FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

Volume II DESIGNING THE FOUNDATION 10
Subsurface exploration typically consists of boring or creating test pits, soils sampling, and laboratory tests.
The Timber Pile Design and Construction Manual (Collin 2002) recommends a minimum of one boring per
structure, a minimum of one boring for every 1,000 square feet of building footprint, and a minimum of two
borings for structures that are more than 100 feet wide. Areas with varying soil structure and profile dictate
more than the minimum number of borings. Again, local geotechnical engineers should be consulted.
The following five types of data from subsurface exploration are discussed in the subsections below: soil
classification, bearing capacity, compressive strength, angle of internal friction, and subgrade modulus.
Soil Classification
Soil classification qualifies the types of soils present along the boring depth. ASTM D2487-10 is a consensus
standard for soil classification. Soil classification is based on whether soils are cohesive (silts and clays) or noncohesive
(composed of granular soils particles). The degree of cohesiveness affects foundation design. Coupled
with other tests such as the plasticity/Atterburg Limits soil classification can identify unsuitable or potentially
problematic soils. Table 10-2 contains the soil classifications from ASTM D2487-10. ASTM D2488-09a is a
simplified standard for soil classification that may be used when directed by a design professional.
Bearing Capacity
Bearing capacity is a measure of the ability of soil to support gravity loads without soil failure or excessive
settlement. Bearing capacity is generally measured in pounds/square foot and occasionally in tons/square
foot. Soil bearing capacity typically ranges from 1,000 pounds/square foot (relatively weak soils) to more
than 10,000 pounds/square foot (bedrock).
Bearing capacity has a direct effect on the design of shallow foundations. Soils with lower bearing capacities
require proportionately larger foundations to effectively distribute gravity loads to the supporting soils. For
deep foundations, like piles, bearing capacity has less effect on the ability of the foundation to support gravity
loads because most of the resistance to gravity loads is developed by shear forces along the pile.
Presumptive allowable load bearing values of soils are provided in the 2012 IBC and the 2012 IRC. Frequently,
designs are initially prepared based on presumed bearing capacities. The builder’s responsibility is to verify
that the actual site conditions agree with the presumed bearing capacities. As a best practices approach, the
actual soil bearing capacity should be determined to allow the building design to properly account for soil
capacities and characteristics.
Compressive Strength
Compressive strength is typically determined by Standard Penetration Tests. Compressive strength controls
the design of shallow foundations via bearing capacity and deep foundations via the soil’s resistance to lateral
loads. Compressive strength is also considered when determining the capacity of piles to resist vertical loads.
Compressive strength is determined by advancing a probe, 2 inches in diameter, into the bottom of the
boring by dropping a 140-pound slide hammer a height of 30 inches. The number of drops, or blows,
required to advance the probe 6 inches is recorded. Blow counts are then correlated to soil properties.
COASTAL CONSTRUCTION MANUAL
10-7