Abstract
The main method for the determination of consolidation parameters in Flanders (Belgium) is still the incremental loading test (IL test). This method can take up to several weeks for some typical Flemish overconsolidated (OC) clays. In theory, the same relationship between settlement and vertical stress can be obtained by performing a constant rate of strain test (CRS test). The main advantages of a CRS test are that the data is continuous and that the test can often be completed considerably faster than an IL test. In this paper, results of both IL and CRS tests on two undisturbed stiff OC clay samples of the same geological formation (Maldegem formation deposited during the Paleogene period) were compared. CRS tests were performed based on ASTM D4186, but constant stress stages were controlled using effective vertical stress instead of total vertical stress as most important adjustment to the standard. In addition, special attention was paid to the development of initial swell pressure and selecting an appropriate rate of strain for this clay with a very high plasticity. Similar values for compressibility and hydraulic conductivity were found using both IL and CRS test results. As the duration of a CRS test on this clay with low hydraulic conductivity can also take up to a few weeks, the time saving aspect of the test was found to be limited for the stiff OC clay tested. The uncertainty in estimating the pre-consolidation pressure and swell pressure was smaller using the continuous CRS test results.
Highlights
In geotechnical engineering projects, settlement of the soil is kept to a tolerable limit based on the results of Incremental Loading (IL) and/or Constant Rate of Strain (CRS) oedometer tests
This paper summarises and compares both IL and CRS results of two undisturbed stiff OC clay samples from the Maldegem formation (Paleogene)
On each sample two incremental loading test (IL test) according to EN 17892-5 [1] and two CRS tests according to ASTM D4186-12 [2] were performed
Summary
Settlement (or swell) of the soil is kept to a tolerable limit based on the results of Incremental Loading (IL) and/or Constant Rate of Strain (CRS) oedometer tests. For a given sample height, the excess pore pressure at a certain strain rate highly depends on the hydraulic conductivity of the soil and as, in most cases, the hydraulic conductivity of the soil sample is not known at the beginning of the test, a number of methods have been developed and described in literature to make a first estimate for a proper deformation rate. The estimation of a suitable strain rate is often based on soil plasticity (liquid limit) [3] or an earlier (IL or CRS) consolidation test [4]. Calculations are made using the steady state equations of ASTM D4186-12 [2] based on the linear model assuming that the soil has a constant coefficient of consolidation and the strain is parabolically distributed over the depth of the sample as elaborated by Wissa et al [5] using the small strain theory
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