Battling the Elements with Helicopters at Elevation 11, 160 feet

Condon-Johnson & Associates – Ty Jahn, P.E.

The current tram at Big Sky Resorts was built in 1995 and could carry 850 people per day to the top of Lone Peak Mountain in Big Sky, Montana. A unique rock feature is present where this tram was constructed called a “rock glacier”. A rock glacier is a distinctive geomorphological landform, consisting either of angular rock debris frozen in interstitial ice, former “true” glaciers overlain by a layer of talus, or something in-between. The rock glacier present at Big Sky is a former glacier which has been overlain by a very thick layer of talus on which the old tram was built on top of. Over 27 years the bottom tram station has moved downhill 16 feet and has tilted several degrees. In order to serve the next generation of skiers and extend the tram services to summertime adventure a new state-of-the-art tram is being constructed which can transport 80 people at a time in single tram car to the peak. The new tram will be the flagship of the most technologically advanced lift system in North America.

The new tram will consist of three structures; a top station, tower, and bottom station. This line will span almost one mile long with a single tower near the top of the mountain to support the line to get the tram cars over a ridge before reaching the top of the mountain at an elevation of 11,260 feet. At that elevation the top of the mountain has snow on it 10 months of the year, the ground is permanently frozen, storms can roll in with little to no warning, and the oxygen levels are less than 13.7% versus 20.9% at sea-level. Additionally, there are not roads which lead to the top of the mountain which means all materials and concrete have to be flown to the tower and top station locations via helicopter.

Condon-Johnson & Associates (CJA) was hired as the general contractor to build the top station and tower foundations which consists of micropiles supporting the axial loading of the structures with 300-kip tiebacks installed to support the top station from rotation and the tower legs from sliding off the mountain. All access to the tower location was with a helicopter or taking the tram to the top of the mountain then a descent off the top of the mountain while repelling down 1,000 feet of rope.

The top station and tower locations were required CJA to perform the following in order to install

·???????over 2,000 yards of rock excavation, blast hole drilling, and blasting to occur to get down to grade and for tram cabin clearance

·???????35 each 242 kip micropiles and 8 each 253 kip tiebacks installed via limited access drill rig which can be taken apart to be flown and moved around the mountain with a helicopter

·???????6 tons of rebar and over 800 cubic yards of concrete flow to the top of the mountain in ? and ? yard buckets

·???????Underpinning of existing tram structure to allow blasting to occur within feet of it

CJA had to come up with innovative solutions to numerous difficulties during the installation. To name of few of the challenges four different helicopter companies were used due to the size of different loads, seasonal storms, diesel equipment operating at 11,160 feet, drilling micropiles in completely frozen ground with ice layers and dealing with grout setup in freezing conditions, working off rappelling ropes, threat of wildfires, massive amounts of snow removal to start and during work, working on a talus cliff with vertical faces, and more.