BEARING CAPACITY ANALYSES FOR EARTHQUAKES - Geotechnical Earthquake Engineering - page 218

TABLE 8.2 Allowable Bearing Pressures

Allowable bearing Maximum allowable

Material type pressure* bearing pressure †

4000 lb/ft 2 (200 kPa) 12,000 lb/ft 2 (600 kPa) Massive crystalline bedrock

2000 lb/ft 2 (100 kPa) 6000 lb/ft 2 (300 kPa) Sedimentary and foliated rock

2000 lb/ft 2 (100 kPa) 6000 lb/ft 2 (300 kPa) Gravel and sandy gravel (GW, GP) ‡

1500 lb/ft 2 (75 kPa) 4500 lb/ft 2 (220 kPa) Nonplastic soil: sands, silty gravel,

and nonplastic silt (GM, SW, SP, SM) ‡
Plastic soil: silts and clays 1000 lb/ft 2 (50 kPa) 3000 lb/ft 2 (150 kPa) §

(ML, MH, SC, CL, CH) ‡

*Minimum footing width and embedment depth equal 1 ft (0.3 m).
† An increase of 20 percent of the allowable bearing pressure is allowed for each additional 1 ft (0.3 m) of width
or depth up to the maximum allowable bearing pressures listed in the rightmost column. An exception is plastic soil;
see last note.
‡ Group symbols from the Unified Soil Classification System.
§ No increase in the allowable bearing pressure is allowed for an increase in width of the footing.

For dense or stiff soils, allowable bearing values are generally conservative. For very loose or very soft soils,
allowable bearing values may be too high.
Source: Data from Uniform Building Code (1997). See also Table 14.6.

the liquefied soil layer (Sec. 8.2.2). The second case uses the traditional Terzaghi bearing
capacity equation, with a reduction in the bearing capacity factors to account for the loss of
shear strength of the underlying liquefied soil layer (Sec. 8.2.3).
4. Special considerations: Special considerations may be required if the structure is
subjected to buoyancy or if there is a sloping ground condition.

8.2.2 Punching Shear Analysis

Illustration of Punching Shear. Figure 8.7 illustrates the earthquake-induced punching
shear analysis. The soil layer portrayed by dashed lines represents unliquefiable soil which
is underlain by a liquefied soil layer. For the punching shear analysis, it is assumed that the
load will cause the foundation to punch straight downward through the upper unliquefiable
soil layer and into the liquefied soil layer. As shown in Fig. 8.7, this assumption means that
there will be vertical shear surfaces in the soil that start at the sides of the footing and extend
straight downward to the liquefied soil layer. It is also assumed that the liquefied soil has
no shear strength and the upper layer remains undamaged (e.g., see Fig. 7.3) during the
earthquake.

Factor of Safety. Using the assumptions outlined above, the factor of safety (FS) can be
calculated as follows:
For strip footings:

P 2 T f

FS R

__ ____

P

(8.1 a )

For spread footings:

P 2 ( B L ) ( T f )

FS R

__ _____________

P

(8.1 b )