encounter all sorts of soil from lighter sandy soil to heavy clay, but whatever the material it is loaded into dump trucks, sometimes using conveyors and chutes, and taken away by lorry. 

We always approach projects with full knowledge about any party wall issues, ground conditions, neighbours, trees, other buildings.Excavating below ground weakens support for the upper part of a building, so has to be undertaken with extreme care. As piling experts, our approach will always leave adequate support in place when we remove soil. We underpin buildings with piling to strengthen and stabilise the site. 

Piling involves boring holes in various ways, and filling the cavities with concrete to provide a strong, permanent support for walls, blocks and beams. For efficiency, we use auger bored piling which continuously drills and excavates soil. Our mini piling rigs can operate in limited spaces, forming mini piles to create a strong and stable foundation. 

If deep foundations are not possible, we use bottom driven piling based on a drop hammer driving a dry concrete mix into steel lining tubes. Rotary piling is used for larger piles. A large auger screw is repeatedly lifted with the soil and rubble being removed each time.

To hold back the soil we build retaining walls of various kinds. The type of wall depends on each specific site.

If there is enough access, we construct reinforced concrete retaining walls, with the concrete pumped over reinforcing steel frames from concrete lorries. For level sites, pre-formed concrete blocks are used to form gravity retaining walls. If access is restricted, we use mini-piling machines to build contiguous piled walls, with piles drilled close together to form a very strong retaining and supporting structure. For large or wet sites and quick installation, we use sheet piling walls consisting of interlocking steel sheets. 

We then get to work with levelling and compacting equipment to allow for floor construction.

Experience shows that one of the most important elements in basement projects is waterproofing. At Simpile we work with leading waterproofing specialists to ensure that our basements stay dry and can withstand potential long term damage from water.

Our basements usually feature cavity drain membrane systems. Polyurethane plastic sheets with dimples are fixed to the walls to create a cavity between the substrate and internal wall. Any water seeping through the wall is collected and drained away, if necessary with the help of pump systems in the floor structure. Alternative waterproofing techniques are cement-based tanking of internal walls, and the use of damp proof membranes on external walls where they are accessible.

To create a level, permanent basement floor we generally use poured concrete. Alternatively we use steel grids as a base material, as well as block and beam construction. Further options include installing underfloor heating and drainage work for basement toilets and washing facilities.

When does a Performance Foundation project finish?

In essence, what we sign off is completion of a waterproof content box. We complete accurate, fully documented pile logs, and ensure that building control approves our work and that the main contractor and client are fully satisfied. 

Next steps for your basement project

We work on basement construction with homeowners, national builders, architects and civil engineers.

Get in touch now to discuss your basement project.

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