BGASCE7-10 Section 4.2

Pressure Computations

Last Revised: 11/04/2014

ASCE 7-05 3.2 treats both lateral soil loads and hydrostatic pressure similarly.  Both increase linearly with depth according to the equation:

lateral pressure at depth h = g h

Where:

• g = Equivalent Fluid Density (62.4 pcf for water; from ASCE 7-10 Table 3.2-1 for some common soils) and
• h = the depth

It is common to have a high water table.  The presence of water is also discussed in ASCE 7-10 3.2.1. When water is present, the density of the soil is decreased by the buoyancy provided by the water.  This will result in a reduction in the lateral pressure exerted by the soil.  However, soil tests need to be preformed to determine the buoyant weight of the soil. It is conservative to use the full density of the soil in conjunction with water.

Figure 4.2.1 illustrates the application of the loads.  Figure 4.2.1a illustrates the expected distribution for a uniform soil without water present.  Figure 4.2.1b illustrates the distribution with water present.

Figure 4.2.1
Soil Loads & Hydrostatic Pressure

Where:

• hs = depth from the surface
• h1 = depth to top of the water
• hwmax is the depth to the bottom of the footing
• gs = the Design Lateral Soil Load per foot of depth
• g's = the buoyant Design Lateral Soil Load per foot of depth
• gw = the unit weight of water

Water Uplift Pressure

For water, you must also consider the upward buoyancy force, g_w h_wmax (see ASCE 7-10 3.2.2).  In essence the structure is acting like a boat when submerged in water.

Active vs. At Rest vs. Passive Pressure

The magnitude of the lateral force exerted by soil is dependent on the movement of the structure in response to the lateral force.  The three possible reactions are that the surface gives way under the load, the surface does not move at all, or the surface is subjected to other forces which causes it to actively push against the soil.

If a surface can be 'pushed' by the pressure--i.e. it can/will deflect significantly in response to the load--then the pressure is said to be 'active pressure' and is not as high as the other forms of pressure. The values given in ASCE 7-10 Table 3.2-1 with the footnote 'b' are all active pressures.  They assume that the surface is able to move in response to the loading.  This is common in cantilevered retaining walls.  It becomes less so if the wall is restrained against movement in response the the load.  Typical restraints are floor and/or wall diaphragms which prevent lateral movement.  It is interesting to read the footnote and commentary about this situation. Walls supported laterally by light floor framing and which are less than eight feet in depth are not considered to be rigid and may use the lower pressures given in the table.

If the surface is unyielding, as is typical of wall restrained by rigid diaphragms, the soil pressures respond more like the normal 'at rest' pressures found in the general soil mass.  These pressures tend to be higher than the active pressures associated with a yielding surface.  The values in ASCE 7-10 Table 3.2-1 with the footnote 'c' are all at-rest soil values and should be used for walls which have significant restraint.

If a surface actively bears on the lateral soil as the result of loads imposed from elsewhere in the structure (generally wind or seismic) then the pressures dramatically increase to 'passive pressure' levels.  These pressures can easily be three time greater than the lower 'active' pressures.  These pressures should be determined in consultation with a geotechnical engineer.