Anchor design includes the selection of a feasible anchor system, estimation of anchor capacity, determination of unbonded length, and consideration of corrosion protection. In determining the feasibility of employing anchors at a particular location, considerations include the availability of underground easements, proximity of buried facilities to anchor locations, and the suitability of subsurface soil and rock conditions within the anchor stressing zone.
Ultimate anchor capacity per unit length may be estimated from Tables 8.9 and 8.10 for soil and rock, respectively. The values are based on straight-shaft anchors
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TABLE 8.9 Ultimate Values of Load Transfer for Preliminary Design of Anchors in Soil
*Values corrected for overburden pressure. fThe presence of mica tends to increase the volume and compressibility of sand and soft deposits due to bridging action and subsequent flexibility under increased pressures. Source: From Standard Specifications for Highway Bridges, 2002, American Association of State Highway and Transportation Officials, Washington, D. C., with permission. |
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TABLE 8.10 Ultimate Values of Load Transfer for Preliminary Design of Anchors in Rock
Source: From Standard Specifications for Highway Bridges, 2002, American Association of State Highway and Transportation Officials, Washington, D. C., with permission. |
installed in small-diameter holes using a low grout pressure. Other anchor types and installation procedures may result in different anchor capacities. Allowable anchor capacity for small-diameter anchors may be estimated by multiplying the ultimate anchor capacity per unit length by the bonded (or stressing) length and dividing by a factor of safety. AASHTO suggests 2.5 for anchors in soil and 3.0 for anchors in rock.
Bearing elements for anchors must be designed so that shear stresses in the vertical wall elements and facing are within allowable limits. The capacity of each anchor should be verified as part of a stressing and testing program.
Determination of the unbonded anchor length should consider the location of the critical failure surface farthest from the wall, the minimum length required to ensure minimal loss of anchor prestress due to long-term ground movements, and the depth to adequate anchoring strata. As shown in Fig. 8.49, the unbonded (or free) anchor length should not be less than 15 ft (4.6 m) and should extend 5 ft (1.5 m) or one-fifth of the design wall height, whichever is greater, beyond the critical failure surface in the soil mass being retained by the wall. For granular soils or drained cohesive soils, the critical failure surface is typically assumed to be the active failure wedge. This wedge is defined by a plane extending upward from the base of the wall at an angle of 45° + ф72 from the horizontal, where ф’ is the effective angle of soil friction. Longer free lengths may be required for anchors in plastic soils or where critical failure surfaces are defined by planes or discontinuities with other orientations.
Selection of anchor inclination should consider the location of suitable soil or rock strata, the presence of buried utilities or other geometric constraints, and constructibility of the anchor drill holes. AASHTO suggests that anchors be located on a minimum inclination of 10° below horizontal and the bonded zone be located a minimum depth of 15 ft (4.6 m) below the ground surface. The component of vertical load resulting from anchor inclination must be included in evaluating the end bearing and settlement of vertical wall elements.
AASHTO suggests that the minimum horizontal spacing of anchors be either 3 times the diameter of the bonded zone or 4 ft (1.2 m), whichever is larger. If small spacings are required, consideration can be given to different anchor inclinations between alternating anchors.
