Anchored Walls.

a. Tiebacks have supplanted both strut and raker systems in many instances to support wide excavations.  The tieback (fig 14-13) connects the wall to an anchorage located in a zone where significant soil movements do not occur.  The anchorage may be in soil or rock; soft clays probably present the only condition where an anchorage in soil cannot be obtained reliably.

In figure 14-13, the distance Lub should extend beyond the "Rankine" zone some distance.  This distance is
necessary, in part, to obtain sufficient elongation in anchored length of rod La during jacking so that soil
creep leaves sufficient elongation that the design load is retained in the tendon.  After jacking, if the soil is
corrosive and the excavation is open for a long time, the zone Lub may be grouted.  Alternatively, the length of tendon Lub is painted or wrapped with a grease impregnated wrapper (prior to placing in position).


Typical tieback details.
Figure 14-13.  Typical tieback details.

b. The tieback tendon may be either a single high-strength bar or several high-strength cables (fy on the order of 200 to 270 kips per square inch) bunched.  It is usually inclined so as to reach better bearing material, to avoid hole collapse during drilling, and to pass under utilities.  Since only the horizontal component of the tendon force holds the wall, the tendon should be inclined a minimum.

c. Tieback anchorages may be drilled using continuous flight earth augers (commonly 4 to 7 inches in
diameter) and may require casing to hold the hole until grout is placed in the zone La of figure 14-13, at which time the casing is withdrawn.  Grout is commonly used under a pressure ranging from 5 to 150 pounds per square inch.  Underreaming may be used to increase the anchor capacity in cohesive soil.  Belling is not possible in cohesionless soils because of hole caving.  Typical formulas that can be used to compute the capacity of tieback anchorages are given in figure 14-14.

Methods of calculating anchor capacities in soil.
Figure 14-14.  Methods of calculating anchor capacities in soil.


d. Exact knowledge of the anchor capacity is not needed as all the anchors are effectively "proof-tested"
(about 120 to 150 percent of design load) when the tendons are tensioned for the design load.  One or more anchors may be loaded to failure; however, as the cost of replacing a failed anchor is often two to three times
the cost of an initial insertion, care should be taken not to fail a large number of anchors in any-test program. 

If the tieback extends into the property of others, permission, and possibly a fee, will be required.  The
tieback tendons and anchorages should normally be left in situ after construction is completed.  See table 14-3 for additional design considerations.

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