Constructing Bridges and Walls, Part 3 - Abutments
Abutments are the supports at the bridge ends. In addition to helping support the weight of the bridge, they also assist with the transition from the bridge to the approach roadway.
Abutments consist of three main parts: endwall, abutment beam (or cap), and piles.
The endwall serves as both a support for the approach pavement and a retainer for the approach soil fill. The abutment beam has raised beam seats on which the bridge beams are positioned. The endwall and abutment beam are made of reinforced concrete.
Piles support the abutment beam which in turn supports the bridge beams. The piles transfer the weight of the bridge to the ground or rock below. Piles can be either steel “H” piles or square concrete piles. The piles for the 28th/31st Avenue Connector Bridge are steel “H” piles driven into rock using a pile hammer. Steel tips are attached to the end of the piles to help cut through the ground and embed the pile into rock.
Two different types of pile hammers were used on the project.
- A Diesel Pile Hammer was used on Abutment 1 located to the south of CSX Rail Line. A diesel hammer operates using a diesel engine. A weight is lifted by a crane and then dropped, striking the pile and driving it into the ground. When the weight strikes the pile, the engine fires and the weight is lifted into the air and the sequence repeats until the pile has reached the load required.
- A Gravity Pile Hammer was used on Abutment 2, located to the north of the CSX Rail Line, because the presence of overhead power lines did not allow use of a diesel pile hammer which requires more clearance. A gravity pile hammer, as its name indicates, uses gravity to achieve its power. A crane lifts a large weight and releases it, striking the pile. The “lift and drop” sequence is repeated until the pile has reached the load required.
Piles are not visible once a bridge is completed. After the piles are driven into place, retaining walls are constructed around them. The retaining walls are backfilled with soil and rock to build up the roadbed.