Lateral pressure from surface loads
The lateral pressure on a wall from surface loads behind it, added to the soil load of ASCE 7-22 3.2.1: wide surcharges, point loads, line loads along or toward the wall, strips and rectangles. Copy the pressure into the design of the wall as the load H of Chapter 2: nothing links them. The type reads Calculated.
Inputs of the surface loads #
- Wall height H: from the ground surface to the base of the part looked at.
- Wall movement: unyielding, or yielding to active conditions, which takes 75 % of the limited loads (DM7.2 4-4.2). A wide uniform surcharge is not reduced, as K already sets it. Your call.
- Coefficient K: for a wide uniform surcharge, at rest for a restrained wall or active. From your soil values.
- Backfill: a row of ASCE 7-22 Table 3.2-1, the values of a geotechnical report entered, or not included. Table 3.2-1 is the code's minimum where no report gives the values. MH, CH, OL, OH are unsuitable as backfill and are not listed.
- Soil pressure: at rest where the top is restrained, active where the wall may move (3.2.1).
- Surface loads: up to 10 by kind. Enter the value that fits: q for uniform, strip and rectangle, Q for a point, Q line for a line.
- x: from the wall to the load, or to its near edge. x2: to its far edge.
- y: along the wall from the section looked at, to a point or a perpendicular line, or the two ends of a rectangle. Line and strip loads along the wall act the same at every section.
Checks performed #
| Result | Basis |
|---|---|
| Uniform: K q | DM7.2 4-4.2.1 |
| Point | DM7.2 Figure 4-18, with fall-off by cos²(1.1 θ) along the wall |
| Line along the wall | DM7.2 Figure 4-19 |
| Line toward the wall | DM7.2 Figure 4-20 |
| Rectangle | Doubled elastic stress for a rigid wall, integrated: Figure 4-17 |
| Strip along the wall | The limit of that rectangle: (2 q / π)(β − sin β cos 2α) |
| Soil load | ASCE 7-22 3.2.1, Table 3.2-1 |
| Total, resultant, height, base moment | Sum at 49 depths, by the trapezoid rule |
Limitations #
- Rigid wall basis: conservative for active pressure, possibly unconservative for passive. A flexible wall may see up to half as much (the elastic value not doubled).
- Level ground, no water. Below free water, 3.2.1 takes buoyant soil plus full hydrostatic pressure: not here. Expansive soil is not here. Compaction pressures (DM7.2 4-4.3) are not here. Surcharge below a wall's base is not carried into a toe.
- Deep footing loads: the equations are for loads on the surface.
- No wall design.
Verification examples #
Verification status: 37 values compared in 24 examples (24 published, 0 hand calculations from the cited equations): 35 within tolerance, 2 recorded deviations. Code basis: the provisions it uses were read in the target edition.
| Example | Source | Values |
|---|---|---|
| Point load, six distances | UFS 3-220-20 Figure 4-18, p. 206 | PH and y/H of the printed table |
| Line load, seven distances | Figure 4-19, p. 207 | PH by the printed formula, y/H of the printed table |
| Rectangle at its corner | Figure 4-17, p. 205 | Influence factors at five values of m, read off the chart |
| Soil loads | ASCE 7-22 Table 3.2-1, p. 72 | Six cells |
No published worked example was found with a whole wall and its loads added up. Those checks test the method against the printed curves, tables and formulas. The Figure 4-17 values are read off a chart by eye, good to about 0.005; the reading at m = 2.0 misses by a hair and is a documented deviation. Figure 4-20 has no printed table: it is checked against a thin rectangle by hand. Other classic sources (Caltrans Trenching and Shoring Manual, NAVFAC DM-7.2 of 1986, AASHTO) use the same equations; only Caltrans's Chapter 4 was read and it does not print them.