DBA has the great fortune to be working with the Alabama Department of Transportation (ALDOT) on a very interesting bridge project in Lacey’s Spring, Alabama just south of Huntsville, Alabama. On February 12 and 13, 2020 a large landslide occurred on SR-53 (US-231) at milepost 301.7 in Morgan County approximately 1.7 miles south of the Laceys Spring Community. The slide completely severed the 4-lane divided highway which is a major commuting route between Huntsville and several communities south of the city. Many of the workers at the U.S. Army Redstone Arsenal, NASA Marshall Space Flight Center, and the contractors and vendors that support these two major installations live in the communities impacted by the closure of the highway. Detours were established on existing state and county roads, but these added 30 to 60 minutes to commute times, depending on time of day. ALDOT was under significant pressure from the impacted communities to quickly solve the problem and reopen the road.
ALDOT drilling crews were immediately mobilized to the site to begin drilling exploratory borings and install slope inclinometer casings for monitoring slide movements. The Department of Civil Engineering at Auburn University was engaged to perform geophysical testing in conjunction with an existing research project for ALDOT. Geotechnical engineering firm TTL also assisted with field investigation efforts.
DBA and ALDOT immediately began evaluating several alternate concepts for stabilizing the slide and reopening the road during the soil and rock exploratory drilling. The design team looked at several retaining wall options, a complete rebuild of the roadway, and bridges. ALDOT selected a solution that removed most of the existing roadway embankments (built in 1947 and 1970) to reduce loading on the slope and then spanning the slide area with bridges built on the existing road alignments, with the bridges designed to withstand future movements of the slope.
The bridges are two-lane structures, one Northbound and one Southbound, each about 947 ft in length. The superstructure is AASHTO BT-72 concrete girders and a concrete deck. There are seven spans in each bridge each about 135ft long. The grading work was begun while the bridge was still being designed to accelerate the schedule and shorten the time the road would be closed.
The foundations for each pier are a pair of 9.5ft diameter, permanently cased drilled shafts with 9ft diameter rock sockets. The sockets are 14ft long into the limestone and shale bedrock. The limestone uniaxial compressive strengths range from 10,820 psi to 28,100 psi.
After much design and analysis in a highly compressed schedule, a bridge contract was let for bid in early May 2020, less than 3 months after the slide occurred. Brasfield & Gorrie was the successful bidder and awarded a $15 million contract that has incentives for finishing early, and disincentives for finishing late.
A.H. Beck (Beck) is the drilled shaft contractor, drilling each shaft, placing reinforcement, and placing concrete. The permanent casing is 5/8 inch wall thickness spiral weld 60ksi steel. The shafts are reinforced with a 1.5inch wall thickness, 8ft diameter, 60ksi steel pipe. These pipes are rolled and welded.
The pair of shafts for each pier is connected by a reinforced concrete grade beam 10ft wide by 7ft high by 46ft long. To connect the shafts to the grade beam, a 14ft long reinforcement cage is placed in each shaft, embedded 8ft into the shaft with 6ft embedded in the grade beam. The cage consists of 28 No.18 Grade 75 bars.
The project includes a robust instrumentation plan with ShapeArray inclinometers installed in each shaft and in the slope, supplemented by traditional inclinometers in the slope and vibrating piezometers to monitor groundwater levels. DBA and ALDOT will monitor the bridge and slope, intending to be able to measure displacement and calculate strain and loads in the shafts should the slope move again in the future.
Foundations are scheduled to be completed by the end of July 2020. The deadline to have the bridge open to traffic is early December, 2020, but Brasfield and Gorrie has an aggressive plan to complete the project early and earn the bonus for early completion. Volkert is the CE&I Consultant on the project for ALDOT, providing construction management and inspection services for the project.