An Evaluation of Incremental, Constant Rate of Strain and Constant Pressure Ratio Consolidation Testing

Consolidation testing is an essential part of geotechnical testing, particularly for settlement analyses and staged construction. Two of the most common tests are incremental loading and constant rate of strain tests. Incremental loading tests hold a particular load over a 24-h duration, typically taking about a week to run, providing limited stress-strain data points, and measures secondary compression characteristics. Constant rate of strain tests holds the rate of strain constant throughout the test, can take about two to three days to run depending on the strain rate, provides a near continuous stress-strain diagram, and limited measurements of secondary compression. This paper presents an alternative to these consolidation testing methods. First proposed in the early 1980s, the constant pressure ratio consolidation test varies the strain rate to maintain the ratio of excess pore pressure to total stress constant throughout the test. To maintain this ratio, a computer is required to loop the strain rate to hold the ratio of the excess pore pressure to total stress to the desired level. An experimental program consisting of 16 re-sedimented Boston blue clay compared the results of the three consolidation test methods. The experimental program also examined conditions unique to the constant pressure ratio method, namely the update time interval. The results for the constant pressure ratio test indicate the test finishes within hours, often taking more time to set up and saturate the sample up than to run the test. The constant pressure ratio test provides similar a near continuous stress-strain and similar coefficients of consolidation across the three tests. The constant pressure ratio test provides limited measurements of secondary compression, like the constant rate of strain test. The update time interval tells the software how often to loop and update the strain rate to maintain the pressure ratio. This paper demonstrates that the constant pressure ratio consolidation test provides results faster than the two common consolidation tests, while providing similar results to both tests.