Foundations: Design Procedures.

Good design must not only be safe but must aim to save construction costs, time and materials. The following procedures should help to achieve this and an ‘educated’ client will recognize the importance of funding this work with a realistic fee.

(1) On the building plan, the position of columns and load-bearing walls should be marked, and any other induced loadings and bending moments. The loads should be classified into dead, imposed and wind loadings, giving the appropriate partial safety factors for these loads.

(2) From a study of the site ground investigation (if available), the strength of the soil at various depths or strata below foundation level should be studied, to determine the safe bearing capacity at various levels. These values – or presumed bearing values from BS 8004 in the absence of a site investigation – are used to estimate the allowable bearing pressure.

(3) The invert level (underside) of the foundation is determined by either the minimum depth below ground level unaffected by temperature, moisture content variation or erosion – this can be as low as 450 mm in granular soils but, depending on the site and ground conditions, can exceed 1 m – or by the depth of basement, boiler house, service ducts or similar.

(4) The foundation area required is determined from the characteristic (working) loads and estimated allowable pressure. This determines the preliminary design of  the types or combination of types of foundation. The selection is usually based on economics, speed and buildability of construction.

(5) The variation with depth of the vertical stress is determined, to check for possible over-stressing of any underlying weak strata.

(6) Settlement calculations should be carried out to check that the total and differential settlements are acceptable.

If these are unacceptable then a revised allowable bearing pressure should be determined, and the foundation
design amended to increase its area, or the foundations should be taken down to a deeper and stronger stratum.

(7) Before finalizing the choice of foundation type, the  preliminary costing of alternative superstructure
designs should be made, to determine the economics  of increasing superstructure costs in order to reduce foundation costs.

(8) Alternative safe designs should be checked for economy, speed and simplicity of construction. Speed and economy can conflict in foundation construction – an initial low-cost solution may increase the construction period. Time is often of the essence for a client needing early return on capital investment. A fast-track programme for superstructure construction can be negated by slow foundation construction.

(9) The design office should be prepared to amend the design, if excavation shows variation in ground condi-
tions from those predicted from the site soil survey and investigation.