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Project Solutions

Big Sky

  • Machine Type Small Boring Unit (SBU)
  • Diameters 30 in (760 mm)
  • Tunnel Type Water Pipeline
  • Tunnel Lengths 216 ft (66 m)
  • Owner The Yellowstone Club
  • Contractor Tunnel Systems Inc.
  • Location Big Sky, Montana, USA

Project Overview

The Yellowstone Club, a private resort in Big Sky Montana, includes an 18-hole championship mountain golf course in addition to miles of ski trails. This golf course is irrigated by 318 ft (96.9 m) of pipeline from a nearby 79 million gallon reservoir.

In 2005, the project owner contracted Tunnel Systems Inc. to bore the pipeline. The contractor started out with an Auger Boring Machine with a Christmas tree’ head attachment. However, they ran into problems 59 feet (18 m) into the dig when they began boring through hard rock. The next two days of the bore cleared only 16 feet (4.9 m). For the rest of the dig, Tunnel Systems Inc. leased a Robbins 30 in (762 mm) diameter Small Boring Unit (SBU).


The section of pipeline through hard rock contained mixed ground conditions including sections of solid rock and mixed rock with soil.


Tunnel Systems elected to utilize a Robbins SBU because they are designed for bores just like this project — in rock with an Unconfined Compressive Strength (UCS) greater than 24,000 kPa (3.5 ksi). They utilized an SBU without stabilizer feet, available on 30 in (762 mm) and 24 in (609 mm) models. Stabilizer feet are standard on all SBU models 36 in (914 mm) in diameter and above, as well as available on some 30 in (762 mm) models. The SBU featured Robbins’ patented disc cutters to obtain the highest advance rates in hard rock. The SBU’s design is based on the same technology as the large-diameter tunnel boring machines.

Tunnel Excavation

At the jobsite, the SBU was welded to the lead pipe casing. In order for the boring to begin, the SBU received thrust from the pipe casing and torque from the Auger Boring Machine (ABM). Muck was then removed through the auger.

Upon restarting the dig, the SBU achieved impressive advance rates of 43-49 ft (13.1-14.9 m) per day. The ABM and SBU bored through solid rock for approximately 197 ft (60 m) and bored through mixed rock and soil for the final 20 ft (6 m).

The SBU finished the project on time despite some setbacks. Harsh weather conditions on the job site of 19 degrees Fahrenheit (-7 degrees Celsius) and 40 mph (65 km/h) winds made it too difficult to work and the bore was forced to halt until the snowstorm passed. The crew completed the project in just a few hours on the following day.