Cranfield University

Appendix
Soil water content 15 % and initial dry bulk density 1.2 g/cm 3
Setting soil water content to 15 % and reducing soil density to 1.2 g/cm 3 gave an average
difference between the track and the tyres of 1.2 %. The resulting increase in soil density is
nearly uniform with depth for the track, whereas for all others the increase decreases with
depth. Interestingly the track is now the worst at the 3 rd layer from the bottom. Close to the
surface all are similar and surface displacement is about 100 mm .
The average difference between the track and the wheel was about 1 % for the increase in
DBD. Surface rut depth was between 4 to 7 mm deeper for the wheels than for the track. No
trend can be recognized for the different treatments at different initial DBD and/or soil water
content. The same is true for varying DBD with respect to the depth from which all treatments
cause the same soil displacement. However, these depths become shallower with an increase
in soil water content.
From a soil mechanics point of view the model supports the hypothesis that soil displacement,
e.g. increase in soil density at the surface is determined by contact pressure whereas at depth
it is determined by the overall load. This cannot be confirmed when looking at the results of
Ansorge (2005). Here the tracks caused less displacement and therefore less increase in soil
density over the entire depth range.
With a reduction in initial soil strength, i.e. an increase in soil water content or a decrease in
initial DBD soil compaction increases. This is in agreement with all findings in literature. For
an initial DBD of 1.27 g/cm 3 the increase in DBD comes into the range measured by Ansorge
(2005). The same is true with an increase in moisture content to 20 %. This puts emphasis on
the fact that the slope and intercept of the virgin compression line should be changed as the
original soil for which the model was designed for must have been stronger than the one used
in the soil bin.
Comparison of prediction behaviour of COMPSOIL for a range of VCL slopes and intercepts
The model SOIL FLEX provides the ability to change critical soil mechanics parameters
embedded in COMPSOIL directly while keeping both initial DBD and soil moisture content
as given in the experiment. From theory an increasing slope of the virgin compression line
means that increases in DBD result from smaller pressure changes. With a decreasing slope
Ph.D. Thesis Dirk Ansorge (2007)
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