Site Investigation

1.1 The preparation of the site will depend on the findings of the site investigation. The site investigation is relevant to Sections 1, 2 and 3 of this Approved Document and also to the requirements of Approved Document A with respect to foundations. The site investigation should consist of a number of well-defined stages:

a. Planning stage. Clear objectives should be set for the investigation, including the scope and requirements, which enable the investigation to be planned and carried out efficiently and provide the required information;

b. Desk study. A review of the historical, geological and environmental information about the site is essential;

c. Site reconnaissance or walkover survey. This stage of the investigation facilitatesthe identification of actual and potential physical hazards and the design of the main investigation;

d. Main investigation and reporting. This will usually include intrusive and non-intrusive sampling and testing to provide soil parameters for design and construction. The main investigation should be preceded by (b) and (c) above.

1.2 The extent and level of investigation need to be tailored to the type of development and the previous use of land. Typically the site investigation should include susceptibility to groundwater levels and flow, underlying geology, and ground and hydro-geological properties. A geotechnical site investigation should identify physical hazards for site development, determine an appropriate design and provide soil parameters for design and construction. British Standard BS 5930:1999 provides comprehensive guidance on site investigations. Guidance on site investigation for low-rise buildings is given in six BRE Digests covering procurement, desk studies, the walk- over survey, trial pits, soil description and direct investigation. Reference should also be made to BS 8103-1:1995.

1.3 Where the site is potentially affected by contaminants, a combined geotechnical and geo-environmental investigation should be considered. Guidance on assessing and remediating sites affected by contaminants is given in Section 2: Resistance to contaminants.

Unsuitable Material

1.4 Vegetable matter such as turf and roots should be removed from the ground to be covered by the building at least to a depth to prevent later growth. The effects of roots close to the building also need to be assessed. Consideration should be given to whether this provision need apply to a building used wholly for:

a. storing goods, provided that any persons who are habitually employed in the building are engaged only in taking in, caring for or taking out the goods; or

b. a purpose such that the provision would not serve to increase protection to the health or safety of any persons habitually employed in the building.

1.5 Where mature trees are present on sites with shrinkable clays (see Diagram 1 and Table 1), the potential damage arising from ground heave to services and floor slabs and oversite concrete should be assessed. Reference should be made to BRE Digest 298. Where soils and vegetation type would require significant quantities of soil to be removed, reference should be made to BRE Digests 240 and 241, and to the FBE (Foundation for the Built Environment) report. The effects of remaining trees on services and building movements close to the building need to be assessed using guidance in NHBC (National House Building Council) Standards Chapter 4.2.

Diagram 1 Distribution of shrinkable clays and principal sulphate/sulphide bearing strata in England and Wales

1.6 Building services such as below ground drainage should be sufficiently robust or flexible to accommodate the presence of any tree roots. Joints should be made so that roots will not penetrate them. Where roots could pose a hazard to building services, consideration should be given to their removal.

1.7 On sites previously used for buildings, consideration should be given to the presence of existing foundations, services, buried tanks and any other infrastructure that could endanger persons in and about the building and any land associated with the building.

1.8 Where the site contains fill or made ground, consideration should be given to its compressibility and its potential for collapse on wetting, and to appropriate remedial measures to prevent damaging differential settlement. Guidance is given in BRE Digest 427 and BRE Report BR 424.

Table 1 Volume change potential for some common clays

Section 1: Clearance or treatment of unsuitable material