When you are faced with a project with limited access with the easiest material access is by helicopter, nearest personnel access is 5 miles (8 km) away, and the closest daylight to the project is a half mile away, the answer to handle all of these issues could be shotcrete. The Poe Tunnel is a 15 mile (24 km) long tunnel in foothills of the Sierra Nevada Mountains and is in an area of steep canyons. The tunnel transports water horizontally from a forebay on the North Fork of the American River to the Poe Powerhouse from which electrical power is generated. The tunnel is almost 20 ft (6 m) diameter and was constructed in the 1950's.
The owner, Pacific Gas & Electric, PG&E, will regularly inspect their tunnel and during the 2016 inspection there were several cracks in the tunnel shotcrete that was cause of concern. In 2016 the General Contractor performed some emergency repairs while a permanent fix was designed and approved. Dees Hennessey, Inc, DHI, was contracted by the General Contractor, Syblon Reid, SRCO, to perform the shotcrete operation for the final repair work. The scope consisted of a new lining of the tunnel for a length of 90 ft (27.5 m). Once the SRCO was able to mobilize on the project for the 2019 season it was decided to increase the scope and perform a shotcrete tunnel lining for 180 lf (55.0 m) of tunnel. This was decided upon two weeks before the shotcrete was scheduled to begin. The tunnel design thickness was 10 in (0.25 m) thick not including overbreak and other repair work. The final shotcrete volume installed was over 500 cy (380 m3).
The difficulties to overcome on the project was access to the site. The location of shotcrete work was 2,300 lf (700 m) from the nearest adit into the tunnel. Also from the adit there was no access to that location by vehicles or pedestrians due to the canyon, a large river at the bottom of the canyon, and a very active railroad in between. The nearest concrete plant was two hours.
We devised a plan that the shotcrete pumps and shotcrete material would be flown by helicopter across the canyon and then the shotcrete material would be pumped 2.300 lf (700 m) to the work location. One of the balancing factors to work with on the project was the weight that a helicopter can carry. It was determined that the most economical and functional way to transport the shotcrete was to use a helicopter that could carry around 0.5 cy (0.4 m3) per flight across the canyon. This provided a good balance with providing shotcrete to the shotcrete pump without overloading it while also emptying the truck within an acceptable duration. It also worked out that the helicopter would have to land to refuel after every other truck was emptied. The crew also took a 5 mi (8 km) buggy ride each day from the nearest portal to and from the work location each day.
We used a highly retarded shotcrete mix to account for the batching and delivery time, the transportation time of the helicopter, and the pumping of the shotcrete mix. We also used a high slump on the mix so that the shotcrete could be pumped 2,300 lf (700 m) from outside the adit to the work location. Once we got to the overhead portion of the shotcrete installation we used Mapequick AFK 888 shotcrete accelerator to rapidly get the shotcrete to setup.
The greatest advantage that shotcrete had for the project was in its flexibility. It provided flexibility in the work sequence, flexibility in the scope of the work, and flexibility being able to stop and starts as needed due to external factors. By using shotcrete all formwork was eliminated and the work sequence could be modified anytime the circumstances changes. This project was done during the winter months so keeping an eye on the weather was always essential and at times the helicopter would fly up to the point that the rain and/or wind would start. Thus using shotcrete the location of construction joints could be moved easily and restarted the next day or the conditions changed. Another example of this is that it was not clear in the drawings that the engineer wanted the invert of the tunnel to be shot in horizontal strips along the tunnel up to the spring line and for the spring line through the overhead portion of shotcrete the tunnel needed to be completed all the way across (remember this is a 20 ft (6 m) diameter tunnel). Because of the flexibility of shotcrete we were able to modify our pour sequences to match the engineers needs with no additional costs or schedule constraints. Because of the flexibility of shotcrete, elimination of formwork, doubling of scope, weather, and project resequencing the project was brought in under the original timeline for the original scope of only 90 lf (27.5 m).
The Poe Tunnel Project was a very unique project with unique problems that shotcrete was able to provide unique solutions. There were many challenges to overcome but with proper planning and execution shotcrete was able to provide the answer to each of the challenges that occurred.
Author
Jason Myers graduated from California Polytechnic University at San Luis Obispo in 1995 with a Bachelor’s Degree in Civil Engineering and from Golden Gate University in 2015 with a Master’s In Business Administration with an emphasis in Project Management. Jason started out his professional career working for an earth retention subcontractor where he learned the importance of budgeting, scheduling, and client relationships. Also during this time he was introduced to the use of shotcrete and its applications. After working for a General Contractor for a couple of years he realized that he enjoyed the tighter knit of working for a subcontractor and the ability to construct projects on a tighter time frame with several going at once. Jason also enjoys the process of handling most of the procedures that go into constructing a project rather then seeing only a small portion of the process. Jason joined Dees Hennessey in 2004 and has been a part owner of the company since 2007. Jason currently serves as the Vice President of Operations as well as the safety director.
