It should come as no surprise that there is a limited supply of water in the deserts of the American southwest, specifically in the Las Vegas Valley. The climate and geography in this part of the Mojave Desert attracts about 4in. of rain/year, and an average of 5200 new residents/month. Las Vegas is the fastest growing city in the country, and the population in the valley is expected to double in the next 20 years to 2.3m. It is already putting noticeable demands on the existing water delivery system.
The Southern Nevada Water Authority (SNWA) is the cooperative state agency formed to manage the area’s water resources and address the present and anticipated water needs in southern Nevada. The capacity of the present water treatment and delivery system is 400m gals/day (mgd). The new delivery system and treatment system, of which this project is a part, will increase that capacity to 600mgd, and includes approximately 83 miles of large diameter pipeline, 11 pumping stations, an ozone treatment facility, two major tunnels and a new intake lake tap.
The expansion is being phased over a 15 year period and is estimated to cost $2.2bn.
Lake Mead Intake No.2, the second lake tap, is a $76m project, due to be completed in early 2000. The scope of work includes: an intake shaft in Lake Mead; an intake tunnel connecting to a forebay chamber, access shaft and surge chamber; excavation of a pad on Saddle Island for a (follow-on contract) pump station; a bay aqueduct pipeline installed from Saddle Island to the mainland; and a drill+blast connector tunnel (the East Tunnel) through a mountain ridge.
Surge chamber/access shaft
To create a pad for the access shaft construction, the forebay well shaft drilling operation and the future pump station, 230 000 yd³ of rock was excavated from Saddle Island next to the existing pump station by a Caterpillar D-10 ripper and a Hitachi EX1800 hydraulic backhoe excavator. All material was loaded out on Caterpillar 777 haul trucks.
The next stage was construction of the surge chamber access adit and the surge chamber itself. The adit portals in at the base of the line drilled highwall next to the pad and curves downwards on an 18% grade. It forms a 120ft long, 12ft x 12ft horseshoe tunnel, with 10ft grouted CT rockbolts and 4in. of shotcrete in the crown and quarter arch for final support.
The surge chamber is a 150ft long x 20ft wide x 16ft high flat arched tunnel that starts at the adit and travels horizontally to intersect the access shaft at the other end. An Atlas Copco hydraulic drill jumbo was used to drill 10ft drift rounds, with emulsion cartridges in both production and trim holes to give stable, smooth wall results. Rock support installed was similar to that in the access adit. Muck was removed with a 3.5 yd³ Wagner Scooptram. The adit and surge chamber work took approximately 19 weeks.
The access shaft was excavated to a 26ft diameter and lined with 4000psi concrete to its finished diameter of 24ft. It is 380ft deep, drilled and blasted in 10ft deep rounds using an Atlas Copco pneumatic 2-boom track drill and an Ingersoll-Rand single-boom track drill.
At the bottom of the access shaft, a temporary sump pumping station was installed to handle up to 2000 gal/min, the anticipated maximum short-term inflows, and two headings were turned under at 180°. The forebay access and forebay are immediately adjacent to and west of the access shaft. In the opposite direction, the intake tunnel extends east from the access shaft to connect the intake shaft on the east side of Saddle Island with the forebay.
Intake tunnel
The intake tunnel is a 1600ft long x 14ft wide x 16ft high horseshoe tunnel. The drill+blast cycle is accomplished using an Atlas Copco 2-boom hydraulic drill jumbo on a full face, 10ft deep round, drilling 60 x 17¼8in. holes and a burn cut. The holes are loaded with cartridge emulsion and typically average 8ft of advance/round. Ground support consists of fully grouted Ingersoll-Rand CT rockbolts (8-12ft lengths), with additional spiling and straps as needed, and 4in. of fibre reinforced shotcrete applied with a shotcrete robot. The flat invert is paved as the tunnel advances, with a ditch left to one side for drainage and final piping installation.
Due to the additional depth of this tunnel and a lake level that is 60ft higher, the present tunnel is being constructed under nearly double the hydrostatic head of the previous construction. The intention for this project was to grout ahead of excavation to achieve a final inflow rate in the range of 500 gal/min. Consequently, water control grouting measures were stipulated in the contract and have become the critical determinant of progress in the tunnel.
A pattern of five primary and five secondary holes are drilled and grouted 80ft in advance of face excavation. Tertiary and quaternary holes are drilled as needed to control heavier inflows. The holes are grouted to refusal in stages using ultrafine cement at 600 psi. The intake tunnel encountered inflows of 600 gal/min in the localised area of Sta 8+95, when the tunnel broke out of the side of the grout cover. The recovery programme required an additional 70 grout holes and 40 days’ delay, but successfully reduced the water inflows in that area to 100 gal/min.
Once the intake tunnel excavation exposes the lower end of the intake shaft, the intake liner will be drained of its ballast water. An elbow connection will be made and finished with shotcrete, and crews will pull out of the tunnel in the final clean-up operation, expected in January 2000.
Intake shaft
The ultimate goal for this unique part of the project was to excavate a 70ft deep shaft under 240ft of water in Lake Mead, then to install and grout in a 150 ton, 12ft i.d. x 70ft long steel liner. This task has been completed within budget and the liner now sits within 6in. of horizontal design CL and 1in. of level across its 12ft diameter. The success of this part of the job is largely due to the consistently high level of teamwork and aggressive problem solving on the part of all involved: Kiewit Engineering Co. (design of fabricated steel structures, as well as grouting and controlled ballasting operations); Saddleback Drilling; Norwesco Marine (dving sub-contractor, ROV and sonar operation); and Parsons Engineering (contract manager).
The work was performed over approximately ten months using the same general construction concept as that of the Lake Roosevelt lake tap in the early 1990s, which provided useful experience for improvements and cost saving measures at Lake Mead.
All intake shaft work was performed from a flexifloat barge anchored over the shaft excavation site, with a smaller dive barge tied alongside. The first of the two work barges was a 125ft x 60ft platform supporting a Manitowoc 4600 dredging crane and was anchored in a series of locations that allowed the underwater slope to be scaled and a level bench to be partially excavated. Loose material and colluvium were easily removed, but sound rock was located much sooner than expected, and it became obvious that a clam bucket was not the correct tool for excavating rock at a 1:1 slope. The dredge barge was moved, a pre-designed bench drilling template arrived and the second flexifloat barge was anchored over the site.
This second barge was a 130ft x 90ft platform with a moon pool at one end through which all drilling and installation operations were performed. A Manitowoc 4100 S2 crane with support equipment was used for the excavation, and was big enough to handle the various picks anticipated for the work see Table 1).
In general, the drill barge was configured for a reverse circulation drill operation. Three 750 cfm compressors powered an Ingersoll-Rand QL120 downhole hammer with a 17.5in. bit. Due to the poor quality of the rock, the bench and shaft were excavated according to the ‘swiss-cheese’ concept: a closely spaced pattern of holes was drilled out, then the rubble created from the inter-hole columns was steadily hammered and airlifted out.
Shaft excavation
The drill stem was located at any position within the 30in. square moon pool by operating the kelly bar through a moveable turntable. Once shaft excavation began, the barge was anchored so that the centre of the moon pool was aligned over the design centreline of the shaft on the lake floor, and the drill bit could be located anywhere necessary for shaft excavation. Various templates were designed for temporary and permanent use on the lake floor to guide the drill bit at the end of a 240ft drill stem hanging below the barge.
A critical factor in the smooth completion of all underwater tasks was the use of divers, and a remote operated vehicle (ROV). Norwesco Marine provided personnel experienced in both drilling, and piloting the ROV. This impressive piece of technology operates on an 1100ft umbilical, and carries a video camera, depth sensor and sonar equipment. The real-time video images allowed the efficient and safe operation of equipment, and the placement of large steel structures at a site 240ft away from the work platform and crew.
In a series of overlapping bench template placements, a 50in. x 50in. level pad was drilled out in the side of the island to elevation 980. This template was then scrapped. A two-sided, 30in. high rock protection fence was installed on the uphill side of the bench to protect the work and the divers from potential rockslides. The permanent template was then placed on the bench, levelled with hydraulic jacks and anchored with pin piles drilled and grouted in each of the corners. Secant piles were installed to support the excavation in a ring through the permanent template using the Concentrix system.
Finally, shaft excavation began through the permanent template in a series of three lifts. The first lift was 25ft deep x 22ft diameter. A steel casing was installed in this hole, hanging from the permanent template on gussets sized to hold it perfectly plumb and level while it was grouted in. Two further lifts totalling 45ft x 14ft diameter were excavated within and below the starter casing.
During this time, the intake shaft liner, fabricated in two pieces by Ameron International, was assembled onshore. When this operation was complete, the entire assembly (basically a hollow can) was picked from the assembly jetty into the water. It was moored in the bay, floating horizontally. When excavation finished, a tugboat pulled the liner to the drill barge, where it was manoeuvered into position in the moon pool. In a carefully controlled ballasting operation, the liner was upended as ballast water was added to the point of neutral buoyancy.
Then, just enough water was added to induce a controlled descent from the moon pool into the excavated shaft, where the support flange of the liner came to rest on the upper rim of the starter casing. As with the starter casing, the liner was tremie grouted in two lifts using a high-strength sanded grout mix containing retarder, super-plasticiser and an anti-washout agent specified in the contract. When work in the intake tunnel exposes the lower steel cap of the liner, the ballast water will be drained through pre-installed valves in the cap, and this lower cap will be removed. The elbow connection will then be completed, the tunnel and shafts flooded, and divers will remove the upper hemispherical cap and replace it with a flared inlet structure.
East Tunnel
The East Tunnel is approximately 2 miles west of the main body of the project on Saddle Island. It is a horseshoe-shaped tunnel, 2576ft long x 12ft. wide x 13ft high, driven at a grade of +5.47% through a dry volcanic ridge, which is well above the water level in Lake Mead. This drill+blast tunnel with installed steel pipe will eventually connect two other sections of overland pipe to tie into the present water distribution system.
Standard drill+blast operations started on a single shift basis on June 1 1998, with a second shift added on July 27. The typical 9ft round consisted of 48 x 1 7/8in. holes drilled with a Tamrock HS-205D 2-boom hydraulic jumbo. Holes were loaded with cartridge emulsion supplied by Austin Powder, and fired with Nonel LP downhole delays. Two rounds/shift were shot and excavated by crews working two shifts, 16h/day, using two Wagner 3.5yd³. Scooptrams. Breakthrough occurred on October 26 1998.
Following the drill+blast excavation, a 109ft i.d. welded steel liner was installed in the tunnel and grouted in with cellular concrete. The steel pipe, manufactured by Ameron International, arrived on site in 40ft sections, each typically weighing 36 200lb, and was installed in the tunnel. To support the pipe in place, five carbon steel I-beams were welded on the pipe before transport. In the tunnel, the remaining space between the beams and the rock face was bolstered with 6in. x 8in. timbers and hardwood wedges.
The final operation in the East Tunnel was filling the annular space with cellular concrete and non-shrink grout. Anning-Johnson, Illinois, placed 7800yd³ of the approved cellular mix containing Voton foam at a rate of 350yd³/day to minimise flotation forces on the pipe. This material was pumped through 3in. grout ports in the arch, and progress was tracked through 2in. ports in the invert, springline and arch. Non-shrink grout was then pumped into holes drilled through each filled grout port in order to fill any remaining voids between the cellular grout and the rock face.
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