Consolidation and Desiccation of Soft Soils

Background:

Significant amounts of slurried waste soils are produced in the mining and dredging operations and deposited hydrulically in large disposal facilities. It is reported that more than 500,000,000 tons of higly plastic phosphatics clays are produced annualy in Florida's Phosphate Industry, and quantities approaching 400,000,000 m3 of soft sediment soils are dredged annualy from the navigable water ways of the United States.

New Desiccation Theory:

Rational new desiccation theory for modelling the overall consolidation and desiccation process of soft soils was developed that eliminates the shortcoming of exsisting studies (Abu-Hejleh, 1995). The theory has four main components: consolidation under one-dimensional compression, desiccation under one-dimensional shrinkage, propogation of vertical cracks, and desiccation under three-dimensional shrinkage. A general form for the finite strain nonlinear partial differential governing equation of the overall consolidation-desiccation process is formulated.

Modelling & Field Applications

The solution for the governing equation of the overall consolidation and desiccation process of soft soils has been implemnted in a very efficient non-linear finite element softwares named CADA12 & CADA3. These programs are the best available softwares in the market for modeling the consolidation and desiccation of soft soils.

These programs model the performance of soft soils with low and high contents of clays, under different deposition conditions: subarerial and subaqueous (with different levels for ponding water), soft soil filling rates and periods, and surface loading varying with time for the following field scenarios (Abu-Hejleh, 1996):

• Self-weight consolidation;
• Seepage consolidation with bottom and intermediate drainage layers;
• Lowering groundwater level;
• Surface drying under different surface evaporation rates
• Surface loading consolidation
• All the above during the instantaneous and continuous slurry filling for different filling periods and times

The predicted variables with time during the fillings and subsequently includes: vertical and lateral deformation, depth and volume of cracks, and profiles of void ratios and pore water pressures along the current height of the soil layer, and the formation of the surface crust.

The Consolidation and Desiccation Constitutive Relations:

• The seepage induced consolidation test was developed to evaluate the soft soils consolidation void ratio-effective stress and void ratio-permeability relations (Abu-Hejleh et al., 1996). This technique was developed for the Florida Institute of Phosphate Research and summarized in Three Reports.
• New Desiccation Experiments (Abu-Hejleh, 1993). The vaccum consolidation tests (suction tests) tests are used to evaluate the desiccation permeability-void ratio and the void ratio-effective stress relationships. The free shrinkage test is used to predict percentage of three-dimensional shrinkage compared to the overall shrinkage. The tensile strength and Ko are used to evluate the nonlinear cracking function.

Major Consulting Projects:

• "Grouse Creek Embankment Dams." For Golder Associates, Denver, CO;
• " Jerritt Canyon Impoundment." For Knight Piesold, Denver, CO;
• "San Manuel Waste Tailings Dams." For SHB Agra, Phoenix, Arizona;
• "Mclaughlin Tailings Impoundment." For D.P. Engineering Inc., Denver, CO; and
• "Cyprus Baghdad Copper Tailings Facility"; For Agra, Inc., Phoenix, Arizona.
• Prepared CADA software for Agra, Inc., Korea University, Hundai Construction, etc.

Typical Results:

• The consolidation and desiccation material characteristics of the slurry soil tailings (compressibility, permeability, shrinkage);
• Using Software CADA to model various and complicated densification methods to prepare design charts showing:
  • The achieved average densification level of the placed slurry soil with time under different deposition conditions (subaerial or subaqueous), different deposition rates and for the appropriate field and environmental conditions. This is needed to estimate the facility storage capacity;
  • The build up of the soil pore water pressure during raising the embankment dams needed to perform the slope stability analysis and assure a safe time rate for slurry tailings filling.
  • Profiles of the densification level, pore water pressure, specific areas f cracks along the current soil layer height and the formation of the desiccated surface crust;
  • The progress of cracks depth and volume with time
• Marketing software CADA package and providing full technical support.

Publications on Soft Soils:

• Abu-Hejleh, A.N., Znidarcic, D., and Barnes, B.L. (1996). "Consolidation Characteristics of Phosphatic Clays." , J. Geotech. Engrg. Div., American Society of Civil Engineers, 122(4), 295-301.
• Abu-Hejleh, A.N., and Znidarcic, D. (1995). "Desiccation Theory For Soft Cohesive Soils." J. Geotech. Engrg. Div., American Society of Civil Engineers, 121(6), 493-502. (Selected by the American Society of Civil Engineering as one of the best 10 Geotechnical Engineering Research Papers for 1996.)
• Abu-Hejleh, A.N., and Znidarcic, D. (1994). " Estimation of the Consolidation Constitutive Relations." Eighth International Conference on Computer Methods and Advances in Geomechanics (IACMAG94), H.J. Siriwardane and M.M Zaman, Eds., Morgantown, West Virginia, pp. 499-504.
• Abu-Hejleh, A.N. and Znidarcic, D. (1992). "User Manual for Program SICTA." Report Prepared for Florida Institute of Phosphate Research , UCB.
• Abu-Hejleh, A.N., Znidarcic, D. and Robertson, A. (1992). "Results of Seepage-Induced Consolidation Tests on Phosphatic Clays." Report Prepared for Florida Institute of Phosphate Research, University of Colorado at Boulder;
• Several consulting reports for the mine tailing projects described above

Updated Research Work:

Modeling the large strain consolidation of two different soil layers was accomplished recently. Modeling the swelling process where the void ratio is function of the current vertical effective stress and over consolidation ratio (e.g. needed for modeling wetting-drying cycles) is under research.

Contact Information

To send any comments or to get any of my papers or computer softwares, call or e-mail:

Dr. Naser Abu-Hejleh, Ph.D, P.E (Colorado DOT Geotechnical Research Engineer).