Cranfield University

Appendix
impact at the surface was ignored due to settlement as explained later. For the maximum
working depth of 350 mm a uniform DBD of 1.49 g/cm 3 was assumed. Below 350 mm it was
similar to before with 1.6 g/cm 3 to 750 mm.
Starting with the sandy loam which received the shallow disc tine cultivation an offset of 15
mm was taken out similar to the clay as both soils received the same cultivation. As obvious
from Figure 19 predicted soil displacement is larger for a concentration factor of 5 compared
to a concentration factor of 4. The influence of the actual pressure calculation when soil
displacement is calculated in the model is marginal, however, the influence of the
concentration factor on the slope and intercept of the VCL causes this difference. Overall a
concentration factor of 4 fits better, however, using the VCL created in the soil bin on the
same soil type with a concentration factor of 5 fits the data best. This is due to the fact that the
VCL from the soil bin contains more data points. The field experiment was conducted at a
moisture content of approximately 20 % whereby the soil bin experiment was conducted at
about 9 %. The good agreement in predicted soil displacement using the soil bin VCL is due
to a similar distance from optimum moisture content for compaction which is at about 15 %
moisture content for a sandy loam soil. Taking the three different VCLs into consideration
which were used to predict soil displacement and created from tyre passes only on shallow
tilled soil, only subsoiled soil and the pooled one, shows the best agreement for the VCL
prediction gained from the shallow tilled plot only. Using the one gained from the subsoiled
plot estimates soil displacement higher and the pooled one (called “both”) predicts
intermediate soil displacement. The three different VCLs are shown in Figure 20.
Predicted vs. measured data for the sandy loam soil subsoiled is shown in Figure 21. A
concentration factor of 4 fits the data for the normal inflation pressure better than a
concentration factor of 5. However, at high inflation pressure a concentration factor of 5
seems more appropriate because it overcomes the under estimated soil displacement for this
passage. Thus on this soil condition no clear recommendation can be given concerning the
concentration factor. Similar to the shallow tilled part the VCL gained from the soil bin gives
the best estimation of soil displacement. Interesting to note is the relation between the
estimation gained from the pooled data (“both”) and the one only utilizing the data gained on
the subsoiled plot (“subsoil”). At a concentration factor of 5, the prediction for “both” is
bigger than for “subsoil” at normal and high inflation pressure. Nevertheless at a
Ph.D. Thesis Dirk Ansorge (2007)
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