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Sick building syndrome, condition in which a building suffers from corrosion due to moisture, can be prevented through protection in the basement walls. All the conventional concrete walls, reinforced masonry walls etc will suffer from moisture penetration from the ground due to porous nature of walls which leads to cracks and eventually structural failures. Therefore waterproofing membrane systems are applied.
The following conditions will cause corrosion:
50% humidity in air. (80% will permanently damage the steel.)
Potable, waste, gray water.
Aggressive soil conditions.
The building value is enhanced by the construction of the basement. Therefore more investments should be made in the site investigations and designing phase to minimize the risks of dampness. The guide will provide identifications to help the contractors and the builders help cope with problems with the help of water, flood resistance, ventilation and other various factors.
The basement construction will increase the usable floor space in a house. A detailed investigation will provide a better understanding of the soil conditions which will lead to better thermal, sound insulations and foundations. The Basement Information Centre is introducing a contractor system which is being handled by the Specialist Concrete Contractors Ltd. This will aid in providing required standards of skills and supervision also solving the shortage of skills on site.
The techniques have advantages and disadvantages with the possible risks involved.
The traditional masonry construction:
The basic construction method is the masonry construction technique, in which a concrete floor slab is created on the base of the excavated area with the sides supported with the help of temporary walls. The basement floor is protected from water barriers coming in contact with the basement floor. The drainage to the exterior walls is installed before the excavated area is filled and the temporary earth support is removed.
The reinforced concrete construction:
Cast-in -situ reinforced concrete is also used for basement construction with the inclusion of water barriers which enables the accessibility of the exterior walls for inspection and facilitates construction of a secondary water barrier and drainage systems. The construction forms will be reinforced with plain wall design. In this case some reinforcements must be applied horizontally to prevent cracking.
This technique consists of permanent polystyrene as a replacement for the temporary formwork. The central core is filled with the in-situ concrete which supports the vertical and the lateral loads.
Domestic basement construction risks:
Reinforced precast modular units are another technique used to construct buildings. The individual units are joined together at the site and then the soil excavation and water insulation is done with another wall against the inside wall. The advantage of this technique is rapid construction along with integrated water barriers. The only problem is that it requires the installation of water barrier in the foundation.
One variety to the precast units is complete precast basement floors walls and ceilings which are put together at the site and joined. The linkages should be checked carefully for leakage of water in this variety.
Steel sheet piles can be used as basement walls. They can act as the water barriers providing that formidable clutches in the form of sliding or welded are present. A ring beam can provide adequate support for further construction of superstructures if they are cast over the steel pile. Latest push-in sheet piles have very reduced sound vibrations. The steel piles are economically viable only for larger constructions.
Contiguous bored pile walls are constructed where the soil is dry. In this case the bores are drilled and concrete is cast in very close proximity so that a perimeter wall is formed. The inside wall is rough which can be face-finished. It is advantageous for deep basements and for higher structures. This is similar to the diaphragm walling both, in terms of cost and features.
Excavation slots are created for the construction of diaphragm walls between the guide walls prior to the excavated walls, with the slots filled with bentonite slurry to keep it opened. After completion, the bentonite is replaced by pouring concrete as the water resistant panel and reinforcements are positioned.
Vibration free installation.
Wall construction with minimum disturbances.
Void of temporary sheeting.
Waterlight but needs support.
The Building Regulations
The 'Approved Documents Basements for Dwellings', constitutes of the guiding principles compiled from various other approved documents, relevant to the basement design. This guide was published in 2004 by the Basement Information Centre.
This is dealt in the first section of the Approved Document F Ventilation, which states that ventilation should be provided through an open space such as a stairwell. But if absent, the basement will be constructed as a separate dwelling with passive or mechanical ventilation systems attached.
This is dealt in Approved Document B Volume 1, which states the necessity of extra provisions for releasing the heat and smoke, with compartmentalization of stairwell access and a fire escape.
This is dealt in Approved Document M Access, which states requirements for access provisions to basement and for cars, in case of a parking area.
It is dealt in Approved Document C, which states that contaminated spoils disposal and site drainage are required and may require additional costs due to the basement depth.
Conservation of fuel and power
It is dealt in Approved Document Part L1A, the conservation of fuel and power was reissued in 2006 with significant changes of just one method in accordance with The Building Regulations, based on SAP 2005.
Flood plain development
Dealt in planning Policy statement 25, states the necessity of material planning in which the local authorities have been granted with the authority to impose according to the specified ground floor level.
Health and safety
Extra precautions are required for the health and safety management to address issues such as:
Performing in excavation.
Moving smaller units.
The primary motive in creating a basement is to increase the floor area, which is particularly useful in places with restrictions on height and in areas where the land value is high and in sloped locations.
Basements can have benefits such as:
Enhanced thermal insulation.
Enhanced sound insulation.
Space for heating alternates.
Technically sound foundation.
Basements provide benefits in planning by:
The provision to increase density without compromising on habitable space.
Provides space for living, car parking in areas where height in construction is restricted.
The housing density may be different in different areas, the calculation of which may depend on:
Quantity of habitable rooms.
Number of bedrooms.
Number of housing units.
Cost and programme
Basements are costly and only be seen with high land value. The planning of the basement should be done beforehand with water resistant and access needs with the planning of depth in the foundation, which may reduce the cost.
Liability and warranties
The standard NHBC arrangements with builders and developers covers the liabilities and warranties with the ASUC+ basement defects insurance effective for retrofit basement.
domestic basement construction risks:
Newly build basements and extensions can only be guaranteed if it meets the ASUC+ criteria of members completing the task.
The soil excavated from the construction sites can be used at the same place because the transport and disposal costs are getting higher.
For avoiding the dampening and water entering the basements, proper and proven designs and construction techniques should be used specially in high water table areas. The water resisting techniques are not very popular among builders so that is why specialists should be used for helping constructing proper water resisting basements.
Certain situations especially constructions in flood-risk areas do not appreciate the building of basements by rules and legislations. Moreover the important spaces in front and rear of the building is often used as access points of corridors.
The document published by 'The Basement Information Centre' named approved documents 'Basements for Dwellings' (2004) covers nearly all the technical issues for constructing the basements including the flow charts and information of fire, ventilation and insulations. The only exceptions are precast modular units and steel piles which are newer techniques
The steel sheets or pile constructions are covered in 'Steel Intensive Basements' by Steel Construction Institute. Special emphasis is given on car parking and office spaces in basements along with explanation on water resistance techniques.
More information is required about the soil in building basements than for ordinary buildings. Quantitative assessments are required. (see approved document C: site preparation and resistance to contamination and moisture' and the BRE digest series on Site investigation for low-rise building.)
The load of the traditional building is near to the load removed at the excavation of soil so that the net increase in the load at the basement is usually very small. However, the basement wall will need to resist the lateral pressure exerted by the surrounding soil (BS 8002). This can be assessed by extra soil data.
It is difficult to predict future water level however following characteristics provide good information for predicting future ground water levels.
° geology ° topography ° history of industrial use ° current industry.
Geological data will tell how much impervious the soil is to water. The topological information can give idea of the water drainage.
Previous industrial use particularly the paper and open cast mining lowers the water level by using or lowering the water and that takes years to come again to its original level. Waterproofing
Site preparation against moisture is stated in the Building Regulations 2000 which states that the walls, roof and the floors are protected against moisture. Issues in water proofing are:
Incompatibility between systems.
Vertical members sealing.
Walls and floors sealing.
Thermal insulations guiding principles are stated in Approved Document L1A of Conservation of Fuel and Power (New Dwellings, 2006 edition). Composite materials should be utilized with insulation, applied externally and internally to the buildings, and act as thermal insulator.
Sound insulation is not required owing to the ground being a good attenuator itself.
Approved Document F-ventilation states that living spaces require sound ventilation which is fulfilled by the provision of forced and natural ventilation.
Minimising risk before construction
Site investigation can reduce the risks in the construction phase, the investigations should cover:
Load bearing potential.
The waterproofing system should be integrated in the structural design to increase building durability and to determine the team composition which can include skilled based builders or specialist contractors.
Basement design is based on geotechnical data which is considered by the contractors and the designers who should provide assistance in determining the:
Groundwater state and future changes in its levels.
Quantitative data for basement, walls and foundations.
Recommendations to guide the water proofing design.
Supervision of the basement construction, retaining walls and foundation.