Dyer, Riddle, Mills & Precourt, Inc.
1505 East Colonial Drive
Orlando, FL 32803
Martin K. Eby Construction Company
2752 Country Club Road
Sanford, FL 32771
Synergy Earth Systems, Inc.
The Central Florida Greeneway, also known as SR 417, is a multi-lane toll road serving the perimeter of Orlando, Florida. Construction of SR 417 has been completed in stages that initially began in the 1990’s. A contract to construct Section No. 2 of the highway, which is located northwest of Orlando near Sanford, Florida, was awarded in 2000 and construction was completed in 2002. The final alignment of SR 417 is such that a significant depth of compressible soils underlies a portion of its 2.6-mile length. Some boring logs indicated that, at the interchange of SR 417 with County Road 46A, silty, organic peat soils extended to a depth of nearly 100 feet. When encountered in their original depositional state, these soils are typically considered problematic due to their weak, compressible nature. To address the stability and settlement issues pertaining to these foundation soils, the project contract required that, for approximately 2,500 linear feet of the proposed alignment, the contractor partially excavate the compressible soils to a depth of 20 feet and then place select backfill in the excavation. Vertical (wick) and horizontal drains and geogrid reinforcing were utilized in conjunction with a surcharge in the design shown in the plans.
The general contractor asked SES to prepare an alternate design that eliminated the requirement for partially excavating the compressible soils. This approach, which had successfully been used in the past by SES, addresses the stability and settlement related issues pertaining to construction over peat by modifying these soils in place. The project engineer had previously indicated that it would approve such an alternate design only if several design criteria could be met. Chief among these criteria was that the alternate design must address the primary and secondary settlement of the compressible soils such that under the proposed finish grade, 100% of the primary consolidation of the compressible soils be achieved, and that the secondary settlement be advanced to the point that not more than 4 inches of post-construction settlement would occur in 10 years. The other primary design criteria established by the project engineer was that the alternate design be completed utilizing a slope stability factor of safety for the long-term condition of 1.5.
After preliminary calculations by SES indicated that the alternate approach was technically and economically viable, the general contractor entered into contract with SES to finalize the design and, pending approval of the design by the owner, to perform certain construction activities. After an exhaustive series of design calculations had been completed, the project engineer approved the alternate design and construction began on the affected portion of the project in June 2000. SES was responsible for installing all wick drains as well as for providing construction management services to the general contractor to oversee installation of all geogrids and instrumentation. The approved alternate design consisted of a biaxial geogrid and multiple layers of uniaxial geogrids, and over 415,000 linear feet wick drains installed on a triangular spacing of six feet. The alternate design also utilized a 5-foot high, 120-day duration surcharge to achieve the required overconsolidation of the underlying compressible soils. The instrumentation plan presented in the contract drawings, including piezometers, settlement plates and slope inclinometers, was not changed by SES and was used to monitor the response of the foundation soils to the construction loading. The embankment and surcharge were successfully constructed by October 2000. In May 2001, after analyses of the instrumentation showed that the foundation soils had achieved the required level of consolidation, the general contractor was given clearance to remove the surcharge. County Road 46A was then paved and opened to traffic while the balance of the work was being completed on the project.