The scope of the geotechnical investigation, including the number, types and depths of borings, the number of test pits or the number of alternative test methods; the equipment used to drill and sample; the in-situ testing; and the laboratory testing program shall be determined by the engineer responsible for the investigation, subject to the requirements of this chapter.
- Borings shall be uniformly distributed under the structure or distributed in accordance with load patterns imposed by the structure.
- As a minimum, investigations for structures shall include:
- One exploratory boring for built-over areas up to and including 750 square feet (69.7 m2).
- Two exploratory borings for built-over areas greater than 750 square feet (69.7 m2) but less than 5,000 square feet (465 m2), and at least one additional boring for each additional 2,500 square feet (233 m2), or part thereof, of built-over areas up to 20,000 square feet (1860 m2).
- At least one boring for each additional 5,000 square feet (465 m2), or part thereof, of built-over areas in excess of 20,000 square feet (1860 m2).
- At a minimum, investigations for retaining walls greater than 10 feet (3.05 m) in height shall include one exploratory boring for every 50 linear feet (15.24 m) of wall.
- Borings shall be taken into bedrock, or to an adequate depth below the top of the load-bearing strata to demonstrate that the foundation loads have been sufficiently dissipated and to evaluate global stability of retaining walls.
- For structures having an average area load (dead plus live) of 1,000 pounds per square foot (47.9 kN/m2) or more, at least one boring for every 10,000 square feet (930 m2) of footprint area shall penetrate at least 100 feet (30 480 mm) below the curb grade or 5 feet (1524 mm) into bedrock of Class 1c or better, whichever is less.
- At least one-half of the borings satisfying this requirement shall be located within the limits of the built-up area and the remainder shall be within 25 feet (7620 mm) of the built-up area limits.
- For structures to be supported on deep foundations, the required number of borings shall be not less than two borings, and based on a minimum of one boring per 2,000 square feet (609.6 m2) for the first 20,000 square feet (1860 m2) and one boring per every additional 4,000 square feet (609.6 m2).
- All boring, sampling, and in-situ testing operations shall be subject to special inspection in accordance with Section 1704.7.4.
Exception: Test pits may be substituted for borings for one and two-story structures, and may be used only to establish the top of rock, where practical, for taller structures. For taller structures, the engineer shall submit a test pit observation report to the commissioner; for one and two-story structures, the registered design professional may submit a test pit observation report to the commissioner.