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Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 
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1 INTRODUCTION 
1.1 Main data 
The Vedeggio-Cassarate road tunnel is part of 
the new Transportation Plan of the city of Lu-
gano (PTL), in the southern part of Switzerland. 
The excavation works started in autumn 2007 
and the tunnel was opened to traffic on July, 
27th 2012. 
The 2.6 km long double way tunnel connects 
the valley of the river Vedeggio at its west por-
tal, with the valley of the river Cassarate at its 
east portal. The main tunnel is coupled on its 
southern side with a safety tunnel and is con-
nected to it every 300 m with escape shafts. The 
distance between both tunnels is 30 m. 
The different geological and geo-
morphological conditions along the tunnel iden-
tify three main sections of the structure travel-
ling from west to east: 
− rock section; 
− soft ground section; 
− cut and cover section. 
The rock section begins directly at the west 
portal of Vedeggio and covers a length of 2,370 
m. The excavation in this part was carried out in 
the relatively sound crystalline rock basement of 
the southern Alps using the drill and blast meth-
od for the main tunnel, while the safety tunnel 
was excavated as the initial underground activi-
ty in 2006, with a TBM dia. 4.50 m, up to 15 m 
before the limit between the rock and the soft 
ground. 
The following soft ground section has a 
length of about 200 m. In this section, the dis-
tance between the main tunnel and the safety 
tunnel decreases progressively up to 15.50 m by 
the east underground portal of Cassarate. The 
excavation in this section was carried out in an 
urbanised area, passing underneath or nearby 
many buildings, among which there was a pub-
lic school. 
The final 60 m of the tunnel was built in cut 
and cover in a trench measuring about 100 m 
long and 28 m wide, supported by pile walls of 
1.2 m dia. In this section, the main tunnel and 
the safety tunnel are joined together in a single 
reinforced concrete structure. 
Are ground settlements really always permanent? 
Something strange happens in the soft ground section of the 
Vedeggio – Cassarate Tunnel 
G. Como 
Lombardi Engineering Ltd, Minusio, Switzerland. 
G. Gubler 
Lombardi Engineering Ltd, Minusio, Switzerland. 
ABSTRACT: The Vedeggio-Cassarate road tunnel is part of the new Transportation Plan of the city 
of Lugano (PTL), in the southern part of Switzerland. A section of about 200 m was excavated in 
soft ground, which presents a very heterogeneous sequence of quaternary glacial lake and fluvial 
sediments, with artesian groundwater horizons at many different independent levels with pressures 
reaching up to 20 m water column above the ground surface. After completing the inner lining, 
which includes a full sealing membrane, the overall water pressure was quickly restored to the levels 
measured before the work had started and the amount of settlement was reduced at the same time 
(uplift of the ground), which in some cases reached up to 40% of the maximum deformations 
measured. The paper will present a possible explanation of this strange phenomenon, which was 
observed over the whole area of the tunnel’s stretch of soft ground. 
Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 
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The present paper describes the behaviours 
of the ground in the aforementioned soft ground 
section. Only this section will be further ana-
lysed in the following chapters. 
1.2 Geology and hydro-geology in the soft 
ground section 
The geology of the soft ground section presents 
a very heterogeneous sequence of quaternary 
glacial lake and fluvial sediments, formed 
mainly by a matrix of silty sands or gravels with 
little clay content in a large consolidation range, 
going from the weak condensed silty sands to 
the quite pure over-consolidated silts. Deposits 
of sand and gravel, in variying quantity and 
thickness, are incorporated in this matrix . The 
overburden in this section increases from 5 m, 
right after the portal, up to 40 m at the rock 
limit. 
The permeability of the ground may be 
generally defined as low, with layers of greater 
permeability along the ancient meanders and 
beds of the river Cassarate. In such a ground 
structure, the ground water may be found in 
artesian conditions at numerous independent 
levels with water columns reaching up to 20 m 
above the ground surface. Generally the 
geotechnical properties may be described as 
being adequate to poor, and the hydro-
geological conditions as extremely complex. 
Figure 1 shows a simplified geological model of 
the ground along the alignment of the main 
tunnel. 
 
Figure 1: Simplified ground model of the soft ground 
section. 
As visible in the Figure 1, three main ground 
typologies may be recognised. Beginning from 
the tunnel portal these are: 
1 a superficial sequence of weak consolidated 
silty sands and gravel with low permeability 
2 a layer of over-consolidated silt with little 
gravel, very stiff and with very low permea-
bility 
3 deep silty gravels till gravelly silts and sands, 
with greater permeability. 
1.3 Excavation Method 
Due to the very heterogeneous and variable 
geological and hydro-geological conditions, 
since the earliest design phases, it was clear 
from the start that an extremely flexible 
excavation method would be required, which 
could be rapidly adjusted on site to the variable 
geological and hydro-geological situations in 
order to ensure sufficient stability of the 
excavated tunnel section and, at the same time, 
reducing the impact of the excavation on the 
surroundings (settlements). 
After the analysis of different possible 
techniques, it was decided to apply the jet-
grouting method for the consolidation of the soil 
as the best solution to fulfil these targets. 
Along the first 30 m from the portal, this type 
of consolidation could be carried out from the 
ground surface (vertical jet grouting) and for the 
remaining stretch the jet grouting was carried 
out directly from the tunnel face (horizontal jet 
grouting). With this method it was possible to 
reduce the overall permeability around the 
tunnel and to improve the geotechnical 
properties of the ground, thus preventing the 
uncontrolled drainage of the groundwater whilst 
Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 
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restraining the deformability of the ground 
around the excavation at the same time. 
The excavation in the over consolidated silts 
was carried out under the protection of a classic 
steel pipe forepoling with fibre reinforced 
anchors in the tunnel face. More information 
may be found in [1] and [2]. 
The construction works have been carried out 
according to the following sequence and 
schedule: 
− Full face excavation of the safety tunnel 
− Staged excavation crown-invert of the main 
tunnel; crown face min. 50 m behind excava-
tion face of safety tunnel 
− After the breakthrough of safety tunnel into 
the rock section, installation of the sealing 
membrane and of the inner lining from inside 
toward the portal 
− After the breakthrough of the main tunnel into 
the rock section, installation of the sealing 
membrane and of the inner lining from inside 
toward the portal 
1.4 Tunnel waterproofing concept 
In the soft ground stretch, the tunnel has a full 
sealed cross section (along the whole perimeter) 
consisting of a double layer of plastic sealing 
membrane, without any textile fleece. In 
addition, sealing rings that were glued to the 
shotcrete primary lining, were placed at regular 
distances between the sealing membrane and the 
same primary lining in order to avoid the 
groundwater flowing along the sealing 
membranetoward the near tunnel portals 
draining the groundwater. 
This system was chosen with the aim of 
allowing at least a partial recovery of the 
groundwater elevations and pressures after the 
completion of the works with the target of 
limiting the value and extension of the long 
term ground settlements. These settlements in 
fact could lead to damages on the existing 
buildings and, in the worst case, to the 
instability and sliding of the entire slope in 
which the tunnel was excavated. 
In fact, despite the previous consolidation of 
the ground with jet grouting, the drainage and 
the consequent water head reduction effect of 
the tunnel excavation on the groundwater could 
not be excluded. 
1.5 Monitoring system 
In such critical geological and hydro-geological 
conditions, with buildings on the ground 
surface, the monitoring and survey system has 
the primary role of guaranteeing the safety of 
the works and most importantly, of the third 
parties involved 
The survey and monitoring system installed 
in the soft ground section was intended to 
survey the following groups of parameters: 
− Group 1: parameters addressed to the safety 
of the excavation works (convergences, tun-
nel levelling, vertical settlements, water table 
variations) 
− Group 2: parameters concerning the exist-
ing buildings in the surroundings and conse-
quently of the safety of third parties (settle-
ments measured manually and with automatic 
theodolite station, slope stability indicators, 
width variation of existing cracks). 
The comprehensive description in the next 
chapters analyses the results of the survey of 
Group 2 and particularly of the manual 
settlement survey. The sole exception will be 
represented in the analysis of the measurements 
of the groundwater levels pertaining to Group 1, 
whilst still representing the most significant 
parameters to be followed in controlling the 
deformation behaviour of the ground in the 
conditions described here. 
A total of 42 target points (manual and 
automatic levelling) were measured in order to 
monitor the development of the settlements of 
the existing buildings and infrastructures during 
the excavation and the finishing works of the 
tunnels. 
The follow-up of the groundwater conditions 
previous to, during and after the tunnel works 
was assured by 17 piezometers installed over 
the whole project area. 
Figure 2 shows the tunnel alignment such as 
the position of the existing buildings and the 
infrastructures as well as the emplacement of 
the above mentioned instrumentation (survey 
targets and piezometers) near to the tunnel. The 
explanations presented in the following chapters 
are limited to this area. 
In order to simplify matters, Figure 2 only 
shows the instrumentation, which will be 
mentioned in greater detail further on. 
 
Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 
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Figure2: Situation in the soft ground section with the 
positions of the instrumentation mentioned in the article. 
2 GROUND AND GROUNDWATER 
BEHAVIOUR 
2.1 Situation before works start 
The whole area of the soft ground section of the 
Vedeggio-Cassarate tunnel is formed by a steep 
slope subject to continuous sliding movements, 
which, in the past, caused some minor to serious 
damages to the existing buildings. 
The existence of artesian groundwater was 
also well known, witnessed by the presence of 
water springs in the form of shifting sands in the 
tunnel portal area (C&C section). 
The piezometers installed in this area show 
the presence of different free and artesian water 
tables in the superficial ground sequence (layer 
1 in Figure 1). A deeper artesian groundwater 
table with a very large surface extension and 
much higher pressures reaching up to 3 bars, 
could be recognised in the deep silty gravels 
(layer 3 in Figure 1), separated from the 
superficial ones from the layer of impermeable 
over-consolidated silts (layer 2 in Figure 1). 
The long-term observation of this deep 
artesian ground water table showed a constant 
decreasing trend of the hydrostatic levels since 
early 2005, two years before the start of any 
activity relating to the tunnel construction in this 
area. The reasons for this trend are not known. 
2.2 Behaviour during tunnel excavation 
The excavation of the safety tunnel started in 
September 2007 after breaking down the portal 
pile wall. Before these works started, during the 
last week of August 2007, several drainage 
holes were drilled from the portal trench, 
through the pile wall, in the lower part of the 
safety tunnel section still to be excavated. 
The target of these drillings was a ground 
layer, among the superficial low consolidated 
silty sands (layer 1 in Figure1), in which high 
groundwater pressures were measured and 
where the upper limit in the area of the portal 
lay only a few metres underneath the bottom of 
the tunnel (see Figure 1). The goal was to 
relieve the groundwater pressures in a localised 
area in order to avoid the danger of ground 
piping in the tunnel during the excavation. 
As shown in Figure 3, an immediate drop in 
the groundwater pressures was observed in the 
piezometer SCI6-31 after the activation of the 
drainage system, despite the relatively low 
permeability of the ground. Figure 3 also shows 
the settlements measured on building 102 (see 
Figure 2), near the piezometer SCI6. 
Please note that the number (in this case “-
31”) following the designation of the 
piezometer (in this case SCI6) indicates the 
depth of the pressure gauge from the ground 
surface. 
Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 
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Figure 3: Immediate drop of groundwater pressures and 
settlements on building 102 after drilling of drainage 
holes. 
After the drainage of this layer, the 
excavation of the safety tunnel could be carried 
out in the superficial silty sands and gravels 
(layer 1 in Figure1) without major problems. 
After about 8 months of excavation, the 
entire surface of the tunnel stretched over the 
over-consolidated silts (layer 2 in Figure 2). 
Figure 4 shows the safety tunnel face in this 
layer. The excavator grooves visible in the 
tunnel face give the idea of the stiffness of this 
type of ground. 
 
Figure 4: Safety tunnel; excavation front fully in the over-
consolidated silts. Note the excavator grooves left in the 
ground. 
The excavation in the over-consolidated silts 
of about 30 to 40 m long, took about 1 month to 
reach from the boundary to the following layer 
of the deep silty gravels (layer 3 in Figure 1), 
where high water pressures were confirmed. 
Before entering this new layer, some 
drainage pipes were drilled from the tunnel face 
(in the same layer) in order to relieve the 
pressure and avoid a possible failure of the 
tunnel face caused by the groundwater pressure 
load. 
Also in this case, the piezometer gauges 
placed in this deep layer measured the same 
immediate dramatic drop of the groundwater 
pressure as observed in the piezometer SCI6 
before the start of the excavation works (see 
Figure 3). This phenomenon is shown in Figure 
5 with the example of the piezometers SCI1-35, 
SCI2-21, and SCI3-45. 
 
Figure 5: Immediate drop of the groundwater pressure 
after drainage drilling in the layer of the deep silty gravels 
and consequences on the ground settlements. 
The same figure shows the consequences on 
the ground settlements, with a noticeable 
acceleration of the settlement on building 102 as 
well as the immediate start of the movements on 
buildings 104 and 107 (position of the 
buildings: Figure 2). 
The almost perfect overlapping of the 
groundwater pressures measured in thethree 
piezometers is quite remarkable, despite the 
quite significant distance between them (see 
Figure 2). This fact gives an idea of the 
extension of this ground layer and of the 
Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 
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homogeneous distribution of the groundwater 
pressure in it. 
It must be further mentioned that the 
piezometers SCI1 and SCI2 have been damaged 
during subsequent work and had to be replaced 
with new piezometers SCI8 (not shown in the 
graph) and SCI9. 
It is further interesting to note the behaviour 
of the water pressure measured from the gauge 
in piezometer SCI6-15, i.e. in one of the 
superficial layers of the silty sands (layer 1 in 
Figure 1). During the whole construction period, 
the groundwater pressure in this upper layer was 
not influenced in any way from the severe 
changes occurring in the lower groundwater 
table. This behaviour demonstrates the strong 
barrier and separating effect created by the layer 
of the very low permeable over-consolidated 
silts. 
The excavation of the safety tunnel in the 
deep silty gravels (layer 3 in Figure 1), until the 
breakthrough into the rock stretch, took about 5 
months (December 2008). 
Until the end of November 2009, the 
following major work has been further 
completed: 
− excavation of the main tunnel; 
− sealing membrane and lining of safety tunnel; 
− sealing membrane and lining of main tunnel. 
Over the whole duration of these works, the 
groundwater level in the layer of the deep silty 
gravels has been maintained at the elevation of 
the tunnel bottom. In this period, the ground 
settlements progressively stopped reaching 
values between 50 and 30 mm, as it can be 
observed in Figure 5. 
2.3 Behaviour after the completion of the 
underground works 
After the finishing of the inner lining of the 
main tunnel, the works in the Cut & Cover 
section (C&C) were carried out and completed 
in June 2010. 
The monitoring activities of the groundwater 
and of the ground deformations continued over 
this period of time and were carried out until the 
opening of the tunnel to the traffic in July 2012. 
A general recovery of the groundwater 
pressures could already be observed during the 
final phase of the lining works of the main 
tunnel, when the draining effect of the tunnels 
was progressively reduced from the laying of 
the sealing membrane and the concrete lining, 
as can be seen in Figure 6. 
 
Figure 6: Recovery of the groundwater pressure in the 
deep silty gravel layer after completion of the 
underground works. 
The further recovery of the groundwater 
pressure levels observed between November 
2010 and February 2011 may be connected to 
the grouting works carried out in the main 
tunnel in order to stop minor water leakages 
appeared during the restore of the groundwater 
levels. 
This behaviour confirms the high sensitivity 
of this type of ground to any change induced to 
the condition of the groundwater pressure. 
When comparing Figure 5 with Figure 6, it is 
interesting to note that the groundwater 
pressures in the deep silty gravels recovered 
practically up to the same level that existed 
before the start of the works pertaining to the 
tunnel construction. On the other hand, the level 
in the upper low consolidated silty sands (layer 
1) was not affected in any way from the changes 
observed during and after these activities. 
The monitoring of the ground movements in 
this same time period showed a very surprising 
behaviour. In fact, the measured settlements 
started to recover following a trend similar to 
the one of the groundwater pressures. 
Several control measures have been carried 
out in order to verify the stability of the 
monitoring benchmarks. All controls confirmed 
the integrity and liability of these stations. 
Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 
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Figure 7 shows the recovery trends of the 
ground settlements together with the ones of the 
groundwater pressure shown in Figure 6. 
The Figure shows the behaviour of the same 
survey targets considered in the previous 
Figures, i.e. of buildings 102, 104 and 107. In 
order to demonstrate the extension of the 
observed phenomenon, The Figure contains 
graphs of the settlements of additional targets, 
namely of the target points 304 and 103. 
 
Figure 7: Recovery of ground settlements following that 
of the groundwater pressure. 
As it can be observed in the Figure, the most 
rapid reactions were registered on points 304 
and 107 (but also on further survey points on 
building 107, which are not displayed), which 
are also the furthest points from the tunnel axis. 
It is also interesting to note that these points 
show the highest recovery rates in absolute and 
relative values from all those that were 
measured. 
In fact, considering the maximal measured 
settlement of these two target points of about 
30 mm, the absolute recovery values reached in 
both cases was about 12 mm, which represents 
about 40% of the maximal settlement. 
The movements of the other displayed targets 
show a very similar trend even if with smaller 
absolute values, between 5 and 10 mm, but with 
relative variations from 20 up to 30%. 
Though the average of the absolute recovery 
values may be considered of the same order of 
size of the monitoring system precision, the 
general uplift trend of the ground in the whole 
area cannot be ignored and must be considered 
as reliable. This fact is confirmed by Figure 8 
here below showing the settlement graphs of a 
few other survey target points in the area. 
 
Figure 8: General uplift trend of the survey target points 
in the site area. 
2.4 Possible explanations 
The layer of the over-consolidated silts acts 
evidently as a watertight barrier, which confines 
the deep silty gravels in all directions, leading to 
the artesian conditions of the groundwater in 
this ground layer. As a reaction, the 
groundwater pressures create a pushing effect 
on the watertight over-consolidated silts. 
The fact that the layer of the silty gravels lays 
under the over-consolidated silts leads us to 
presume that a certain degree of natural 
consolidation also took place in the deep silty 
gravels. 
Taking into account these considerations, it 
may be concluded that the settlements observed 
on the ground surface could have resulted from 
the overlapping of the following phenomena: 
− plastic deformations caused by the tunnel ex-
cavation (stress changes in the ground), 
− consolidation of the ground caused by the 
drainage of the groundwater (in the deep silty 
Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 
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gravels attenuated by the consolidated nature 
of the ground), 
− "elastic" lowering of the over-consolidated 
silts layer caused by the drop of the support-
ing groundwater pressures in the lower silty 
gravel layer. 
According to the ground behaviour observed 
during and after the tunnel excavation works 
described in the previous chapters, it seems that 
the last illustrated phenomenon, i.e. the 
lowering of the over-consolidated silts, could 
have been partially recovered through the 
recovery of the groundwater pressure in the 
deep silty gravels. This pressure, acting on the 
very tight lower surface of the over-
consolidated silts, has probably led to an 
extended uplift of this ground layer and those on 
top of it. 
This effect has been observed as a recovery 
of the ground settlements. 
It can be supposed that this uplift started in 
the areas with less overburden, i.e. with lower 
ground load, dragging the remaining part of the 
layer upward also in theother areas, eventually 
with less intensity. The spatial development of 
the phenomenon should be further studied. 
2.5 Final results 
Despite the significant settlements observed 
during the construction of the tunnels, with 
values up to 50 mm, the buildings nearby the 
excavation site didn’t suffer any major damage. 
In fact, the settlements measured on the single 
buildings were almost even and no significant 
differential settlement was observed. 
The only damages to the buildings that was 
recorded pertained to the limited opening of 
already existing fissures. The dimensions 
reached by the fissures did not represent any 
danger and could be easily repaired. 
The tunnel excavation was completed within 
the time schedule and the estimated costs. 
3 CONCLUSION 
The experience gained during the excavation of 
the Vedeggio-Cassarate tunnel in southern 
Switzerland has shown that complex geological 
and hydro-geological conditions can lead to 
unexpected ground behaviour during and after 
underground excavation works. 
The present case has shown that ground 
layers carrying artesian groundwater with 
relatively high pressures, confined by other 
extremely watertight ground layers, could result 
in extremely sensitive behaviour following any 
minor change in the pressure of the 
groundwater. 
With regards to the drainage of the artesian 
groundwater (independently from the amount of 
water drained), the ground behaviour may show 
an immediate start and a strong increase of the 
ground settlements followed by a partial 
recovery of the measured surface settlements 
after restoring the initial pressure of the artesian 
groundwater following the completion of the 
underground works. 
When work is to be carried out in similar 
geological and hydro-geological conditions, the 
risks which could be related to such ground 
behaviour should be taken into account at the 
design stage of the work. 
ACKNOWLEDGEMENTS 
Department of Construction of the Canton of 
Tessin, Switzerland. 
REFERENCES 
Como G., Ferrari A., Sidler M. 2008. Lockergesteins-
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(IGT) of the Swiss Federal Institute of Technology 
(ETHZ). Zurich. 
G. Como, Ferrari A., Gubler G. 25.04.2008. Tunnel 
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