GIBE II TUNNEL PROJECT - ETHIOPIA - 40 BARS OF MUD ACTING ON THE TBM

Transcription

1 GIBE II TUNNEL PROJECT - ETHIOPIA - 40 BARS OF MUD ACTING ON THE TBM IL FUTURO DI SELI SPECIAL DESIGNS AND MEASURES IMPLEMENTED TO FACE ONE OF THE MOST DIFFICULT EVENT IN THE HISTORY OF TUNNELING Condivisione di obiettivi e traguardi

2 The Gilgel Gibe II Hydro-Power Plant is located approximately 240km southwest of Addis Ababa, in Ethiopia, between the Gibe and the Omo Rivers. The 25,8 km long tunnel has been excavated by two 7 m diameter Double Shield TBMs. Tab.1 Water Conveyance project main data PURPOSE TUNNEL LENGHT INTERNAL TUNNEL DIAMETER DESIGN DISCHARGE DESIGN INTERNAL PRESSURE Tab.2 Lining geometrical Characteristics SEGMENT TYPE SEGMENT THICKNESS SEGMENTAL LINING OUTSIDE DIAMETER SEGMENTAL LINING FINAL INTERNAL SEGMENT WIDTH HPP (432MW) 25,8 km 6,3 m 100 m 3 /s min.2 bar max.7 bar n.4 HEXAGONAL 25 cm mm mm mm

3 Figure 1. Typical lining section

4 FORESEEN GEOLOGY- OMO BASALSTS AND JIMA VOLCANICS Figure 2. Geological profile

5 EVENT AT THE CHAINAGE FROM INTAKE HEADING At the end of October 2006, the TBM was pushed back as consequence of the sudden extrusion and collapse of the tunnel face against the cutterhead and the front shield. The tunnel face moved towards the TBM mm/hour. The TBM has been pushed back more than 60 cm and displaced laterally more than 40 cm. As consequence, severe damages occurred to the shields, the cylinders and the last 7 segment rings installed behind the TBM.

6 Delta Right (old point) Delta Left Delta Right (new point) [ m m ] /1/07 9/3/07 28/4/07 17/6/07 6/8/07 25/9/07 14/11/07 3/1/08 22/2/08 Day Figure 17. Distometer measurements of Gripper Shield backing Figure 18. Topographic monitoring of TBM displacements

7 Figure 3. Tail TBM Shield damaged and bent

8 Figure 4. Lining segments moved and severely damaged

9 Figure 5. Overview of the TBM Shield left part and installed lining segments

10 Some boreholes through the front shields after the event, were performed with extreme difficulties, in some cases the rod bars were pushed out by the high pressure of the rock (clay and mud poured out with a temperature of about 40 and the pressure raised to 40 bar Figure 6. Mud poured out from one of the borehole

11 BACK CHAMBER DESIGN (BY LOMBARDI CONSULTING ENGINEERS ) Figure 8. Back Chamber Layout

12 Figure 10. State of the intervention on May 2007 (ch )

13 In June 2007 a collapse of the front face started at the left Exploratory Adit, producing three controlled leaching of about 3500 m3 of mud, which filled the all Adit and about 80m of the power tunnel (Figure 11 to 16). Figure 11. Extruding mud in the Exploratory Adit

14 Figure 12. Extruding mud in the Exploratory Adit

15 Figure 13. Extruding mud at the Adit Portal

16 Figure 14. Extruding mud in the Power Tunnel

17 Figure 15. Consolidated mud filling the Power Tunnel

18 The major three events represent the most important leaching of mud in the left Exploratory Adit. Each event was immediately followed by a sudden reduction of the acting pressure and convergence. Important pressure reductions were also obtained after August 2007, when the new boreholes crossed the fault zone, draining water. Water pressure (manometer BH n 2 Back Chamber) 26,0 p [bar] 24,0 22,0 20,0 18,0 16,0 14,0 12,0 1st Event BH-SM-1 BH-SM-3 BH-SM-9 BH-SM-11 BH-SM-15 BH-SM ,0 8,0 6,0 4,0 2,0 2nd Event 3th Event 4th Event 0,0 12/5 22/5 1/6 11/6 21/6 1/7 11/7 21/7 31/7 10/8 20/8 30/8 9/9 19/9 29/9 9/10 19/10 29/10 8/11 18/11 28/11 8/12 18/12 28/12 7/1 17/1 27/1 days Figure 26. Pressure variation measured in the Back Chamber

19 A new strategy was then defined: the Mud was removed from the power tunnel and partly only from the left Exploratory Adit; the Back-Up was extracted from the tunnel; a more powerful drilling equipment was positioned behind the TBM shields (having removed the Back-Up of the TBM); the construction of a new Exploratory Adit on the Right side (40m behind) then started.

20 A new attempt to reach the TBM cutterhead was carried out from the right side Exploratory Adit but the monitoring of loads and deformations suggested renouncing (Figures 20).

21 On December 2007, monitoring measurements have shown lowering of the rock stresses, while the right Exploratory Adit was crossing the fault zone A further new strategy was considered: Completion of the Back Chamber through a concave shot-concrete wall, reinforced with horizontal steel ribs HEB200 (see figure 21) TBM Dismantling inside the Back Chamber and carrying outside Refurbishing and reconstruction of the TBM (in the external yard) Construction of a new assembly chamber and TBM launching chamber Segmental lining dismantling and casting of a concrete plug in the Power Tunnel Resuming of the Intake drive excavation along a new alignment at ch

22 Figure 21. 3D Model of the Rescue Chamber built to recover the TBM

23 The new Power Tunnel alignment was chosen maintaining a minimum distance from the explorative Right Adit. (Figure 22). Figure 22. Position of TBM assemblylaunching Chamber and By-pass general layout

24 To facilitate TBM steering at the excavation resuming and to reinforce the pillar between the two tunnels, in the transition zone the old tunnel has been filled with concrete (Figure 23) Figure 23. Restarting of excavation and plug in the transition zone

25 The TBM has been refurbished and re-assembled in the tunnel. The excavation diameter was enlarged from 6980mm to 7074mm Peripheral cutter housings (from n 37 to n 44) were re-positioned (figure 24) Figure 24 TBM cutting profile after modification The TBM was re-assembled (Figure 25) excavation along the new alignment restarted on August 1st, 2008 Figure 25. TBM cutterhead

26 The TBM crossed the fault zone along the new alignment during the month of October 2008, performing later excellent production. Figure 28 resumes the monthly TBM excavation from August to December Figure 28. TBM production chart after the excavation resuming

27 The Tunnel excavation has been completed on 6 of June 2009 when the two TBMs meet at the ch.: 8+520,8

28 FINAL CONSIDERATIONS The success of the intervention was possible only after the releasing of the pressure and the stresses acting in the area and surrounding the TBM. Some numbers are significant for the Event n 19: 3500 m3 of mud flowed and removed m3 of water drained during the intervention 230 m of Exploratory Adit excavated with traditional method 1600 m of investigation boreholes