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Figure 1. Bearing walls constitution 
Rehabilitation and underpinning of a building classified as a 
national patrimony 
 
Sabrime BOUBAKER 
Terrasol Tunisie, Tunis, Tunisia, s.boubaker@terrasol.com 
 
SUMMARY: Located in the center of a dense urban area of the Tunisian capital, the building to be 
modified is a construction that dates from the early twentieth century and classified as a national 
heritage. The purpose of this project is to preserve intact the original building facade by 
underpinning the internal structure and foundation system. The site on which is based the building is 
characterized by the presence of a highly compressible soil over several tens of meters. As there is 
no information about the initial foundation system of the building, a specific campaign of surveys 
was conducted to analyze the structural elements present in the basement. The purpose of this study 
will be to create a new level of basement and to insure the stability of the building for the next 
decades. 
In this paper will be detailed the project elements and the results of all structural and geotechnical 
investigations. Then will be exposed the different undertaken stages for the operation of 
underpinning of the building. 
 
KEYWORDS: underpinning, basement, inclusion, settlement. 
 
1 INTRODUCTION 
The building to be rehabilitated is compound of 
a first floor and three levels built on a 580 m² 
corner lot. The construction is situated in a 
dense urban area at the most important street of 
Tunis: the Habib Bourguiba Str (geographic 
coordinates: 36°47’58’’N, 10°11’1’’E). It is 
delimited on the North and West sides by two 
roads. The East side is adjacent to a similar old 
building and in the south side is next to a lot 
where has been constructed another similar 
building been demolished since several years. 
 The project consists in removing and 
changing all the internal existing structure of 
the building to create a new hotel compound of 
a first floor and four levels with an additional 
level of basement. The creation of such a new 
basement will require an excavation of about 3 
m depth. 
 All this work will have to be undertaking 
while keeping intact the initial facades 
considered as a national heritage. 
 
2 PROJECT CONTEXT. 
2.1 Building structure and fondation 
prospection 
The construction is established on external 
bearing walls of big thicknesses built in 
freestones. The central core, including the 
stairwell, is constituted by big supporting walls 
in masonry rubble stones and inserted bricks 
(cf. Figure 1). 
 Floors are shaped in full brick-built jack 
arches and metallic profiles. The whole is bond 
by a complex system of floors – metallic 
stringers which insures the equilibrium of the 
structure by conferring it a monolithic behavior. 
 
 
 
 
 
 
 
 
 
 
 The building dates from the early XXth 
century; no documentation on the foundation 
details was preserved since the time. So, and to 
make a clearer idea about the existing system of 
foundation, we proceeded to prospecting works 
in various areas of the construction. 
 Therefore, an excavation was opened up 
against a bearing wall on a surface of 1.35 x 
1.37 m² on 2.1 m depth (cf. Figure 2). 
 The auscultation of the underground structure 
let take the following hypotheses on the existing 
foundation mode: 
- Bearing walls rest on stone bases of a 
trapezoid shape until the depth of 2.1 m; 
- This pad lets the transfer of loads to a 
cyclopean raft foundation (which thickness 
is about 1.6 m according to a borehole). 
This raft would be generalized on all the 
surface of the building. 
- The whole parts constitute a tight system 
(flow rate not exceeding 2.10-5 l/day and 
the initial water level not restored even 
after a week). 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Besides, according to a nearby similar old 
building where basement extension works and 
excavations were made, it was discovered a 
similar foundation system made of trapezoid 
shaped stone bases laying on a cyclopean raft 
foundation. The subgrade was improved by a 
regular mesh of rigid inclusions. These 
elements are made of eucalyptus trunks of 120 
to 150 mm diameter. 
 Furthermore, the topographic surveys have 
shown that the building would have swing to 
the South-East side according to maximal 
amplitude of 46 cm. This tipping would be due 
to the location of the building next to the nearby 
constructions. Indeed, the building had been, 
according to its history, bounded on two sides 
by roads and on other sides (South and East) by 
constructions. Besides, the system of foundation 
such as it was explored supposes that all the 
adjoining constructions rest on a structure 
equivalent to a common raft foundation whose 
settlement is more important in the center (cf. 
Figure 3). 
 On the other hand, and according to 
excavation works inside the building, it was 
noticed the presence of an old flooring at 1 m 
under the ground level, buried on all the surface 
of the building. This give evidence of a long-
term generalized settlement that the 
construction would have sudden: it is about soil 
consolidation. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 2. Foundation system prospecting 
Figure 3. Settlement of the group of buildings 
Figure 4. Principal of the existing foundation system 
 
 Therefore, and according to these diagnostics, 
we had concluded that the foundation system 
layout would be as shown in the Figure 4. 
 
2.2 Soil investigation results 
The analysis of the results of the geotechnical 
investigations made on the site, allows 
highlighting the following geological levels: 
- Level 1: (from 0 to 3.7 m) sandy fill put 
on a layer of calcareous blocks with joints 
of sand and gravel (the existing raft 
foundation); 
- Level 2: (from 3.7 to 18 m) gray muddy 
clay with shell debris the first ten meters; 
- Level 3: (from 18 to 20 m) sandy 
greenish-yellow with shell debris; 
- Level 4: (from 20 to 29 m) grayish brown 
clay with traces of silt and shell debris; 
- Level 5: (from 29 to 32 m) yellowish gray 
laminated clay with traces of sand; 
- Level 6: (from 32 to 45 m) grayish brown 
clay with silty crossings and cemented 
concretions; 
- Level 7: (from 45 to 48 m) silty fine sand 
with traces of cemented concretions on 
the first meter; 
- Level 8: (from 48 m) compact brownish 
clay with traces of silt. 
 
 The following table summarizes the average 
pressuremeter values measured. 
Table 1. Soil mechanical properties 
Level Z (m) Em (MPa) Pl* (MPa) Em/pl* 
1 0 – 3.7 1.9 0.21 9 
2 3.7 – 18 3.4 0.41 8 
3 18 – 20 16.1 1.6 10 
4 20 - 29 6.2 0.7 9 
5 29 - 32 11.3 1.2 9 
6 32 - 45 9.3 0.97 10 
7 45 - 48 30 2.7 11 
8 > 48 26.7 2.2 12 
 
 The water level is almost situated at 0,8 m 
depth. 
 Several laboratory tests were made on some 
samples of level 1 to 6, it consists on oedometer 
and triaxial tests with Atterberg limit 
determination. 
 However, in level 4 and 5 no eodometer tests 
were done. These parameters are very important 
to estimate the consolidation settlement of the 
new structure. Therefore, and while having an 
idea about the initial project loads and its long 
term settlement, we have tried to make a model 
calibration of the soil based on the following 
hypothesis: 
- Project loads are composed of a general 
stress of 50 kPa and a linear load due to the 
walls weight estimated at 300 kN/m. 
- The similar neighboring buildings would 
have the same load distribution. 
- The building settlement consists on a 
general settlement of about0,5 m with a 
differential settlement of 0,4 m to the 
South-East side; 
- All soil layers are supposed to be normally 
consolidated; 
- The ratio between the compression and the 
recompression index is considered equal to 
10 (Cc/Cs = 10) ; 
 These entire hypotheses, coupled with the 
laboratory tests, let clayey soil oedometric 
calibration as following. 
Table 2. Soil oedometric parameters 
Level γh (kN/m3) OCR Cc/(1+e0) Cs/(1+e0) 
1 17 
1 
0.18 0.018 
3 19 0.12 0.012 
4 20 0.05 0.005 
5 20 0.07 0.007 
 
 
3 UNDERPINNING SOLUTION DESIGN 
3.1 Principal of the underpinning solution 
Given the diagnostic results of the building, 
previously presented, and to ensure a new 
redevelopment of the building on a basement 
level, the foundation system will be inspired 
from the existing system and will be directed to 
a mixed foundation that consists of a 
generalized slab resting on a quasi-regular mesh 
of micropiles. 
 Underpinning works of the current system 
will begin by creating an internal rigid portal 
frame (main framing structure of the future 
building) provisionally based groups of 30 m 
long micropiles. This structure will retain the 
original structure of the building (including the 
front walls) before demolition of interior walls 
and floors. 
 
 Once all elements of internal structure 
demolished, earthworks to create the basement 
will be started gradually and in parallel with the 
works of final raft construction. This raft will be 
sealed to the foundation members newly created 
in previous step (micropiles groups) and will 
ensure “rigid box” behavior in the basement. 
 The principal of this underpinning system is 
shown in the Figure 5 below. 
 
3.2 Design of the temporary retaining system 
In order to begin earthworks without disturbing 
the behavior of the original facade structure, it 
will be realized at the future columns positions, 
behind the outside bearing walls, a retaining 
system of reinforced concrete walls, based on 
micropiles. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Foundation micropiles will be 30 m long 
equipped with tubular steel frames of 114 mm 
diameter and 7.5 mm thick. They will be sealed 
by injection into boreholes of 250 mm diameter. 
 
3.3 Design of the temporary foundation system 
The temporarily foundation system will let 
maintaining the portal frame structure during 
demolition phases of interior walls and roofs 
before placing permanent foundation system. 
This system is compound of groups of four 
micropiles at each future column, realized from 
pre-excavation of 0.8 to 1 m depth. These 
micropiles will be sized according loads 
corresponding to the own weight of the beams 
and columns of the portal frame. 
 Bearing capacity of micropiles is normally 
counted from 4 m depth. Moreover, and in 
order to add additional security to the 
foundation system, we have considered only 
30% of the effect of lateral friction throughout 
the thickness of soft muddy clay (Level 2). 
 Thus, as these micropiles will be included in 
the final foundation system, we have considered 
elements of 30 m in length, with a bearing 
capacity of isolated micropile given by the 
following table. 
 
Table 3. Bearing capacity of micropiles 
L (m) QSLS (kN) 
QSLS (kN) 
QULSfd (kN) 
QULSac (kN) 
30 404.7 495.7 578.4 637.2 
 
 These results are supposed to be confirmed by 
at least one static axial load test (control test) on 
a micropile foundation. 
 
3.4 Design of the final foundation system 
The final system will be a mixed type. It will 
consist of a foundation raft 40 cm thick resting 
on a semi-regular mesh of micropiles of 250 
mm in diameter and 25 to 30 m depth. 
Micropiles used for temporary foundation will 
be an integral part of the final mesh. 
 The settlement under the foundation raft will 
have two components: 
- Elastic settlements: immediate settlement 
consumed during the construction work 
progress; 
- Oedometer settlement: consolidation 
settlement consumed very long term after 
dissipation of excess pore pressures 
generated in the clay layers. 
 Elastic settlements are estimated using a 3-
dimensional simulation of the raft foundation. 
The soil is modeled as an elastic multilayer 
(horizontal layers characterized by Young's 
modulus and Poisson's ratio). Micropiles are 
introduced by their equivalent stiffness. The 
maximum elastic settlement at the foundation 
raft is about 1.7 cm. 
 The calculation of oedometric settlement take 
into consideration the effect of over-
consolidation of the soil due to the load of the 
initial embankment of 0.5 m and the existing 
slab 1.6 m thick, placed over the entire area of 
the building for several years. The impact of 
unloading will be modeled by OCR values 
based on a model initially normally 
consolidated as follows. 
Figure 5. Principal of the solution to be undertaken 
 
Table 4. OCR values retained for oedometer settlement 
calculation 
Level OCR 
I 1.63 
III 1.25 
IV 1.19 
V 1.14 
 
 The maximum calculated settlements are 
about 3.5 cm. 
 On the other hand, as the foundation system is 
floating (micropiles arrested at -25 and -30 m), 
it will be necessary to estimate the subsidence 
that may occur under micropiles. Thus, it was 
considered the load transfer of the project to the 
deep compressible layers by the foundation 
system. This led to an additional settlement of 
about 1.3 cm. The total long term settlements 
would then not exceed the 5 cm. 
 
4 PHASING OF WORK 
In order to reduce the impact of earthworks on 
the elements of the original structure, including 
the foundation elements, following phasing was 
recommended (Figure 6): 
1. Generalized excavation of 0.8 to 1 m 
depth; 
2. Achievement of temporary foundation 
micropiles (groups of 2 to 3 under each 
column) and final micropiles (regular 
mesh under final raft); 
3. Portal frame structure construction; 
4. Demolition of interior walls and roofs; 
5. Slabs construction for different levels 
 All this phasing has to be undertaken after 
securing the exterior by a peripherally 
confinement system to avoid any disturbance of 
the existing structure 
 
5 CONCLUSION AND PROSPECTS 
When editing this paper, underpinning works 
were not yet undertaken. Several additional 
investigations have to be made to fix most soil’s 
parameters and to confirm the study hypothesis. 
 Moreover, micropiles’ loading tests will be 
conducted to estimate the real bearing capacity 
of these foundation elements. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 6. Underpining works phasing 
External side 
Embakement 
NG 
External side 
Embakement 
NG 
External side 
Embakement 
NG 
External side 
Embakement 
NG 
External side 
Embakement 
NG 
 
Another alternative of the solution was 
proposed providing more space in the basement. 
The main idea was to partially demolish the 
existing foundation pads so that basement’s 
peripheral structure can be executed just under 
the first floor’s columns. However, the 
excavation works could not be undertaking 
before performing trial tests in small areas of 
the existing building (not more than 2 m x 2 m 
surfaces). 
 
REFERENCES 
Fascicule 62 – Titre V : « Règles techniques de 
conception et de calcul des fondations des ouvrages 
de génie civil », LCPC-SETRA. 
« L’usage des modules de déformation en 
géotechnique », O. COMBARIEU. 
NF P94-262 « Justification des ouvrages 
géotechniques », national application of the standardEurocode7, July 2012.

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