4.4 Strip and trench fill foundations
SCOPE
DESIGN STANDARDS
Design that follows the guidance below will be acceptable for both strip foundations and trench fill foundations.
STATUTORY REQUIREMENTS
Design should be in accordance with relevant Building Regulations and other statutory requirements
REQUIREMENT FOR FOUNDATIONS
Elements requiring foundations include the following:
- external walls
- separating (party) walls
- chimney breasts
- piers
- internal loadbearing walls.
SLEEPER WALLS
In Scotland, a sleeper wall is also defined as a loadbearing element and must be provided with a suitable foundation.
In England, Wales, Northern Ireland and the Isle of Man, sleeper walls should not be built off oversite concrete:
- on shrinkable clay soils where heave could take place
- where infill below the oversite concrete is greater than 600mm
- which is less than 100mm thick.
In these situations, suitable foundations will be required.
SAFE TRANSMISSION OF LOADS
Items to be taken into account include:
(a) dead and imposed loads
Dead and imposed loads should be calculated in accordance with BS 6399 and BS 648.
Appendix 4.4-A shows suitable foundation dimensions and gives minimum widths of strip foundations for different sub-soil and wall loadings. Strip foundations should be 150mm to 500mm thick. Trench fill foundations should be greater than 500mm thick.
(b) stability of the dwelling and any associated constructions
Where appropriate, reference should be made to BS 8103.
Unless there are reasons for doing otherwise, foundations should be symmetrical beneath loadbearing elements.
Strip and trench fill foundations should be continuous throughout the building, including integral garages, porches, conservatories, bay windows, etc. The foundations should be of sufficient width throughout to avoid overstressing the ground, especially where the foundation is required to support piers or columns.
Reference should be made to Chapter 4.2 'Building near trees' where:
- soil is shrinkable
- trees have been, or are being, removed
since heave is possible in these situations special precautions are necessary.
The width of the foundation will depend on the loadbearing capacity of the sub-soil and the loads from the building. However, the foundation width should not be less than the wall thickness, plus at least 50mm each side, to ensure that the foundation is not oversailed by any part of the wall.
(c) stability of any adjoining dwelling or construction
Foundations adjoining those of an existing building may require special design. If taken to a greater depth, such foundations will usually need to be Engineer designed and carefully supervised to check the standard of workmanship. Where necessary, allowance should be made in the design for differential movement.
DESIGN BY AN ENGINEER
Details of hazardous ground to be taken into consideration are given in Chapters:
4.1 'Land quality - managing ground conditions', and
4.2 'Building near trees'
Foundations should be designed by an Engineer in accordance with Technical Requirement R5 where:
- buildings exceed 3 storeys in height
- retaining walls are required for habitable rooms below ground.
Where hazardous ground has been identified, NHBC must be notified before work starts. Hazardous ground is defined in Chapter 4.1 'Land quality - managing ground conditions'.
NHBC Rules state:
"If a Home is to be constructed on a Hazardous Site you must before making an Application for Inspection notify the NHBC in writing of the particular hazards which arise. You must do this at least 8 weeks before work begins on the site."
SITE CONDITIONS
Items to be taken into account include:
(a) the results of site appraisal
All relevant information about the nature and loadbearing capacity of the ground should be available before the foundations are designed.
Information about ground conditions and the past history of the site may be available from a number of sources. These include NHBC, Local Authorities and the area offices of the Gas, Water and Electricity Companies. Aerial photographs, Ordnance Survey maps and geological maps and surveys may often be studied at local Public Libraries and Record Offices.
Site assessment surveys may require supplementary site investigations involving trial pits and borings. Details are given in Chapter 4.1 'Land quality - managing ground conditions'.
(b) dwelling design and layout
Foundation design is governed by the shape and size of the dwellings as well as the site conditions. Foundations for terraced dwellings may require special precautions to prevent damage from differential settlement.
(c) site levels
Stepped foundations or suspended floors may be needed for sloping sites. Reference should be made to Clause D9 for stepped foundations and to Chapter 5.2 'Suspended ground floors' (Design).
FOUNDATION DEPTH
Items to be taken into account include:
(a) soils with volume change potential
In shrinkable soils that are classified as containing more than 35% fine particles (clay and silt), and have a modified Plasticity Index of 10% or greater, the minimum foundation depth should be as in the following table:
| Modified Plasticity Index | Volume change potential | Minimum depth (m) |
| 40% and greater | High | 1.0 |
| 20% to less than 40% | Medium | 0.9 |
| 10% to less than 20% | Low | 0.75 |
(b) frost susceptible soils
To avoid damage from frost action, the depth to the underside of the foundation in frost susceptible ground, eg chalk, should be at least 450mm below finished ground level.
This depth should also be used when construction is undertaken during cold weather. Alternatively, precautions should be taken to prevent freezing of the ground.
(c) suitable bearing strata
The depth of foundations should be such as to give a clean, firm and adequate bearing for the design loads.
Trench fill foundations greater than 2.5m in depth must be designed by an Engineer in accordance with Technical Requirement R5.
STEPPED FOUNDATIONS
Sloping ground may require stepped foundations.
Where foundations are stepped, the height of the step should not exceed the thickness of the foundation, unless it forms part of a foundation designed by an Engineer in accordance with Technical Requirement R5.
For details of stepped foundations, reference should be made to Sitework Clause 4.4 - S13(b).
SERVICES AND DRAINAGE
Items to be taken into account include:
(a) ground water drainage
Provision should be made for adjusting any existing ground water drains affected by excavation work.
(b) existing services
Precautions should be taken to accommodate the effects of settlement, where drains run under or near a building.
(c) access for services
Where services are to pass through or under foundations, provision should be made for suitable ducts or lintels to enable them to be installed later.
Reference should be made to Chapters 8.1 'Internal services' (Design and Sitework) and 5.3 'Drainage below ground' (Design and Sitework) for further details.
MOVEMENT JOINTS
Where movement joints are specified in foundations, they should be continuous with those in the superstructure.
PROVISION OF INFORMATION
It is important that all relevant information needed for the completion of the sitework is stated clearly and unambiguously and is readily available to all concerned.
All necessary dimensions and levels should be indicated and related to:
- at least one benchmark, and
- reference points on site.
All necessary details of junctions, steps, movement joints and, where necessary, any critical sequences of construction should be provided.
Both designers and site operatives need to be aware of the ground conditions and, in particular, any features requiring special attention, such as any existing sewers or other services, levels of water table and the presence of any deleterious substances, especially sulfates.
Information on ground conditions, the results of site investigation and the foundation design can be requested by NHBC, even for those sites which are not classified as hazardous.
Where toxic materials (or materials likely to present a health hazard) are found, all available information should be supplied to NHBC, together with proposals for dealing with the hazard.
