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1 St. John s Tunnel Rehabilitation A Demonstration of Successful Fast- Track Design and Construction Christopher P. Russell, P.E. Shannon & Wilson Inc., Staff Engineer Gregory R. Fischer, Ph.D., P.E. Shannon & Wilson, Inc., Senior Vice President Ronald J. Sanchez, P.E. Colorado Springs Utilities, Managing Engineer Jackie M. Chambers Colorado Springs Utilities, Project Leader Presentation Outline Site Description and Tunnel Conditions Tunnel Construction & Rehabilitation History Project Purpose Conclusions 1

2 Site Description & Tunnel Conditions Site Description & Tunnel Conditions Located at 11,000 feet, access via several miles of winding dirt roads. 2,200 feet long, 6.5 feet x 6 feet on average. Houses 30-inch diameter steel pipe for water conveyance. Very difficult access. 2

3 Site Description & Tunnel Conditions Mostly unlined and mined through Pike s Peak Granite. Timber-lined section from Sta to Site Description & Tunnel Conditions Access from the west portal through 80-foot lined vertical shaft. Shotcrete and steel set supports from Sta to

4 Site Description & Tunnel Conditions Access at east portal at ground level. East portal lined with reinforced concrete box culvert sections and steel sets. Tunnel Construction & Rehabilitation Tunnel originally excavated from 1903 to Pipeline installed in the 1930s to provide water flow control following a collapse near east portal. East portal, west shaft, and western 80 feet of tunnel rehabilitated from 1992 to 1993 to repair collapsed zones. 4

5 Tunnel Construction & Rehabilitation 1992 and 1993 rehabilitation work. Tunnel Construction & Rehabilitation Design-Build project initiated in 2003 to reduce rockfall hazard in the tunnel. Shannon & Wilson recommended installing rock bolts in several unlined sections of the tunnel, and replacing the timber-lined section of the tunnel. Limited funding, spread mitigation over maximum tunnel length possible (rock bolting). Installed mechanical rock bolts through 850 linear feet of the tunnel. 5

6 Project Purpose Colorado Springs Utilities obtained additional funding in the summer of Decided to complete rehabilitation work in the tunnel that was recommended in Replace timber-lined section of the tunnel with steel sets, backfill behind steel sets with concrete. Install rock bolts through additional 240 linear feet of tunnel (4-foot long epoxy-grouted #7 bars). Project had to be complete so money could be spent by end of fiscal year (December 20). No time for invasive subsurface investigation behind the timber sets. One-day reconnaissance to verify that original recommendations were still valid. 6

7 High angle joints, tight, rough. Shear Zone ¼ to 2-inch wide, slicken-sided, clay infilling. Rotted timber sets. Rock altered to soil-like material. 7

8 Rock Mass Rating using the Q-system (Barton & Lunde, 1974) suggested rock mass was very poor. Modeled loading conditions using method by Proctor & White (1988) for tunneling through crushed rock. Verified loading conditions by calculating loads that would result in failure of the timber sets. Loads of 1.2 ksf estimated: W4x13 steel sets bent at 2.5-foot radius. Concrete backfill optional and dependent on funding. To save time, Colorado Springs Utilities opted to procure and galvanize the steel sets before project went to bid. To address uncertainty in how ground would respond during timber set removal, specifications written to reduce likelihood of running ground and collapse: - Probe holes - Install temporary spiling if needed - Removal of only one timber set at a time - Fully block steel sets with wood cribbing 8

9 Contract awarded to Drill Tech Drilling & Shoring out of Antioch, California. Tight schedule of design phase did not allow construction to start until November 17. Adverse weather conditions prevented use of east portal, all materials lowered through west shaft. Equipment and materials transported through tunnel on wheeled cart with very difficult access. 9

10 Due to potentially unstable rock, Contractor required to install rock dowels in areas between west shaft and timber-lined section first. Rock dowel installation and proof testing complete by Dec. 1; then steel set erection and timber set replacement began. 10

11 Once first two steel sets were installed, fully lagged, and blocked, demolition of the timber sets began. Contractor carefully removed one set at a time, probing the roof and wall in front to evaluate ground conditions and determine need for spiling. 11

12 Each steel set was fully lagged and supported with wood blocking to transfer roof loads before Contractor was permitted to remove the next timber set. Shear Zone Shannon & Wilson staff mapped the rock mass conditions behind the timber sets as they were removed to verify design assumptions and need for spiling. 12

13 Constant communication between the Contractor, Design Engineer, and Owner was critical during construction. Once determination was made that spiling was not needed, the remaining schedule and costsavings allowed Springs Utilities to move forward with backfilling behind the steel sets with concrete. Contractor installed bulkhead formwork for the concrete pour by December

14 Weep holes for drainage and grout ports installed during steel set erection. Grout mix consisted of 500 psi, sand, cement, and fly ash mixture. Contractor installed PVC pipe into roof grout ports to verify required 3 feet of backfill above the steel sets. 14

15 Final walk-through completed December 21. Project completed on time and within budget. Conclusions Project completed on time and within budget. Work completed safely and in accordance with Contract Drawings and Specifications with no change orders. Communication between the designer, contractor, and owner throughout the project was key to success, maintaining schedule, and saving money. This rehabilitation project demonstrates that challenging tunnel projects can be completed using fast-track design and construction. 15