TM 5-818-1 / AFM 88-3, Chap. 7
the order of 200 to 270 kips per square inch) bunched. It
is usually inclined so as to reach better bearing material,
a. Tiebacks have supplanted both strut and
to avoid hole collapse during drilling, and to pass under
raker systems in many instances to support wide
utilities. Since only the horizontal component of the
excavations. The tieback (fig 14-13) connects the wall to
tendon force holds the wall, the tendon should be
an anchorage located in a zone where significant soil
inclined a minimum.
movements do not occur. The anchorage may be in soil
c. Tieback anchorages may be drilled using
or rock; soft clays probably present the only condition
continuous flight earth augers (commonly 4 to 7 inches in
where an anchorage in soil cannot be obtained reliably.
diameter) and may require casing to hold the hole until
In figure 14-13, the distance Lub should extend beyond
grout is placed in the zone La of figure 14-13, at which
the "Rankine" zone some distance. This distance is
time the casing is withdrawn. Grout is commonly used
necessary, in part, to obtain sufficient elongation in
under a pressure ranging from 5 to 150 pounds per
anchored length of rod La during jacking so that soil
square inch. Underreaming may be used to increase the
creep leaves sufficient elongation that the design load is
anchor capacity in cohesive soil. Belling is not possible
retained in the tendon. After jacking, if the soil is
in cohesionless soils because of hole caving. Typical
corrosive and the excavation is open for a long time, the
formulas that can be used to compute the capacity of
zone Lub may be grouted. Alternatively, the length of
tieback anchorages are given in figure 14-14.
tendon Lub is painted or wrapped with a grease
d. Exact knowledge of the anchor capacity is
impregnated wrapper (prior to placing in position).
b. The tieback tendon may be either a single
high-strength bar or several high-strength cables (fy on
U. S. Army Corps of Engineers
Figure 14-13. Typical tieback details.