Page 33 - Forensic Structural Engineering Handbook

1.14 DESIGN AND CONSTRUCTION PRACTICES

Subgrade
The following inspection and testing of the subgrade should be carried out.
Borings
• Subsurface investigations are a requirement for major structures. Alternatively, a “judge-
ment call” may be made by the structural engineer of record (SER) or the geotechnical
engineer of record (GER) for minor structures particularly if evidence of the subgrade
conditions is available at adjoining sites.
• National building codes, more specifically, international building codes require at least
2
one boring for 2500 ft for all buildings which are more than three stories or 40 ft in height.
Whenever it is proposed to use float, mat, or any type of deep foundation, there shall be at
least one exploratory boring to rock or to an adequate depth below the load-bearing strata.
• Some building codes, such as that of New York City, require the soil boring investiga-
tions to be under the supervision of a licensed professional engineer (preferably a geot-
echnical engineering consultant) registered in the state of the project.
• Standard test borings should be taken in accordance with ASTM D-1586. 1
• Subgrade investigations should ascertain the presence and the elevation of natural ground
water through the installation of piezometers for observation.
Inspections
• A geotechnical engineer should perform physical and laboratory tests to verify the safe
soil bearing value for the specified foundation design.
• A geotechnical engineer should also perform inspection of the preparation of the sub-
grade prior to the placement of foundation concrete.
• Pile driving operations, installation of caissons, and other deep foundations should be
under the observance of a geotechnical engineer.

Testing
• It may be necessary to validate the safe bearing value of the subgrade by a soil bearing
plate test method. As an example, the New York City Building Code, Subchapter 11,
Article 5, provides the requirements and interpretation of a soil load-bearing tests.
• Sieve analysis, plasticity, moisture content, and other tests should be performed by a
qualified testing laboratory.
• Pile loads tests, when required, should be performed per ASTM D-1143. 1
• Inspection of caisson bottoms should be conducted either visually by a soils technician
or by use of TV equipment.
• Compaction tests during backfill operations should be done in accordance with ASTM
D-1557. 1

Reinforced Concrete
Formwork Inspection. Inspection of concrete work is particularly important because
once the concrete has hardened, corrections to any structural problems is extremely diffi-
cult and costly.
• Formwork should be checked for proper dimensions, alignments, and tolerances.
• Formwork bracing and shoring is the contractor’s responsibility, therefore the designer
is not involved.