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Section 1802 Geotechnical Investigations and Material Classifications
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Geotechnical 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 1802.7. An engineer shall scope, supervise and approve the subsurface investigation and the classification of the soil and rock encountered.
A geotechnical investigation shall be conducted for:
- New structures;
- Horizontal enlargements;
- Vertical enlargements or alterations necessitating new foundations or resulting in additional loading that exceeds 5 percent of the existing foundation design capacity; or
- As required by the commissioner or applicant of record.
Where a structure is determined to be in Seismic Design Category C in accordance with Section 1613, the geotechnical investigation 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. Peak ground acceleration for use in liquefaction analyses shall be determined in accordance with Section 1813.2.1.
Where a structure is determined to be in Seismic Design Category D in accordance with Section 1613, the requirements for Seismic Design Category C, given in Section 1802.2.1, shall be met. In addition, the following shall be conducted:
- A determination of lateral pressures on basement, cellar, and retaining walls due to earthquake motions. Peak ground acceleration for use in lateral pressure analyses shall be determined in accordance with Section 1813.2.1.
- 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. Mitigation measures shall be addressed. 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.
Soil and rock classification shall be based on materials disclosed by borings, test pits or other subsurface exploration methods. Soil classifications shall be determined in accordance with ASTM D 2487 (refer to Table 1802.3) and the supplemental definitions contained herein. Rock classifications shall be determined in accordance with generally accepted engineering practice and the supplemental definitions contained herein. Laboratory tests shall be conducted to ascertain these classifications where deemed necessary by the engineer responsible for the geotechnical investigation or the commissioner.
BEDROCK.
SANDY GRAVEL AND GRAVELS. Consists of coarse-grained material with more than half of the coarse fraction larger than the #4 size sieve and contains little or no fines (GW and GP). The density of these materials shall be determined in accordance with the following:
GRANULAR SOILS. These materials are coarse-grained soils consisting of gravel and/or sand with appreciable amounts of fines, and gravel. Soil types include GM, GC, SW, SP, SM, and SC. The density of granular materials shall be determined in accordance with the following:
CLAYS. For soil types SC, CL and CH in the absence of sufficient laboratory data, the consistency of clay materials shall be determined in accordance with the following:
SILTS AND CLAYEY SILTS. For soil types ML and MH in the absence of sufficient laboratory data, the consistency of silt materials shall be determined in accordance with the following:
BEDROCK.
- Hard sound rock (Class 1a). Includes crystalline rocks, such as gneiss, granite, diabase and mica schist. Characteristics are as follows: the rock rings when struck with pick or bar; the rock does not disintegrate after exposure to air or water; the rock breaks with sharp fresh fracture; cracks are unweathered, less than 1/8-inch (3.2 mm) wide, and generally no closer than 3 feet (914 mm) apart; and the RQD (rock quality designation) with a double tube, NX-size diamond core barrel is generally 85 percent or greater for each 5-foot (1524 mm) run; or core recovery with BX-size core is generally 85 percent or greater for each 5-foot (1524 mm) run.
- Medium hard rock (Class 1b). Includes crystalline rocks of paragraph (1) of this subdivision, plus marble and serpentinite. Characteristics are as follows: all those listed in paragraph (1) of this subdivision, except that cracks may be 1/4-inch (6.4 mm) wide and slightly weathered, generally spaced no closer than 2 feet (610 mm) apart; and the RQD with a double tube, NX-size diamond core barrel is generally between 50 and 85 percent for each 5-foot (1524 mm) run; or core recovery with BX-size core is generally 50 to 85 percent for each 5-foot (1524 mm) run.
- Intermediate rock (Class 1c). Includes rocks described in paragraphs (1) and (2) of this subdivision, plus cemented shales and sandstone. Characteristics are as follows: the rock gives dull sound when struck with pick or bar; does not disintegrate after exposure to air or water; broken pieces may show moderately weathered surfaces; may contain fracture and moderately weathered zones up to 1 inch (25 mm) wide spaced as close as 1 foot (305 mm) apart; and the RQD with a double tube, NX-size diamond core barrel is generally 35 to 50 percent for each 5-foot (1524 mm) run; or a core recovery with BX-size core of generally 35 to 50 percent for each 5-foot (1524 mm) run.
- Soft rock (Class 1d). Includes rocks described in paragraphs (1), (2), and (3) of this subdivision in highly weathered condition, plus talc schist and poorly cemented shales and sandstones. Characteristics are: rock may soften on exposure to air or water; may contain highly weathered zones up to 3 inches (76 mm) wide but filled with stiff soil; and either the RQD with a double tube, NX-size diamond core barrel is less than 35 percent for each 5-foot (1524 mm) run or core recovery with BX-size core of generally less than 35 percent for each 5-foot (1524 mm) run, or a standard penetration resistance more than 50 blows per foot (0.3 meters).
SANDY GRAVEL AND GRAVELS. Consists of coarse-grained material with more than half of the coarse fraction larger than the #4 size sieve and contains little or no fines (GW and GP). The density of these materials shall be determined in accordance with the following:
Dense (Class 2a). These materials have a standard penetration test N-value greater than 30 blows per 1 foot (0.3 meter).
Medium (Class 2b). These materials have a standard penetration test N-value between 10 and 30 blows per 1 foot (0.3 meter).
Loose (Class 6). These materials have a standard penetration test N-value less than 10 blows per 1 foot (0.3 meter). These materials shall be considered nominally unsatisfactory bearing materials.
Medium (Class 2b). These materials have a standard penetration test N-value between 10 and 30 blows per 1 foot (0.3 meter).
Loose (Class 6). These materials have a standard penetration test N-value less than 10 blows per 1 foot (0.3 meter). These materials shall be considered nominally unsatisfactory bearing materials.
GRANULAR SOILS. These materials are coarse-grained soils consisting of gravel and/or sand with appreciable amounts of fines, and gravel. Soil types include GM, GC, SW, SP, SM, and SC. The density of granular materials shall be determined in accordance with the following:
Dense (Class 3a). These materials have a standard penetration test N-value of greater than 30 blows per 1 foot (0.3 meter).
Medium (Class 3b). These materials have a standard penetration test N-value of between 10 and 30 blows per 1 foot (0.3 meter).
Loose (Class 6). These materials have standard penetration test N-value of fewer than 10 blows per 1 foot (0.3 meter). These materials shall be considered nominally unsatisfactory bearing materials.
Medium (Class 3b). These materials have a standard penetration test N-value of between 10 and 30 blows per 1 foot (0.3 meter).
Loose (Class 6). These materials have standard penetration test N-value of fewer than 10 blows per 1 foot (0.3 meter). These materials shall be considered nominally unsatisfactory bearing materials.
CLAYS. For soil types SC, CL and CH in the absence of sufficient laboratory data, the consistency of clay materials shall be determined in accordance with the following:
Hard (Class 4a). Clay requiring picking for removal, a fresh sample of which cannot be molded by pressure of the fingers; or having an unconfined compressive strength in excess of 4 TSF (383 kPa); or having a standard penetration test where the N-value is greater than 30 blows per 1 foot (0.3 meter).
Stiff (Class 4b). Clay that can be removed by spading, a fresh sample of which requires substantial pressure of the fingers to create an indentation; or having an unconfined compressive strength of between 1 TSF (96 kPa) and 4 TSF (383 kPa); or having a standard penetration test where the N-value is between 8 and 30 blows per 1 foot (0.3 meter).
Medium (Class 4c). Clay that can be removed by spading, a fresh sample of which can be molded by substantial pressure of the fingers; or having an unconfined compressive strength of between 0.5 TSF (48 kPa) and 1 TSF (96 kPa); or having a standard penetration test where the N-value is between 4 and 8 blows per 1 foot (0.3 meter).
Soft (Class 6). Clay, a fresh sample of which can be molded with slight pressure of the fingers; or having an unconfined compressive strength of less than 0.5 TSF (48 kPa); or having a standard penetration test where the N-value is fewer than 4 blows per 1 foot (0.3 meter). This material shall be considered nominally unsatisfactory bearing material.
Stiff (Class 4b). Clay that can be removed by spading, a fresh sample of which requires substantial pressure of the fingers to create an indentation; or having an unconfined compressive strength of between 1 TSF (96 kPa) and 4 TSF (383 kPa); or having a standard penetration test where the N-value is between 8 and 30 blows per 1 foot (0.3 meter).
Medium (Class 4c). Clay that can be removed by spading, a fresh sample of which can be molded by substantial pressure of the fingers; or having an unconfined compressive strength of between 0.5 TSF (48 kPa) and 1 TSF (96 kPa); or having a standard penetration test where the N-value is between 4 and 8 blows per 1 foot (0.3 meter).
Soft (Class 6). Clay, a fresh sample of which can be molded with slight pressure of the fingers; or having an unconfined compressive strength of less than 0.5 TSF (48 kPa); or having a standard penetration test where the N-value is fewer than 4 blows per 1 foot (0.3 meter). This material shall be considered nominally unsatisfactory bearing material.
SILTS AND CLAYEY SILTS. For soil types ML and MH in the absence of sufficient laboratory data, the consistency of silt materials shall be determined in accordance with the following:
Dense (Class 5a). Silt with a standard penetration test where the N-value is greater than 30 blows per 1 foot (0.3 meter).
Medium (Class 5b). Silt with a standard penetration test where the N-value is between 10 and 30 blows per 1 foot (0.3 meter).
Loose (Class 6). Silt with a standard penetration test where the N-value is fewer than 10 blows per 1 foot (0.3 meters). This material shall be considered nominally unsatisfactory bearing material.
Medium (Class 5b). Silt with a standard penetration test where the N-value is between 10 and 30 blows per 1 foot (0.3 meter).
Loose (Class 6). Silt with a standard penetration test where the N-value is fewer than 10 blows per 1 foot (0.3 meters). This material shall be considered nominally unsatisfactory bearing material.
TABLE 1802.3 UNIFIED SOIL CLASSIFICATION
(Including Identification and Description)
(Including Identification and Description)
An engineer shall scope and supervise the geotechnical investigation. The geotechnical investigation shall be sufficient for evaluating soil and rock conditions including but not limited to material classification, stratigraphy, groundwater, slope stability, soil and rock strength, adequacy of load-bearing soils and rock, the effect of moisture variation on soil-bearing capacity, compressibility, liquefaction and expansiveness. The investigation shall comply with Sections 1802.4.1 through 1802.4.4.
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.
At the request of the engineer responsible for the geotechnical investigation, the 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 geotechnical 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 geotechnical investigation shall determine the existing groundwater table.
In areas containing compressible soils, the geotechnical investigation shall determine the extent of these soils in the plan area of the structure and shall be subject to the requirements of Section 1802.3.