Strona 2_redak - Instytut Agrofizyki im. Bohdana Dobrzańskiego ...

50
of the curve defined by A, where A =σ x /σ z0 = ν/(1-ν) was determined. Values of
A for different granular materials were estimated using linear regression
procedure applied to experimental values of stresses (see figure 6.4). Knowing A,
values of Poisson’s ratio ν were calculated as:
A
ν = (6.11)
1+ A
Values of modulus of elasticity E were estimated using relationship ε z (σ z0 ) (Eq. 6.10)
with experimental values of ε z and σ z0 , and ν determined as described above.
The apparatus utilized was an uniaxial tester whose walls were formed by two
semicircular halves cut along the axis (see figure 6.4). The two semicircular
halves were connected with four load cells installed in pairs on the two
connection lines, restoring cylindrical shape of the wall. Bottom and top plates of
the chamber transmitted the vertical load through the load cells. The experimental
setup allowed for the determination of mean lateral pressure σ x , mean vertical
pressure on the bottom σ z , and the mean vertical pressure acting on the top plate
σ z0 . The surface of the cylinder walls was smooth while the surfaces of the top
and bottom plates were rough.
During testing the granular material was poured into the test chamber,
without vibration or any other compacting action. The test sample was 80 mm
high and 21 cm in diameter. The bedding was loaded in compression to the
reference vertical stress, σ z0 of 100 kPa using a universal testing machine at
a constant loading rate of 0.35 mm min -1 . The displacement of the sample was
measured using an inductive transducer having an accuracy of 0.01 mm. Loading
was followed by unloading which took place at the same speed of deformation
until σ z0 of 0 kPa was reached.
7. STRENGTH PARAMETERS
7.1. Methods and apparatus
Two test methods are recommended by Eurocode 1 [50] for the estimation of
strength parameters (angle of internal friction ϕ and cohesion c): direct shear test
and triaxial compression test.
7.1.1. Direct shear test
For this test reference may be made to the ASTM D6128 [9], but the
parameters derived following that standard are not identical with those defined in
Eurocode 1. The test apparatus is a cylindrical shear cell as shown in figure 7.1.