For new sites of largely unknown soil conditions, preliminary borings on
approximately 200 foot centers are appropriate. For detailed exploration,
borings are taken at the corners of the proposed structure with intermediate
borings as necessary for refinement of the soil profile.
Depth of Borings. All borings must extend through soft,
compressible or otherwise unsuitable material into firm strata. A general
rule of thumb for boring depth is to carry borings to such depth that the net
increase in soil stress under the weight of the structure is less than 10% of
the average load of the structure. In applying this guideline, the
increased soil stress can be calculated using Boussinesq type equations given
in NAVFAC DM-7.1. Generally all borings should be no less than 30 feet in
depth. Where deep excavations are anticipated, borings must be extended to
at least 1.5 times the expected depth of excavation.
Procedures. The most frequently used procedures for completing
borings and obtaining soil samples in the field are detailed in two American
Society for Testing and Materials (ASTM) standards.
So called "standard" sampling, in which disturbed samples are obtained for
Reference 2). Undisturbed samples suitable for soil shear strength and
consolidation testing must be obtained using a thin-walled sampler. One
frequently used standard procedure is described in ASTM D 1587 (see
LABORATORY TESTS. Detailed information on the Unified soil
provided in NAVFAC DM-7.1. The laboratory testing program must be directed
by a qualified geotechnical engineer.