Soil-Structure Interaction Seminar - Foundation Performance ...

Geotechnical Foundation Design Criteria
Foundations for a residential structure should satisfy two independent design criteria. First, the
maximum design pressure exerted at the foundation base should not exceed the allowable net
bearing pressure based on an adequate factor of safety with respect to soil shear strength.
Secondly, the magnitude of total and differential settlements (and shrink and swell) under
sustained loads must be such that the structure is not damaged or its intended use impaired.
It should be noted that properly designed and constructed foundation may still experience distress
• from improperly prepared bearing soils and/or expansive soils which will undergo volume
change when correct drainage is not established or an incorrectly controlled water source
becomes available.
The design of foundations should be performed by an experienced structural engineer·using a
soils report 'from an experienced soils engineer. The structural engineer must use a lot/site
specific soils report for the foundation design. The structural engineer should not use general
subdivision soils reports written for underground utilities and paving for the slab des:gn.
Furthermore, he should not design slabs with disclaimers, requiring future soils reports to verify
his design. The designers or architects should not provide clients with foundation design
drawings with generic foundations details. All of the foundation drawings should be site and
structure specific and sealed by a professional engineer.
Recommended Scope of Geotechnical studies
Soil testing must be performed on residential lots before a foundation design can be developed.
The recommended number of borings should be determined by a geotechnical engineer. The
number of borings and the depths are a function of the size of the structure, foundation loading,
site features, and soil conditions. As a general rule, a minimum of one boring for every four
lots should be performed for subdivision lots. This boring program assumes that a
conventionally-reinforced slab or a post-tensioned slab type foundation is going to be used.
Furthermore, many lots will be tested at the same time so that a general soils stratigraphy can
be developed for the entire subdivision. In the event that a drilled footing foundation is to be
used, a minimum of one boring per lot is recommended. In the case of variable subsoil
conditions, one to two borings per lot should be performed. A minimum of two borings is
recommended for custom homes or a single in-fill lot. A minimum boring depth of 15-feet is
recommended.
GEOTECH ENGINEERING AND TESTING, INC. -------------·
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