Consolidation Testing with Back Pressure

It is important that laboratory tests on soils saturated in situ be performed on fully saturated specimens. The use of back pressure, common in triaxial testing, has been extended to consolidation testing. A new consolidometer developed for tests with and without back pressure is fully described and back pressure testing procedures are outlined. Comparative tests on one organic silt indicate that the coefficients of consolidation from the back pressure test are lower than those from the standard test below the preconsolidation load; above this load they are equal. Tests on a second organic silt indicate values from the back pressure test are higher throughout the load range. Differences are attributed to air bubbles in the pore water of standard test specimens. Bubbles decrease permeability and alter the rate of pore pressure dissipation. Use of back pressure causes the air to be dissolved in the water and eliminates the initial compression commonly found in time-compression curves from standard tests. Additional research is needed to substantiate these findings.