John Burland stands before the soil-extracting device that
recently gave the Leaning Tower the lean it had 30 years ago.
Where it Stands Today
As of June 1999, the Leaning Tower of Pisa leaned an inch less than it had at
the beginning of the year. It may sound like a small step for the tower, but
it's a giant leap for the Pisa Commission in charge of saving the dangerously
tilted structure. Here John Burland, a Commission member and professor of soil
mechanics at London's Imperial College of Science, Technology, and Medicine,
tells how perilously close the tower has come to toppling over, and about the
painstakingly careful procedure to remove soil from beneath the north side of
the tower to help ease it slightly back toward the vertical—and toward a
stability it has not known for 300 years.
NOVA: You've recently finished the preliminary soil extraction. Would you
consider it a success?
Burland: The whole operation has been very successful. The original intention
was to see if we could move the tower a very small amount, about six
millimeters. In fact, we moved it over 27 millimeters, or about an inch. Not
only has the tower moved northwards, but the south side of the foundation has
come up a little, which is a very positive result.
If all goes to plan, the
Leaning Tower will lean about a foot and a half less at the top than it does
NOVA: To the layman, one inch may not seem like very much, but to you it's an
enormous amount, right?
Burland: In a sense, the amount is immaterial. That's not what we were looking
for. Since the soil-extraction method is a new thing, the uncertainty was:
Would we get a positive response at all? We've got a tower that's literally on
the point of falling over. We did everything we possibly could beforehand to
check the method: by numerical analysis, closed-form analytical solutions,
model tests, and then a large-scale trial. They all pointed to the fact that we
would get a positive response. Nevertheless, we had to ask: Are we prepared to
go in and try it on the tower? The Tower of Pisa is notorious for springing
surprises on those who try and stabilize it. We bit the bullet, and the result
has been positive. In fact, we moved the tower five times as much as we had
NOVA: How many years will you gain if all goes to plan?
Burland: If we manage to reduce the inclination by a half a degree, or about
half a meter at the top, which is our intention, then that would add at least
300 years to the life of the tower, assuming there were no other untoward
NOVA: How does soil extraction work?
Burland: It involves a special drill 200 mm in diameter, which drills down into
the very soft soil just beneath the tower's foundations on the north side. The
drill is designed so there's no disturbance to the ground on the way in, but
when we pull it back out a bit, it leaves a cavity, which we have found closes
very gently. As a result, the ground above it subsides a little bit and takes
the tower with it.
NOVA: How much soil do you actually extract?
Burland: Each time we go in, we extract between 15 and 20 liters of soil. We
can keep going into the same spot, or we can go in to other spots around the
tower, which allows us to control exactly how much it moves each time. It's a
very flexible, repeatable method.
In the so-called "full
intervention," workers will use up to 40 tubes like these to extract soil from
beneath the tower's north side, thereby causing the tower to lean a bit
NOVA: So you can actually steer the tower?
Burland: Yes, we are literally steering it. If it goes a little bit to the
east, we can take a little bit of soil out on the west side, and it comes west.
Indeed, one of the great successes of this preliminary intervention was that we
were able to control the east/west movement of the tower exactly as we wanted
to. We wanted to bring it back to the west a couple of millimeters, and that's
what we did.
NOVA: What will the full intervention entail?
Burland: It will be exactly the same method, but instead of using only 12
tubes, which is what we're using now—we can drill in 12 locations, which is
only over a width of five meters or so—the full intervention would involve
something like 40 tubes over the entire width of the tower, so that we could
then evenly extract soil over its full width. That way, we can get a much
larger response. We're aiming at something on the order of half a meter
reduction in tilt. The present tilt is four and a half meters, so we'd be
reducing it by about 10 percent.
NOVA: Why wouldn't you go any further than that?
Burland: Because you'd then start seeing it. Ten percent is barely visible, so
tourists, unless they were making incredibly careful measurements, wouldn't
really notice it. The Leaning Tower's not going to appear to be leaning any
less. But if we took it much more than that, then both people living there and
tourists would start saying, "Look, they're reducing the inclination of the
It would be "quite
wrong and quite inappropriate," Burland says, if the tower were to lose its
lean. The Pisa Commission only seeks to lessen the angle by a small amount.
NOVA: And the tourists would stop coming?
Burland: Well, not quite that. But we'd start getting complaints, I think, and
we don't want that. We think half a meter would be enough to stabilize the
tower for hundreds of years at least, if not permanently, but it wouldn't be
noticeable. And that's very important, that we don't really change the
character of the monument. That would be quite wrong and quite
NOVA: What is the rough cost of the full intervention?
Burland: I think we're probably talking about two or three million pounds,
perhaps a bit more.
NOVA: So what are the Commission's long-term plans—in other words, after the
Burland: Well, there are a few things that still have to be done. We want to
strengthen the masonry, which we'll probably do during the full
under-excavation. If we left the tower alone after reducing its inclination,
it's possible that it might start moving again. But there are ways of
permanently stabilizing it after we've brought it back.
We found that what is causing the tower to continue to move is a water table in
the ground very near the surface that fluctuates seasonally—it goes up and
down. We could develop a scheme to stabilize the water table. We would put
what's called a diaphragm in the ground around the tower, some distance away,
which would isolate the ground immediately beneath the tower from the water
outside the diaphragm. It would create a sort of water-tight compartment around
the tower, and that way, the water under the tower would not move seasonally
because it would be trapped inside the diaphragm, and its level would be
controlled by pumps and wells and so on. That's not an easy operation. It's not
hazardous, but it's quite expensive and a bit time-consuming.