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Stabilization of Phosphatic Clay Using Lime Columns



The phosphate mining industry has created over 85,000 acres of phosphatic waste clay ponds in central Florida, with approximately 5,000 acres of additional ponds created annually. Techniques have been developed to drain, crust, and reclaim these areas; however, these reclaimed areas are generally only suitable for agriculture or highly specialized use (such as wastewater effluent disposal areas). Construction of buildings or utilities on these lands by conventional methods is not possible because, just a few feet below the surface crust, the low strength clay remains very soft and highly compressible due to a high water content. For this reason, a need exists to develop economical and practical techniques to remediate these areas to a level at which the land can be utilized for a broad range of purposes, such as suburban housing, light commercial building, and utilities.

The lime column method has been used in other countries, primarily in Scandinavia and Japan, to provide additional bearing capacity and reduced settlements for soft clays, and appears to be suitable for use with phosphatic waste clay. Lime columns are constructed in-situ by intimate mixing of clay and finely pulverized quicklime (CaO). These columns reduce plasticity, increase permeability and strength, and lower the water content through hydration and pozzolanic reaction.

In this report, field scale experimental programs that were done on two 40 meter by 40 meter (125 feet by 125 feet) test plots, are described. The field test program was completed to compare the settlement magnitudes and rates of the phosphatic clay due to surface loadings for plots with and without lime columns, as well as demonstrate the feasibility of dry mixing lime with very soft soils.

Strength measurements of the lime columns correlated well with the laboratory data obtained from BCl’s study in 1987. Results show that the shear strength of lime columns has increased more than ten times over untreated clay within 300 days of mixing. The solids content of the lime columns has increased significantly from about 30 percent up to 65 percent, after being mixed with lime. The solids content of the clays between the columns has also increased from about 30 to 35 percent, after column installation. The permeability of amended clays increased one to two orders of magnitude as compared to untreated phosphatic clays. Consolidation of waste phosphatic clays appears to be accelerated by the installation of lime columns, while at the same time reducing the anticipated total magnitude of settlement.