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Section 1802 Foundation and Soils Investigations
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Foundation and soils investigations shall be subject to special inspections in accordance with Sections 1704.7, 1704.8 and 1704.9 and be conducted in conformance with Sections 1802.2 through 1802.6. An Engineer shall scope, supervise and approve the classification and subsurface investigation of soil.
Where the safe load-bearing capacity of the soil is in doubt, or where a load-bearing value superior to that specified in this code is claimed, the commissioner shall require that the necessary investigation be made. Such investigation shall comply with the provisions of Sections 1802.4 through 1802.6.
Where a structure is determined to be in Seismic Design Category C in accordance with Section 1616, an investigation shall be conducted, and shall include an evaluation of the following potential hazards resulting from earthquake motions: slope instability, liquefaction and surface rupture due to faulting or lateral spreading.
Where the structure is determined to be in Seismic Design Category D in accordance with Section 1616, the soils investigation requirements for Seismic Design Category C, given in Section 1802.2.6, shall be met, in addition to the following:
- A site-specific analysis in accordance with Sections 1813.2, 1813.3, and 1813.4. Site specific response shall be evaluated for site peak ground acceleration magnitudes and source characteristics consistent with the design earthquake ground motions.
- A determination of lateral pressures on basement, cellar, and retaining walls due to earthquake motions.
- An assessment of potential consequences of any liquefaction and soil strength loss, including estimation of differential settlement, lateral movement or reduction in foundation soil-bearing capacity, and shall address mitigation measures. Such measures shall be given consideration in the design of the structure and shall include, but are not limited to, ground stabilization, selection of appropriate foundation type and depths, selection of appropriate structural systems to accommodate anticipated displacements or any combination of these measures. Peak ground acceleration shall be determined from a site-specific study taking into account soil amplification effects, as specified in Section 1615.2.
Soil and rock classification shall be based on materials disclosed by borings, test pits or other subsurface exploration methods and shall be determined in accordance with Tables 1804.1 and 1804.2 and Section 1804.2. Additional laboratory tests shall be conducted to ascertain these classifications where deemed necessary by the engineer responsible for the investigation or the commissioner.
Soil classification shall be based on observation and any necessary tests of the materials disclosed by borings, test pits or other subsurface exploration made in appropriate locations. Additional studies shall be made as necessary to evaluate stratigraphy, slope stability, soil strength, adequacy of load-bearing soils, the effect of moisture variation on soil-bearing capacity, compressibility, liquefaction and expansiveness.
The scope of the soil investigation, including the number, types and depths of borings or test pits; 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. Borings shall be uniformly distributed under the structure or distributed in accordance with load patterns imposed by the structure. As a minimum, investigations shall include two exploratory borings for built-over areas up to 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). For built-over areas in excess of 20,000 square feet (1860 m2), there shall be at least one boring for each additional 5,000 square feet (465 m2), or part thereof. 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. 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 pile foundations the required number of borings shall be increased by 30 percent.
The engineer responsible for the investigation shall have a qualified representative on the site inspecting all boring, sampling, and in-situ testing operations.
Suitable borings, test pits, probings, and the logs and records that were obtained as part of earlier exploration programs and that meet the requirements of this section may be used as partial fulfillment of the requirements of this section, subject to the approval of the commissioner. Additional borings shall be made at the direction of the engineer responsible for the investigation when uncertainty exists as to the accuracy of the available information or specific new project or loading conditions indicate the need for additional information.
The subsurface soil investigation shall determine the existing ground-water table.
In areas that have compressible soils, the investigation shall determine the extent of these soils in the plan area of the building and shall determine the pre-consolidation pressure and consolidation parameters of the deposit using appropriate laboratory tests. The information shall be used in the building's foundation design.
The soil boring and sampling procedures and apparatus shall be in accordance with ASTM D 1586 and D 1587 and generally accepted engineering practice. Where liquefaction assessment is performed, the investigation shall be in accordance with ASTM D 6066. The rock coring, sampling procedure and apparatus shall be in accordance with ASTM D 2113 and generally accepted engineering practice. Rock cores shall be obtained with a double-tube core barrel with a minimum outside diameter of 27/8 inches (73 mm). With the approval of the engineer responsible for the investigation, smaller diameter double-tube core barrels may be used under special circumstances such as telescoping casing to penetrate boulders, or space limitations requiring the use of drill rigs incapable of obtaining large diameter cores.
Where the foundation design relies on rock to support footings, piles or caisson sockets, a sufficient number of rock corings shall extend at least 10 feet (33048 mm) below the lowest level of bearing to provide assurance of the rock soundness. Where foundations are to rest on bedrock and such rock is exposed over a part or all of the area of the building, borings are not required in those areas where rock is exposed, provided the following requirements are met:
- The presence of defects or the inclination of bedding planes in the rock are of such size and location so as not to affect stability of the foundation.
- The foundation is not designed for bearing pressures exceeding those permitted in Table 1804.2.
The engineer responsible for the investigation may engage specialized technicians to conduct alternative investigative methods such as cone penetrometer probing. Data from these investigations may be used to: 1) supplement soil boring and rock coring information, provided there is a demonstrated correlation between the findings, and 2) determine material properties for static and seismic or liquefaction analyses. Subject to the approval of the commissioner, alternate exploration methods may replace borings on a two for one basis, but in no case shall there be less than two standard borings for every 10,000 square feet (930 m2) of footprint area. All the borings shall penetrate at least 100 feet (30 480 mm) below the curb grade or 5 feet (1524 mm) into rock when the average area load equals or exceeds one thousand pounds per square foot (48 kPa).
Soil and rock samples shall be maintained in an accessible location, by the permit holder or owner and made available to the engineer responsible for the investigation and to the department, until the foundation work has been completed and accepted, or until one year after the investigation is complete, whichever is longer.
The soil classification and design load-bearing capacity shall be shown on the construction documents. Where required by the commissioner, the engineer responsible for the investigation shall sign, seal and submit a written report of the investigation that includes, but need not be limited to, the following information:
- A description of the planned structure
- A plot showing the location of test borings and/or excavations.
- A complete record of the soil sample descriptions.
- A record of the soil profile.
- Elevation of the water table, if encountered.
- Results of in-situ or geophysical testing
- Results of laboratory testing.
- Recommendations for foundation type and design criteria, including but not limited to: bearing capacity of natural or compacted soil, mitigation of the effects of liquefaction (if applicable), differential settlement and varying soil strength; and the effects of adjacent loads.
- Expected total and differential settlement.
- Pile and pier foundation recommendations and installed capacities
- Special design and construction provisions for footings or foundations founded on expansive soils, as necessary.
- Compacted fill material properties and testing in accordance with Section 1803.5.
For pile or pier foundations, the report shall also include:
- Special installation procedures.
- Pier and pile load test requirements.
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