GEO-TECHNICS AMERICA, INC

Seismic Retrofit for Historic Charleston Courthouse

The Charleston, South Carolina Courthouse was built in 1742 and appears on the National Register of Historic Places. The structure suffered minor damage during the earthquake of 1886 and from Hurricane Hugo in 1994. Closely watched by local historians, the Courthouse is undergoing a $12 million restoration to return it to its 1792 condition.

Because 60' (18.2m) of loose sands underlie Charleston, the major concern during the design of the seismic retrofit plan was the potential for liquefaction of these soils during the 100-year earthquake event. During their evaluation of retrofit options, the Geotechnical Engineers were concerned that traditional ground improvement methods (jetted gravel drains and/or injection grouting) would be disruptive to on-going restoration efforts, would result in intolerable differential settlements of the fragile structure, and would be not be feasible due to economic constraints. The project was made more difficult by limited access and low-overhead conditions.

When consulted on the project, Geo-Technics America, Inc. proposed that a system of 144 E-QUAKE^TM Drains be installed beneath and extending 10' (2.3m) outside of the footprint of the structure to the bottom of the liquefiable layer. E-Quake Drains prevent liquefaction by providing a shortened drainage path for the dissipation of seismically-induced excess pore pressures as fast as they are generated during an earthquake. Water expelled from the sand flows radially into the drains and then vertically into a reservoir above the ground water table. By maintaining the excess pore pressure ratio, r_u, less than 0.6, liquefaction and intolerable seismic settlements are prevented.

The design of the E~QUAKE Drain system for this project was performed using the finite element computer program FEQDrain (Report No. UCB/EERC-97/15) distributed by the Earthquake Engineering Research Center, College of Engineering, University of California at Berkeley. Based on the magnitude of the design earthquake and the permeability and compressibility of the liquefiable soils at this site, E~QUAKE drain spacing for the project was on 8' (2.43 m) centers.

The installation of the E-QUAKE Drains was performed with limited access/restricted headroom drilling equipment. Steel casing was drilled approximately 60' (18.2 m) through the liquefiable sands to the underlying marl. The drains were inserted through the center of the casing and the casing was withdrawn leaving the E-QUAKE Drains in place. The installation of the E-QUAKE Drains on this project was performed in conjunction with the on-going restoration and did not effect the construction schedule. In addition, the installation was performed in one third of the time and at one quarter of the cost of the next closest ground improvement option (jetted gravel drains).
		Low -headroom installation equipment		Inserting E-QUAKE Drain

For additional information about the Charleston Courthouse project, E-QUAKE Drains, other GTA contracting services, or for assistance with your next ground improvement project fax us at (770) 719-1907 or email us at RussJoiner@msn.com