Island was eroded such that the lighthouse now sits surrounded by the sea. A review of USACE documents revealed that the foundation of the lighthouse consisted of 264 yellow pine wooden piles — 15 in (381 mm) in diameter, of unknown length, but possibly as long as 56 ft (17 m). On top of these piles, a “grillage” was constructed, and a concrete cap, approximately 11 ft (3.4 m) in thickness was built to transfer the weight of the lighthouse to the piles into the bearing strata below. The original lighthouse was designed so that the wooden piles would always be below the existing water table to ensure the piles would not rot or deteriorate. The scouring of the soil surrounding the structure resulted in the piles being exposed to salt water, and, more importantly, to water born parasites, such as shipworms or tereda worms. These organisms caused severe deterioration of the original support piles. 1996 with its sole purpose of facilitating saving the MIL structure, which represents an intricate and permanent place in the history and local culture. The repairs completed through 2008 represented Phase I of the restoration work, which essentially cleaned up decades of early efforts and repairs aimed at delaying the inevitable deterioration around the structure. In fall 2009, STL solicited RFPs to develop a design-build concept to preserve and strengthen the existing foundation. Palmetto Gunite Construction Co. in teamwork with Cape Romain Contracting Co. (marine support), WPC/Terracon (geotechnical engineering and construction services) and Stantec Engineering (structural engineering) was awarded the $2 million design-build contract from STL. Design Analysis Before a design could be performed, numerous unknown project parameters had to be determined, including: • Condition of original foundation • Condition of the tower structure • Design intent of original foundation • Weight of the entire structure • Strength of existing foundation materials and components In addition, the team had to determine qualitatively the impact of the intended repairs on the existing structure, and to anticipate other conditions that could become evident as the project continued. An initial exploration of the structure yielded key information about the structure, including: • Weight of the structure was about 4,400 tons (4,000 tonne) Schematic of foundation beneath MIL “Save the Light” The deterioration of the lighthouse structure and foundation led to the current effort to save the lighthouse from the harsh Atlantic environment. Although the lighthouse’s function was discontinued by the U.S. government in 1962, an organization, known as Save the Light (STL), purchased the structure in 1999, and has been spearheading the effort to save the lighthouse. STL was formed in 94 • DEEP FOUNDATIONS • NOV/DEC 2017 • Average strength of concrete founda- tion was about 3,000 psi (20.7 MPa) • Average strength of the bricks was about 2,300 psi (15.9 MPa) • Poor condition of existing wood members As important, the team was able to obtain a general understanding of the original design intent and the general condition of the original support structure. Although many of the properties of the existing foundation remained a mystery, the design- Good quality existing concrete and bricks build team concluded that the existing foundation piles were deteriorated to the point that they could not be counted on to provide continued support to the structure. Foundation Design The team decided that micropiles would be designed to support the entire weight and loading f rom the s t ructure, thus discounting any residual contribution from the original foundation piles. To provide the required support, 68 micropiles, each with a diameter of 8 in (203 mm) and a design capacity of 75 tons (667 kN) in compression, were installed to a depth of about 65 ft (19.8 m). Ischebeck 52/26 TITAN bars were selected as the reinforcing and load carrying element of the micropile, which are continuously threaded, hollow reinforcing bars. The micropiles were spaced in a staggered pattern along two concentric rows in an octagonal shape around the base of the lighthouse structure to ensure that no eccentric loading would result, which could be destabilizing on the lighthouse. The length of the micropiles was calculated based on data collected during the exploration, and a prepro- duction load testing program would be used to confirm the design assumptions. A neat grout mix consisting of Portland cement and water was designed for a minimum compressive strength of 5,000 psi (34.5 MPa). In the Charleston area, micropiles have been tested to more than 150 tons (1,335 kN) in compression, 50 tons (445 kN) in tension, and 11 kips (50 kN) laterally.
