When tens of thousands of visitors descend on the 4.6 million square foot Las Vegas Convention Center, it can take 45 minutes to walk across the property. Now it can be a mere two minute ride below ground.
The project consisted of two tunnels weaving through the Convention Center Campus connecting the new main hall with the existing exhibit halls. The alignment is approximately 4,000 lineal feet. Each tunnel is 13.5-foot O.D. (12-foot I.D.) wide and located at a depth between approximately 20-feet below ground surface (bgs) and 35-feet bgs. The tunnels were constructed using precast concrete segments. Six of the concrete segments were configured to form one 5-foot-long tunnel “ring.” The segments were reinforced with steel fibers, along with monofilament polypropylene. The crown (top) and invert (bottom) of the tunnel are no shallower than 20 feet and 35 feet below the land surface, respectively.
The tunnel included three access shafts along the alignment. Each access shaft covered an area of approximately 200-feet by 80-feet and extended to a depth of up to approximately 60 feet below existing grade (bgs). Stations were constructed using common excavation and shoring systems in several phases. Stations were initially built using temporary support of excavation such as secant pile walls or sheet pile walls prior to and/or during tunneling, and final station structural supports were designed after tunnel construction is completed. Permanent waterproofing was installed inside the stations.
COMPANY SCOPE
GES provided a geotechnical data report and a design level geotechnical interpretive report for each station and each phase of the project. Prior to this project, GES drilled over 100 borings for the Convention Center Phase 2 Expansion and utilized much of that previously obtained information for this project. GES installed several monitoring wells at the convention center site and along the tunnel alignment. Slug testing and groundwater analyses were performed to evaluate flow rate and develop temporary dewatering recommendations.
Two-dimensional shear wave velocity testing was performed generally along the entire 4,000-foot tunnel alignment. A 200 to 600-foot line was used with geophones spaced every 20 to 25 feet. The data was collected using ambient noise from the area. Pressuremeter testing (PMT) was performed during mud rotary drilling in borings B-3 and B-4. The purpose of PMT is to determine the modulus (stiffness) and material strength properties of the tested soils. Testing was performed at three intervals in two borings to a depth of 60 feet. In general, Eagle Drilling prepared a test pocket of 5-foot length using mud rotary techniques. GES, through Eagle Drilling, attempted to core cemented zones encountered during drilling. Seven core runs were attempted with mixed results.
