Section 1808 Foundations
Fill or other surcharge loads shall not be placed adjacent to any building or structure unless such building or structure is capable of withstanding the additional loads caused by the fill or the surcharge. Existing footings or foundations that will be affected by any excavation shall be underpinned or otherwise protected against settlement and shall be protected against detrimental lateral or vertical movement or both.
Exception: Minor grading for landscaping purposes shall be permitted where done with walk-behind equipment, where the grade is not increased more than 1 foot (305 mm) from original design grade or where approved by the building official.
Foundations placed on or within the active zone of expansive soils shall be designed to resist differential volume changes and to prevent structural damage to the supported structure. Deflection and racking of the supported structure shall be limited to that which will not interfere with the usability and serviceability of the structure.
Foundations placed below where volume change occurs or below expansive soil shall comply with the following provisions:
- Foundations extending into or penetrating expansive soils shall be designed to prevent uplift of the supported structure.
- Foundations penetrating expansive soils shall be designed to resist forces exerted on the foundation due to soil volume changes or shall be isolated from the expansive soil.
Where expansive soil is removed in lieu of designing foundations in accordance with Section 1808.6.1 or 1808.6.2, the soil shall be removed to a depth sufficient to ensure a constant moisture content in the remaining soil. Fill material shall not contain expansive soils and shall comply with Section 1804.5 or 1804.6.
Exception: Expansive soil need not be removed to the depth of constant moisture, provided that the confining pressure in the expansive soil created by the fill and supported structure exceeds the swell pressure.
In general, buildings below slopes shall be set a sufficient distance from the slope to provide protection from slope drainage, erosion and shallow failures. Except as provided in Section 1808.7.5 and Figure 1808.7.1, the following criteria will be assumed to provide this protection. Where the existing slope is steeper than one unit vertical in one unit horizontal (100-percent slope), the toe of the slope shall be assumed to be at the intersection of a horizontal plane drawn from the top of the foundation and a plane drawn tangent to the slope at an angle of 45 degrees (0.79 rad) to the horizontal. Where a retaining wall is constructed at the toe of the slope, the height of the slope shall be measured from the top of the wall to the top of the slope.
Where concrete is placed through a funnel hopper at the top of a deep foundation element, the concrete mix shall be designed and proportioned so as to produce a cohesive workable mix having a slump of not less than 4 inches (102 mm) and not more than 8 inches (204 mm). Where concrete or grout is to be pumped, the mix design including slump shall be adjusted to produce a pumpable mixture.
|FOUNDATION ELEMENT OR CONDITION||SPECIFIED COMPRESSIVE STRENGTH, f 'c|
|1. Foundations for structures assigned to Seismic Design Category A, B or C||2,500 psi|
|2a. Foundations for Group R or U occupancies of light-frame construction, two stories or less in |
height, assigned to Seismic Design Category D, E or F
|2b. Foundations for other structures assigned to Seismic Design Category D, E or F||3,000 psi|
|3. Precast nonprestressed driven piles||4,000 psi|
|4. Socketed drilled shafts||4,000 psi|
|5. Micropiles||4,000 psi|
|6. Precast prestressed driven piles||5,000 psi|
For SI: 1 pound per square inch = 0.00689 MPa.
The concrete cover provided for prestressed and nonprestressed reinforcement in foundations shall be not less than the largest applicable value specified in Table 1808.8.2. Longitudinal bars spaced less than 11/2 inches (38 mm) clear distance apart shall be considered to be bundled bars for which the concrete cover provided shall be not less than that required by Section 18.104.22.168.4 of ACI 318. Concrete cover shall be measured from the concrete surface to the outermost surface of the steel to which the cover requirement applies. Where concrete is placed in a temporary or permanent casing or a mandrel, the inside face of the casing or mandrel shall be considered to be the concrete surface.
|FOUNDATION ELEMENT OR CONDITION||MINIMUM COVER|
|1. Shallow foundations||In accordance with Section 20.6 of ACI 318|
|2. Precast nonprestressed deep foundation elements|
|Exposed to seawater||3 inches|
Not manufactured under plant conditions
|Manufactured under plant control conditions||In accordance with Section 22.214.171.124.3 of ACI 318|
|3. Precast prestressed deep foundation elements|
|Exposed to seawater||2.5 inches|
|Other||In accordance with Section 126.96.36.199.3 of ACI 318|
|4. Cast-in-place deep foundation elements not enclosed by a steel pipe, tube or permanent casing||2.5 inches|
|5. Cast-in-place deep foundation elements enclosed by a steel pipe, tube or permanent casing||1 inch|
|6. Structural steel core within a steel pipe, tube or permanent casing||2 inches|
|7. Cast-in-place drilled shafts enclosed by a stable rock socket||1.5 inches|
For SI: 1 inch = 25.4 mm.