Cranfield University

Soil Compaction Models
be specific to that particular soil type. The same holds true for water content as the method
simply can be repeated at any given moisture content and a water content specific in-situ
VCL will be derived. This is a great simplification taking the work of Wheeler and Siva-
kumar (1995), Toll and Ong (2003), Blatz and Graham (2003) and many others into ac-
count trying to link critical state soil mechanics parameters to soil moisture content. It was
possible to account for water content not with soil stresses but as an influence on critical
state soil mechanics i.e. soil physical parameters as found by Hettiarachi and O’Callaghan
(1980), Hettiarachi (1987) and Kirby (1989).
The robustness of the entire approach against the concentration factor introduced by Fröh-
lich (1934) was surprising, but agrees with the findings of Etienne and Steinmann (2002)
who are able to predict soil stresses independent of soil type with one concentration factor.
Interestingly their concentration factor is 2 and the one most commonly used in this study
is 5. However, for the work here, 5 is most appropriate.
With respect to the physics of granular media, the framework of critical state soil mechan-
ics theory embedded in COMPSOIL is regarded as an approach which is sensitive enough
to describe soil density changes accurately by means of a physical concept (critical state
soil mechanics theory, Schofield and Wroth, 1960), while at the same time it is stable
against variations at the grain level. Much work has been conducted to describe factors
influencing the physics of granular media at the grain level, yet it is virtually impossible to
account for all of them in one model. The orientation of particles changes mechanical be-
havior according to Oda et al. (1997), particle size determines the strength of particle ac-
cording to Ning et al. (1997), Delie and Bouvard (1997) found that spherical inclusions are
easier to compact than angular inclusions. All these effects are averaged and thus do not
affect the predicted soil displacement utilizing the in-situ VCL.
At the onset of the in-situ approach it was mentioned that the error originating from disre-
garding the elastic recovery of the soil when deteriming the in-situ VCLs would be ignored
as the error is small. The entire section proved this assumption to be correct due to its high
accuracy. The validation in the field even confirmed the validity of the whole approach on
a different soil type. Now there is a method available which has been validated on the two
of the most opposing soil types found in arable agriculture – a sandy loam and a clay soil.
Ph.D. Thesis Dirk Ansorge (2007)
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