One Dimensional consolidation in IL and CRS oedometer: A study of strain rates and time dependency

Abstract

The main aim of this work was to investigate effect of strain rates and time on the compressibility of soft soil. Two clays have been used in the study: one from Stjørdal and another from Eberg. The main testing equipment has been the CRS oedometer testing equipment. Strain rates varying from 3%/hr to 0.3%/hr have been used in the CRS testing programme. Incrementally loaded oedometers have also been run at 24-hr intervals and 1-hr intervals in which pore pressure was measured. The main aim of the incrementally loaded (IL) oedometers at 24-hr intervals was to obtain creep parameters that were later compared to those from CRS oedometer test. The 1-hr interval oedometer tests were conducted to investigate the influence of loading time on the compressibility characteristics of soft soil. Also, attempts into automating the IL oedometer scheme have been initiated at loading intervals of 1-hr using the CRS test equipment.The study has revealed that:

• The strain rates of 3%/hr and 2.3%/hr were high for both the Stjørdal and Eberg clay since the pore pressure-load ratio was still higher than 0.05 while passing the preconsolidation pressures

.• Strain rates greatly influence the yield pressures (preconsolidation pressure) of the soft clays. The preconsolidation pressures will increase with increasing strain rate. The modulus numbers in the NC-region are unaffected by strain rate while those in the OC-region showed some variation. The coefficients of consolidation and permeabilities reduce with decreasing strain rate due to creep that develops in the soil skeleton. Therefore excessively slow and fast CRS tests are not desirable; the very fast tests set up high pore pressure gradients in the soil specimens while the very slow ones lead to excessive creep development in the soil skeleton

.• Strain rates once carefully selected with controlled pore pressure build-up in the soil specimens, should not have much influence on the compressibility characteristics needed in practical Engineering. However the preconsolidation pressures from CRS oedometer need correction to compare with those from the conventional IL oedometer systems.

• Creep resistance numbers obtained from the CRS oedometer were reasonable compared to the Norwegian experience. In some instances, they were in close agreement with those from the conventional 24-hr step load tests in IL oedometer. The outstanding challenge is to obtain a suitable strain rate in the CRS oedometer that will give comparable results to those from IL oedometer systems for which majority of the practitioners hold enormous experience.

• The longer the loading interval in an IL oedometer, the more creep develops. This has influence on the soil stiffness. Therefore testing time should be considered when designing oedometer testing programmes, in addition to the Engineering problem at hand

.• Automation of the IL oedometer test procedure, though only initiated, has shown great potential. More studies need to be done to exploit its numerous advantages.