Rammed Aggregate Piers are a patented/proprietary ground improvement system that uses compacted aggregate to create stiff pier elements. The installation process modifies the soil surrounding the piers. Piers are installed by drilling 24-inch to 36-inch diameter holes into the foundation soils and ramming thin lifts of well-graded aggregate within the holes to form stiff, high-density aggregate piers. The drilled holes typically extend from 7 to 30 feet below grade. The first lift is often open-graded aggregate and forms a bulb below the bottom of the pier and acts as a stabilized layer for compaction of subsequent lifts which are typically 12 inches in thickness. Natural and recycled aggregates have been used (e.g., aggregate from crushed Portland cement concrete). A high-energy beveled tamper mounted on excavator equipment is used to compact the aggregate. The ramming action both compacts the aggregate and forces the aggregate laterally into the sidewalls of the hole. This action increases the lateral stress in the surrounding soil, further stiffening the stabilized composite soil mass. As a result of the lateral pre-stressing, the matrix soils surrounding the piers are partially densified. Piers have been installed primarily in clays, silty clays, and silty sands. Because the installation method requires drilling a hole, cohesionless soil types and high ground water conditions create some difficulties, although casing can be used in the construction operations. Design parameters include pier length, spacing, pier stiffness, and stress concentration ratio. Applications for transportation infrastructure include embankment foundation ground improvement, mechanically stabilized earth wall foundation support, bridge abutment backfill reinforcement, pavement foundation reinforcement, and slope reinforcement. Quality control operations consist of monitoring drill depth, number and thickness of aggregate lifts, compaction time per lift (normally 15 seconds), bottom stabilization tests, and dynamic cone penetration index tests. Quality assurance can consist of a full-scale load test to verify the design pier stiffness. Performance specifications have been used to allow this proprietary product to be bid against other systems.
The Collin Group was the principal investigator for the HITEC evaluation of the Geopier Rammed Aggregate Pier System and provided peer reviews of designs using this system during the early development of the technology. We have also utilized rammed aggregate piers as the column type for numerous column supported embankments.