A new group of engineering materials, called locked sands, has been identified and described. The criteria which differentiate locked sands from dense sands and sandstones are: absence of cohesion, highly quartzose mineralogy, high strength, steeply curved failure envelopes, low porosities, and considerable geological age. Locked sands are also characterized by a lack of interstitial cement, brittle behaviour, residual shear strengths of 30°–35°, and exceptionally large dilation rates at failure. These materials may be common in non-metamorphosed sedimentary strata.
The locked sands were identified during strength testing of Athabasca Oil Sands, a quartzose sand with interstitial bitumen. The St. Peter Sandstone in Minneapolis, Minnesota and the Swan River Sandstone from Manitoba were also found to be locked sands. Tests on artificial dense sands and on redensified material indicate that in situ densities cannot be reproduced in the laboratory. The characteristic behaviour of locked sands is the result of dilation, and the dilation is the result of an interpenetrative fabric identifiable by microscopic examination. This fabric and grain-surface rugosity develop through the action of diagenetic processes of crystal overgrowth and solution.