CONSOLIDATION OF THE QUEBEC CITY FORTIFICATION WALLS
By François LeBlanc
(Published in 1994)
consolidation of the Quebec City fortification walls is a project
which began in the late 1970s and was completed in the early 1980s.
Much technical research was done to determine the most appropriate
consolidation technique, and many solutions were developed and applied.
However, only one technical component of the project will be examined
here: the consolidation of the masonry walls by injecting cement
Recently, several colleagues have asked me what materials and techniques
should be used for masonry consolidation. I was surprised to discover
that there is not a great deal of technical information available
on this subject. Therefore, I thought that the experience of consolidating
the Quebec City fortification walls might be interesting for the
readers of the Bulletin.
Quebec City was founded in 1608 by Samuel de Champlain. Built on
a high rocky cliff, about 200 kilometres (160 miles) east of Montreal,
it controlled the mouth of the St. Lawrence River. It did not take
long before the first French military engineers came over to fortify
the city. Several masonry fortification systems were built, modified,
demolished or rebuilt by both French and, later, British engineers.
Today, Old Quebec City is completely encircled by fortification
walls. It is the only remaining city north of Mexico with intact
surrounding walls. In fact, it is so exceptional, that it was put
on the World Heritage List.
More than 5,000 linear feet of walls surround the Old City. They
average 12 metres high (40 ft.), and 1.5 m (4 ft.) to 3.5 m (7 ft.)
in thickness. The exposed face of the walls is a green and grey
granite that came from the Quebec region. The wall's inner face
is composed of a local limestone; in between is rubble.
THE CAUSES OF DETERIORATION
Numerous causes account for the deterioration of the walls:
- heavy traffic;
- accumulation of water due to poor drainage;
- effect of freeze-thaw cycles;
- differential thermal expansion;
- air pollution;
- lack of maintenance; and
- weak bonding between the wall's outer and inner skins.
Before any serious work was done to repair and stabilize the walls,
archaeological investigations were carried out. They confirmed the
presence of hidden buttresses and the poor condition of the masonry.
Further engineering investigations determined the causes of deterioration,
soil conditions and properties, water table, bedrock and the percentage
of voids in the walls.
CEMENT GROUT INJECTIONS AND RESTORATION TECHNIQUE
The restoration technique involved removing the fill behind the
fortification walls, installing a drainage system, repointing the
sound masonry, replacing the rotten stones, drilling vertical and
horizontal injection holes at regular intervals, inserting steel
reinforcing bars in the holes, and injecting a special cement grout.
First the injection holes were drilled using an air-track percussion
and rotation machine. It drills 6 cm <R>(2.5 in.) holes at
every metre (3 ft.). The depth averages 13.5 m (45 ft.), that includes
1.5 m of boring into the solid rock at the foot of the wall. The
machine is linked to a 600 cfm air compressor.
Then, the fill behind the walls was removed to expose the masonry.
At the same time, steel beam shoring was put into place. It had
to be strong enough to support both the wall and the heavy equipment
(3 to 4 tons) used to drill the horizontal injection holes.
As the air-track machine progressed and the scaffolding was erected,
the masons began repointing the walls. In this case, the mortar
had to match the original in texture and colour. Some cement was
added, however, to improve its water repellency. The mix used was:
- 1 part (volume) Portland cement;
- 1 part (volume) hydrated lime;
- 5 parts (volume) sand.
This was a hard mortar because we were dealing with a very hard
stone. Now, with our current knowledge of mortars, we would probably
exchange the Portland cement with white cement, and add another
part of sand to make it a 1-1-6 mix. We would insist that the lime
be slaked, and we would probably add an air entrainment agent.
During the repointing operation, the masons installed preliminary
injection tubes. These plastic tubes, 13 mm (1/2 in.) in diameter
were placed every 60 cm (2 ft.) on both sides of the walls. They
were pushed in as far as the masons could insert them into the wall,
and left to protrude by 30 cm (1 ft.). They were used for low pressure
injection prior to the main grout injection.
We discovered that it was preferable to bore the holes first, repoint
second and inject third.
Once a main injection hole (6 cm/2.5 in.) was bored, a steel reinforcing
rod was slid into it (generally a no.8). A 13 mm (1/2 in.) plastic
tube was previously taped to the rod to evacuate the air from the
bottom of the hole when the grout was injected. The hole was then
temporarily plugged with a mix of Sika No. 4A and Portland cement
to prevent anything from falling into it. If dirt or debris did
fall into the hole, it could be forced out by using compressed air.
Sometimes several weeks would elapse between the time a hole was
bored and the grouting took place.
The cement grout was mixed in two big cylindrical tanks - one was
mixed while the other supplied mixed grout to the pump. The mix
- 1 bag of Portland cement type III;
- 0.5 kilo (1 pound) of Intraplast N; and
- 20.5 litres (5 gallons) of water.
The Intraplast N reduced shrinkage and liquified the mix. In some
instances, sand, equal in quantity to the cement, was added to the
mix. Cement, sand and water had to meet CSA Standard A23.
The grout mix can be modified according to site conditions. Generally,
the minimum compression standard for the grout was set at 17,900
kilopascal (2,600 psi) at 28 days.
The grout was then pumped into the masonry walls. Injection pressure
was not to exceed 170 to 240 kilopascal (25 to 30 p. sq. in.). A
mix of Portland cement and Sika 4A was used to stop leaks.
The use of a tubular junction "cross" permitted the injection
of several holes at the same time.
An injection team consisted of three persons:
- one to operate the pump;
- one at the "cross";
- one to clog the leaks.
The equipment had to be thoroughly cleaned every day: it took about
a half hour to start it up in the morning, and an hour to clean
up and shut down at the end of the day.
The pump caused the most problems. It is, therefore, good practice
to specify that the contractor always has a spare pump on the site.
Without getting into details, it is possible to say that the average
cost for consolidating the Quebec City walls was $3,500 per linear
metre ($1,000 per linear foot) in 1976 dollars. This cost covered
all work except the professional services to prepare the plans and
specifications and do the site supervision.
THINGS TO WATCH CAREFULLY
Noise: the drilling equipment is very noisy; all efforts should
be made to try to reduce the volume.
Stains: the drilling equipment often drips oil and grease that
might stain the masonry. It is advisable to protect it with polyethylene
sheeting to avoid staining.
The level of competence and skill of the operators is paramount.
Very often during a day's work, immediate and important decisions
have to be taken by the operators. Their qualifications should,
therefore, be submitted for the approval of the architect or engineer.
It is also wise to hold a short training session for the operators
before the work begins.
Accurate records should be kept for every step of the operation:
depth of holes, number of men working, quantity of work done each
day, volume of grout injected, problems encountered, etc.
Grouting the Quebec City walls is not a new technique. In "Specification
of Work and Materials Required for Taking Down and Rebuilding Sundry
Portions of Fortifications Walls, Quebec 1879," it stated "grout
with liquid mortar at every two feet."
What is new and interesting about the past decade's work is the
special effort taken to test and obtain scientific data on every
step taken. Most interesting, though inconclusive, was the attempt
to precisely determine the flow of the grout through the masonry,
and the quantity of voids filled. Ultra sound, radar waves, infrared
scanning and many other technologies were explored. Furthermore,
several approaches, devised by an "in house" government
team for the private contractors selected through a public tendering
process, were adopted to prepare, execute and supervise the work.
What is unfortunate is that all the scientific data accumulated
for this project rests in unpublished government reports, and is,
therefore, not accessible to the professionals who might need it.
North American professionals tend to finish a project and then move
on to the next one. Publishing results or taking the time to share
them with students seems to be thought of as a loss of time as opposed
to an investment or a professional duty.
European colleagues, on the contrary, consider it a privilege to
be published or to teach. It is deemed important to pass on knowledge
and experience to future generation of young professionals, especially
experience that is unique. I believe that this is a lesson we might
draw from our European colleagues.