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 Slurry
walls 26"-thick, 56'-deep, formed the walls for the 3-level
basement (base of the excavation at El. -21'3"ft CCD). Tiebacks,
rakers, and corner braces
were used to brace the slurry
walls during excavation. On the north wall a tie-rod and deadman
sheet pile system
was used in place of the simple tieback bracing because of space
restrictions. Tiebacks at 6' depth, inclined at10° from the
horizontal and prestressed rakers to 100 tons placed at the second
level provided additional support for the east and north slurry
walls. The south slurry wall was braced by tiebacks inclined 45°
at El. -10ft CCD at the slurry wall face, with fixed length within
the hardpan stratum.
Steel H-Beams placed at regular intervals reinforced the slurry
wall together with reinforcing steel cages that provided additional
tensile strength. Drilled caissons extending to the limestone
bedrock were used to support the superstructure loads from the
60-story tower.Geologically the site lies in a portion of the
city that was once below the lake level. The soil profile generally
consists of fill, underlain by fine sand, stiff clay, soft to
medium clay and stiff to very stiff clay extending to a sandy
clay "hardpan" at El. -70ft approximately. Extremely
dense silt with variable amounts of cobbles and boulders was found
above the surface of bedrock to elevations in the range of El.
-90 to El. -100 ft. A deep groundwater table (El. -60'CCD) exists
at the site and is associated with the water level in the rock
and the dense silt below the hardpan. A shallower perched water
level within the sand soils ranged from El. 0' to El. -3'.
During slurry
wall and caisson construction soil moved towards the slurry
filled trenches by as much as 0.25" inches. During excavation
construction, soil moved towards the excavation by as much as
0.5". There was very little wall bending observed in this
project, with the wall also slightly translating towards the excavation
by 0.15" at its base.
In one inclinometer 25' away from the excavation (In-1A), the upper few feet of
soil moved up to 0.75" towards the excavation while 5' from the excavation
they reached 0.3". This effect was caused by the deadman sheetpile and tie
configuration that was used along the northern project side.
Horizontal soil movements slowly increased while the excavation progressed.
Generally most of the movement happened after the soil berm was left in place
and the first level ties were tensioned. The final inclinometer reading was
taken when most of the rakers were jacked.
Slurry wall construction
faced only one major problem regarding the tilt of the slurry
wall reinforcing steel H-members during or after panel concreting.
Reinforcing cages had to be shortened in many cases because of
the large bending of steel H-beams in slurry
walls during or after concreting. With the exceptions of some
sand pockets in the sand upper sand layer slurry
wall construction encountered no other difficulties. There
were no records of any slurry
wall leaks or any other special observations regarding slurry
wall quality found.
Overall, the earth retention system performed very well since
the basement excavation caused very small horizontal movements
and probably small settlements as well. Actual settlements although
not measured but must have been small as well if we consider that
maximum lateral soil movements reached up to 0.75".
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The Prudential
Two is a 60-story high-rise located in the southwestern corner
of Stenson Avenue & Lake Street. The new building was constructed
next to the pre-existing 41-story Prudential building at Randolph
& Stenson Streets. Five levels of parking were constructed
under the tower portion of the site. Three out of these parking
levels are below the lower street level at Stenson Street (eastern
side). Stenson Street has three levels with the lower being at
El. +6'2"CCD, the mid-level at El. +26'CCD, and the upper
at varying elevation close to El. +46'ft. To the north of the
project there is the 80-story AMOCO (Standard Oil) building constructed
during the 70s with its basement constructed using 