Managing Agency: Lane County, Oregon, Public Works Department Primary Contractor: West Coast Contractors, Inc. Primary Consultant: OBEC Consulting Engineers Nominated By: APWA Oregon Chapter Classification: Less than $5 Million Category: Transportation
Sweet Creek Road is located near the community of Mapleton, Oregon, approximately 15 miles east of the Pacific Ocean, and is a vital transportation link between State Route 126 and large tracts of public and private forest lands. This includes 30,000 acres of National Forest System Lands where approximately five million board feet of timber is harvested annually. The road also provides the only all-season access route to 68 rural residences.
The section of road between Mileposts 1.90 and 2.20 has been a constant maintenance challenge for Lane County due to settling pavement, sunken grades and a failing retaining wall which was constructed in the early 1970s. Maintenance issues were compounded by a roadway only 22 feet wide and bordered on either side by a sheer rock bluff and the steep banks of the Siuslaw River. In January 2011, one lane of the roadway sunk severely, resulting in a huge crack in the roadway and making the westbound lane impassable. Access through this section of the roadway was limited to one-way traffic, with Lane County installing traffic signals to control traffic flow.
The Sweet Creek Road Retaining Wall Replacement project was designed to replace the failing retaining wall and reconstruct both lanes of this vital connector for two-way commercial, residential and life-support traffic. Lane County secured a grant from the Federal Highway Administration through the Western Federal Lands Highway Division (WFLHD) to construct two tieback retaining walls connected with a 243-feet pre-stressed concrete bridge. WFLHD committed to fund $5,865,200 with Lane County contributing the remaining portion from its Road Fund. West Coast Contractors, Inc. (WCC) from Coos Bay, Oregon, was awarded the contract with a low bid of $4,753,049 in June 2012.
As referenced earlier, the Sweet Creek Road Retaining Wall Replacement project was initially designed as a 243-foot-long pre-stressed bridge, together with a 906-foot-long tieback retaining wall. The project was completed as a 1,130-foot-long pre-stressed concrete bridge structure and 40 feet of retaining wall. The construction limitations initially imposed by the river, the rock bluff and the narrow and heavily utilized roadway were compounded by soil and bedrock conditions discovered during construction. Eight change orders were issued during construction which resulted in significant modification of the project as it was initially designed. The bridge bents, bent caps and concrete slab plans were developed as the project was being constructed, resulting in “just in time design.”
A number of issues led to the changes, the first being the difficulty in designing a shoring plan that would allow deep excavations for retaining wall construction while maintaining the traffic flow on half of the existing roadway. The shoring plan, as designed, called for the use of steel sheet pile running the length of the deep excavations at three different locations through the existing roadway cross-section.
The contractor proposed a Value Engineering Plan (VEP) to resolve these issues. Based on the VEP, two long sections of the tieback retaining wall were replaced with bridge structures. The original 243-foot bridge was expanded to 740 feet in length and a second bridge of 150 feet was added.