Lowering the Site 11 Feet Reduced Shoring Design and Expedited the Schedule
The Iowa Department of Transportation selected Hawkins Construction Company to replace the Eastbound I-80 highway bridge in Council Bluffs, Iowa. The new steel girder bridge was planned to be constructed adjacent to the existing eastbound bridge. The new bridge piers have footings situated in a soft clay stratum with driven H-pile foundations that extend through an underlying course alluvium and into shale bedrock. Given the flow within Mosquito Creek and concern for scour, one of the new bridge piers extends almost 27-ft. below grade. Since the work zone is adjacent to the creek and the terminal depth of the planned excavation was below the groundwater table an impermeable excavation support system was required. Due to the relatively soft clay soils at the site and high groundwater conditions, the original shoring design concept shown on the Contract Drawings required the installation of the permanent H-piles foundations in the wet and placement of a concrete tremie seal to dewater the excavation.
Brierley Associates was retained by Hawkins after recognizing an alternative shoring design might be viable. After much consideration, Hawkins and Brierley determined the site could be lowered 11-ft prior to installation of the sheet pile walls with only a minor increase in risk of flooding over the duration of the pier construction.
Removing the overburden and lowering the top of the shoring walls significantly reduced: loading on the excavation support, size and length of the sheeting, and number of internal brace levels. With the shallower excavation depth, Brierley determined that only a single level of internal bracing was required and could be removed after casting the pier footing to allow unobstructed access for construction of the pier wall, eliminating the need for additional construction joints or penetrations. Lowering the top of the excavation decreased the design hydrostatic loading, eliminating the need for a concrete tremie seal to dewater the excavation, and further eliminating the need to drive piles with a follower. Brierley and Hawkins collaboration led to reduction of overall construction schedule by approximately 1.5 months and over $320,000 in bid item savings in comparison to other bidders. Brierley will also be retained for the second phase of I-80 Westbound, in Council Bluffs, IA, by Hawkins Construction for a similar excavation and dewatering scope.
To discuss constructability or alternative analysis of shoring designs, contact Russ Lutch at rlutch@brierleyassociates.com.