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1804.2 Allowable Foundation Pressure
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The allowable foundation pressure for supporting soils at or near the surface shall not exceed the values specified in Table 1804.1, unless data to substantiate the use of a higher value are developed and contained in the engineer's geotechnical report and subject to the commissioner's approval. Allowable bearing pressure shall be considered to be the pressure at the base of a foundation in excess of the stabilized overburden pressure existing at the same level prior to construction operations.
Soil materials shall be classified and identified in accordance with Table 1804.2. In addition, refer to Sections 1804.2.1 through 1804.2.4 for supplementary definitions.
a. Hard sound rock (Class 1a). Includes crystalline rocks such as gneiss, granite, diabase, and schist. Characteristics are: the rock{:} rings when struck with pick or bar; {the rock} do not disintegrate after exposure to air or water; {the rock} break with sharp fresh fracture; cracks are unweathered and less than 1/8-inch (3.2 mm) wide, generally no closer than 3 feet (914 mm) apart; 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.
b. Medium hard rock (Class 1b). Includes crystalline rocks of paragraph (a) of this subdivision, plus marble and serpentinite. Characteristics are: all those listed in paragraph (a) 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; 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 fifty to eighty-five percent for each 5 foot (1524 mm) run.
c. Intermediate rock (Class 1c). Includes rocks described in paragraphs (a) and (b) of this subdivision, plus cemented shales and sandstone. Characteristics are: the rock gives dull sound when struck with pick or bar; does not disintegrate after exposure to air or water; broken pieces may show weathered surfaces; may contain fracture and weathered zones up to 1 inch (25 mm) wide spaced as close as 1 foot (305 mm); 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.
d. Soft rock (Class 1d). Includes rocks described in paragraphs (a), (b) and (c) of this subdivision in partially weathered condition, plus poorly cemented shales and sandstones. Characteristics are: rock may soften on exposure to air or water; may contain thoroughly weathered zones up to 3 inches (76 mm) wide but filled with stiff soil; 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 thirty-five percent for each 5 foot (1524 mm) run and 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 containing little or no fines (GW and GP). The density of these materials shall be determined in accordance with the following:
Dense (Class 2a). Those materials having a Standard Penetration Test N-value greater than 30 blows per one foot (0.3 meter).
Medium (Class 2b). Those materials having a Standard Penetration Test N-value between 10 and 30 blows per one foot (0.3 meter).
Loose (Class 6). Those materials having a Standard Penetration Test N-value less than 10 blows per one foot (0.3 meter).
GRANULAR SOILS. These materials are coarse-grained soils consisting of gravel and/or sand with appreciable amounts of fines, and gravel (GM, GC, SW, SP, SM, and SC). The density of granular materials shall be determined in accordance with the following:
Dense (Class 3a). Those materials having a Standard Penetration Test N-value greater than 30 blows per one foot (0.3 meter).
Medium (Class 3b). Those materials having a Standard Penetration Test N-value between 10 and 30 blows per one foot (0.3 meter).
Loose (Class 6). Those materials having Standard Penetration Test N-value less than 10 blows per one foot (0.3 meter).
CLAYS. In the absence of sufficient laboratory data, the consistency of clay materials (SC, CL, and CH) 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 Standard Penetration Test N-values greater than 30 blows per one 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 between 1 TSF (96 kPa) and 4TSF (383 kPa), or Standard Penetration Test N-values between 8 and 30 blows per one 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 between 0.5 TSF (48 kPa) and 1 TSF (96 kPa), or Standard Penetration Test N-values between 4 and 8 blows per one foot (0.3 meter).
SILTS AND CLAYEY SILTS. In the absence of sufficient laboratory data, the consistency of silt materials (ML and MH) shall be determined in accordance with the following:
Dense (Class 5a). Silt with a Standard Penetration Test N-values greater than 30 blows per one foot (0.3 meter).
Medium (Class 5b). Silt with a Standard Penetration Test N-values between 10 and 30 blows per one foot (0.3 meter).
Loose (Class 6). Silt with a Standard Penetration Test N-values less than 10 blows per one foot (0.3 meters).
Organic silts, organic clays, peats, soft clays, loose granular soils and varved silts. The allowable bearing pressure shall be determined independently of Table 1804.1 subject to the following:
1. For varved silts, the soil bearing pressure produced by the proposed building shall not exceed two tons per square foot (192 kPa), except that for desiccated or over consolidated soils, higher bearing pressures are allowed subject to approval by the commissioner.
2. For organic silts or clays, soft clays, or for loose granular soils, the engineer responsible for the investigation shall establish the allowable soil bearing pressure based upon the soil's specific engineering properties. This may require that the soils be preconsolidated, artificially treated, or compacted.
3. A report prepared, signed and sealed by the engineer is required to be filed to substantiate the design soil pressures to be used on soil materials and it shall contain, at a minimum:
3.1. Sufficient laboratory test data on the compressible material to indicate the soil strength and the preconsolidation pressure, coefficient of consolidation, coefficient of compressibility, permeability, secondary compression characteristics, and Atterberg limits.
3.2. Where the design contemplates improvement of the natural bearing capacity and/or reduction in settlements by virtue of preloading: cross sections showing the amount of fill and surcharge to be placed, design details showing the required time for surcharging, and computations showing the amount of settlement to be expected during surcharging and the estimated amount and rate of settlement expected to occur after the structure has been completed, including the influence of dead and live loads of the structure.
3.3. A detailed analysis showing that the anticipated future settlement will not adversely affect the performance of the structure.
3.4. Where strip drains, sand drains, or stone columns are to be used, computations showing the diameter, spacing, and anticipated method of installation of such drains.
3.5. Records of settlement plate elevations and pore pressure readings, before, during, and after surcharging.
Fills shall be considered as satisfactory bearing material of the applicable class when placed in accordance with the following procedures and subject to the special inspection provisions of Chapter 17:
- Area to be filled shall be stripped of all organic materials, rubbish and debris.
- Fill shall not be placed when frozen or on frozen or saturated subgrade.
- The engineer, or the engineer's representative, shall approve the subgrade prior to fill placement.
- Fill material shall consist of well graded sand, gravel, crushed rock, recycled concrete aggregate, or a mixture of these, or equivalent materials with a maximum of 10 percent passing the #200 sieve, as determined from the percent passing the #4 sieve.
- Fill shall be placed and compacted in lifts, not exceeding 12 inches (305 millimeters), at its optimum moisture content, plus or minus two percent, and to not less than a density of ninety-five percent of the optimum density as determined by ASTM D 1557.
- Fill density shall be verified by in-place tests made on each lift.
- The allowable bearing value of controlled fill shall be limited to 3 tons per square foot (383 kPa) providing the underlying soil is not weaker than the controlled fill.
Fills other than controlled fill may be considered as satisfactory bearing material of applicable class, subject to the following:
- The soil within the building area shall be explored using test pits at every column. All test pits shall extend to depths equal to the smaller width of the footing and at least one test pit shall penetrate at least 8 feet (2438 mm) below the level of the bottom of the proposed footings. All test pits shall be backfilled with properly compacted fill. Borings may be used in lieu of test pits, provided that continuous samples of at least 3 inches (76 mm) in diameter are recovered.
- The building area shall be additionally explored using one standard boring for every twenty-five hundred square foot (232.3 m2) of building footprint area. These borings shall be carried to a depth sufficient to penetrate into natural ground, but not less than 20 feet (6096 mm) below grade.
- The fill shall be composed of material that is free of voids and free of extensive inclusions of mud, organic materials such as paper, wood, garbage, cans, or metallic objects, and debris.
- The allowable soil bearing pressure on satisfactory uncontrolled fill material shall not exceed two tons per square foot (192 kPa). One-and two-family dwellings may be founded on satisfactory uncontrolled fill provided the dwelling site has been explored using at least one test pit, penetrating at least 8 feet (2438 mm) below the level of the bottom of the proposed footings, and that the fill has been found to be composed of material that is free of voids and generally free of mud, organic materials such as paper, garbage, cans, or metallic objects, and debris. Test pits shall be backfilled with properly compacted fill.
Nominally unsatisfactory soil materials that are artificially compacted, cemented, or preconsolidated may be used for the support of buildings, and nominally satisfactory soil materials that are similarly treated may be used to resist soil bearing pressures in excess of those indicated in Table 1804.1. The engineer shall develop treatment plans and procedures and post-treatment performance and testing requirements and submit such plans, procedures, and requirements to the commissioner for approval. After treatment, a sufficient amount of sampling and/or in-situ tests shall be performed in the treated soil to demonstrate the efficacy of the treatment for the increased bearing pressure.
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