Department of Civil Engineering, N-W.F.P UET, Peshawar
Design of Retaining Walls
Soil parameters used in the analysis of retaining walls: Table 1 gives representative values for γs (unit weight of soil) and φ (angle of internal friction of soil) often used in engineering practice. (Note that the φ values do not account for probable additional pressures due to pore water, seepage, frost, etc). The table also contains values for the coefficient of friction “f” between concrete and various soils. The values of φ for soils 3 through 5 maybe quite unconservative; under saturated conditions, clays and silts may become entirely liquid (that is, φ = 0). Soils of type 1 or 2 should be used as backfill for retaining walls wherever possible. Table 1: Unit weight (γ), effective angles of internal friction (φ), and the coefficient of friction with concrete (f) Soil 1. Sand or gravel without fine particles, highly permeable 2. Sand or gravel with silt mixture, low permeability 3. Silty sand, sand and gravel with high clay content
Unit Weight (γs), pcf
φ (degree)
f
110 to 120
33 to 40
0.5 to 0.6
120 to 130
25 to 35
0.4 to 0.5
110 to 120
25 to 30
0.3 to 0.4
4. Medium or stiff clay
100 to 120
25 to 35
0.2 to 0.4
5. Soft clay, silt
90 to 110
20 to 35
0.2 to 0.3
Earth pressure for normal conditions of loading: In computing earth pressures on walls, three common conditions of loading are most often met: (1) Horizontal surface of fill at the top of the wall (figure 3a), (2) Inclined surface of fill sloping up and back from the top of the wall (figure 3b), (3) Horizontal surface of fill carrying a uniformly distributed additional load (surcharge), such as from goods in a storage yard or traffic on a road (figure 3c). The increase in pressure caused by uniform surcharge s (case 3) is computed by converting its load into an equivalent imaginary height of earth (h') above the top of the wall such that, h′ = s / γs and measuring the depth to a given point on the wall from this imaginary surface.
Prof Dr. Qaisar Ali (http://www.eec.edu.pk)
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