1 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

Transcription

1 Rock Engineering Practice & Design Lecture 15: Case Histories 1 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

2 Author s Note: The lecture slides provided here are taken from the course Geotechnical Engineering Practice, which is part of the 4th year Geological Engineering program at the University of British Columbia (Vancouver, Canada). The course covers rock engineering i and geotechnical design methodologies, building on those already taken by the students covering Introductory Rock Mechanics and Advanced Rock Mechanics. Although the slides have been modified in part to add context, they of course are missing the detailed narrative that accompanies any lecture. It is also recognized that these lectures summarize, reproduce and build on the work of others for which gratitude is extended. Where possible, efforts have been made to acknowledge the various sources, s with a list of references r being provided d at the end of each lecture. Errors, omissions, comments, etc., can be forwarded to the author at: erik@eos.ubc.ca 2 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

3 Case History #1: Campo Vallemaggia Campo Vallemaggia, CH Geology - gneisses & schists Mechanism translational ti l slide Surface Area ~6km 2 Total Volume ~ 800,000,000 m 3 Average Velocity ~ 5 cm/year Maximum Depth ~ 300 m Background: For more than 200 years, the villages of Campo Vallemaggia and Cimalmotto have been slowly moving atop a deep-seated rockslide in the southern Swiss Alps. Over this time, numerous mitigation measures have been carried out to stabilize the rockslide but with limited to no success. These works largely focussed on minimising erosion at the toe of the landslide. More recently, the need to stabilize the slope was becoming critical as with each passing year the two villages were being pushed closer to the edge of a 100-m high erosion front at the foot of the rockslide. 3 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

4 Case History #1: Campo Vallemaggia Uncertainty t in stabilizing i the rockslide came about from two competing arguments as to the cause of the slope movements. Opinion #1: Massive erosion at the toe of the slide acts to reduce passive resistance. Opinion #2: Deep artesian water pressures act to reduce the effective strength along the slide surface. Solution: Erosion protection. Solution: Deep drainage. 4 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

5 Campo Vallemaggia Field Investigations Borehole and seismic investigations indicated the basal sliding surface reached depths of up to 300 m. Bonzanigo et al. (2007) The slide was also seen to be divided into two main bodies, separated by a large fault running the length of the rockslide. 5 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

6 Campo Vallemaggia Geology Bonzanigo et al. (2007) 6 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

7 Campo Vallemaggia Subsurface Monitoring Bonzanigo et al. (2007) 7 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

8 Campo Vallemaggia Displacement Monitoring Surface geodetic and subsurface inclinometer measurements indicated that the eastern half of the slide (Campo) was moving freely towards the valley along well-defined shear planes. Bonzanigo et al. (2007) In contrast, the western half (Cimalmotto) was inhibited in its movement by the neighbouring Campo block resulting in a more diffused behaviour with depth. 8 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

9 Campo Vallemaggia Slide Kinematics Target: deep drainage. Bonzanigo et al. (2007) Target: erosion protection. 9 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

10 Campo Vallemaggia Integrating Data Sets Geod detic Movem ment (m) Velocity (m mm/day) Head (m) Borehole critical threshold at 1390 m Comparisons between periods of slope acceleration and pore pressures at depth showed a high h degree of correlation Bonzanigo et al. (2007) 10 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

11 Campo Vallemaggia Deep Drainage Mitigation Pre-drainage Post-drainage Flow at depth was seen to be controlled by fracture permeability. Eberhardt et al. (2007) 11 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

12 Campo Vallemaggia Deep Drainage Mitigation Geod detic Movem ment (m) drainage adit opened Velocity (m mm/day) Head (m) Borehole Eberhardt et al. (2007) With construction ti of the drainage adit, the pore pressures at depth were seen to drop significantly and the slide ceased moving. 12 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

13 Campo Vallemaggia Deep Drainage Mitigation BEFORE Drainage mm/year AFTER Drainage mm Vertical component Total settlement with drainage Eberhardt et al. (2007) geodetically measured surface displacements showing down-slope displacements before deep drainage, and the development of a settlement trough (i.e. consolidation) after deep drainage. 13 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

14 Campo Vallemaggia Competing Mitigation Works Eberhardt et al. (2007) Despite the apparent success, the effectiveness of deep drainage was called into question given the low outflows (<30 litres/s) for the large volume targeted. It should be noted, that at the same time the drainage adit was constructed, so was a diversion tunnel to redirect the river. Thus proponents of the erosion protection solution could also claim their solution stabilized the slide. 14 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

15 Campo Vallemaggia Numerical Analysis To better understand the stabilizing influence of the two mitigation measures carried out, and to argue that the drainage tunnel was effective and therefore should be maintained, numerical modelling was undertaken. 15 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

16 Campo Vallemaggia Numerical Analysis Simulation of influence of erosion protection in the form of non-removal of buttressing material at toe of slide. Eberhardt et al. (2007) 16 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

17 Campo Vallemaggia Numerical Analysis t Simulation of deep drainage was carried out through a coupled hydromechanical distinct-element analysis, using measured borehole pore pressures to constrain the model. t Eberhar rdt et al. (20 007) 17 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

18 Campo Vallemaggia Conclusions 007) Eberha ardt et al. (2 X - Dis splacemen nts (m) without drainage adit drainage adit opened without pore pressures (i.e. dry slope) Time Steps with drainage adit Distinct-element models verified that very little drainage is required (approximately 10 l/s) to significantly reduce pore pressures and to stabilize the slope. Fracture permeability corresponds to low storativities, therefore large water outflows through drainage are not necessary to achieve significant reductions in head. 18 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

19 Case History #2: Gotthard Tunnel Subsidence Zan ngerl et al. (2 2008a) Background: A routine levelling survey in central Switzerland revealed up to 12 cm of surface subsidence several hundred metres above the Gotthard highway tunnel. Perplexingly, subsidence of this nature is not normally associated with deep tunnels in hard rock. In light of the 57-km long Gotthard Base Tunnel, currently under construction through similar rock conditions, understanding these processes is key to avoiding significant differential displacements under sensitive concrete structures on surface (for example thin arch dams, bridges, abutments, etc.). 19 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

20 Case History #2: Gotthard Tunnel Subsidence Spatial and temporal relationships between the measured settlements and the nearby Gotthard highway tunnel pointed to causality between tunnel drainage and surface deformation. Zangerl et al l. (2008a) Excavated in granitic gneisses, steeply inclined brittle fault zones acted as the primary drainage conduits into the tunnel. 20 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

21 Subsidence in Fractured Rock - Precedence 800 m Lom mbardi (1998 8) km Investigation adit driven through a confined, fractured, limestone aquifer near the Zeuzier dam, Switzerland. Recorded settlements of approximately 13 cm were recorded at the dam site, which subsequently led to cracks appearing in the dam. 21 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

22 Subsidence in Fractured Rock - Implications key dams The potential for subsidence over the Gotthard highway tunnel was completely unforeseen. The implications are thus significant for the 57-km long Gotthard Base Tunnel as its trajectory passes through similar rock mass conditions and close to several important concrete dams. 22 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

23 Tunnelling & Subsidence In tunnelling, concerns related to tunnel drainage, timedependent consolidation and surface subsidence are almost exclusively focussed on shallow tunnels excavated in soft, unconsolidated soils. Ground Collapse Consolidation Anagnostou (2002) Biot s 3-D consolidation theory: ij p ij p ij 1 ij ij kk ij 2G 1 3K p ij Conventional methods used to calculate subsidence are largely based on continuum mechanics. Attewell (1988) 23 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

24 Case History #2: Gotthard Tunnel Subsidence Coupled hydro-mechanical continuum analysis. linear poroelasticity: Zangerl et al. (2008b) ij = ij - p ij ' ij p ij ij Effective stress Ttl Total stress Biot's coefficient Pore pressure Kroenecker's delta 24 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

25 Consolidation Subsidence Continuum Analysis Pore Pressures Vertical Displacements Zangerl et al. (2008b) Continuum results were able to reproduce the maximum subsidence measured but could not reproduce the asymmetric shape of the subsidence profile. 25 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

26 Gotthard Tunnel - Geology Predominantly crystalline rock (granites & gneisses). Porosity of intact rock < 0.5%. Overburden ranges between 500 and 1500 m. Rock mass is fractured (i.e. jointed and faulted), and hence the permeability, consolidation and subsidence must also be largely l fracture controlled!! 26 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

27 Consolidation Settlements in Fractured Rock Zangerl et al. (2008a) Consolidation of a set of sub-horizontal fractures could be seen as directly contributing to vertical displacements. However, mapping of the major conductive fault zones in the Gotthard region showed that the majority of these structures were steeply inclined (i.e. sub-vertical). 27 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

28 Gotthard Tunnel - Geology Zangerl et al. (2006) 28 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

29 Case History #2: Gotthard Tunnel Subsidence Coupled hydro-mechanical discontinuum analysis. constant t Zangerl et al. (2003) joint fault zone n = n- f p n constant normal deformation (i.e. closure) vertical slip/shear Poison ratio effect (i.e. horizontal block expansion/vertical n/ contraction) 29 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

30 Coupled H-M Discontinuum Analysis Zangerl et al. (2 2008c) Zangerl et al. (2008b) 30 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

31 Consolidation Subsidence Discontinuum Analysis Discontinuum models were able to reproduce the asymmetry of the measured subsidence trough, but fracture consolidation could not explain the total vertical displacements observed alone. Zangerl et al. (2008b) 31 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

32 Case History #2: Gotthard Tunnel Subsidence Thus, large-scale l consolidation i resulting from tunnel drainage and pore pressure changes in the fractured crystalline rock mass around the Gotthard Tunnel was seen to be related to both the coupled hydromechanical behaviour of the fractures, as well as that of the rock matrix through poroelastic strains. Already, small surface displacements have been recorded close to the Nalps dam in relation to the Gotthard Base Tunnel excavations near Sedrun. If such displacements develop towards critical values, then tunnelling operations may be forced to cease causing expensive delays and intervention measures like high pressure grouting. Ebneter (20 006) 32 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

33 Aiding the Judgment Process The more complex the model, the more input parameters it requires and the harder it becomes to determine these parameters without extensive, high quality (and of course, expensive) field investigations and laboratory testing; As such, we should always begin by using the simplest model that can represent the key behaviour of the problem, and increase the complexity as required. Everything should be made as simple as possible but not simpler. - Albert Einstein Numerical modelling should not be used as a substitute for thinking, but as an aid to thought and engineering judgment 33 of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

34 Lecture References Anagnostou, G (2002). Urban tunnelling in water bearing ground - Common problems and soilmechanical analysis methods. In Proceedings of the Second International Conference on Soil Structure Interaction in Urban Civil Engineering, Zurich, pp Attewell, PB (1988). An overview of site investigation and long-term tunnelling-induced settlement in soil. In Engineering Geology of Underground Movements. Geological Society: London, Engineering Geology Special Publication No. 5, pp Bonzanigo, L, Eberhardt, E & Loew, S (2007). Long-term investigation of a deep-seated creeping landslide in crystalline rock Part 1: Geological and hydromechanical factors controlling the Campo Vallemaggia landslide. Canadian Geotechnical Journal 44(10): Eberhardt, E, Bonzanigo, L & Loew, S (2007). Long-term investigation of a deep-seated creeping landslide in crystalline rock Part 2: Mitigation measures at Campo Vallemaggia and numerical modelling of deep drainage. Canadian Geotechnical Journal 44(10): Ebneter F (2006). Geodätische Überwachung Sedrun-Faido. In Geologie und Geotechnik der Basistunnels am Gotthard und am Lötschberg; Proceedings of the Symposium Geologie AlpTransit, Zürich. vdf Hochschulverlag AG: Zurich, pp Lombardi, G (1992). The FES rock mass model - Part 2: Some examples. Dam Engineering 3: Zangerl, C, Eberhardt, E & Loew, S (2003). Ground settlements above tunnels in fractured crystalline rock: numerical analysis of coupled hydromechanical mechanisms. Hydrogeology Journal 11(1): of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition

35 Lecture References Zangerl, C, Loew, S & Eberhardt, E (2006). Structure, geometry and formation of brittle discontinuities in anisotropic crystalline rocks of the Central Gotthard Massif, Switzerland. Eclogae Geologicae Helvetiae 99(2): Zangerl, C, Evans, KF, Eberhardt, E & Loew, S (2008a). Consolidation settlements above deep tunnels in fractured crystalline rock: Part 1 Investigations above the Gotthard highway tunnel. International Journal of Rock Mechanics and Mining Sciences 45(8): Zangerl, C, Eberhardt, E, Evans, KF & Loew, S (2008b). Consolidation settlements above deep tunnels in fractured crystalline rock: Part 2 Numerical analysis of the Gotthard highway tunnel case study. International Journal of Rock Mechanics and Mining Sciences 45(8): Zangerl, C, Evans, KF, Eberhardt, E & Loew, S (2008c). Normal stiffness of fractures in granitic rock: A compilation of laboratory and in-situ experiments. International Journal of Rock Mechanics and Mining Sciences 45(8): of 35 Erik Eberhardt UBC Geological Engineering ISRM Edition