Civil Engineering Reference
In-Depth Information
(1173 & Ongoing)
The Tower of Pisa (Figure 1-1) in Italy is about 60 m (200 ft) tall from foundation to
belfry, 20 m (66 ft) in diameter and weighs approximately 145 MN (14,500 tons). It
was constructed in three phases. Four floors were built over a 5 year period from
1173 to 1178. Following an almost 100 year hiatus, in the second phase of
construction, three additional floors were constructed between 1272 and 1278. The
third construction phase occurred more than 80 years later between 1360 and 1370
when the bell tower was added. The tower's foundation is inclined at almost 5.5
degrees to the south; the tower overhangs the ground about 6 m (20 ft) out of plumb.
The value corresponding to the eccentricity on the loads on the foundation is 2.3 m
(7.5 ft).
Evidence indicates that the phased construction was mandated by the
performance of the structure as it was being built. Further, records indicate that
during construction the tower appeared to move sufficiently so that the builders used
obliquely cut stones in an effort to maintain the floor of each successive story
approximately horizontal. It is interesting to note that the obliquely cut stones were
used by the Pisa Commission, entrusted with gathering and collating relevant data for
an international competition organized to identify a method to stabilize the tower in
1972, to reconstruct the pattern of movements throughout the first two phases of the
tower's construction. Their calculations showed that at the end of the first
construction phase the tower had begun to lean towards the northwest. During the
second and third phases, the angle of inclination increased and the principal direction
of tilt shifted first to the northeast and then to the south.
By 1993 the tower's maximum horizontal tilt had progressed to 5.2 meters (17
ft). In June of that year an attempt was made to reduce the tower's tilt. Specially
fabricated lead counterweights were placed on top of the north side of a tensioned
concrete ring built around the base of the tower. The tilt was reduced by
approximately 12 seconds of arc over a 6-week period, as about 1.3 MN (130 tons) of
lead were placed. Over the course of the remediation, until January 1994, a total of
6.9 MN (690 tons) was applied and by July 1994 the tower had righted its position
toward the north a full 52 arc seconds. The Pisa Commission decided to replace the
lead counterweights with an anchored cable system. In 1995 they began freezing the
ground with liquid nitrogen in preparation for installing the cables. As soon as the
freezing stopped the tower began to once again lean southward at a rate of four arc
seconds per day. The operation was halted. They continued a search for a permanent
solution.
In March of 1996 engineers successfully completed a test of a soil-extraction
method to reduce the tower's lean. In this method an inclined drill was used to create
cavities that gently closed due to the pressure of the overlying soil. The method was
fully employed three years later. In 1999, after the installation of temporary cables,
which could be tensioned to steady the tower if detrimental movements occurred,
engineers drilled a dozen boreholes over a width of approximately 5.5 m (18 ft).
They slowly removed underlying soil at a rate of approximately 0.02 cubic meters
