|
The 2.0'-thick slurry walls were typically 40' to 45' deep with a final toe
embedment, 15' below the base of excavation (BOE). Four levels of tiebacks
provided temporary lateral support along the western wall. Tieback inclinations
of 12°, 5°, 30°, and 30° were used for the 1st, 2nd, 3rd, and
4th levels respectively. Floor bracing provided permanent lateral support.
Tiebacks were spaced at 5 ft intervals horizontally, and a minimum of 10 ft
vertically. Numerous additional tiebacks were used due to the poor performance
of the original tiebacks so that the final tieback spacing was even smaller.
Rakers were used together with tiebacks along St. James Street for the same
reasons. The top figure shows a typical north-south excavation profile.
In this project, settlement control was very important along Clarendon and
Boylston Streets. Lateral wall movements were the greatest along Clarendon
Street where there was significant lateral soil straining as well near the
Church. The maximum horizontal slurry wall deflection was dW=3.3" along
Clarendon (I-10) whereas other walls deflected to a much smaller extend.
Inclinometer I-2 in front of Trinity Church and opposite of I-10 across
Clarendon had a maximum deflection of close to dH=2". Ties were not
successful in reducing wall movements since their tensioning was not able to
reduce inward wall deflections. Wall movements were smaller along St. James
Street, where rakers were used in place of the three lower levels of tiebacks.
Walls translated up to 1" at their bases even though they all extended
into stiff overconsolidated clay. Such translation movements should be expected
under such conditions where the wall is floating and soft base material is
present.
Considerably larger than expected surface settlements along Clarendon and St.
James Streets were observed during the excavation. Maximum settlement was dV =
5.2" along Clarendon Street and 3" along St. James whereas the
Boylston Street settlements were insignificant. Settlements along St. James are
puzzling since rakers were used in place of the three lower levels of tiebacks.
Possibly the groundwater lowering below minimum levels that went on for three
or more weeks might be responsible for these settlements.
Wall deflections were smaller than surface settlements along Clarendon Street
(same was true for other project sides). This indicates that there were
possible soil losses through the tiebacks or soil disturbance caused by the
tieback installation procedure. Tieback load deficiencies are also to blame for
the excessive movements. Major parts of the effective length of tiebacks were
in the organic silt, resulting in load shedding and excessive creep.
Furthermore when additional tiebacks were installed adjacent previously locked
tiebacks could not sustain their initial lock-off loads. Wooden pile extraction
in the preliminary excavation construction phase may be responsible for part of
the induced movements. Building settlements were small, with Trinity Church
settling up to dV=0.62". The excavation did not affect other buildings,
supported by pile foundations.
|
