SR-509, Port of Tacoma Road Interchange
Fife, Washington

PanGEO, Inc. recently completed value-engineering services for Balfour Beatty Construction, Inc. that resulted in $600,000 cost saving for the SR-509 Port of Tacoma Road Interchange project. PanGEO's knowledge of the cost reduction incentive proposal (CRIP) process of the Washington State Department of Transportation (WSDOT) helped expedite the review and approval of the proposal. Our services included optimizing the geometry of the expanded polystyrene (EPS) bridge abutment backfills to reduce the overall project cost.

The project site is underlain by complex subsurface conditions including compressible soft silts and organic soils, and liquefiable sediments. Timely completion of construction was of vital importance to WSDOT to reduce congestion and improve freight mobility in this industrialized area. The short construction window posed geotechnical challenges related to the potential delays associated with the settlement of the soft foundation soils under the weight of the approach fills. To expedite the schedule, WSDOT elected to take advantage of the extremely low unit weight of EPS blocks to construct a lightweight backfill behind the bridge abutments to reduce the total settlement of the embankment fills and mitigate the potential for high downdrag forces on the abutment piling.

Through the WSDOT CRIP process, a concept was developed to reduce the quantity of EPS blocks by using site-specific embankment settlement data from several approach fills that had already been constructed with settlement monitoring instruments. The settlement data collected from the site was used by PanGEO to refine estimates of settlement and downdrag potential for the bridge abutment piling. The analysis concluded that the less costly conventional granular fills could be moved closer to the abutments thereby reducing the required quantity of EPS and reducing construction costs.

The quantity of EPS was further reduced by incorporating a steep geogrid reinforced slope at the transition from the EPS abutment backfill to conventional granular approach fill. This reinforced slope was designed in accordance with WSDOT and AASHTO requirements for mechanically stabilized earth structures. To reduce the potential effects of downdrag on the piling, a second lightweight fill material was recommended for backfill of the geogrid reinforced slopes. This material was a by-product of coal-fired power plants called bottom ash. Bottom ash has a unit weight (density) about two-thirds that of ordinary gravel borrow.

The steepened transition allowed further cost savings by decreasing the number of pile-supported, pre-cast concrete panels specified for facing of the EPS backfill.