Lake Mead inundation - TunnelTalk
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Inundation at Lake Mead Intake No 3 Jul 2010
Shani Wallis, TunnelTalk
Connection with a geological fault has flooded the underground works at the Lake Mead Intake No 3 project in Nevada. A run of ground ahead and above the face of the TBM starter tunnel at the bottom of the 600ft (182m) access shaft triggered an inflow of water and material that had seven crew members withdrawing back to the shaft bottom to evacuate the works and has since risen about 150ft into the shaft where it is being held.
The drill+blast heading was about 250ft into the 350ft long starter tunnel and about 500ft from the lake's edge when the fallout occurred on Monday night 28th June. Crews were advancing the 26ft horseshoe tunnel and installing ribs and steel lagging and steel-fibre reinforced shotcrete support Ground continued ravelling however after the initial face fallout and culminated in a break in of ground and water on Thursday morning July 1st that halted all work and required the underground works to be evacuated by about 5am.
Pic 1

Design of the deep level intake tunnel project

Material carried by the water filled the two sumps at the bottom of the shaft and over came the pumps. Water rose during the following hours to about 150ft into the 27ft (9m) i.d. shaft. Back up pumps installed at the relay station in the shaft are working at about 800gpm capacity to hold the water at just below the equilibrium level of the lake. Some equipment had been lifted to the surface but the heavier pieces of main excavation equipment remain behind in the flooded works, which includes the 200ft long assembly chamber and the 85ft tail tunnel.
All workers underground at the time were brought safely to the surface in the shaft bucket. Access into the works is now cut off until the inflow is stemmed, the flood water can be pumped out, and the situation is recovered. Drilling from the surface to investigate the extent of the ground loss at tunnel level and to prepare for grout injection to stabilize the area has started.
Work continues on other parts of the contract including on the marine works where potable water for the city of Las Vegas and the Southern Nevada region will be drawn from a new intake at some 600ft (182m) below the current level of the lake. A 7.2m diameter Mixshield from Herrenknecht is on site and being prepared to excavate the 3-mile (4.8km) connection to the intake structure.
The intake tunnel is being constructed by Impregilo of Italy and its US subsidiary SA Healy of Chicago. The $447 million design-build tunnel contract was awarded to the Vegas Tunnel Constructors JV (VTC) in March 2008. Arup is engaged as the contract’s lead designer and engineer for the TBM tunnel and marine works, and Brierley Associates is the geotechnical consultant and designer of the conventional excavations and their support. MWH/CH2M-Hill JV prepared the 10% design contract documents and the GBR, and Parsons has the Program Construction Management contract. The tunnel contract is part of a total $700 million new water intake program by SNWA (the Southern Nevada Water Authority).
"We had been in adverse ground conditions for about 100ft or so," said Jim McDonald, Project Manager for VTC. "Earlier instability between the ribs had been caught and supported. The fallout in the face created a hole of about 2m2 that hit a fault with a lot of water and initiated the inrush. The two pit bottom sumps had a total pumping capacity of 2,400gpm and some 1,200-1,300gpm of this was managing the inflow before the pumps were overcome by washed in material and the decision was made to let it fill up.
"We are optimistic to get on top of this," said McDonald. "Drilling equipment on the surface, outside the perimeter fence between the work site and the lake's edge is probing the area to assess the extent of the ground loss void. The recovery work is complicated by the fact that it is 600ft deep and there is no knowing the exact situation underground until we regain access. On the positive side, nobody was hurt and there is nothing on the surface to suffer damage or restrict access to begin the recovery process.
"It is difficult to know where we are on the schedule. We had delays sinking the shaft due to problems installing the cut-off grouting. Some associated works have been started early to mitigate those delays," said McDonald. "The TBM arrived on site in October last year and preassembly is progressing before final assembly in the bottom of the shaft. A launch was scheduled for late this summer but now by the current recovery process."
Marc Jensen, Director of Engineering for SNWA said: "The tunnel hit a pocket of unstable ground that resulted in an incursion of material and water that chased the crew out of the works. The tunnel face is not beneath the lake but it is below the water level. Water inflow of about 800gpm was being managed in the starter tunnel before the event. Inflow of about 1,300gpm as a result of the event and of the chocked off sump pumps took several hours to flood the works. The starter tunnel excavation was going slower than expected and in ground that was less stable than we would have liked. This is a serious situation but it is a temporary set back. The key is progressing the grouting from the surface. The contract is running a few months behind schedule, but 2013 remains the program end date for the project."
In speaking to other sources associated with the project, who asked to remain unidentified, reports were of workers swimming back to the shaft for rescue. This was refuted by McDonald who said that crew members had already withdrawn from the heading and were in the shaft area when the water began to rise.
There were reports of differing opinions about grouting the faults, about coping with unstable ground, and about adjusting the excavation and support cycles to respond to changing geological conditions in the starter tunnel and on other parts of the drill+blast work. McDonald explained that a clay gouge in faults in the starter tunnel made it difficult to achieve a good take-up of injected grout.
Reports of a claim by an anonymous tunnel worker of unsafe conditions to the State of Nevada OSHA are not confirmed. Jensen and McDonald both said they knew nothing of the report and had heard of no formal enquiry into the event.
Both managers said that the SNWA and the VTC JV are working cooperatively to assess the situation and advance the recovery process.
References
Lake Mead No 3 intake tunnel awarded - TunnelTalk, June 2008
Lake Mead TBM designed for the extreme - TunnelTalk, Nov 2009
New Lake Mead intake for Las Vegas - TunnelTalk, Sept 2006

           

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