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Often the tiebacks are used only for temporary excavation support,
while the basement floors provide permanent lateral earth support.
In such projects the tiebacks are detensioned when the basement
floors have gained sufficient strength. The basement floors should
be designed to resist permanent lateral earth pressures, since
stress transfer from the tiebacks to the floor system will take
place when the ties are detensioned. This stress transfer has
reportedly caused long-term cracking of many the basement floors.
Tieback installation follows a predetermined sequence as to minimize
soil movements and speed the excavation construction (Fig. 3).
The excavation is carried a couple of feet below the tieback to
enable access for the drill rig. Further excavation occurs only
after prestressing and proof-testing of the anchors. As Figure
4 illustrates, the process can be repeated for additional levels
of tiebacks. Building codes require that all tiebacks are proof-tested
to an excess percentage of their final lock-off load, which usually
ranges from 120 to 150% of the final lock-off load. Regroutable
tiebacks are most commonly used because their capacity can be
increased by regrouting (to meet test requirements) without having
to drill a new anchor hole.
A tieback is made by first drilling a hole with an auger and
then placing a bar (tendon) in the hole, concrete is then poured
in the hole and the connection with wall is made (Figure 3). Different
types of augers are used to drill the tieback holes. The choice
of the drilling method depends on the soil/rock conditions on
the site.
Drilling should be done carefully since inadequate procedures
can cause significant soil losses. The biproduct of drilling is
removed by flushing the hole with either air, water, or slurry.
Air is most efficient in dry ground, but it requires special attention
because it can become entrapped during drilling, building up zones
of high pressure in the soil that can eject material for several
feet and at high speeds (potentially injuring workers). Water
flushing is best used in sticky clayey soil, and it also cleans
the sides of the hole by its sweeping action, providing a stronger
bond at the grout-anchor interface. Bentonite slurry flushing
works the best since it keeps particles in suspension, while the
sealing action keeps the hole from collapsing.
Significant soil losses through the tiebacks cause significant
settlements even if the retaining walls do not move towards the
excavation. In granular soils the drilled hole must be cased to
avoid collapse.
Some tieback creep can be expected especially if the ties are
very short and the fixed length of the tie is within soft ground.
For stability reasons, the fixed anchor should be located beyond
the active zone of movements. As a result, tieback anchors may
not be an option at sites congested where there are adjacent underground
utilities or when adjacent owners do not grant permission to drill
them under their properties.
Special attention should be given to the waterproofing details
at the anchor heads and at the tieback holes. Significant leakage
can be caused by inadequate waterstopping details at these locations.
Figure 1: Picture from a tieback slurry wall excavation (World
Bank Project Washington)
Figure 2: Tieback slurry wall excavation (Dana Farber Tower, Boston).
Figure 3: Tieback configuration, free and fixed lengths (Adapted
from Schnabel, 1982)
Figure 4: Steps in making a tieback: (a) hole drilled; (b) bar
placed in hole; (c) concrete poured for anchor; (d) wall connection
made (Adapted from Schnabel, 1982).
Figure 5: Steps in making a multilevel tieback excavation, (A)
first level of tiebacks installed and second level of tiebacks
drilled, (B) second level of tiebacks installed.
Figure 6: Looking underground at
an anchored wall with 3DEEP software
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