Description

A marginally stable slope adjacent to a bridge abutment along the I-495 Capital Beltway had a low factor of safety against sliding.

Challenge

An existing slope along the I-495 HOT Lanes had a factor of safety (FS) of 1.2.  Due to the critical location of the slope a FS of 1.5 or greater was required.  Traditional methods, such as retaining walls or overexcavation and replacement, to improve the FS were considered to be too costly and time-consuming due to the required amount of earthwork and maintenance of traffic.

The existing slope had an inclination of approximately 1.5H:1V and was 25 ft high.  Soil conditions consisted of firm to stiff silt with fine sand and silty sand fill extending to depths of approximately 28 ft below the ground surface.

Solution

ECS Mid-Atlantic, LLC recommended the Geopier SRT™ system to raise the factor of safety above the Virginia Department of Transportation (VDOT) standard of 1.5 for critical structures.  The SRT system installation could occur without disruption to traffic and with minimal disturbance to the surrounding slope.  Installation was completed in less than a week, thereby meeting a strict construction schedule requirement for overall project completion.

The Geopier SRT™ system designed for this project consisted of plate piles installed 4 ft on-center in the horizontal direction (i.e. parallel to the slope) and 5 ft on-center in a staggered up-slope pattern.   An excavator-mounted impact hammer was used to install the 13 ft long black steel plate piles.