Slope Stabilization Using DYWIDAG Strand Anchors at Sellwood Bridge in Oregon
Built in 1925, the existing Sellwood Bridge over the Willamette River south of Portland, Oregon was very narrow and no longer suitable for heavy traffic. Furthermore, the bridge was not designed to withstand major earthquakes, and the west abutment sits on fill material that caused cracks in the structure. Multnomah County, the owner of the bridge, approved the replacement with a new steel deck arch bridge, the construction of which began in 2012.
The west abutment was prone to landslides and needed stabilization for the duration of the entire time the new bridge was under construction. The purpose of the stabilization project was to prevent any landslide movement larger than 1mm (0.04") per month. The geotechnical engineer chose a combination of concrete shear piles with top grade beams and 3 x 3m (10ft x 10ft) precast concrete anchor blocks, both of which were tied back with long DYWIDAG Strand Anchors. An elaborate system of instrumentation monitoring was also included in the project. Due to the proximity of Willamette River to the installation site, the drilled holes were filled with water, so the anchors were installed through steel casing that was removed as grout was installed.
DSI convinced the engineer that DYNA Force® Elasto-Magnetic Sensors would be a more accurate and cost effective replacement for the specified strain gauges. 56 sensors were installed on anchors at DSI’s warehouse. 6 test anchors with 15 and 23 strands in lengths of up to 43m were manufactured first. The engineer wanted to know exactly how much load would be transferred along the very short 3 and 4.6m (10ft, 15ft) bonded length. 3 of these anchors had five sensors placed at about 1m spaces along the bonded length, and the sixth sensor was located about 1.5m above the bonded length.
Initial test anchors results were inconclusive; consequently DSI produced two additional 26 strand test anchors with 7 sensors on the bonded length and the eighth placed 1.5m above the bonded length. For better sensor protection during installation, 2 temporary anchors were fully encapsulated inside corrugated HDPE sheathing, and the sensor group cables were placed into a smaller plastic tube running along the anchor length. All sensor cables were connected to a readout unit through a multiplexer for easier load monitoring during proof and performance tests. This time, the readouts provided better results that helped the engineer determine the final length of the production strand anchors.
DSI produced 82 anchors with 8 to 27 strands in lengths of up to 52m (170ft). 15 of these anchors had two DYNA Force® Sensors placed on their unbonded length about 1-1.5m below the bearing plate. Experience dictates that is always better to have at least 2 sensors installed per anchor for better control readings. Sensor cables were connected to 15 readout units placed inside waterproof containers together with 24V batteries for power supply. The readout units with storage software automatically record the anchor load several times a day. So far, reading results have not shown any ground movements, thus ensuring a safe continuation of the construction work at the bridge.
The installation of the DYWIDAG Strand Anchors was successfully completed in 2013, and the new bridge is scheduled for completion at the end of 2015.