1806A Presumptive Load-Bearing Values of Soils

1806 A .1 Load combinations. The presumptive load-bearing values provided in Table 1806 A .2 shall be used with the allowable stress design load combinations specified in ASCE 7, Section 2.4 or the alternative allowable stress design load combinations of Section 1605 A .2. The values of vertical foundation pressure and lateral bearing pressure given in Table 1806 A .2 shall be permitted to be increased by one-third where used with the alternative allowable stress design load combinations of Section 1605 A .2 that include wind or earthquake loads.

1806 A .2 Presumptive load-bearing values. The load-bearing values used in design for supporting soils and rock near the surface shall not exceed the values specified in Table 1806 A .2 unless data to substantiate the use of higher values are submitted and approved. Where the building official has reason to doubt the classification, strength or compressibility of the soil or rock, the requirements of Section 1803 A .5.2 shall be satisfied.

Presumptive load-bearing values shall apply to materials with similar physical and engineering characteristics. Mud, organic silt and organic clays (OL, OH), peat (Pt) and undocumented fill shall not be assumed to have a presumptive load-bearing capacity unless data to substantiate the use of such a value are submitted.

Exception: A presumptive load-bearing capacity shall be permitted to be used where the building official deems the load-bearing capacity is adequate for the support of lightweight or temporary structures.

TABLE 1806A.2—PRESUMPTIVE LOAD-BEARING VALUES

CLASS OF MATERIALS	VERTICAL FOUNDATION PRESSURE (psf)	LATERAL BEARING PRESSURE (psf/ft below natural grade)	LATERAL SLIDING RESISTANCE	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 = 0.0479 kPa, 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_A_.3.2.

1806 A .3 Lateral load resistance. Where the presumptive values of Table 1806 A .2 are used to determine resistance to lateral loads, the calculations shall be in accordance with Sections 1806 A .3.1 through 1806 A .3.4.

1806 A .3.1 Combined resistance. The total resistance to lateral loads shall be permitted to be determined by combining the values derived from the lateral bearing pressure and the lateral sliding resistance specified in Table 1806 A .2.

1806 A .3.2 Lateral sliding resistance limit. For clay, sandy clay, silty clay, clayey silt, silt and sandy silt, the lateral sliding resistance shall not exceed one-half the dead load.

1806 A .3.3 Increase for depth. The lateral bearing pressures specified in Table 1806 A .2 shall be permitted to be increased by the tabular value for each additional foot (305 mm) of depth to a value that is not greater than 15 times the tabular value.

1806 A .3.4 Increase for poles. Isolated poles for uses such as flagpoles or signs and poles used to support buildings that are not adversely affected by a 1/2-inch (12.7 mm) motion at the ground surface due to short-term lateral loads shall be permitted to be designed using lateral bearing pressures equal to two times the tabular values.

SOILS AND FOUNDATIONS