Cranfield University

Appendix
Rel. Density
2
1,9
1,8
1,7
1,6
1,5
1,4
1 10 100 1000
Pressure (kPa)
System Compressibility not accounted System Compressibility accounted for
Figure 27: VCL with and without compensation for water compressibility
11.1.5.2 Axial and Radial Pressure
As there was poor agreement for the VCL from the soil bin gained by the tyre passes and the
VCLs created with radial pressure in the triaxial test apparatus the idea rose to apply radial
and axial pressure simultaneously replicating stress conditions implied by Eq. 1 which has
been discussed earlier. Radial pressure has been set to 25 kPa and axial load was applied
replicating a tyre pass in the soil bin. According to Eq. 1 the resulting � 1 should be
approximately 130 kPa because of the relation of � 1 to the sum of � 2 and� 3 being
approximately 0.38 as shown in Table 6. The resulting virgin compression line is shown in
Figure 28. The density change was calculated from the change in cell volume as before and
additionally from the height change of the sample assuming that the entire height is taken up
by the sample and the sample does not squeeze sideways. For the 1.8 s in which axial load
was applied no change in cell volume was measured and therefore measuring the height
change was the only suitable measurement parameter. However, when looking at the sample
afterwards it failed sideways during the test which resulted in a VCL weaker than in reality
(this will be shown later).
Figure 29 shows the loading cycle for the virgin compression line plotted in Figure 28 with
� 1 against � 2 and� 3 , i.e. axial load vs. radial load. It can be seen that � 1 does not get close
to 130 kPa and is only in the second half of the test about 4 times larger than both � 2 and� 3 .
Due to the slow response time of the cell volume/pressure controller the radial pressure could
Ph.D. Thesis Dirk Ansorge (2007)
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