A new rational analysis of the overall consolidation-desiccation process of soft mine and dredging wastes during and after filling in confined disposal sites has been developed and implemented into a computer program. This program has been prepared to evaluate a wide variety of disposal and reclamation schemes under various field, environmental, and filling conditions. The results of the analysis for low and high clay content waste slurry soils deposited in subaqueous and aubaerial environments are presented and compared. The benefits of self-weight and seepage consolidation, i.e., when bottom and or intermediate drainage layers exist, are evaluated for various combinations of filling rates and periods. The influence of the initial slurried void ratio and the level of surface ponding water are demonstrated. The feasibility of the reclamation of soft disposal sites by surcharge loading after a surface crust is formed is investigated. The benefits of surface drying and a bottom drainage layer in term of the degree of densification and the development of desiccated crust during the intermediate stages of filling as well as for the final reclamation of the disposal site are evaluated. These benefits are found to be a function of the material type, the evaporation rate, and the filling rate.
This paper presents a rational analysis for evaluating the benefits of a wide variety of disposal and reclamation management schemes for low and high clay content waste soils prior to their trial implementation in the field. The benefits of mixing clay with sand for these schemes can be determined by comparing the results of Agrico, which consists of 100 % fines, and Sunbeam, which consists of 50 % fines-50% sand. Agrico undergoes large volumetric changes under a very small densification rate with a maximum average dry density of 54 pcf for an effective stress of 50000 psf. Sunbeam undergoes much less volumetric changes under relatively higher densification rate with a maximum average dry density of 100.2 pcf for an effective stress of 6774 psf. Consequently, the benefits of sand mixing are to improve both the compressibility and permeability characteristics of clays and to decrease the void ratio that corresponds to zero effective stress, eo. For example, eo and its corresponding permeability control solely 30 % of the initial settlement of soft soils undergoing self-weight consolidation, implying also that the void ratio-effective stresses relation has not any influence on the settlement response in the low range of effective stresses.
For the subaqueous deposition, the benefits of seepage consolidation, i.e, when there are bottom and intermediate drainage layers, are to accelerate the consolidation process and promote a a small increase in the level of densification. This would increase the degree of densification during filling, especially for Sunbeam, and reduce the time required to terminate long term settlements, especially for Agrico. The presence of drainage layer is important for Agrico since its consolidation rate would be very slow under the self-weight consolidation. An increased height of the level of water that is maintained on the top of the slurry soil during and after filling would increase the benefits of seepage consolidation. An analysis of the consolidation process that does not account for the filling rates and periods (e.g. based on instantaneous filling) could produce acceptable results for the self-weight consolidation but will result in significant errors in the design capacity of the disposal site as well as the settlement response after filling for the seepage consolidation.
The benefits of surface drying during the subaerial deposition can be evaluated with two limits for the evaporation rates: a minimum evaporation rate, S_min, that would desiccate the slurry soil during filling and a maximum evaporation rate, S_max, that would produce a fully consolidated crust during filling. S_min is 0.99 ft/year for Agrico and is dependent on the filling rate for Sunbeam with a value of 9.87 ft/year for the high filling rates. For practical filling rates, the chances to desiccate Agrico and develop surface crust during filling are more for Agrico than Sunbeam.S_max can be reduced by decreasing the filling rate and eo. Depending on the surface evaporation rate, practical, relatively large, filling rates could be used to produce a fully consolidated crust during the filling of Sunbeam but extremely low filling rates are required for Agrico. The presence of a bottom drainage layer maximizes the densification benefits of surface drying, especially in the lower portion of the soil layer. Finally, the reclamation methods that are based on desiccating the material as well as the application of surface loading have been evaluated in this report. For Sunbeam, the ultimate benefits of surface drying across the entire depth of the soil layer \par can be reached in a relatively short period of time. The presence of a bottom drainage increases the rate of densification and must be considered when the filing and evaporation rates are high to promote an increased level of densification in the lower portion of the soil layer that might remain unaffected by surface drying. For Agrico, the influence of surface drying can penetrate to significant depths in the soil layer only if the evaporation rate is low. However, when the evaporation rate is medium or high, the benefits of surface drying will be restricted to the uppermost portion of the soil layer where a very small thickness surface crust will be developed quickly. In this case, the presence of a bottom drainage layer must be included in any reclamation plan to densify the soft soil zone beneath the surface crust that will remain largely unaffected by surface drying. One way to extend the influence of surface drying through the soil layer is to prepare the disposal site during filling with intermediate drainage layers and our analysis can \par predict the suitable location for these layers. The reclamation of Agrico disposal site that has a bottom drainage layer by the application of surface loading could produce results in term of the rate of densification better than surface drying under any evaporation rate and in term of the final degree of densification better than surface drying under the medium and high evaporation rates.