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Section 1804 Allowable Load-Bearing Values of Soils and Rock
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The allowable bearing pressures provided in Table 1804.1 shall be used with the allowable stress design load combinations specified in Section 1605.3.
TABLE 1804.1 ALLOWABLE BEARING PRESSURES
TABLE 1804.1 ALLOWABLE BEARING PRESSURES
| CLASS OF MATERIALS |
(Notes 1 and 3)
| MAXIMUM ALLOWABLE BEARING |
| MAXIMUM ALLOWABLE BEARING |
|1. Bedrock (Notes 2 and 7)|
|1a Hard sound rock||60||5,746|
|1b Medium rock||40||3,830|
|1c Intermediate rock||20||1,915|
|1d Soft rock||8||766|
|2. Sandy gravel and gravel (GW, GP) (Notes 3, 4, 8, and 9)|
|3. Granular soils (GC, GM, SW, SP,SM, and SC) (Notes 4, 5, 8, and 9)|
|4. Clays (SC, CL, and CH) (Notes 4, 6, 8, and 9)|
|5. Silts and silty soils (ML and MH) (Notes 4, 8, and 9)|
|6. Nominally Unsatisfactory Bearing Materials:||See 1804.2.1||See 1804.2.1|
|Loose granular soils|
|Soft clay soils|
|• All organic silts, organic clays, peats, soft clays, granular soils and varved silts|
|7. Controlled and uncontrolled fills||See 1804.2.2 or 1804.2.3||See 1804.2.2 or 1804.2.3|
|1. Where there is doubt as to the applicable classification of a soil or rock stratum, the allowable bearing pressure applicable to the lower class of material to which the given stratum might conform shall apply.|
|2. The tabulated values of allowable bearing pressures apply only for massive rocks, or for sedimentary or foliated rocks where the strata are level or nearly so, and then only if the area has ample lateral support. The allowable bearing pressure for tilted strata and their relation to nearby slopes or excavations shall be evaluated by the engineer responsible for the geotechnical investigation. The tabulated values for Class 1a materials (hard sound rock) may be increased by 25 percent provided the engineer performs additional tests and/or analyses substantiating the increase.|
|3. For intermediate conditions, values of allowable bearing pressure shall be estimated by interpolation between indicated extremes.|
|4. Footing embedment in soils shall be in accordance with Section 1805.3 and the width of the loaded area shall not be less than 18 inches (457 mm), unless analysis demonstrates that the proposed construction will have a minimum factor of safety of 2.0 against shear failure of the soil.|
|5. Estimates of settlements shall govern the allowable bearing pressure, subject to the maximums given in this table, and as provided in Section 1804.2.|
|6. Allowable bearing pressure of clay soils shall be established on the basis of the strength of such soils as determined by field or laboratory tests and shall provide a factor of safety against failure of the soil of not less than 2.0 computed on the basis of a recognized procedure of soils analysis, shall account for probable settlements of the building and shall not exceed the tabulated maximum values.|
|7. Allowable bearing pressure may be increased due to embedment of the foundation. The allowable bearing pressure for intermediate rock (1c), medium hard rock (1b), and hard sound rock (1a) shall apply where the loaded area is on the rock surface. Where the loaded area is below the rock surface and is fully confined by the adjacent rock mass and provided that the adjacent rock mass above the bearing surface is of the same rock class or better, and the rock mass has not been shattered by blasting or otherwise is or has been rendered unsound, the allowable bearing pressure may be increased 10 percent of the base value for each 1 foot (0.3 meters) of embedment below the surface of the adjacent rock surface in excess of 1 foot (0.3 meters), but shall not exceed 200 percent of the values.|
|8. The allowable bearing pressure for soils of Classes 2, 3, 4, and 5 determined in accordance with Notes 3, 4, and 5 above, shall apply where the loaded area is embedded 4 feet (1219 mm) or less in the bearing stratum. Where the loaded area is embedded more than 4 feet (1219 mm) below the adjacent surface of the bearing stratum, and is fully confined by the weight of the adjacent soil, the allowable bearing pressure may be increased 5 percent of the base value for each 1-foot (305 mm) additional embedment, but shall not exceed twice the values. Increases in allowable bearing pressure due to embedment shall not apply to soft rock, clays, silts and soils of Classes 6 and 7.|
|9. The allowable bearing pressure for soils of Classes 2, 3, 4, and 5 determined in accordance with this table and the notes thereto, may be increased up to 1/3 where the density of the bearing stratum below the bottom of the footings increases with depth and is not underlain by materials of a lower allowable bearing pressure. Such allowable bearing pressure shall be demonstrated by a recognized means of analysis that the probable settlement of the foundation due to compression, and/or consolidation does not exceed acceptable limits for the proposed building.|
|10. The maximum toe pressure for eccentrically loaded footings may exceed the allowable bearing pressure by up to 25 percent if it is demonstrated that the heel of the footing is not subjected to tension.|
The allowable bearing pressure for supporting soil and rock 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 the commissioner approves such value. 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.
Organic silts, organic clays, peats, soft clays, loose granular soils, loose silts, and varved silts shall be considered nominally unsatisfactory bearing material. The allowable bearing pressure shall be determined independently of Table 1804.1 subject to the following:
- For varved silts, the soil bearing pressure produced by the proposed building shall not exceed 2 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.
- For organic silts or clays, peats, soft clays, loose granular soils, or loose silts, the engineer responsible for the geotechnical 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.
- A report prepared, signed and sealed by the engineer is required to be filed with the department to substantiate the design soil pressures to be used on soil materials and shall contain, at a minimum:
- 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.
- 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.
- A detailed analysis showing that the anticipated future settlement will not adversely affect the performance of the structure.
- 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.
- 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 special inspection agency shall approve the subgrade prior to fill placement.
- Fill material shall consist of gravel, crushed rock, recycled concrete aggregate, well-graded sand or a mixture of these, or equivalent materials with a maximum particle size of 3 inches (76.2 mm ) and a maximum of 10 percent passing the #200 sieve.
- Fill shall be placed and compacted in lifts, not exceeding 12 inches (305 mm), at its optimum moisture content, plus or minus 2 percent, and to not less than a density of 95 percent of the optimum density as determined by ASTM D 1557.
- Fill density shall be verified by in-place tests made on each lift.
Provided the capacity of the underlying soil is not exceeded, the allowable bearing pressure of controlled fill shall be limited to:
- 6 tons per square foot (766 kPa) for gravel and crushed rock.
- 3 tons per square foot (383 kPa) for recycled concrete aggregate and well-graded sand.
Fills other than controlled fill may be considered as satisfactory bearing material of applicable class, subject to the following:
- Where spread footings will be used, the soil within the built-up 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. Where mat foundations will be used, one test pit or minimum 3 inch (76 mm) diameter sampler boring shall be provided for every 1,000 square feet (232.3 m2) of building footprint area. For continuous concrete footings, one test pit or minimum 3 inch (76 mm) diameter sampler boring shall be provided for every 25 linear feet (7.62 m).
- The building area shall be additionally explored using one standard boring for every 2,500 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 and 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 2 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 the fill has been found to be composed of material that is free of voids and generally free of mud and 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.