Section 1807A — California Building Code (Title 24, Part 2)

FOUNDATION WALLS, RETAINING WALLS AND EMBEDDED POSTS AND POLES

1807 A .1 Foundation walls. Foundation walls shall be designed and constructed in accordance with Sections 1807 A .1.1 through 1807 A .1.6. Foundation walls shall be supported by foundations designed in accordance with Section 1808 A .

1807 A .1.1 Design lateral soil loads. Foundation walls shall be designed for the lateral soil loads determined by a geotechnical investigation, in accordance with Section 1803A.

1807 A .1.2 Unbalanced backfill height. Unbalanced backfill height is the difference in height between the exterior finish ground level and the lower of the top of the concrete footing that supports the foundation wall or the interior finish ground level. Where an interior concrete slab on grade is provided and is in contact with the interior surface of the foundation wall, the unbalanced backfill

TABLE 1806 A .2 PRESUMPTIVE LOAD-BEARING VALUES

CLASS OF MATERIALS	VERTICAL FOUNDATION PRESSURE (psf)	LATERAL BEARING PRESSURE (psf/ft below natural grade)	LATERAL SLIDING RESISTANCE
CLASS OF MATERIALS	VERTICAL FOUNDATION PRESSURE (psf)	LATERAL BEARING PRESSURE (psf/ft below natural grade)	Coefficient of frictiona	Cohesion (psf)b
1. Crystalline bedrock	12,000	1,200	0.70	—
2. Sedimentary and foliated rock	4,000	400	0.35	—
3. Sandy gravel and gravel (GW and GP)	3,000	200	0.35	—
4. Sand, silty sand, clayey sand, silty gravel and clayey gravel (SW, SP, SM, SC, GM and GC)	2,000	150	0.25	—
5. Clay, sandy clay, silty clay, clayey silt, silt and sandy silt (CL, ML, MH and CH)	1,500	100	—	130

For SI: 1 pound per square foot = 0.0479kPa, 1 pound per square foot per foot = 0.157 kPa/m. a. Coefficient to be multiplied by the dead load. b. Cohesion value to be multiplied by the contact area, as limited by Section 1806.3.2.

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height shall be permitted to be measured from the exterior finish ground level to the top of the interior concrete slab.

1807 A .1.3 Rubble stone foundation walls. Not permitted by DSA-SS, DSA-SS/CC or OSHPD.

1807 A .1.4 Permanent wood foundation systems. Not permitted by DSA-SS, DSA-SS/CC or OSHPD.

1807 A .1.5 Concrete and masonry foundation walls. Concrete and masonry foundation walls shall be designed in accordance with Chapter 19 A or 21 A, as applicable.

1807 A .2 Retaining walls. Retaining walls shall be designed in accordance with Sections 1807 A .2.1 through 1807 A .2.3. Freestanding cantilever walls shall be designed in accor- dance with Section 1807A.2.4.

1807 A .2.1 General. Retaining walls shall be designed to ensure stability against overturning, sliding, excessive foundation pressure and water uplift.

1807 A .2.2 Design lateral soil loads. Retaining walls shall be designed for the lateral soil loads determined by a geo- technical investigation in accordance with Section 1803A and shall not be less than eighty percent of the lateral soil loads determined in accordance with Section 1610A. For use with the load combinations, lateral soil loads due to gravity loads surcharge shall be considered gravity loads and seismic earth pressure increases due to earthquake shall be considered as seismic loads . For structures assigned to Seismic Design Category D, E, or F, the design of retaining walls supporting more than 6 feet (1829 mm) of backfill height shall incorporate the additional seismic lateral earth pressure in accordance with the geotechnical investigation where required in Section 1803 A .2.

1807 A .2.3 Safety factor. Retaining walls shall be designed to resist the lateral action of soil to produce sliding and overturning with a minimum safety factor of 1.5 in each case. The load combinations of Section 1605 A shall not apply to this requirement. Instead, design shall be based on 0.7 times nominal earthquake loads, 1.0 times other nominal loads, and investigation with one or more of the variable loads set to zero. The safety factor against lateral sliding shall be taken as the available soil resistance at the base of the retaining wall foundation divided by the net lateral force applied to the retaining wall.

Exception: Where earthquake loads are included, the minimum safety factor for retaining wall sliding and overturning shall be 1.1.

1807A.2.4 Freestanding cantilever walls. A stability check against the possibility of overturning shall be per- formed for isolated spread footings which support free- standing cantilever walls. The stability check shall be made by dividing Rp used for the wall by 2.0. The allow- able soil pressure may be doubled for this evaluation.

Exception: For overturning about the principal axis of rectangular footings with symmetrical vertical loading and the design lateral force applied, a triangular or trapezoidal soil pressure distribution which covers the full width of the footing will meet the stability require- ment.

1807 A .3 Embedded posts and poles. Designs to resist both axial and lateral loads employing posts or poles as columns embedded in earth or in concrete footings in earth shall be in accordance with Sections 1807 A .3.1 through 1807 A .3.3.

1807 A .3.1 Limitations. The design procedures outlined in this section are subject to the following limitations:

The frictional resistance for structural walls and slabs on silts and clays shall be limited to one-half of the normal force imposed on the soil by the weight of the footing or slab.
Posts embedded in earth shall not be used to provide lateral support for structural or nonstructural materials such as plaster, masonry or concrete unless bracing is provided that develops the limited deflection required.

Wood poles shall be treated in accordance with AWPA U1 for sawn timber posts (Commodity Specification A, Use Category 4B) and for round timber posts (Commodity Specification B, Use Category 4B).

1807 A .3.2 Design criteria. The depth to resist lateral loads shall be determined using the design criteria established in Sections 1807 A .3.2.1 through 1807 A .3.2.3, or by other methods approved by the building official.

1807 A .3.2.1 Nonconstrained. The following formula shall be used in determining the depth of embedment required to resist lateral loads where lateral constraint is not provided at the ground surface, such as by a rigid floor or rigid ground surface pavement, and where lateral constraint is not provided above the ground surface, such as by a structural diaphragm.

d = 0.5 A {1 + [1 + (4.36 h / A )] ^1/2 } (Equation 18 A -1)

where: A = 2.34 P/(S 1 b) . b = Diameter of round post or footing or diagonal

dimension of square post or footing, feet (m).

d = Depth of embedment in earth in feet (m) but not

over 12 feet (3658 mm) for purpose of computing lateral pressure.

h = Distance in feet (m) from ground surface to point

of application of “ P .”

P = Applied lateral force in pounds (kN). S 1 = Allowable lateral soil-bearing pressure as set

forth in Section 1806 A .2 based on a depth of onethird the depth of embedment in pounds per square foot (psf) (kPa).

1807 A .3.2.2 Constrained. The following formula shall be used to determine the depth of embedment required to resist lateral loads where lateral constraint is provided at the ground surface, such as by a rigid floor or pavement.

or alternatively

(Equation 18 A -2)

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(Equation 18 A -3)

SOILS AND FOUNDATIONS

overturning effects is by equivalent lateral force analysis or modal analysis, the proportioning shall be in accordance with Section 12.13.4 of ASCE 7.

1808 A .3.2 Surcharge. 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.

1808 A .4 Vibratory loads. Where machinery operations or other vibrations are transmitted through the foundation, consideration shall be given in the foundation design to prevent detrimental disturbances of the soil.

1808 A .5 Shifting or moving soils. Where it is known that the shallow subsoils are of a shifting or moving character, foundations shall be carried to a sufficient depth to ensure stability.

1808 A .6 Design for expansive soils. Foundations for buildings and structures founded on expansive soils shall be designed in accordance with Section 1808 A .6.1 or 1808 A .6.2.

Exception: Foundation design need not comply with Section 1808 A .6.1 or 1808 A .6.2 where one of the following conditions is satisfied:

The soil is removed in accordance with Section 1808 A .6.3.
The building official approves stabilization of the soil in accordance with Section 1808 A .6.4.

1808 A .6.1 Foundations. 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.

1808 A .6.2 Slab-on-ground foundations. Moments, shears and deflections for use in designing slab-onground, mat or raft foundations on expansive soils shall be determined in accordance with WRI/CRSI Design of Slab- on-Ground Foundations or PTI DC 10.5. Using the moments, shears and deflections determined above, non

where: Mg = Moment in the post at grade, in foot-pounds

(kN-m). S 3 = Allowable lateral soil-bearing pressure as set

forth in Section 1806 A .2 based on a depth equal to the depth of embedment in pounds per square foot (kPa).

1807 A .3.2.3 Vertical load. The resistance to vertical loads shall be determined using the vertical foundation pressure set forth in Table 1806 A .2.

1807 A .3.3 Backfill. The backfill in the annular space around columns not embedded in poured footings shall be by one of the following methods:

Backfill shall be of concrete with a specified compressive strength of not less than 2,000 psi (13.8 MPa). The hole shall be not less than 4 inches (102 mm) larger than the diameter of the column at its bottom or 4 inches (102 mm) larger than the diagonal dimension of a square or rectangular column.
Backfill shall be of clean sand. The sand shall be thoroughly compacted by tamping in layers not more than 8 inches (203 mm) in depth.
Backfill shall be of controlled low-strength material (CLSM).