At Churchill Falls, the rocks comprise a relatively homogeneous, high-quality gneissic assemblage. Modulus testing was performed on intact laboratory specimens and on in situ mass rock at a depth of 1000 ft; standard equations of elasticity were used to calculate the moduli from the tests, although deviance from elastic behavior was noted, especially with the in situ plate jack tests. Plate jack tests were strongly affected by the compressibility of blast-damaged rock around the underground openings, which resulted in inelastic behavior and a low modulus for rock near the surface of the opening, but a high modulus and approximately elastic behavior for rock at depth. These effects are particularly important in the design of pressure conduits.

The results of modulus tests reflect the effects of structural discontinuities in, and the size of, the specimen, as well as its in situ and applied stress environment. Accordingly, the testing apparatus and methods must be carefully selected to accurately measure rock behavior relative to the expected mode of behavior in design.