By: Jim Sanneman
As part of a project to build a new regional hospital in Owensboro, Ky., surrounding roads Pleasant Valley Drive and Daniels Lane required reconstruction. However, the project’s contract had no provision for stabilizing soft soils below the conventional pavement sections. Proof rolling the prepared subgrade resulted in rutting between one and two in. Also, the conventional pavement sections had only four in. of aggregate base course (ABC) to place prior to the first lift of asphalt. This prompted the contractor—Scott and Murphy Inc.—to contact American Engineers Inc. to evaluate the site and design and propose a solution to construct the pavement.
The solution
Richard Tutt, vice president of American Engineers Inc., evaluated the site soil conditions, reviewed the conventional pavement sections, reanalyzed the traffic data upon which the original sections were designed and then proposed alternative pavement sections utilizing the Spectra roadway improvement system featuring TriAx geogrid. He reduced the amount of ABC by developing a section that utilized a mechanically stabilized layer (MSL) with TriAx geogrid and making the MSL thicker than the original ABC section, resulting in a constructible pavement section over existing soils that met the required design loading. Furthermore, it did not force the contractor to increase costs by undercutting and replacing subgrade soils.
Tutt worked with Tensar Intl. Corp. to provide an alternate section that followed American Association of State Highway and Transportation Officials (AASHTO) and Kentucky Transportation Cabinet (KYTC) guidelines. By incorporating an MSL with TriAx geogrid into the design, he was able to meet and exceed the original pavement section structural number (SN) and, therefore, loading. It also allowed for the immediate construction of a pavement over a subgrade that was rutting up to two in. This design maintained the original pavement block thickness as closely as possible to account for set elevations. As a bonus, no additional costs were assessed to the contractor to construct the new pavement design.
The results
The revised design for Pleasant Valley Road converted 10 ¾ in. of asphalt over 4 in. of aggregate base course to 6 ½ in. of asphalt over an MSL (12 in. of crushed stone base on a layer of TX5 geogrid). The revised design for Daniels Lane consisted of 4 ¾ in. of asphalt over a 12 in. MSL. The project saved over $100,000 by utilizing the Spectra roadway improvement system instead of the original pavement section that was planned for the reconstruction.
