Abstract :
[en] Soil strength is defined as the resistance which as to be overcome to obtain a given
soil deformation. Amongst the numerous methods developed to measure soil strength,
two are classically used. On one hand, a laboratory method based on triaxial tests of
undisturbed soil samples allows the estimation of cohesion and internal friction angle
by the Mohr-Coulomb equation. On the other hand, measuring soil penetration resistance
by pushing a cone into a soil is a widely used technique. Both techniques deliver
discontinuous field information and are not suited to produce digital soil mapping.
The objectives of this paper are to present a sensor able to continuously measure soil
strength variations.
The sensor was constituted of a thin blade pulled in the soil at a constant depth and
speed and a beam which transferred the soil-blade forces to a transducer fixed on
a vehicle. The transducer measures the draft force Fx, the vertical force Fz and the
moment My thanks to an octagonal ring dynamometer. A measurement chain was
developed to acquire simultaneously the signals provides by the soil strength sensor
and those of a DGPS. Signal processing was notably based on geostatistics and allows
soil mapping [1].
Four fields representative of the soils used in silty areas for arable production in Belgium
were selected. The measurements were repeated several times during 1999-2003.
Targeted test plots were chosen in each field to perform reference measurements,
namely granulometry, cohesion, friction angle, pF, water content, dry bulk density,
and cone index. The within-field studies revealed high variability caused by texture,
history, traffic, etc., and showed a correlation between the sensor signals and physical
parameters, such as cone index and soil moisture, as long as no over-consolidation of
the soil occurred [2].
To assess the similarity of soil strength between the fields, the data Fx, Fz and My
were classified by using canonical variates (CV). The two first CV represented 95.9
% of the variability, which means that two main variables contain the essential part
of the information. In a plane (Fz, Fx), three clusters could be distinguished. The
first one (trials 1 and 5), characterized by a low draft and a high Fz, corresponded to
trials performed in March on soils ploughed during the winter, naked or covered with
small vegetation, and characterized by small values of cone index. The second one
(trials 2, 3, 4, 7) with high values of Fx and Fz grouped measurements done just after
wheat harvest in August. The third cluster (trials 6) corresponded to measurements
performed during wheat growth.
It may be concluded that the signals from the sensor treated by suited statistical analysis
have the potential to differentiate soil structures at a field scale.
REFERENCES
[1] Sirjacobs D., Hanquet B., Lebeau F., Destain M.-F. (2002). On-line mechanical
resistance mapping and correlation with soil physical properties for precision agriculture.
Soil and Tillage Research 64, 231-242.
[2] Hanquet B., Sirjacobs D., Destain M.-F., Frankinet M., Verbrugge J.-C. (2004).
Analysis of soil variability measured with a soil strength sensor. Precision Agriculture,
5, 227-246.
