VIBRO REPLACEMENT, STONE COLUMNS

IMPROVEMENT OVERVIEW

Stone columns are a ground improvement method which involves replacing in-situ material with an aggregate such as 1-1/2 inch to 3/4 inch crushed rock. This replacement process not only densifies the surrounding material through displacement, but also through settlement due to vibration. During an earthquake, seismic loading is resisted by the combination of stone reinforcement and densified soils. Additionally, the dynamically induced increase in pore water pressure dissipates through the permeable stone columns thus preventing liquefaction.

GROUND CONDITIONS

This Port of Tacoma project raises Lincoln Avenue over key railroad tracks in the Port area, removing the at-grade conflict between rail activities and heavy vehicular traffic. The bridge design incorporated large earthen embankments founded on reclaimed lands composed of highly variable fill soils and landfill.

DESIGN

Malcolm Drilling installed approximately 120 dry, bottom-feed stone columns to a 55 foot depth at the planned locations of bridge abutments. The purpose of the stone column ground improvement was to reduce the liquefaction-induced settlements of bridge abutments and embankments during a seismic event.


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CONSTRUCTION

Malcolm Drilling utilized a dry, bottom-feed B-27 Vibroflot featuring state-of-the-art technology to construct the stone columns on this project. Powered by a 180-hp electric motor, the B-27 Vibroflot generates in excess of 25-tons of eccentric force. The Vibroflot is suspended from a crawler crane and is modular in nature enabling construction at depths from near surface to greater than 100 feet and diameters ranging from 30 to 48 inches. A robust automatic data acquisition package is mated to the system to compliment quality control measures. Offering an economical and convenient package to the General Contractor, Malcolm also installed drilled shafts to support each bridge pier along the eight bents.

QUALITY ASSURANCE

MDCI maintained an extensive QA/QC program, recording the following parameters: injection pressure at the pump, injection pressure at the hole collar, injected volume per stage, and grout termination criteria per stage (pressure, heave, or volume). Post-grouting CPT tests were performed to verify that the design criteria for densification was met.

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