Observations on the behaviour of clay fill containing occluded air bubbles

Abstract

Clayey soils can be saturated, and they can exist with continuous pore air and water phases. An intermediate stage occurs when air is present but only as occluded bubbles. In both the continuous air and the occluded air state there are two pore pressures, water and air: the latter is higher by capillarity. Three aspects of the occluded air state are considered. First, the difference between the air and water pressures decreases as the degree of saturation increases. It is shown that this can be due to the way in which the pore structure of the soil constrains the bubbles. Second, a fill in the field may start with continuous air and change to the occluded air state as it is loaded. While air remains, continuous air drainage is likely. Once it becomes occluded, air drainage ceases. Laboratory tests are usually undrained throughout. Tests on samples of a clay compacted in the field show that initial air drainage has a measurable effect on subsequent behaviour but not a dominant one. Third, occluded air may develop as a result of flooding a compacted fill that initially has continuous air. Tests on specimens compacted in the field show that the pore pressure parameter Bw = Δuw/Δσ then becomes very close to 1, even though the fill is not saturated. The fill will then behave as though it is saturated in, say, drawdown of a clay embankment slope, even though it is not. Measurement of Bw = Δuw/Δσ = 1 is not a proof of saturation.