Number and Depths of Boreholes

It is practically impossible and economically infeasible to completely explore the whole project site. You have to make judgments on the number, location, and depths of borings to provide sufficient information for design and construction. The number and depths of borings should cover the zone of soil that would be affected by the structural loads. There is no fixed rule to follow. In most cases, the number and depths of borings are governed by experience based on the geological character of the ground, the importance of the structure, the structural loads, and the availability of equipment.

Building codes and regulatory bodies provide guidelines on the minimum number and depths of borings. The number of boreholes should be adequate to detect variations of the soils at the site. If the locations of the loads on the footprint of the structure are known (this is often not the case), you should consider drilling at least one borehole at the location of the heaviest load. As a guide, a minimum of three boreholes should be drilled for a building area of about 250 m² (2500 ft²) and about five for a building area of about 1000 m² (10,000 ft²).

Some guidelines on the minimum number of boreholes for buildings and for due diligence in subdivisions are given in the Table 1. 

Some general guidance on the depth of boreholes is provided in the following:

• In compressible soils such as clays, the borings should penetrate to at least between 1 and 3 times the width of the proposed foundation below the depth of embedment or until the stress increment due to the heaviest foundation load is less than 10%, whichever is greater.

• In very stiff clays and dense, coarse-grained soils, borings should penetrate 5 m to 6 m to prove that the thickness of the stratum is adequate.

• Borings should penetrate at least 3 m into rock.

• Borings must penetrate below any fills or very soft deposits below the proposed structure.

• The minimum depth of boreholes should be 6 m unless bedrock or very dense material is encountered. General guidelines for the minimum number or frequency of boreholes and their minimum depths for common geotechnical structures are shown in the Table 2.