DEVELOPING ON MINED LAND – PERILS AND SOLUTIONS

By: Larry D. Madrid, P. E., Principal

Madrid Engineering Group, Inc.

     Central Florida, as the phosphate capital of the world, has an abundance of land that has been previously mined for phosphate. This land is typically highly variable as concerns its soil profile to a depth of 30 feet or more – the typical depth of strip mining. These lands are increasingly being used for development, such as agricultural, residential, commercial, and industrial. However, many difficulties can arise from the variability and loose or soft nature of the land. The following are the types of land that are left over after a mine has completed its effort in the area:

Unmined areas: Portions of the land, such as office complex, storage areas, repair and maintenance shops, etc., are left unmined. In addition, those areas with poor phosphate recover may not be mined. These areas typically have a natural soil profile consisting of surficial sand underlain by stiff clay and then limestone at depth. These areas would be generally considered for "normal" construction.
Reclaimed uplands: These are areas that were mined for phosphate, but have been re-contoured through land reclamation and earth moving techniques. Reclaimed uplands consist of mixed sand and clay, which is often very loose or very soft in consistency, as the typical placement is with a dragline.
Lakes: Certain areas where the water table is near the surface, or the overlying soil above the phosphate is relatively thin are reclaimed as lakes. Phosphate pits (lakes) are relatively deep and are generally underlain by a thin layer of very loose soil, and then natural soils that are denser.
Sand tailings: During the mining process, the sand and clay are separated from the phosphate pebble. The sand is often transported by pipeline to its final resting place, in sand tailings mounds that may be as thick as 40 feet. These areas have uniform sandy soils, with almost no fines in them. Sand tailings are generally very loose to loose in consistency, as they are hydraulically placed.
Clay tailings, also called "slimes": These are the clays left over after phosphate minings. These clays are highly plastic, very soft, and very compressible. They are the most problematic of the soil types. Because they have been mixed with additional water, they are generally placed at a very low solids content (very high water content), and are commonly placed in impoundments.
A mixture of any of the above soil types.
 

 
     Fortunately, many methods have been used by geotechnical engineers to improve the land for agricultural, residential, or commercial use. The first thing is to have the land checked by a competent geotechnical engineer with local experience. If soft or loose soils are encountered during the preliminary exploration, many methods can be used to improve the land. They include the following methods

 
Densify the near-surface soils.
De-water loose sand tailings to densify them.
Design the foundations of the building with wider footings to reduce the stress imposed on the soil. In addition, extra steel reinforcement may be warranted.
Monolithic slabs with bell footings can also be designed to lower the effective stress.
Pre-load the soil. Surcharging involves placing typically 5 to 10 feet of soil over compressible waste phosphatic clay to consolidate the clay over time. It is important to note that it takes several months to several years to consolidate the clay such that the land is usable. After the surcharge has taken effect, remove the load and replace with the load of the building, which is equal to or lower than the previous loading.
Design the foundations with a post-tensioned slab. A post-tensioned slab should be designed by a structural engineer, and acts as a "raft" to spread the load out.
Pilings. If all else fails, the load of a building or structure can be transferred to competent soils below using pilings or other deep foundation technique.
 

 
     Other less common methods are also available, depending on loading, thickness of the deposit, availability of materials, cost, etc. Madrid Engineering Group has 20 years of experience in Polk County and has completed research with the Florida Institute of Phosphate Research. We have recently developed a technique called Phos-Crete™, a method using chemicals to stabilize the very soft clay and/or sand/clay mix.

     This is a proprietary methodology and can only be obtained through Madrid Engineering Group. For utilities and roads, many of the above methods can be used. The most common approach has been to excavate and replace soft clays with sands, but if soft clays remain below, there can still be settlement. Geotextiles have been used to minimize movement under pavements and pipelines. Geotextiles are man-made (polyethylene or polypropylene) materials that systematically take tensile forces to help spread out loads and minimize settlement. Finally, chemical stabilization, such as Phos-Crete™, can stabilize roads, parking lots, and pipe backfill.