Cranfield University

Ancillary Experiments
The results in general agree with Koolen and Vaandrager (1984) who did not detect lateral
soil movement at a relationship of � �� � 0.
5 which is close to the relationship assumed
3
1
for minor and major principal stresses in Section 6.3.2 for a concentration factor of five
and equal to the relationship at a concentration factor of four.
In an investigation of Gibson (1967) the surface did not settle outside the loaded area.
Chancellor and Schmidt (1962) report a circular pattern of soil movement whereby with
increasing rut depth the soil is pushed more sideways, but none or only small amounts are
pushed above the original surface. Nevertheless Chancellor and Schmidt (1962) state as
well, that a high percentage of volume reduction was caused by vertical displacement and
suggest to measure rut depth to measure soil displacement supporting the procedure de-
scribed to gain the increase in DBD in Section 6.3. In their case contact pressures were
higher and thus they observed sideways soil movement. Issensee (2007) reported the same
finding from horizontal penetrometer resistance measurements whereby no increase in re-
sistance could be detected outside the loaded area. All this agrees with the findings of
Kirby et al. (1997) who detected the largest vertical shear displacement underneath the tyre
edge and ties back to the effect of the LPPL derived in Section 7.3.
Hence it has been possible to clearly explain the nature of the vertical soil failure with two
independent soil mechanics theories, plate sinkage and passiv earth pressure theory, re-
spectively. Therefore it can be concluded that if the pressure bulbs of Söhne (1953) reach
outside the loaded area, they at least do not to influence the direction of soil movement.
Results from Lamade et al. (2006) suggest that these pressure bulbs do not reach outside
the contact area.
7.6 Conclusions on Ancillary Experiements
� The benefit of the tracks could be attributed to the dense layer created at the soil
surface and their small contact pressure. This layer is caused by an alternating
backward forward soil movement below a track. Overall backward soil movement
was more pronounced below a track whereby the surface 150 mm of soil tilted sig-
nificantly backwards (-4 mm). In comparison to this the passage of a tyre tilted the
Ph.D. Thesis Dirk Ansorge (2007)
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