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Section 1804 Allowable Load-Bearing Values of Soils
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The presumptive load-bearing values provided in Table 1804.1 shall be used with the allowable stress design load combinations specified in Section 1605.3 and Section 1801.2.
|CLASS OF MATERIALS
(Notes 1 and 3)
|MAXIMUM ALLOWABLE FOUNDATION
|MAXIMUM ALLOWABLE FOUNDATION
|1. Bedrock (Notes 2 and 7)
1a Hard sound rock—gneiss, diabase, schist
1b Medium hard rock—marble, serpentine
1c Intermediate rock—shale, sandstone
1d Soft rock—weathered rock
|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. Organic silts, organic clays, peats, soft clays, loose
granular soils and varvedsilts
|See 1804.2.1||See 1804.2.1|
|7. Controlled and uncontrolled fills||See 1804.2.2 or 1804.2.3||See 1804.2.2 or 1804.2.3|
- Where there is doubt as to the applicable classification of a soil stratum, the allowable bearing pressure applicable to the lower class of material to which the given stratum might conform shall apply.
- 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. Tilted strata and their relation to nearby slopes or excavations shall receive special consideration. The tabulated values for Class 1a materials (hard sound rock) may be increased by 25 percent provided the geotechnical engineer performs additional tests and/or analyses substantiating the increase
- For intermediate conditions, values of allowable bearing pressure shall be estimated by interpolation between indicated extremes.
- Footing embedment in soils shall be in accordance with Section 1805.2 and the width of the loaded area not less than 2 feet (610mm), unless analysis demonstrates that the proposed construction will have a minimum factor of safety of 2.0 against shear failure of the soil.
- Estimates of settlements shall govern the allowable bearing value, subject to the maximums given in this table, and as provided in Section 1804.2.
- The bearing capacity 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.
- Increases in allowable bearing pressure due to embedment of the foundation. The allowable bearing values for intermediate to hard rock shall apply where the loaded area is on the surface of sound rock. Where the loaded area is below the adjacent rock surface and is fully confined by the adjacent rock mass and provided that the rock mass has not been shattered by blasting or otherwise is or has been rendered unsound, these values 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 val ues.
- The allowable bearing values for soils of Classes 2,3,4, and5 determined in accordance with Notes three, four and five 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, these values may be increased 5 percent of the base value for each 1-foot (305mm) 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.
- The allowable bearing values for soils of Classes 2, 3, 4, and 5 determined in accordance with this table and the notes thereto, may be increased up to one - third 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 values 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.
- The maximum toe pressure for eccentrically loaded footings may exceed the allowable bearing value by up to 25 percent if it is demonstrated that the heel of the footing is not subjected to tension.
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.
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