Inspection and rehabilitation of the Daly-Judge Tunnel, Park City, Utah

CIM Bulletin, Vol. 1, No. 4, 2006
M.G. Nelson
The file is a zipped PDF document.The Park City mining district has produced about $5 billion in polymetallic ores. Mining in the claims owned by the Daly-Judge Mining Co. began in 1881. In 1885, the Anchor shaft was begun, but heavy water flow halted work in 1887, and a drainage tunnel was started. Its original length was 2,011 m, and it was timbered with a standard, four-piece tunnel set, with the sills hitched into the ribs. The tunnel was eventually extended to 3,292 m, and became known as the Daly-Judge Tunnel. It passes near two shafts and through two major faults. The tunnel requires maintenance because it provides part of the potable water supply for several users in the area. The water quality is excellent, but roof falls can increase particulate content so that the water must be diverted. In August 2001, it was decided to rehabilitate the tunnel so that its contribution to the area’s water supply during the 2002 Olympic Winter Games would be uninterrupted. Previous tunnel maintenance included replacement of some of the original timber sets with steel. Timber lagging was used on the back and ribs between steel sets. All of the steel sets were in good condition, and no indications of failure were observed. Four types of failure were observed in the old timber sets: 1) failure of posts in compression; 2) failure of cap timbers in shear; 3) failure of posts in shear; and 4) failure of lagging in shear. The failure in post and cap timbers occurred when the dry rot in the timbers was severe.It was concluded that the tunnel could best be rehabilitated by installing steel sets similar to those previously used. However, to ensure that the rehabilitation work would be long-lasting, the maximum loads taken by the timber sets (in failure) were calculated and compared with the safety ratings for the steel sets.The table shows design values for steel beams and failure values for timbers in analogous positions. The design values for steel are considerably smaller than the failure values for timber. However, the observed timber failures included dry rot, and the failure loads were estimated to be two to three times less than those calculated. Further, the standard design values for steel are four to ten times less than the loads at failure. Thus, the design for the steel sets was deemed adequate, particularly because it allowed for the use of any one of three beam sizes, depending on ground conditions.A contract mining company was hired, and deployed two crews. The day crew fabricated steel and installed new lagging; the night crew installed steel sets. Where the timber sets were visibly loaded, a steel set was installed immediately adjacent, so that the timbers could be removed after the load had transferred to the steel set. Twenty-five timber sets were removed, 86 steel sets were installed, and lagging was installed or replaced over about 229 m of tunnel length.Conclusions

Because of dry rot, tunnel sets made of untreated timber are unsuitable for long-term use in dry climates.
Timber tunnel sets can conveniently be replaced with sets made of steel I-beams. Wide-flange beams are recommended; the size of those beams will normally be 6 x 5, 6 x 20, or 6 x 25, and should be determined by conditions at each location. (These sizes are nominal dimensions in inches, equivalent to 150 x 375 mm, 150 x 500 mm, or 150 x 625 mm.)
Timber lagging is recommended for the sides and back in areas supported by steel sets, to allow inspection and removal of loose material when required.
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