UTAH 700 SOUTH CAPACITY UPGRADES
History
The MP12A-700 South Capacity Upgrades project provides additional sewer capacity to the northwest quadrant of Salt Lake City. It replaces an existing 5 MGD pump station and will discharge to existing gravity lines with a 7.5 MGD Pump Station and two new 24-inch HDPE force mains discharging to an existing gravity line 2 miles away.
Challenges
Three of the four new tunnels were installed under railroads, while the fourth and longest crossing of 700-FT was under the Bangerter Highway, a State Route in Utah that connects commuters to Salt Lake City International Airport. The purposed crossings were constructed in the Salt Lake Valley, which contains a variety of historic lake bottom deposits. The tunneling soils contained interlayers of clays, silts, and sands of a wide range of densities and stiffnesses. In addition to the soft soils, groundwater in the area was extremely shallow, anywhere from 2 to 8 feet below grade, and it affected the behavior of the soil and its likelihood to be running in pockets.
Solutions
The wet soils combined with the potential for running sands, silts, and clays prompted our design team to choose pipe ramming and microtunneling (MTBM) as the installation methods. For the shorter railroad crossings BTrenchless used a guided pipe ramming method to install the sanitary lines. This method lined up well with the soil conditions and had a very low probability of surface subsidence (a major concern for under railroads). For the 700-FT drive, BTrenchless skinned up our Akkerman SL60 microtunneling machine to match the 72-inch Permalock casing. The MTBM was a suitable choice for this crossing because of it's continuous face support and ability to handle long distances. The combination of these methods proved successful and BTrenchless installed a total of 1,258-FT on line and grade.