rock is water-tight when non-absorbent of water, but that a soil is not water-tight unless it will absorb an enormous quantity of water.
This would seem to indicate that super-saturation and not pressure is necessary to increase the water-tightness of earth materials.
Again, in a recent discussion regarding the saturation and percolation of water through the lower half of a reservoir embankment, it was remarked, that
the more compact the material of which the bank is built, the steeper will be the slope of saturation.
Exception was taken to this, and the statement made, that
with compact material, the sectional area of flow is larger below a given level with porous material, and as the bank slope is one determining factor of the line of saturation, this line tends to approach the slope line; while with porous material in a down-stream bank, the slope of saturation is steeper and the area of the flow less.
In reply to this, it was said,
that it is obvious that if the embankment below the core wall is built of material so compact as to be impervious to water, no water passing through the wall will enter it, and the slope of saturation will be vertical. If it be less compact, water will enter more or less according to the head or pressure, and according to its compactness or porosity, producing a slope of saturation whose inclination is dependent on the frictional resistance encountered by the water. And the bank will be tight whenever the slope of saturation remains within the figure of the embankment.
Further,
that it was necessary to distinguish between the slope assumed by water retained in an embankment and that taken by water passing through an embankment made of material too porous to retain it; where the rule is clearly reversed and where the more porous the material the steeper the slope at which water will run through it at a given rate.