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Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 1 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. 2 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. 3 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. 4 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. 5 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. 6 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. 7 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. 8 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- strecke des Vedeggio – Cassarate Tunnels: Bauhilfs- massnhamen im Mittelpunkt des Bauvorhabens, In: Bauhilfsmassnahmen im Tunnelbau, Intervention at the annual colloquium of the geotechnical Institute (IGT) of the Swiss Federal Institute of Technology (ETHZ). Zurich. G. Como, Ferrari A., Gubler G. 25.04.2008. Tunnel Vedeggio – Cassarate: Tunnelvortrieb in wasser- gesättigtem Lockergestein. In: Seminar of the Swiss Society for Soil and Rock Mechanics SGBF. Lugano
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