|
Slurry walls were used in conjunction with steel piles were used to construct
the basement for the AMOCO building. The piles were used as load bearing
elements for the slurry wall and not as wall reinforcing. The slurry wall
scheme was adopted after caisson construction had already been built when a
restraining order was issued to halt construction. The slurry walls were
21" thick and extended from El. +8.0ft to El. -32ft CCD (Chicago City
Datum), and bracing consisted of one level of temporary tiebacks at El. +1.5ft
(Fig. 5.10).
Movements at the top of the slurry wall reached up to 4.6" at one station
(Sta. #2) while other inclometers measured deflections close to 2" at the
top of the casing (Fig. 5.11). Cunningham and Fernandez [1972] reported
deflection data 10' below the top of the casing in order to minimize surface
effects. Soil movements started as soon as the center of the site was excavated
to El -26ft CCD even while a 25' wide berm was left in place. Apparently the
berm was not sufficient to restraint deformations since movements increased by
as much as 0.8" during this period. Movement continued as the perimeter
was excavated to El. -15ft CCD. Movements did not stabilize until some time
after removal of the berm was complete. Although tiebacks were restressed on
several occasions, actually pushing the wall back somewhat, the average rate of
movement did not seem to change. This indicates that re-stressing of the
tie-backs provided only temporary restraint to the movement that was initiated
by a change in the stress equilibrium of the soil mass, and the movement
stopped only after a new state of equilibrium was reached (Cunningham &
Fernandez, 1972). Most of the slurry walls translated and rotated about their
toe, with very little to no wall bending evident. The horizontal toe
translation was in the order of 1.0".
Considerable difficulties were experienced in compacting the backfill between
the slurry wall and the deadman sheetpile wall (installation of tieback rods),
due to freezing weather and/or water. This was particularly true along the east
and south walls, and resulted in relatively poor compaction of the backfill
with the exception of a 20-ft strip in front of the deadman wall, where select
and relatively dry material was utilized. It is possible that large movement
along the east wall may have been partly due to this condition.
Significant lateral movements were observed to a considerable depth. In
stations 2, 3, and 4 the slurry wall had clearly rotated about its base since
there was no sign of bending at any elevation higher than El. -32ft CCD. This
clearly indicates that the embedment of the slurry wall was not enough to
restraint movement of the stiff slurry wall. As it is shown in Figure 5.10, the
slurry walls were embedded in medium clay; clearly this clay did not possess
enough strength to resist the lateral loads imposed on the slurry wall.
|
