Strengthening and reconstruction of Hugo Treffner Gymnasium building was carried out from 1994 to 1997 in four stages. The mandate of KAREG Consulting Engineers entailed designs of temporary and permanent structures as well as development of a monitoring system and examining the condition of the structure. The foundations on the yard side walls were placed two meter higher than the remaining of foundations of the structure. As a consequence, the above mentioned findings led to forming cracks and brought about the walls of the building to incline. The project involved corner walls and interior walls stabilization, exterior walls strengthening and a new basement area construction. A new reinforced slab was poured, designed using the yield line theory analysis. The use of the selected strengthening options and technique for structural intervention of the walls was found to be a suitable technique to achieve the desired objective.

1. 19th IABSE Congress Stockholm, 21-23 September 2016
Challenges in Design and Construction of an Innovative and Sustainable Built Environment
1
Strengthening and Reconstruction of the Hugo Treffner Gymnasium
building in Tartu, Estonia
Kari Avellan, Erika Belopotocanova
KAREG Consulting Engineers, Helsinki, Finland
Contact: kari.avellan@kareg.com
Abstract
Strengthening and reconstruction of Hugo Treffner Gymnasium building was carried out from
1994 to 1997 in four stages. The mandate of KAREG Consulting Engineers entailed designs of
temporary and permanent structures as well as development of a monitoring system and
examining the condition of the structure. The foundations on the yard side walls were placed two
meter higher than the remaining of foundations of the structure. As a consequence, the above
mentioned findings led to forming cracks and brought about the walls of the building to incline.
The project involved corner walls and interior walls stabilization, exterior walls strengthening and
a new basement area construction. A new reinforced slab was poured, designed using the yield
line theory analysis. The use of the selected strengthening options and technique for structural
intervention of the walls was found to be a suitable technique to achieve the desired objective.
Keywords: strengthening; structural intervention; stabilisation; foundations; piles; cracks;
settlement; walls; concrete slab.
1 Introduction
Hugo Treffner Gymnasium (HTG) is a secondary
school located in Tartu, Estonia with special
emphasis on science education. Founded by a
renowned scholar Hugo Treffner, it was the only
large secondary school in 19th century in Estonia
with dominantly Estonian students and no age
restrictions. During the Estonian national awakening
(1850 – 1918) the school greatly contributed to the
number of Estonian intellectuals. In November 2011
HTG became the partner school of UNESCO.
Today, HTG is a modern and comfortable
educational institution and a leading co-educational
public school for students of age 16 to 19 (Figure 1).
Figure 1. Hugo Treffner Gymnasium today.
1.1 Hugo Treffner Gymnasium building
The building has served as a school for centuries.
Founded in 1883, HTG boasts a combination of a

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Challenges in Design and Construction of an Innovative and Sustainable Built Environment
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historic building and modern facilities in the heart
of Tartu.
In the course of extensive restoration work the
gymnasium has retained its historic aura (Figure
2). The majority of original details have been
successfully preserved, e.g. wooden ceiling beams
in the common room, or ceiling paintings in the
classroom on the street level 1.
Figure 2. HTG. Basement library and study.
Nowadays HTG building consists of four parts built
at different times. The building on Munga 12 is the
oldest part of the HTG building ensemble. It was
used as a schoolhouse since 1804, and it is
certainly one of the most eye-catching buildings in
the historical Jaani quarter of Tartu (Figure 3).
Figure 3. HTG building and Jaani Church.
The house by Rüütli Street was specifically
renovated for the school in 1830 by architect
G.F.W. Geist. An interesting fact is that this part of
the building complex is locally known and called
“the Princess House” as in 1784 it belonged to
Hedwig E. Biron, the Princess of Courland.
2 Mandate and scope of work
Strengthening the foundations and deteriorated
walls required considerable expertise in this
project.
The leading authority in this project was the city of
Tartu, represented by Mr. H. Jogi. The work was
performed as a joint venture of Stinger As and
KAREG Consulting Engineers in four stages during;
1994 – 1995, 1995 – 1996, 1997 and in 1999
(Figure 4).
Figure 4. HTG. Project stages. Layout.
The mandate of KAREG Consulting Engineers was
for the design-build, project and technical
management. It entailed structural and
geotechnical designs of temporary and permanent
structures as well as designing a monitoring
system, undergoing geotechnical investigation of
the site and examining the condition of the
structure. The designs were implemented for the
foundation work as well as for the concrete
structures from the basement up to the first floor.
A project of such complex proportions as this
requires diligent approach and sensitive time-

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management on site. It was crucial to oversee
operations on a day-to-day basis to ensure that
work is done safely, on time and to the right
quality standards. At times there were several
project stages ongoing simultaneously 2.
3 Site condition and investigation
A series of ground and site investigations was
performed by KAREG Consulting Engineers to
obtain information on the physical properties of
ground surrounding the site to design earthworks
and strengthening for the structure and for repair
of distress to earthworks and the structure caused
by subsurface conditions.
There were several unexpected findings during
the site investigation. A major revelation from the
results of the performed pit tests was the fact that
the old foundations of the house laid directly on
soil that consists mostly of silty clay and silt. The
uneven settlement of substrata was caused by the
ground water level decrease due to
industrialization.
Another important outcome of subsurface
exploration was the fact that the foundations of
the yard side walls were placed two meter higher
than the remaining of foundations of the structure
(Figure 6).
As a consequence, the above mentioned findings
led to forming cracks and brought about the walls
to incline.
For that reason, before actual foundation
strengthening could be safely initiated, stabilizing
the exterior walls by temporary shoring using
heavy timber struts needed to be performed in
order to retain and support the structural stability
of the building (Figure 5).
4 Foundations strengthening
Underpinning, as a process of strengthening
existing foundations, is chosen and done for
several purposes. In this project of stabilizing,
strengthening and reconstruction of HTG building,
underpinning was necessary for the following
reasons:
Figure 5. Shored exterior inclined wall.
 To strengthen existing foundations and walls
that have suffered from cracks, deterioration,
and settlements
 To deepen a part of the existing foundations
Underpinning by piling in pits was accomplished
by extending the foundations in depth so they rest
on a more supportive soil stratum.
4.1 Project overview
The selected strengthening methods were
deemed appropriate and reflected the condition
of the substructure and superstructure. Factors
such as the unstable and degrading basement
walls, uneven settlements, cracks or inclined walls
were carefully analysed and taken into
consideration.
The phases of strengthening combined different
ways and manners of using jack piles as structural
members (Figure 6).
Figure 6. Building section. Phases of project.

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The project entailed the following phases and
stages: strengthening foundations by piling in pits,
corner walls and interior walls strengthening and
stabilization, exterior walls strengthening using
piles jacked through openings and cantilever steel
beams, new basement area construction using
jacked piles as soldier piles and reconstruction,
stabilization and expansion of interior premises of
the building 2.
5 Phases and stages of
strengthening, stabilization and
reconstruction of building
Preserving a structure of great historic importance
by strengthening its foundations and stabilizing
the walls requires significant expertise, know-how
and suitable selection of strengthening methods.
Due to uneven settlement of the structure and
overall fragile, understrength condition of the
degrading walls, strengthening by piling using
drilled spiral piles and jacked piles was deemed as
the most appropriate, economic, quick and least
invasive method.
Jacked piles consisted of sections of 1 m long
pipes. Every added part was spliced after the
previous part was jacked down. The piles had
closed toes and once embedded in soil they were
filled with concrete.
The first section of the drilled spiral pile was 1 m
long and had a spliced spiral = 25 mm x 10 mm.
The spiral piles were embedded into the soil by
twirling.
Each pile was pre-stressed in accordance with the
end jacking procedure. Drilled spiral piles were
used first as temporary tension piles and
afterwards as permanent compression piles.
5.1 Strengthening of foundations, corner
walls and interior walls
This stage of work consisted of excavation pits
beneath old foundations, reinstalling steel packing
plates with concrete beneath old foundations and
establishing a temporary support until the bound
was strong enough to keep the steel plate in its
position. Therefore the position of the first piling
pits was selected in the manner so that the
immediate wall over it could structurally work as
an arch.
Before digging works could start in this section of
the building, it was necessary to anchor the corner
walls of Jaani and Lübeck Street by horizontal
anchors in both directions.
That was achieved by installation of tension rods
of ø = 20 mm on each side of the corner walls in a
height about 4.5 m from the bottom end of the
old foundations.
The allowable load was 350 kN. Every pile was
pre-tested by end jacking procedure.
Strengthening of the interior walls was executed
using needle beams and two jacked piles. First,
two intersecting holes of ø = 300 mm were bored
in order to install universal beams. Afterwards,
the piles were jacked under needle beams. This
technique required two jacks working
simultaneously.
5.2 Exterior wall strengthening using piles
jacked through openings and cantilever
steel beams
This phase of the project was carried out on the
exterior wall opposite Jaani Church. The below
described method was selected as most
appropriate and economic taking into
consideration the unstable condition of the wall.
Works started by boring two intersecting holes of
ø = 400 mm into the wall and erecting pre-spliced
channels with partly up and down welded steel
plates. The following step was to pour and
reinforce a concrete slab of 600 mm thickness
having the channels in the concrete and living
space between the channels for jacking the piles.
The slab was designed as a two-way slab
supported by needle beams. There were four
tension rods located above the channels (4 pieces
M36 grade 8.8 threatened rods) together with a
thick steel plate. The jack above the slab was
buckled against the steel plate by the piles. The
channels were then supported by the jacked pile
working as cantilever beams loaded by the old
wall. The counterweight was then made of the old
walls and partly of the new reinforced concrete
slab (Figure 7).

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Figure 7. Wall strengthening. Piles jacked through
openings.
5.2.1 Settlement measurement and values
Settlement during strengthening works was
monitored by leveling. Based on the
measurements, there was no noticeable
settlement recorded in this section of the building.
The measured settlement after strengthening
ranged between 2…4 mm. It was assumed that the
applied method of strengthening using pre-testing
and end-jacking yielded to such low settlement
values 2.
5.3 New basement area construction using
jacked piles as soldier piles
The foundations under the yard wall on axis 5
were laid 2 m higher than the other exterior walls
of the structure, therefore there was no
basement. The work was done by archeological
digging carried out in the interval manner.
Following the same principle, reinforced concrete
polings were poured 3.
Since the jacked piles used as soldier piles worked
structurally also as bended and compressed piles,
the allowable jacking force was only 250 kN per
pile.
Due to the earth pressure against the poured
polings between the soldier piles, the old wall and
some of the soldier piles had to be shored
horizontally against the old walls.
Finally, an insulated new reinforced concrete slab
was poured as a basement floor 4.
5.4 Reconstruction, stabilization and
expansion of interior premises
This phase of the project consisted of necessary
stages and steps of design, construction,
supervising and monitoring work towards making
large open areas both in the basement and on the
street level.
The process started off by safely removing partial
interior load-bearing walls in the basement. In this
area there were subsequently erected four drilled
spiral piles (1 – 4) that were embedded into the
soil by twirling (Figure 8).
Figure 8. Strengthening plan design. New
basement/open area.
The joints of the drilled spiral piles were spliced
together. Because of the lack of counterweight,
the two of the piles had first to work as tension
piles (1, 4) so that the remaining two (2, 3) could
be end-jacked. Once this process was completed,
the first set of the piles (1, 4) was end-jacked.
Finally, each of the four piles was checked by the
end-jack procedure. Only after this they were
filled with concrete.

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Figure 9. Street level. Piles 1 – 4 as temporary
shoring. New R/C beams.
In the basement area, a new concrete column was
constructed around the drilled spiral piles
incorporated inside the column as compressed
bars by bound. The column was positioned in the
middle of a new slab that was designed using the
yield line analysis for slabs and virtual work.
The slab together with the exterior walls it had
the function of a load-bearing member. The slab
and the old walls were then connected by dowels.
At the end, the reminding interior load-bearing
walls were removed.
Figure 10. HTG. New open area on the street level.
In the area on the street level (first floor), the
drilled spiral piles worked also as temporary
support during the process of pouring new
concrete beams around the middle wall. At the
final stage, the interior walls and the remaining
part of the middle wall were safely removed
(Figure 9).
This phase of the project resulted into creating
two large open areas; in the basement it was
about 95 m2
(below the slab), and on the street
level the open area was around 100 m2
(above the
slab). This outcome met the request of the client
and requirements of all safety precautions.
Nowadays the areas are used as a common room
for important school events and as the auditorium
(Figure 10).
6 Conclusions
 The strengthening scheme presented was an
economic and aesthetic solution to the
reconstruction of understrength foundations
and unstable walls of the building.
 Time management, minimal disruptions,
precise execution was not only an economic
and effective, but also inevitable factor in
order to follow technical specifications and
technological process that would lead the
project to successful completion.
 The use of the selected strengthening options
and technique for structural intervention of the
walls was found to be an appropriate
technique to achieve the desired objective and
at the same time being minimal invasive.
7 References
[1] Website. Hugo Treffneri Gümnaasium.
2016;http://www.htg.tartu.ee/.
[2] Avellan K., KAREG Consulting Engineers.
Internal archive. Helsinki: Finland; 1994-
1999.
[3] Ympäristöministeriö. Betonikrakenteen B4.
Ohjeet.Suomenrakentamismääräyskokoelm
a (Ministry of Environment, Concrete
structures B4, Guidelines. The National
Building Code of Finland. 2005; Finland. (in
Finnish).
[4] Johansen K.W. Pladeformler. Polyteknisk
forlag. 1968; Copenhagen; Denmark (in
Danish).