Cave Buttes Dam Modification Project – Auxiliary Outlet Tunnel Construction Services CMAR
Location: Phoenix, AZ
Method: Tunnel Boring Machine
Description: The Cave Buttes Dam project represented not only SSC’s largest project undertaking but also the first time it was part of a joint venture partnership, as well as the lead on a CMAR. SSC partnered with Coffman Specialties to form a joint venture and was awarded the CMAR contract for this project that consisted of SSC building a 932’ under Dike 1 of the Cave Buttes Dam in northwest Phoenix to install a 60” steel pipe that would serve as a secondary outlet for water impoundment at the dam.
Our JV partner, Coffman Specialties, performed earthwork to construct an inlet channel at the upstream end of the pipe and an impact basin at the downstream end, along with building an intake structure with a hydraulic gate and an outlet structure.
The tunnel was constructed using a 68” tunnel boring machine and disc cutter head. 3 Intermediary Jacking Stations were installed at points along the tunnel to help with jacking the pipe if needed but ended up not being used. The pipe selected for installation was Permalok steel pipe, it provided the advantage of quick joint assembly along with meeting the requirements to be the final drainage pipe as no carrier line would be installed.
Soil conditions varied greatly along the length of the bore from soft, fractured rock to clay to extremely hard, solid rock – ground water was even encountered – not common in the desert! Due to the range of soil conditions, production rates also fluctuated from inches per shift to up to 40’ per shift.
After the tunnel was complete, an extensive grouting operation took place to fill the void between the pipe and the earth – the excavation diameter was 68” and the pipe installed was 60” – in addition to voids that were created during the tunnel operation. Keller was hired as a subcontractor to help with the operation to pump an extremely flowable mix that would penetrate all voids. Pumping was done in several stages to ensure the pipe did not float, and monitoring was also used to ensure that the pipe did not float.
The overall project length was approximately 18 months, with the tunnel and grouting taking about 7 months of that time. The entire project team worked extremely well together and the outcome was successful for all, including the key stakeholders of the Maricopa County Flood Control District and the US Army Corps of Engineers.