Spring 2001

An Underground Operation

The answer was the Geo Jet earth retention wall system.

Geo Jet is a method in which the soil is drilled while at the same time, injected with grout that mixes with the soil and creates a column in the ground. Each column is drilled with a six-inch overlap of one another. The process, developed in Italy, has been used primarily on the “Big Dig” and only a handful of times for buildings in the Eastern United States.

“One major advantage of the Geo Jet wall, as opposed to slurry wall construction,” notes Project Manager Marc Buchan,” is that the process results in minimal vibration of the site and surrounding structures.” Geo Jet also allows for the construction of conventional footing and foundation wall installation along with a waterproofing system that’s applied to the new footing and foundation. This results in smooth walls rather than the rough moisture-laden walls associated with slurry wall construction.

“All the options were carefully reviewed and Geo Jet was the way to go,” adds Project Superintendent Don Cook. “It was the most cost effective and the most practical method given the site and surrounding conditions.”

In order to fully control and manage any vibration to surrounding structures—two of which are on wood pile construction—two seismographs were installed in the Isabella Stewart Gardner Museum to monitor any movements. Additionally, water table wells within the museum are checked daily.

Installation of the earth retention system and tiebacks, as well as the excavation of approximately 48,000 yards of material from within the footprint of the garage is scheduled to be completed in four months. Placement of all the concrete within the garage is expected to be completed in another four months. The new building at One Palace Road was designed by Goody, Clancy & Associates.

Drilling the moist soil

Dry cement, stored in an on-site silo, is mixed with water and fed into an agitation tank. The mixture is then pumped across the site through hoses to the 125-ton drill rig, and into the rig’s 80’ mast. The grout flows down the mast to the 30’ auger which is boring its way through the earth. Injectors in the auger dispense the grout into the soil. While the auger churns its way down, the grout and soil mix together.

A total of 400 30-inch holes will be drilled around all four sides of the One Palace Road site. The holes are 30 inches wide and overlap by 6 inches on each side. “Primary holes” are drilled first and alternate every other hole until the grout is set. Drilling the holes consecutively would damage the previous hole, therefore, holes are drilled in this “every-other” fashion. “Secondary holes” are drilled in the same manner between the primary holes with the only difference being that an H pile is placed in each secondary hole.

A variety of H pile sizes were engineered for this particular project. The heaviest are along Palace Road where the piles are W24 x 104 and 58’ long. The wall of the new building is 54.5’ in length at its lowest depth of elevation which is 41.8’. The existing grade being at elevation 16.00+/-.

The drill rig is equipped with an on-board computer that stores data for every hole drilled. The data contains such information as drilling rate, grout pressure, grout volume and grout flow. An average drilled hole contains approximately 900 gallons of grout.