Twenty years ago every mining-geotechnical conference was dominated by papers on slope stability. At the SME in St Louis, March 2006, it seemed as though every mining-geotechnical sessions was dominated by talk of unsaturated flow. Every speaker trotted out their own moisture retention curve and the results of their own unsaturated flow evaluation whether it be through a cover, a heap, a liner, or the vadose zone around a mine.

My old professor always maintained we are better engineers the more frequently we return to basics. So first a return to basics: InfoMine has just posted a classic paper “Unsaturated Seepage Modeling Made Easy” written by Murray D. Fredlund from the archives of BiTech and Geotechnical News.

Fredlund describes his first forays into unsaturated seepage analyses and tells how he used hand-drawn estimates of the moisture retention curves. He notes that “several thousands of dollars of software was limited in value by the lack of proper methods to determine the input information.” If that were all the paper contained it would hardly be worth a word. However, Fredlund proceeds to describe how to measure a soil's volumetric water content versus soil matric potential and also the soil’s hydraulic conductivity versus soil matric potential characteristics. He surveys practical methods of establishing the unsaturated hydraulic curves for soils given only their grading curves or their soil moisture retention characteristics. Valuable review of a non-intuitive topic.

The bad news is a paper from the 7th ICARD in St Louis, March 2006 by Milczarek, Van Zyl, Peng, and Rice. They note the obvious: “The accurate determination of saturated and unsaturated hydraulic properties of mine waste and cover material is critical for predicting the long-term drainage behavior and closure performance.” They further define the problem thus: “The rock fragments typically found in mine waste and borrow materials complicate laboratory hydraulic property measurement. Many hydraulic testing laboratories address this issue by removing all material greater than 4.75 mm in diameter, repacking the remaining fine-earth sample in small diameter cores and “correcting” the resulting measurements for the gravel content using published correction factors.”

They describe sophisticated testing to establish the influence of gravels on the hydraulic properties of mine wastes. They find that the practice of testing only the fine-earth fraction and using correction factors can result in orders of magnitude errors in the mine waste’s hydraulic conductivity characteristics. On the basis of their findings, they conclude that if gravels are removed from the sample that:

Ø The flux through a gravelly cover material could be underestimated due to the prediction of lower unsaturated hydraulic conductivities than may exist.

Ø The long-term drainage rates and the volume of drainage from a heap leach facility could be over-estimated due to the prediction of more negative air entry values than may actually occur.

Ø The amount of moisture storage in a waste rock facility available before flow occurs could be underestimated due to prediction of lower residual moisture contents than may actually occur.

They recommend conducting saturated hydraulic conductivity and moisture retention characteristic measurements in large diameter cores for materials containing more than 15% gravels, and if possible, conducting direct unsaturated flow measurements on very gravelly samples such as waste rock and heap leach materials. Core diameters should be at least 6 times the largest particle size diameter.