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LDG’s first GRS-IBS bridge design project was recently completed for Albany Township Supervisors, Bradford County. The Geosynthetic Reinforced Soil – Integrated Bridge System (GRS-IBS) consists of three main parts: the Reinforced Soil Foundation (RSF), the Geotextile Reinforced Soil abutment (GRS), and Integrated Bridge System (IBS).
GRS-IBS bridge design and construction offers an economical way for local municipalities to construct bridge abutments with their own forces because they are constructed with commonly available materials using ordinary construction equipment and do not require experienced bridge labor.
GRS-IBS structures can accommodate a variety of different superstructure types, offer a proven alternative to deep foundations on loose granular soils, and can easily be adapted in cases where unforeseen field conditions arise during construction. The GRS abutments are estimated to be 40-60% less costly than conventional reinforced concrete abutments and take about one third of the time to construct.
GRS-IBS structures can also help to reduce maintenance costs by eliminating joints at the ends of the bridge, and by eliminating the potential for differential settlement that often occurs between the bridge abutments and approach roadway.
In most cases, the abutments are constructed of 8″ x 12″ blocks similar to masonry blocks, but at a much higher strength. The blocks are dry laid with alternating layers of high strength geotextile and compacted fill placed behind the block face. The process is repeated until the desired height is reached. Blocks are filled with concrete for added strength when used over streams with higher water velocities.
This recent project was completed by a local contractor, and the bid cost for the GRS-IBS Structure was approximately 70% less than conventional abutments. GRS-IBS structures are not recommended for streams with velocities exceeding 12 ft./sec. Many mountain streams in north central Pennsylvania exceed this maximum velocity, so municipalities should consult with an engineer before proceeding with this type of structure. On this project, the old abutments were utilized for scour protection. This can be considered as long as the old abutments are in satisfactory condition.
