Abstract:
The city of Boiling Spring Lakes is located about 14 miles southeast of Wilmington, North Carolina. A series of earthen embankment dams were constructed in the 1960s as part of the development of a lake-centered community. During Hurricane Florence in September 2018, four of the structures in the Boiling Spring Lakes dam system failed or suffered major damage. The city of Boiling Spring Lakes elected to reconstruct and repair these dams.
Subsurface investigations and a liquefaction potential analysis revealed that all four dams were founded on loose sands with low fine contents. The cohesionless foundation soils have the potential to experience cyclic liquefaction and lose strength when subjected to seismic ground motions. If the soils were to liquefy during the design seismic event the dams would not meet the minimum factor of safety requirements for post-earthquake slope stability. The proposed foundation liquefaction mitigation measures include soil-cement panels using mix-in-place (MIP) construction method.
The objective of the MIP panel design was to increase shear strength of the liquefiable foundation soils and improve the factor of safety for global slope stability. MIP panels were designed in accordance with guidance provided by FHWA and included checks for both internal and external stability. The specification method was performance based in order to allow the contractor the greatest degree of freedom in choosing the most appropriate means-and-methods. Construction was preceded by a bench scale lab testing program and field validation program to allow for evaluation and refinement of soil-cement mix designs. Construction of the MIP panels was completed late in 2023.Â
This presentation will include a summary of the challenging geologic conditions, the risk analysis performed to decide which dams needed soil-cement shear panels, the MIP panel design, challenges encountered during construction, and a comparison of assumed design parameters versus the data collected during construction.