dry and submerged state. The backfill carries a surclsarge of 320 lb/ft^2 (15.3 kPa). Apply-
ing Rankine's theory, compute the total pressure on this structure between A and C.Calculation Procedure:- Compute the specific weight of the soil in the submerged state
The lateral pressure of the soil below the water level consists of two elements: the pres-
sure exerted by the solids and that exerted by the water. The first element is evaluated by
applying the appropriate equation with w equal to the weight of the soil in the submerged
state. The second element is assumed to be the full hydrostatic pressure, as though the
solids were not present. Since there is water on both sides of the structure, the hydrostatic
pressures balance one another and may therefore be disregarded.
In calculating the forces on a bulkhead, it is assumed that the pressure distribution is
hydrostatic (i.e., that the pressure varies linearly with the depth), although this is not
strictly true with regard to a flexible wall.
Computing the specific weight of the soil in the submerged state gives w=114-(l-
0.42)62.4 - 77.8 lb/ft^3 (12.22 kN/m^3 ). - Compute the vertical pressure at A, B, and C caused by the
surcharge and weight of solids
Thus, pA = 320 lb/ft^2 (15.3 kPa); pB = 320 + 5(114) = 890 lb/ft^2 (42.6 kPa); pc = 890 +
12(77.8) = 1824 lb/ft^2 (87.3 kPa). - Compute the Rankine coefficient of active earth pressure
Determine the lateral pressure at A 9 B and C. Since the surface is horizontal, Eq. 171 ap-
plies, with </> = 34°. Refer to Fig. lib. Then C 0 = tan^2 (450 - 170) = 0.283; pA =
0.283(320) = 91 lb/ft^2 (4.3 kPa);p 5 = 252 lb/ft^2 (12.1 kPa);/?c= 516 lb/ft^2 (24.7 kPa). - Compute the total thrust between A and C
Thus, P =^1 / 2 (5)(91 + 252) + V 2 (12)(252 + 516) = 5466 Ib (24,312.7 N).
CANTILEVER BULKHEAD ANALYSIS
Sheet piling is to function as a cantilever retaining wall 5 ft (1.5 m) high. The soil weighs
110 lb/ft^3 (17.28 kN/m^3 ) and its angle of internal friction is 32°; the backfill has a hori-
zontal surface. Determine the required depth of penetration of the bulkhead.
Calculation Procedure:
- Take moments with respect
to C to obtain an equation for
the minimum value of d
Refer to Fig. 12a, and consider a 1-ft (30.5-
cm) length of wall. Assume that the pres-
sure distribution is hydrostatic, and apply
Rankine's theory.
The wall pivots about some point Z near
the bottom. Consequently, passive earth (a> Cantilever (b) Assumed pressures and
pressure is mobilized to the left of the wall bulkhead resultant forces
betwen B and Z and to the right of the wall FIGURE 12