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Crust Formation and Desiccation Characteristics for Phosphatic Clays: User’s Manual for Computer Program CONDES0



Although, many new disposal/reclamation approaches for the phosphatic clays have been proposed and investigated, the conventional method still dominates. The traditional phosphatic clay disposal procedure is quite simple. Twenty- to sixty-foot high dikes are constructed around areas 300 to 800 acres in extent. The clay slurry (3 to 5 pct solids) is pumped into the impoundments at a rate of 20,000 to 80,000 gpm. During natural settling, most clays consolidate to a 12 to 15% solids level within 3 to 30 months. After the initial settling, surface water is drained from the settled material, which allows the solids to desiccate and form a crust. Frequently, sand tailings and/or overburden materials are used to cap the clays to promote further consolidation and compaction.

As a result of two decades of extensive research, several computer programs, based on the finite strain consolidation theory, have been introduced to the phosphate industry. Several mathematical models describing the consolidation behavior of the phosphatic clays are in routine use and have proved to be helpful tools for planning the size of required settling areas and as well as for predicting when reclamation is technically feasible. However, none of the existing models addresses the desiccation process which has a significant effect on the eventual volume of consolidated phosphatic clays.

Recently, a new theory for modeling desiccation of soft soil has been developed at the University of Colorado at Boulder. This new theory involves four components: (1) consolidation under one dimensional compression, (2) desiccation under one dimensional shrinkage, (3) propagation of desiccation vertical cracks with depth, and (4) desiccation under three dimensional shrinkage.

This project was originally designed to overlay the new desiccation theory onto an existing phosphatic clay consolidation model. It consisted of three steps. First, existing literature as well as field and laboratory observations related to the desiccation of phosphatic clays will be thoroughly reviewed in the light of the new theory, to make sure that the numerical model will properly simulate field conditions and observations. The present theory will then be modified, if necessary, to reflect the desiccation characteristics specific to the behavior of phosphatic clays. Finally, the consolidation model SLURRY will be critically examined to correct its deficiencies, and a computer program will be formulated to incorporate the new theory into SLURRY.

As it turned out, a completely different model from the SLURRY was developed for two major reasons: 1) it was technically difficult to overlay the new clay desiccation and crusting theory onto SLURRY, and 2) the SLURRY was not very user friendly.

FIPR recently sponsored a workshop for a hands-on training on the model. The participants were impressed by the user friendliness of the new model. Those who had experience in the SLURRY agreed that the new model has many advantages over SLURRY. This product can be used as a stand-alone consolidation model. It could also model both consolidation and desiccation, if the required desiccation parameters are properly determined.


CONDES0 is a numerical algorithm for the analysis of the one-dimensional large strain consolidation and desiccation of soft fine-grained soils using implicit finite difference method. It solves a non-linear second order partial differential equation formulated for one-dimensional compression, three-dimensional shrinkage, and propagation of vertical cracks in soft fine-grained soils. It provides the one-dimensional time-dependent solutions of void ratio distribution (solid content distributions), layer thickness and gives information on propagation and volume of cracks.

CONDES0 is written in FORTRAN. It can be used on any IBM compatible system. A PC with at least 8MB RAM is required and a 486DX2-66 processor is recommended for better performance. The executable file “DOSXMF.EXE” is required for running CONDES0.