Continuous consolidation tests (CRS)

Continuous consolidation tests (CRS)

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The principle of the apparatus and the calculations of the CRS consolidation test.

A serious disadvantage of the usual consolidation on clay, in which the loading is applied incrementally, is the length of time required for com-pletion A test in an ordinary oedometer cell (Terzaghi) can take up to weeks CRS is an alternative method, which reduces the testing period

to within a day or two, or even a few hours This procedure makes use

of a steady increase of effective stress, instead of applying stress increments The test depends on the measurement of pore water pressure at the base of the sample

Load is gradually applied to the sample by increasing the axial displace-ment at a constant rate The resulting increase in vertical stress and pore water pressure at the base are monitored while drainage takes place from the top surface

Constant Rate of Strain test

GDS

force controller

EXPERTS IN SOIL, WATER AND THE ENVIRONMENT

2003-02-01/JCL

Product sheet

Geo

Maglebjergvej 1

2800 Lyngby

Denmark

Tel.: +45 4588 4444

Apparatus:

The sample is confined in a loose steel ring, which prevents horizontal deformation, and reduces friction in the apparatus The test is carried out

on samples with diameter, D, and height, H, where DxH = 60 x 30 mm

A GDS actuator controls the vertical stress and strain A force transducer placed at the end of a piston measures the force Pore pressure is measured at the base filter stone where the drainage valves are closed

Procedure

Load is gradually applied to the sample by increasing the axial displace-ment at a constant rate The resulting increase in vertical stress and pore water pressure at the base are monitored while drainage takes place from the top surface Some assumptions must be made at the outlet in order to select a suitable rate of strain, which should generally give excess pore pressure in the range 3% to 20%

Calculation

The CRS test is based on Wissa's solution (Wissa et al 1971) Where the average internal strain rate, r, is calculated from the external strain:

where Deaverageis the mean strain corrected for the self deflection of the apparatus The vertical mean effective stress,sv,average, is:

The constrained modulus is determined as a tangent parameter, Kt,from the increase in the effective stress and strain:

The coefficient of permeability, k, is calculated from:

The coefficient of consolidation, cv, is calculated using k and Kt:

Reference:

Wissa, A.E.Z., Christian, J.T., Davis, E.H & Heiberg, S ( 1971):

Consolidation at Constant Rate of Strain ASCE Vol.97, No.SMIO, pp

1393-1413 and "Swedish Standard SS 027126 CRS Oedometer Test on Cohesive Soil"

u bottom

GDS

force controller

Deaverage

r =

Ds’v,average

Kt=

average

2

s ’v,average= sv= u

bottom

3

2ubottom

kK

cv =

3

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