7. EVALUATION OF THE SOCOMO DECISION SUPPORT SYSTEM
7.2 Methods
The following subsection describes the trial site, the vehicle description, how the soil pressure measurements were obtained and a description of the simulations run using the DSS.
7.2.1 Site description
Field testing took place on a forestry site near Richmond in KwaZulu-Natal, South Africa (29° 51' S; 30° 12' E; 1075 m above sea level). The site was on a slope of 11%
with recently harvested 15-year old Eucalyptus Smithii. The soil properties for this site are presented in Table 7.1.
Soil samples were analysed by the South African Sugar Research Institute (SASRI) to determine soil texture and organic matter. Core samples were taken to determine the dry bulk density of the soil before and after the SC event.
Table 7.1 Description of Richmond soil properties
Texture: Sandy Loam Clay (%)
Silt (%) Sand (%)
Organic matter (%)
19 14 67 6.42
Soil moisture content (%) Internal angle of friction (<9in ° ) Cohesion (C in kPa)
Preconsolidated soil strength (SS in kPa) Dry Density (p in kg.m"01)
44.1 21 10 150 796
Three undrained triaxial tests were performed on three undisturbed soil samples of height 75 mm and diameter 38 mm, taken from the upper 0.3 m of the soil. At the time, the average moisture content of the soil was 44.1%. The range of dry densities was between 725 - 991 kg.m"3. The deformation rate used in the triaxial tests was 1.25 mm.min"1. This would give failure for the simple test at 20% strain in about 20 minutes.
The cell pressures used were 25, 44 and 60 kPa. The pre-consolidated strength (SS) of this soil is known to be much higher than the shear strength and was therefore not considered (Smith, 2004).
7.2.2 Vehicle description
The timber was placed in bundles of approximately 4 tons and then loaded onto a self- loading trailer. The trailer was hitched to an agricultural tractor. Figure 7.1 illustrates the tractor and trailer combination, note the smooth tyre on the trailer, which was used to apply the SC. A smooth tyre was selected to reduce any possible effects lugged tyres may have. The tyre was a Firestone 10.5/16/ZS. It was a 14 ply radial, had a diameter of approximately 1 m, a contact width of approximately 280 mm and was slightly over- inflated to 424 kPa and therefore would not deflect the required 20%. The weight on the wheel was measured as 2148 kg.
Figure 7.1 Self-loading trailer used to apply the compaction
7.2.3 Soil pressure measurements
Fluid filled soil pressure sensors were designed and developed by Eweg (2005). One of the objectives of the work completed by Eweg (2005) was to develop a simple and cheap sensor that could be inserted into the soil without changing its structure significantly. Figures 7.2 and 7.3 illustrate the components of the sensor. The sensor is made up of a latex bulb attached to a hydraulic tube. A "T" piece is attached at the other end of the tube which allows two ports, one to which a Motorola MPX 5700DP pressure transducer was attached, while the other port had a tap to allow the user to bleed and pre-pressurise the bulb.
Figure 7.2 Diagram of the fluid filled pressure sensor made out of latex (Eweg, 2005)
Figure 7.3 Diagram illustrating (A) the "T" piece, (B) the Motorola MPX 5700DP pressure transducer and (C) the tap used to pre-pressurise the bulb (from
The fluid filled sensors were inserted into the soil using the insertion technique and frame described by Eweg (2005). Sixteen sensors were placed in a nest in the soil perpendicular to the direction of travel. Figure 7.4 illustrates the orientation of Sensors 1 to 16 in the nest.
»pth
Q
1
Ground level
: ix —*;. 4100mm
i i 1 • i
i t • i i
30 * 4 ty -"€>---
400 Q- <^ - < $ - -
150mm
Figure 7.4 Diagram showing sensor orientation in the soil.
Once the sensors were inserted into the soil, the trailer was reversed over the nest of sensors causing the SC. The pressure changes in the sensors as a result of the event were logged by a computer and a data acquisition cord. These results are shown in Section 7.3.
7.2.4 SOCOMO DSS simulation runs
SOCOMO Simulations were carried out to evaluate and verify the DSS. The simulations involved trying various options that were available in the DSS, for example geotechnical inputs vs. easy inputs, the two contact area scenarios and the two loading scenarios (see Sections 6.3 and 6.4). Descriptions of the simulations are summarised in Table 7.2.
The results from the simulations were displayed graphically and compared to the pressure values and the measured bulk density values obtained in Section 7.3. Plots of
residuals (simulated minus observed) were drawn to illustrate the accuracy of the model. Linear trend lines were fitted through the data, and the slope and R of these lines was also compared. The root mean square error (RMSE), the mean error (ME), the maximum error (MAXE) was used to compare measured and simulated data.
Table 7.2 Description of the SOCOMO simulations carried out for the Richmond trial.
The different references and concepts are explained in Section 6.3 and 6.4
Code Al 1 A I 2 A l l 1 A II 2 B I 1 B I 2 B l l 1 B II 2
Soil input Measured
geotechnical inputs Measured
geotechnical inputs Measured
geotechnical inputs Measured
geotechnical inputs
"Easy"input based on soil texture
"Easy"input based on soil texture
"Easy" input based on soil texture
"Easy" input based on soil texture
Contact area equation O'Sullivan et al. (1999) O'Sullivan et al. (1999) Koolen and Kuipers (1983)
Koolen and Kuipers (1983)
O'Sullivan et al. (1999) O'Sullivan etal. (1999) Koolen and Kuipers (1983)
Koolen and Kuipers (1983)
Loading pattern Uniform pressure distribution Parabolic loading based on Johnson and Burt (1990) Uniform pressure distribution Parabolic loading based on Johnson and Burt (1990)
Uniform pressure distribution Parabolic loading based on Johnson and Burt (1990) Uniform pressure distribution Parabolic loading based on Johnson and Burt (1990)