Project Summary/Scope:

Constructed in four phases over a seven-year timeframe, the 450-acre County Line site in Hialeah, Florida has become a major industrial hub in South Florida as a home to over two dozen warehouses.  

Decades of uncontrolled dumping left the majority of the site covered with about 15 feet of municipal solid waste (MSW) landfill material sitting on top of the surficial limestone formation.  To facilitate a shallow foundation system for the proposed buildings, a robust dynamic compaction program was implemented across the entirety of the site in various phases.  Following completion of the work, a three-foot thick engineered fill soil mat was established across the surface to provide a cushion for the foundation and slab-on-grade system. 

The dynamic compaction program consisted of two high-energy passes of a 16-ton weight dropped from a height of 60-feet, utilizing a 12-foot grid spacing with 10 drops completed at each point.  In calculating the depth of influence of the weight/height combination to be used, an n-value of 0.35 was chosen, reflective of the landfill material in place and to be improved.  Overall, a total applied energy of 982 kJ/m3 was imparted to the soils at the site.   

Complementary Technologies Used: 

None

Alternate Technologies:

None

Performance Monitoring: 

Ground response (i.e. crater depth) was monitored during the course of the work, along with vibration monitoring to assess the impact on off-site structures and utilities.  Post-improvement testing consisted of embankment load testing to assess the magnitude of settlement that could be expected under construction loads; the results of multiple tests indicated an anticipated post-construction settlement of less than one inch. 

Cost Information: 

The dynamic compaction work was completed on a unit rate basis of approximately $1.15/sf; cost of the earthwork support is unavailable. 

Case History Author: 

Chris Woods, P.E., G.E., BC.GE, P.Eng, CEng MIEI, F.ASCE

Project Technical Paper: 

Woods, C.B., Shaffer, R.A., and Drumheller, S.J. (2023).  “Dynamic Compaction: A Proven Ground Improvement Alternative Method for Landfill Sites.” In proceedings for GeoCongress 2023, Los Angeles, California, March 26-29.