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Determination of the Degree of Saturation above the Water Table from CPTu Probing in Tailings

Tailings and Mine Waste Conference (2023 : Vancouver, B.C.); University of British Columbia. Norman B. Keevil Institute of Mining Engineering

The degree of saturation in tailings above the water table has been a subject of much research in recent times. Such an assessment is important when determining, for instance, the extent of the liquefiable zone in a tailings profile. Full saturation below the water table and within the capillary fringe is well understood. Above the capillary fringe, layers of finer (silty) and coarser (sandy) tailings often exist within the profile. Some of the finer layers remain saturated, in a process of desiccation, with pore water held in suction (negative pressures). These layers have no hydraulic connection with the groundwater profile below. The saturated layers remain saturated until the suctions exceed the air entry limit of the material. A large database spanning several decades of CPTu probing has been used to develop a method for identification of these saturated layers. Induced dynamic pore pressures in saturated layers above the water table dissipate to ambient negative values in classical pore pressure dissipation tests similar to those found below the water table. These results prove saturation of the layers because such pore pressure dissipation behaviour would not be possible in unsaturated material. Additionally, suction probe measurements carried out in these layers to measure the suctions and air entry values has been part of ongoing research to assess the limiting (air entry) properties of the tailings. Using the pore pressure dissipation and suction measurement data, the dynamic pore pressure trace in the tailings layers above the water table has been investigated. In unsaturated materials the dynamic pore pressure response closely follows the zero value; i.e., there is practically no dynamic response in unsaturated tailings. When probing through saturated layers, the dynamic pore pressure response is either positive (in contractive material) or negative (in dilative material). Using the dynamic pore pressure traces in saturated layers, where pore pressure dissipation results have proven saturation, a methodology is proposed to identify saturated and unsaturated layers in the entire profile. This methodology is the subject of this paper.

Text

2024-02-15

Attribution-NonCommercialNoDerivatives 4.0 International

http://hdl.handle.net/2429/87449

Unreviewed

http://creativecommons.org/licenses/by-nc-nd/4.0/