Nisenan Bridge is a 150-foot-long cable suspension bridge spanning Deer Creek as part of the Deer Creek Tribute Trail in historic Nevada City, California. NV5 provided geotechnical engineering and construction materials testing services in support of this trail improvement construction project.
During the geotechnical investigation, NV5 observed and recorded the subsurface soil and rock conditions revealed by excavation at the abutment locations. Accessing the site for subsurface exploration was challenging because the trails leading to the site were approximately 2 to 3 feet wide – equivalent to a single track running and bicycle trail – and all subsurface work had to be performed by hand. Due to access restrictions, foundation design criteria were based on observation of shallow subsurface soil and rock conditions. To verify the foundation design criteria, we performed pull testing of rock anchors during construction of the bridge abutments. Additionally, NV5 worked with a local surveyor to prepare a topographic map of the proposed bridge alignment. The survey referenced nearby property boundary markers and included the proposed abutment locations and elevations, creek bed and water surface elevations, and the location of prominent site features pertinent to bridge design and construction. We also worked with the bridge designer to design footing and tower anchors; four were installed as soil anchors through a concrete abutment and two were installed as rock anchors for the tower supports. Anchor loads were tested to 105,000 pounds and locked off to 80,000 pounds. We observed bolt installation and also load tested the anchor bolts to 1.33% of the design loads.
NV5 also provided observation of concrete and reinforcing steel as part of the construction materials testing. A technician documented the concrete mix design and observed rebar placement. We prepared and tested two sets of concrete cylinders for compressive strength testing, including the observation of rock anchor installation and pull testing for rock anchors. We then installed six rock anchors at each abutment. Since vehicle access to the abutments were not feasible, bridge construction and transportation of pull testing equipment was performed by ropeway winch system, transporting materials across the creek.