Nongravity cantilevered walls are those that provide lateral resistance through vertical elements embedded in soil, with the retained soil between the vertical elements usually supported by facing elements. Such walls may be constructed of concrete, steel, or timber. Their height is usually limited to about 15 ft (4.6 m), unless provided with additional support anchors.
8.6.1 Earth Pressure and Surcharge Loads
Lateral earth pressure can be estimated assuming wedge theory using a planar surface of sliding as defined by Coulomb’s theory. For permanent walls, effective stress methods of analysis and drained shear strength parameters for soils can be used for determining lateral earth pressures. Alternatively, the simplified earth pressure distributions shown in Figs. 8.45 and 8.46 can be used. Nomenclature and notes for Fig. 8.45 are given in Table 8.8.
|
FIGURE 8.45 Simplified earth pressure distributions for permanent flexible cantilevered walls with discrete vertical wall elements. (a) Embedment in soil; (b) embedment in rock. Note: Refer to Table 8.8 for general notes and legend. (From Standard Specifications for Highway Bridges, 2002, American Association of State Highway and Transportation Officials, Washington, D. C., with permission) |
1. Determine the active earth pressure on the wall due to surchage loads, the retained soil, and differential water pressure above the dredge line.
2. Determine the magnitude of active pressure at the dredge line (P*) due to surcharge loads, retained soil, and differential water pressure, using the earth pressure coefficient Ka2.
3. Determine the value of x = P*/[(Kp2 — Ka2)y2] for the distribution of net passive pressure
in front of the wall below the dredge line.
4. Sum moments about the point of action of F to determine the embedment (D0) for which the net passive pressure is sufficient to provide equilibrium.
5. Determine the depth (point a) at which the shear in the wall is zero (i. e., the point at which the areas of the driving and resisting pressure diagrams are equivalent).
6. Calculate the maximum bending moment at the point of zero shear.
7. Calculate the design depth, D = 1.2 D0 to 1.4 D0, for a safety factor of 1.5 to 2.0.
(a) Pressure distribution (b) Simplified design procedure
N°tes: (1) Surcharge and water pressures must be added to the above earth pressures.
(2) Forces shown are per horizontal foot of vertical wall element.
FIGURE 8.46 Simplified earth pressure distributions and design procedures for permanent flexible cantilevered walls with continuous vertical wall elements. (From Standard Specifications for Highway Bridges, 2002, American Association of State Highway and Transportation Officials, Washington, D. C., with permission)
For temporary applications in cohesive soils, total stress methods of analysis and undrained shear strength parameters apply. The simplified earth pressure distributions shown in Figs. 8.46 and 8.47 can alternatively be used with the following limitations:
1. The ratio of overburden pressure to undrained shear strength must be less than 3. This ratio is referred to as the stability number N = yH/c.
2. The active earth pressure must not be less than 0.25 times the effective overburden pressure at any depth.
Nomenclature and notes for Fig. 8.47 are given in Table 8.8.
Where discrete vertical wall elements are used for support, the width of each vertical element should be assumed to equal the width of the flange or diameter of the element for driven sections, and to equal the diameter of the concrete-filled hole for sections encased in concrete.
For permanent walls, Figs. 8.45 and 8.46 show the magnitude and location of resultant loads and resisting forces for discrete vertical elements embedded in soil and rock. The procedure for determining the resultant passive resistance of a vertical element assumes that net passive resistance is mobilized across a maximum of 3 times the element width or diameter (reduced, if necessary, to account for soft clay or discontinuities in the embedded depth of soil or rock). Also, a depth of 1.5 times the width of an element in soil, and 1 ft (300 mm) for an element in rock, is ineffective in providing passive lateral support.
Legend:
y’ = effective unit weight of soil b = vertical element width
l = spacing between vertical wall elements, center to center S = undrained shear strength of cohesive soil s = shear strength of rock mass Pp = passive resistance per vertical wall element P = active earth pressure per vertical wall element p = ground surface slope behind wall 1 + for slope up from wall
p’ = ground surface slope in front of wall J — for slope down from wall Ka = active earth pressure coefficient; refer to Art. 8.2.3
K = passive earth pressure coefficient; refer to Standard Specifications for Highway Bridges,
P AASHTO, 2002. ф’ = effective angle of soil friction
Notes:
1. For temporary walls embedded in granular soil or rock, refer to Fig. 8.45 to determine passive resistance and use diagrams on Fig. 8.47 to determine active earth pressure of retained soil.
2. Surcharge and water pressures must be added to the indicated earth pressures.
3. Forces shown are per vertical wall element.
4. Pressure distributions below the exposed portion of the wall are based on an effective element width of 3b, which is valid for l > 5b. For l < 5b, refer to Figs. 8.46 and 8.48 for continuous wall elements to determine pressure distributions on embedded portions of the wall.
Source: From Standard Specifications for Highway Bridges, 2002, American Association of State Highway
and Transportation Officials, Washington, D. C., with permission.
|
FIGURE 8.47 Simplified earth pressure distributions for temporary flexible cantilevered walls with discrete vertical wall elements. (a) Embedment in cohesive soil retaining granular soil; (b) embedment in cohesive soil retaining cohesive soil. Note: Refer to Table 8.8 for general notes and legend. (From Standard Specifications for Highway Bridges, 2002, American Association of State Highway and Transportation Officials, Washington, D. C., with permission) |
(a) Embedment in cohesive soil (b) Embedment in cohesive soil
retaining granular soil retaining cohesive soil
Notes: (1) For walls embedded in granular soil, refer to Fig. 8.46 and use above diagram for retained cohesive soil when appropriate.
(2) Surface and water pressures must be added to the above earth pressures.
(3) Forces shown are per horizontal foot of vertical wall element.
FIGURE 8.48 Simplified earth pressure distributions for temporary flexible cantilevered walls with continuous vertical wall elements. (From Standard Specifications for Highway Bridges, 2002, American Association of State Highway and Transportation Officials, Washington, D. C., with permission)
The design lateral pressure must include lateral pressure due to traffic, permanent point and line surcharge loads, backfill compaction, or other types of surcharge loads, as well as the lateral earth pressure.
