poster - International Conference of Agricultural Engineering

Cylindrical containers (191mm diameter, 300mm height) for sampling (Figure 4) were made of polyvinyl
chloride (PVC). PVC is a resistant material withstands temperatures above 50° without suffering
deformations
Figure 1: Dry soil Figure 2: Wetting of the soil sample Figure 3: Distribution of water Figure 4: Sample container
Containers were filled of three layers of soil, with moisture of 15% RH. Each layer’s thickness was
approximately of 8 mm, and they were compacted successively with equal loads in order to achieve
reasonable compaction uniformity throughout the confined volume. A hydraulic press (Figure 5) fed with a
gear pump was used for the compaction. The press had a flow rate of 18 l/min in an open circuit working
from 0 to 40 bars and the section of the press piston was of 40 mm 2 . Compaction loads applied ranged
from 500 N to 5 kN which meant pressures on the soil from 17 to 175 kPa. Eight laboratory samples were
prepared from different loads of soil compaction, resulting in the following bulk densities: 1.46, 1.40, 1.36,
1.34, 1.21, 1.15, 1.12 and 1.09 (in g/cm 3 ).
The samples´ wetting process was carried out by capillary rise until reaching the field capacity
(saturation level of the soil) levels which means values around 30% of RH. For this purpose, the cylinders
were immersed in sealed containers of a greater capacity, sheets of water were added as it was absorbed
by capillary rise through the soil matric potential, until a darker color of the upper layer of soil or even the
presence of water showed up. The elimination of excess water was accomplished by drain.
2.3 Test
Each cylindrical sample was measured (Figure 6) by an Eijkelkamp penetrometer with a one cm 2 cone
for each of the obtained moistures. To attempt the progressive reduction in the moisture content in between
SR measurements, the cylinders were successively introduced in a drying oven at 40°C (Figure 7) and
weighted in a digital scale. SR was measured repeatedly in each cylindrical container (Figure 8) from field
capacity (about 30%) to wilting point (5-8% RH approx.).
Figure 5: Hydraulic press for
compaction
Figure 6: Measurement of SR by a
cone penetrometer
Figure 7: Containers in drying
oven
Figure 8: Control of moisture by
weight difference
SR values recorded at approximately 25 cm soil depth varied between 0.14 and 4.0 MPa with a standard
deviation of 0.94 MPa. SR variation was directly proportional to bulk density and it was observed that
decreasing soil moisture led to increasing the soil strength
3. Results
The function defined in (da Silva and Kay, 1997) (equation 1) was particularized to the clay content
(22%) and soil organic matter (0.8%) sampled, obtaining the three parameter values (lnc, d and e) showed
in equation 2.
ln SR = ln c + d ln θ + e ln D b [1]
ln SR = - 6.23 – 3.04 • ln θ + 5.97 • ln D b [2]