The Job
A stone arch bridge constructed in the 1800s was experiencing significant scour at the bases of the arch. This caused differential settlement of the structure, and deterioration of the wing walls.
The bridge, which was adjacent to a historic mill structure, could not be removed and replaced with a modern structure, it had to be rehabilitated in a fashion which preserved its historic nature.
The bridge was constructed on shallow stone foundations.
The Challenge
As part of rehabilitating the structure, structural engineers designed two new mass footings in the stream bed to underpin the edges of the stone arch. Then, to strengthen the arch designed a 12″ thick reinforced concrete arch overlay on top of the stone arch for the full length/width of the bridge.
In order to compensate for this additional weight, designers determined that a lightweight fill material was needed to reduce the chances of inducing settlement.
Designers also evaluated remedial deep foundations but determined that lightweight fill was the fastest and most economical solution.
The Solution
After the structure had been rehabilitated, CJGeo installed 325CY of 30lb/cuft cellular concrete to backfill the structure. Placed in lifts up to 4 feet thick, the work was completed over a period of three days.
By using 30lb/cuft non-permeable cellular concrete, the dead load on the underlying soil decreased by up to 935lbs/sqft relative to compacted soil backfill.
Because cellular concrete is self-consolidating and can easily be pumped hundreds of feet, single side access was not a problem, and no compaction equipment was necessary.
A crew of two workers completed the pour using continuous wet batch generation. Wet batch generation was chosen due to the relatively small daily volume of material placement.
