Flexible cantilevered walls should be dimensioned to ensure stability against passive failure of embedded vertical elements using a factor of safety of 1.5 based on unfactored loads. Vertical elements must be designed to support the full design earth, surcharge, and water pressures between the elements. In determining the depth of embedment to mobilize passive resistance, consideration should be given to planes of weakness (such as “slickensides,” bedding planes, and joint sets) that could reduce the strength of the soil or rock from that determined by field or laboratory tests. AASHTO recommends that for embedment in intact rock, including massive to appreciably jointed rock, which should not be allowed to fail through a joint surface, design should be based on an allowable shear strength of 0.10 to 0.15 times the uniaxial compressive strength of the intact rock.
Structural design of individual wall elements may be performed by service load or load factor design methods.
The maximum spacing L between vertical supporting elements depends on the relative stiffness of the vertical elements and facing, the design pressure P, and the type and condition of soil to be supported. Design the facing for the bending moment Mmx at any level, as determined by the following equations:
Simple span (no soil arching):
PL2
M = —— (8.22)
max 8
Simple span (soil arching):
PaL2
M = —— (8.23)
max 12
Continuous:
PaL2
M = —— (8.24)
max 10
Equation (8.22) is applicable for simply supported facings where the soil will not arch between vertical supports (e. g., in soft cohesive soils or for rigid concrete facing placed tightly against the in-place soil). Equation (8.23) is applicable for simply supported facings where the soil will arch between vertical supports (e. g., in granular or stiff cohesive soils with flexible facing, or rigid facing behind which there is sufficient space to permit the in-place soil to arch). Equation (8.24) is applicable for facings that are continuous over several vertical supports (e. g., reinforced shotcrete).
