Ground Loss and Static Soil–Structure Interaction during Urban Tunnel Excavation: Evidence from the Excavation of the Athens Metro
Ground Loss and Static Soil–Structure Interaction during Urban Tunnel Excavation: Evidence from...
Kontogianni, Villy;Stiros, Stathis C.
2020-07-31 00:00:00
infrastructures Article Ground Loss and Static Soil–Structure Interaction during Urban Tunnel Excavation: Evidence from the Excavation of the Athens Metro 1 , 2 Villy Kontogianni * and Stathis C. Stiros Hellenic Survey for Geology and Mineral Exploration (HSGME—former IGME), Sp. Loui 1, Olympic Village, 13677 Acharnes, Greece Department of Civil Engineering, Patras University, 26500 Patras, Greece; stiros@upatras.gr * Correspondence: villy@igme.gr Received: 23 June 2020; Accepted: 28 July 2020; Published: 31 July 2020 Abstract: Ground settlement above urban tunnels is a threat for nearby buildings, because it may lead to their dierential settlement, tilting, and damage, depending on their structural characteristics, on ground conditions, and on the excavation procedure. Still, for few cases only details on ground settlement are known. In this article we analyze ground subsidence data during the excavation of Lines 2 and 3 of the Athens Metro. Based on this evidence, and in comparison with previous studies, we show that observations of elevation changes and of tilting of buildings may underestimate the amount of ground loss; this is because part of the ground deformation may be compensated by the stiness of buildings or accommodated by internal deformation of sizeable buildings hosting measuring benchmarks. This eect can be described as static soil–structure interaction (sSSI), in analogy to the dynamic SSI produced during earthquakes. sSSI can produce bias in monitoring data above an advancing tunnel front, leading to skew and not to symmetric subsidence curves if observations are made on one side on buildings and on the other side in open spaces (‘greenfields’). Furthermore, we show that ‘bowls’ of increased subsidence are observed along subsidence troughs during excavation; such ‘bowls’, not infrequently underestimated because of sSSI, may conceal a potential for sinkholes and other types of failure. Isolated towers on the contrary describe well ground subsidence and tilting. Keywords: ground settlement; geodetic monitoring; tunnel excavation; building damage; soil structure interaction 1. Introduction Tunnels represent important civil infrastructures, tend to be excavated in increasingly adverse conditions, and their construction is not without problems. The reason is that tunnel excavation produces a rearrangement of stresses around the void produced by the excavation and deformation of its surrounding rock mass, usually reaching the ground surface and known as ‘ground loss’. In terms of topography, ground loss takes the form of a subsidence trough, with maximum above the axis of a shallow (up to 30 m deep) tunnel and attenuating away from it, at distances usually 2–3 times the tunnel diameter [1]. Such ground deformation is obviously a threat for overlying buildings, and for this reason, since the experience gained by the excavation of the Chicago Metro by Karl Terzaghi (see [2]), a main task during tunnel excavation is to model, observe, control, and minimize surface ground subsidence, including time-dependent deformation. In fact, there are numerous examples of structural deformation and damage especially of flexible masonry buildings during excavation of nearby tunnels [3,4], even in the case of small ground movements [5]. The eects of tunneling are even more apparent at high, slender structures, such as towers, chimneys, or minarets. The most known Infrastructures 2020, 5, 64; doi:10.3390/infrastructures5080064 www.mdpi.com/journal/infrastructures Infrastructures 2020, 5, 64 2 of 13 example is the controlled tilting of the Big Ben Clock Tower in London during the excavation of the Jubilee Metro Line extension, passing at a distance of 30 m from the Clock Tower [6]. Still, modeling and prediction of ground loss proved not easy. Theoretical modeling by closed functions proved not successful, numerical models are limited by the uncertainty and variability of ground strength parameters in urban environment, while the quasi-empirical model of a Gauss-type curve proposed by Peck in 1969 [1], or its variations [7], remains the most popular method for ground loss modeling. On the other hand, despite the large number of tunneling projects combined with geodetic measurements of ground subsidence [4,6,8–14], the available information on the ground loss is limited. This is due to two reasons: (1) in each project, the amount and distribution of monitoring points is limited, usually confined to buildings at risk (Figure 1a), mainly for cost-suppression and practical reasons (for example benchmarks on the ground above or near the tunnel axis are frequently destroyed during the project) and (2) monitoring data are rarely available because of their legal and financial implications in case of failure (cf. [15]). Such limitations are reflected in two critical, interrelated problems. First, measurements of ground subsidence of large buildings usually indicate moderate amounts of subsidence and tilting [4,14,16], but on the contrary, measurements on benchmarks on free-field (or greenfields), on posts and fences, and on isolated towers show higher, even extreme amounts of tilting and conspicuously of ground subsidence (Figure 1b; [6]). This was clearly detected during excavation of the Athens Metro at the Gazi area indicating a complex pattern of surface ground deformation and tilting of up to 0.75% of the two 30 m high old brick chimneys at the site (Figure 2, [17]), in contrast, minor deformation was recorded at adjacent massive buildings. This contrast, however, is not limited to tunneling, but it tends to indicate a more general eect: for example, in Venice, widespread dierential ground subsidence produces important tilting to isolated towers, but not to nearby massive buildings (Figure 1c). Second, in several cases of tunneling, observations on buildings provided evidence of moderate ground deformation and absence of structural damage in buildings, but sinkholes have been produced in open, nearby spaces, mostly roads. Such an example is an 8 m-wide sinkhole in Ottawa city in 2016, during the construction of the light rail tunnel through weak ground [18]. A similar sinkhole had been reported much earlier during the Athens Metro construction (Figure 3, [19]). Such eects in many cases probably indicate accumulation of ground deformation not identified by monitoring on nearby buildings [20]. Hence, studies of ground loss based on observations on bulky buildings tend to underestimate the true ground loss, because of the potential of buildings to aect their deformation, as a result of the stiness or of the internal deformation of the structures. In analogy to soil–structure interaction (SSI) in earthquake engineering (for example [21]), this eect can be regarded as static Soil-Structure Interaction, sSSI [3,22]; a complicated, geotechnical eect, with high impacts for tunneling. In this article we present and analyze monitoring data from dierent projects of the excavation of Lines 2 and 3 of Athens Metro about 15 years ago. These data, until recently confidential, can provide not only a qualitative, but also a quantitative approach to the role of sSSI and of the associated greenfield (free-field) eect in the modeling of ground loss during tunnel excavation. Infrastructures 2020, 5, 64 3 of 13 Infrastructures 2020, 5, 64 3 of 13 Infrastructures 2020, 5, 64 3 of 13 Figure 1. The static soil–structure interaction (sSSI) and the ‘greenfield eect’ manifested as contrast Figure 1. The static soil–structure interaction (sSSI) and the ‘greenfield effect’ manifested as contrast Figure 1. The static soil–structure interaction (sSSI) and the ‘greenfield effect’ manifested as contrast in in the amplitu the amplitude de of tilting betw of tilting between een whole bu whole buildings ildings (ground loss an (ground loss and d tilt tilting ing partly concealed) and partly concealed) and in the amplitude of tilting between whole buildings (ground loss and tilting partly concealed) and isolated towers (local tilting evident): (a) monitoring points (indicated by circles) in the facades of isolated towers (local tilting evident): (a) monitoring points (indicated by circles) in the facades of isolated towers (local tilting evident): (a) monitoring points (indicated by circles) in the facades of Amsterdam buildings for the metro excavation study. 2-D displacement only (settlement and tilting Amsterdam buildings for the metro excavation study. 2-D displacement only (settlement and tilting Amsterdam buildings for the metro excavation study. 2-D displacement only (settlement and tilting down-to-the-street) can be recorded, but not tilting parallel to the street because it is compensated down-to-the-street) can be recorded, but not tilting parallel to the street because it is compensated down-to-the-street) can be recorded, but not tilting parallel to the street because it is compensated by adjacent buildings. (b) Evidence of ground subsidence on the road pavement and on a stone wall by adjacent buildings. (b) Evidence of ground subsidence on the road pavement and on a stone wall by adjacent buildings. (b) Evidence of ground subsidence on the road pavement and on a stone wall (fence) at the plain of Thessaly, Greece, due to water overpumping. The rigid wall responded to (fence) at the plain of Thessaly, Greece, due to water overpumping. The rigid wall responded to ground (fence) at the plain of Thessaly, Greece, due to water overpumping. The rigid wall responded to ground subsidence with some hysteresis; on the contrary, the building made with stiff concrete subsidence with some hysteresis; on the contrary, the building made with sti concrete foundations ground subsidence with some hysteresis; on the contrary, the building made with stiff concrete foundations showed no significant evidence of damage (after [20]). (c) Regional ground subsidence showed foundations sh no significant owed no signif evidence icant evidence of damage (after of damage (after [20]). ( [20]). (c) Regional c) Regional ground ground subsidence subsin idence Venice in Venice produces limited tilting in massive buildings (palazzos and churches), but high tilting in produces in Venice produces limited t limited tilting in mas ilting in massive build sive buildings (palazzos ings (palazzos and churches and churches), but high ), but high tilting in tilting in bell towers bell towers (campanile), such as that of San Giorgio dei Greci (tilting indicated by red arrow). bell towers (campanile), such as that of San Giorgio dei Greci (tilting indicated by red arrow). (campanile), such as that of San Giorgio dei Greci (tilting indicated by red arrow). Figure 2. Contrasting evidence of ground loss during the excavation of the Athens Metro Line 3in Figure Figure 2. 2. Contrasting Contrasting evidence evidence of of gr g ound round l loss oss during during the the excavation excavation of of the the Athens Metro Line 3in Athens Metro Line 3in the the Gazi area from buildings and chimneys, based on geodetic monitoring. (a) Benchmarks on Gazi the Gazi area area from bui from buildin ldings andgs chimneys, and chimneys based, bas on geodetic ed on geod monitoring. etic monitoring. (a) Benchmarks (a) Benchmarks on on buildings buildings (black and red symbols) indicate limited ground loss, but benchmarks on the 30 m-high buildings (black and red symbols) indicate limited ground loss, but benchmarks on the 30 m-high (black and red symbols) indicate limited ground loss, but benchmarks on the 30 m-high chimneys chimneys indicate a 3-D deformation, summarized in (d), (e) and tilting up 0.75%. The tunnel path is chimneys indicate a 3-D deformation, summarized in (d), (e) and tilting up 0.75%. The tunnel path is indicate a 3-D deformation, summarized in (d), (e) and tilting up 0.75%. The tunnel path is shaded, shaded, and the predicted influence limits are shown by purple and green lines. (b) Aerial view of shaded, and the predicted influence limits are shown by purple and green lines. (b) Aerial view of and the predicted influence limits are shown by purple and green lines. (b) Aerial view of the Gazi the Gazi area (map of the Hellenic Cadastre) of the area in (a). The excavation route is marked by the Gazi area (map of the Hellenic Cadastre) of the area in (a). The excavation route is marked by area (map of the Hellenic Cadastre) of the area in (a). The excavation route is marked by two yellow two yellow curves. (c) A simplified sketch of the area around chimneys K1 and K2 showing the two yellow curves. (c) A simplified sketch of the area around chimneys K1 and K2 showing the curves. (c) A simplified sketch of the area around chimneys K1 and K2 showing the advancement of advancement of the tunnel front. Predicted limits of the tunnel influence zone are marked by advancement of the tunnel front. Predicted limits of the tunnel influence zone are marked by the tunnel front. Predicted limits of the tunnel influence zone are marked by continuous and dotted continuous and dotted lines. (d,e) Summary of observations of movement of the center of gravity of continuous and dotted lines. (d,e) Summary of observations of movement of the center of gravity of lines. (d,e) Summary of observations of movement of the center of gravity of chimneys K1, K2 near chimneys K1, K2 near their top. (f) Side view of the K2 chimney; in the background, some of the chimneys K1, K2 near their top. (f) Side view of the K2 chimney; in the background, some of the their top. (f) Side view of the K2 chimney; in the background, some of the buildings which provided buildings which provided evidence of minimal subsidence. Based on [17] and unpublished data. buildings which provided evidence of minimal subsidence. Based on [17] and unpublished data. evidence of minimal subsidence. Based on [17] and unpublished data. Infrastructures 2020, 5, 64 4 of 13 Infrastructures 2020, 5, 64 4 of 13 (a) (b) Figure 3. The problem of confidentiality in tunnel excavation data: (a) A sinkhole 8 m wide and 12 Figure 3. The problem of confidentiality in tunnel excavation data: (a) A sinkhole 8 m wide and 12 m m deep opened up at the construction site of Ottawa’s light rail tunnel in 2016 [18]. (b) Α 10 m wide deep opened up at the construction site of Ottawa’s light rail tunnel in 2016 [18]. (b) A 10 m wide and 15 m deep sinkhole opened up during tunneling beneath Doukissis Plakentias Ave., Athens for and 15 m deep sinkhole opened up during tunneling beneath Doukissis Plakentias Ave., Athens for the Metro Line 3 in 2003 [19]. In both cases poor ground conditions have been reported but no other the Metro Line 3 in 2003 [19]. In both cases poor ground conditions have been reported but no other technical technical infor information mation is is avai available. lable. 2. Theoretical Modeling of Ground Loss 2. Theoretical Modeling of Ground Loss The simplest and most ecient way to describe ground loss was proposed by Peck [1]. A settlement The simplest and most efficient way to describe ground loss was proposed by Peck [1]. A trough is predicted on the ground surface along the tunnel axis. The 2-D deformation of the ground settlement trough is predicted on the ground surface along the tunnel axis. The 2-D deformation of surface S (x) in a section normal to the tunnel axis is described by Equation (1): the ground surface Sv(x) in a section normal to the tunnel axis is described by Equation (1): S (x) = S exp