3. SOIL INVESTIGATIONS FOR PHYSICAL AND
MECHANICAL PROPERTIES OF SOIL
The variety of rocks ranges from Paleozoic to tertiary in Pisa city. Due to
tectonic activities, the city’s strata have undergone severe deformation.
Moreover, there is a fault passing through the bedrock beneath the town of
Pisa.
The town of Pisa is located on the alluvial sediments and the mean sea level is
only 3-4 m.
Soil profile below the tower consists of three horizons.
Horizon-A consists mainly of estuarine deposits. This stratum is laid under
tidal conditions. Therefore, the amount of sandy and clayey silts varies under
the strata. The thickness of horizon-A is 10 m.
4. Horizon-B consists of marine clay and sandy
soil. Sandy soil is sandwiched between the
layers of clayey soil. The upper clay layer is
very sensitive, whereas the lower clay layer
is stiff and less sensitive. The thickness of
horizon-B is 40 m.
Horizon-C consists of dense sand. The
thickness of horizon-C extends to a
considerable depth.
The amount of silt and clay is more on the
south side of the tower. Also, the sand layer
is much thinner than on the north side of the
tower. This is one of the reasons that led to
the tilting of the tower towards the south
side.
The natural groundwater level is 1-2 m
below the ground surface area. However, in
the past, water extraction from the lower
sand has led to downward seepage from the
horizon-A layer. As a result, the pore
pressure distribution is slightly below
hydrostatic pressure.
5. CAUSES OF TILTING OF TOWER
The rapid increase in the leaning process of any building or tower towards the end of
the construction process is known as leaning instability. Leaning instability of a tall,
narrow structure occurs at a critical height when the overturning moment generated
by a small increase in inclination is equal to, or larger than the
corresponding resisting moment generated by the foundations.
Leaning instability is not due to lack of strength of the ground but due to insufficient
stiffness. It is apparent that the combination of the very soft ground and the
geometry has actually resulted in the tower of Pisa reaching its critical height.
The factors that have contributed towards the leaning of the Pisa tower are:
1. Change in the groundwater table causing subsidence in the alluvial sand.
2. Heavy rainstorms.
3. Temperature variation in the summers caused the change in the leaning stability of
the tower.
4. Stabilization process of the tower.
7. TEMPORARY STABALIZATION
The following points describe the temporary stabilization measures taken to improve
the stability of the tower:
Marble cladding around the outer wall of the tower started showing signs of cracking.
Therefore, to reduce the failure of marble cladding, temporary lightly pre-stressed
plastic-covered steel tendons were installed at a suitable interval up to the second
story in the year 1992.
The tower's northern side was gradually rising as well as the southern side, which was
tilting with time. Therefore, the experts recommended placing emphasis on the
masonry foundation's northern side. As a result, it would stop the tower's constant
tilting and perhaps even lessen it somewhat.
A temporary pre-cast concrete ring was built around the plinth level of the tower for
the aforementioned reason. Lead ingots were positioned on the pre-cast concrete
ring for loading at regular intervals.
8. • Between July 1994 and January 1995,
there were three lead ingot
placements total. The tower began to
tilt towards the north one month
after the application of loads.
• The tower eventually settled at a
distance of around 2.5 mm from the
surrounding ground level,
nevertheless, as a result of the
application of loads.
• In order to monitor the tower's response, the weight on the northern side
was applied in four stages.
9. Permanent Stabilization
The committee tested a number of techniques in an effort to find a long-
term solution for rotating the tower back toward the north. These
techniques included loading the earth surrounding the north foundation
using a pressing slab loaded by ground anchors, consolidation below the
north foundation using electro-osmosis, and draining below the north
foundation using wells. None of these approaches, however, turned out to
be successful.
10. • The installation of several soil extraction tubes directly beneath the
north side of the foundation is done using this technique.
• On the trial footing that was set up close to the tower location, this
approach was tested out initially. It was successful to rotate the trial
footing by 0.25°.
The committee decided to adopt the induced subsidence method on the
northern side of the tower. The following points describe the induced
subsidence method to rotate the tower back in the northern side:
11. • On the tower's northern base, the true
permanent stabilizing work began in
August 1998. The northern side of the
foundation initially had a little amount of
earth removed from it, creating a hollow.
The overburden pressure caused the cavity
to begin closing automatically, which led to
a modest amount of surface sinking.
• The whole northern side of the foundation
was built using the aforementioned
procedure.
• After the fully induced subsidence, the
tower significantly rotated towards the
northern side.
13. Firstly, At the foot of the tower, a 0.8 m
thick cement-conglomerate ring was
constructed and attached to the
tower's foundation using steel
reinforcement. The ring was also
reinforced using the post-tensioning
technique. As a result, the foundation's
useful area expanded, increasing both
safety and resistance to leaning
instability.
CEMENT-CONGLOMERATE RING
14. Secondly, to reduce the groundwater
level fluctuation during the rainy
season, a drainage system was
installed. This system consists of three
wells. These wells were constructed
with radial sub-horizontal drains
running beneath the north side of the
foundation. The water level in the
outlet pipes control the water level in
the wells. The installation of this
drainage system induced a further
northward rotation of the tower.
THREE-WELL DRAINAGE SYSTEM
