Basement excavation
This article needs more work, to help develop this article, click ‘Edit this article’ above.
Contents |
[edit] Introduction
A basement is part of a building that is either partially or completely below ground level. Approved document B of the UK building regulations, Fire Safety, Volume 1 Dwellinghouses, defines a ‘basement storey’ as:
‘A storey with a floor which at some point is more than 1,200 mm below the highest level of ground adjacent to the outside walls.’
Basements can be constructed using brick or concrete block, poured concrete, pre-cast concrete, or even treated wood.
Basements generally become more expensive as the depth increases. However, in prime locations, the cost of land may justify multi-story basements or even below-ground parking garages. This is becoming increasingly common in central London where even larger ‘iceberg basements’ have provoked much controversy as they can cause significant disruption to and disturbance of neighbours.
See Basement Excavation (Restriction of Permitted Development) Bill and Planning (Subterranean Development) Bill.
Concerns about the disruption caused by the construction of basements, and the trend for 'iceberg' basements have resulted in attempts to introduce restrictions, such as the Basement Excavation (Restriction of Permitted Development) Bill and the Planning (Subterranean Development) Bill.
In August 2016, the City of Westminster introduced a new code of construction practice with an average levy of £8,000 on the construction of basements and a new ‘subterranean squad’ to help reduce the impact of basement construction works.
[edit] Excavation process
Depending on the size of the property, the amount of excavation and building work required and its complexity, the work for constructing a basement to a house will last typically for 12-20 weeks.
The first stage is for hoarding to be erected and usually for a timber shelter to be constructed in front of the house or around the opening to the excavation. An initial external opening to the basement area is created and temporary weather-proofing and supports are installed where necessary. A backhoe or excavator can then dig down, with excavated material brought to the surface and into a skip. Existing foundations may be underpinned if necessary as the excavation proceeds. Existing building services may need to be re-routed.
[edit] Types of construction
Very broadly, there are three generic types of basement construction:
[edit] Poured concrete
The external structure of the basement is poured into formwork, with reinforcing steel bars included if required. Forms are then stripped away after curing. This type of construction tends to be stronger than others and is much more resistant to water infiltration.
[edit] Block, masonry walls
This tends to be the most economical option for small-scale basement construction and often requires less time. However, concrete blocks are not suited to soil conditions that are prone to swelling as this applies lateral pressures to the basement wall which can weaken joints.
[edit] Precast panels
Precast concrete panels can be transported to the site and assembled on footings. This method is not as common but can be economical where there are multiple basements being constructed at the same time. If the joists between panels are not properly sealed then moisture can get trapped in the panels or penetrate to the interior and cause problems.
[edit] Waterproofing
BS 8102: 2009 Code of practice for protection of structures against water from the ground, provides three categories of basement waterproofing:
[edit] Tanking
A continuous waterproof barrier is applied to the inside or outside of the basement structure. The most common form is a bituminous ‘stick-on’ plastic sheet. Whilst this is relatively inexpensive, it can lose adhesion and is easily damaged during backfilling.
Alternatively, an external membrane can be painted or sprayed onto the external surface which can be covered by a drainage board to allow provide protection from the backfill.
[edit] Structurally integral protection
This is chemically-enhanced water-resistant concrete which can be used in combination with a waterproof membrane.
[edit] Drained cavity
Cavities are formed within between the internal and external wall and floor constructions to collect and drain away water entering the basement using a sump and pump. As well as the internal drain, a perimeter drain may also be included. This runs around the external perimeter of the building just below the level of the foundation, removing water from the building’s external face.
[edit] Insulation
As basements are surrounded by earth, their temperature has a tendency to remain fairly even throughout the year. There are several methods of insulating a basement, such as foam insulation, rigid insulation boards, or fibreglass batts. These allow for the retention of heat inside the basement and the prevention of condensation on walls which will eventually lead to mould growth. It is particularly important that basements are properly ventilated as they may have a vapour-impermeable construction and are unlikely to have openable windows.
Building services are likely to supply the basement from above, and drainage from the basement is likely to require pumping.
[edit] Risks
Compared with building higher, building down can involve more risks. The complexity of basement excavation increases with the depth, a higher water table, and a more congested site.
Prior to construction, good site information and soil tests are critical in terms of being able to adequately plan and forecast potential difficulties that may arise.
- Drainage difficulties and risks of flooding due to poor weather.
- Poor ground conditions and natural ground heave or settlement.
- Various obstructions, such as tunnels, existing services, mining works, archeological remains, and so on.
- Boundary issues that may prove to be contentious, most commonly the foundations of nearby properties (see party wall act and right of support).
- Loads from adjacent buildings and roads.
- Failure of waterproofing, insurance, and guarantees.
[edit] Related articles on Designing Buildings
- Basements in buildings.
- Basement Excavation (Restriction of Permitted Development) Bill.
- Basement impact assessment.
- Basement v cellar.
- Basement waterproofing.
- Compensated foundation.
- Diaphragm wall.
- Excavation.
- Party Wall Act.
- Pile wall.
- Planning (Subterranean Development) Bill.
- Right of support.
- Safe room.
- Substructure.
- Trench.
- Underground car park.
[edit] External references
- Building – Whole life costs basements
- Self-build – Basement extensions: Pros and cons
- DIY Doctor – Dig a basement
- About Home Design
Featured articles and news
RTPI leader to become new CIOB Chief Executive Officer
Dr Victoria Hills MRTPI, FICE to take over after Caroline Gumble’s departure.
Social and affordable housing, a long term plan for delivery
The “Delivering a Decade of Renewal for Social and Affordable Housing” strategy sets out future path.
A change to adoptive architecture
Effects of global weather warming on architectural detailing, material choice and human interaction.
The proposed publicly owned and backed subsidiary of Homes England, to facilitate new homes.
How big is the problem and what can we do to mitigate the effects?
Overheating guidance and tools for building designers
A number of cool guides to help with the heat.
The UK's Modern Industrial Strategy: A 10 year plan
Previous consultation criticism, current key elements and general support with some persisting reservations.
Building Safety Regulator reforms
New roles, new staff and a new fast track service pave the way for a single construction regulator.
Architectural Technologist CPDs and Communications
CIAT CPD… and how you can do it!
Cooling centres and cool spaces
Managing extreme heat in cities by directing the public to places for heat stress relief and water sources.
Winter gardens: A brief history and warm variations
Extending the season with glass in different forms and terms.
Restoring Great Yarmouth's Winter Gardens
Transforming one of the least sustainable constructions imaginable.
Construction Skills Mission Board launch sector drive
Newly formed government and industry collaboration set strategy for recruiting an additional 100,000 construction workers a year.
New Architects Code comes into effect in September 2025
ARB Architects Code of Conduct and Practice available with ongoing consultation regarding guidance.
Welsh Skills Body (Medr) launches ambitious plan
The new skills body brings together funding and regulation of tertiary education and research for the devolved nation.
Paul Gandy FCIOB announced as next CIOB President
Former Tilbury Douglas CEO takes helm.
UK Infrastructure: A 10 Year Strategy. In brief with reactions
With the National Infrastructure and Service Transformation Authority (NISTA).
Comments
When you are constructing a basement you should consider:
[edit] Level of water table
[edit] The cost