100 mm below the ground surface (Turner and Raper, 2001) 25 Figure 3.9 Pressure exerted by a rubber-tracked skidder, as recorded by a.
1 INTRODUCTION
The basic process of soil compaction can be defined as a volume change for a given mass of soil (McKibben, 1971). Soil compaction can only be managed if it can be measured and predicted; therefore be careful.
2 MEASURING THE EFFECTS OF SOIL COMPACTION ON RELEVANT SOIL PROPERTIES
Measuring Strain in Soils
Strain gauges attached to a telescopic rod with plates or anchors at each end can be used to measure deformation in the soil. The plates move with the soil and therefore the displacement of the telescopic rod is equivalent to the deformation in the soil (Freitag, 1971).
Static cone penetrometers
Acoustic measurements are essentially empirical measures of the effects of soil compaction (Tekeste et al., 2002). Changes in the amplitude of sound waves in a frequency range can be related to a change in the degree of ground strength.
3 MEASURING PRESSURE IN SOILS
Residual pressure can be described as the pressure that remains in the soil some time after the compaction event. The measured vertical pressure was found to be 1-3% higher than the pressure exerted by the slab on the ground surface. This was thought to be due to stresses concentrating around the pressure sensor because it was firmer than the ground around it.
From the results shown in Figure 3.8 and Figure 3.9, the highest peak pressure recorded was approximately 120 kPa. This is thought to be due to the higher stiffness of SSTs, which does not allow them to deform somewhat and respond in a similar way to the ground (Turner and Raper, 2001). The relatively small size of the sensor reduced disturbance to the soil structure during sensor installation.
4 DESIGN AND INITIAL EVALUATION OF DIFFERENT SOIL PRESSURE SENSORS
Calibration
Since the sensors were open to the atmosphere, it was not possible to calibrate them in a pressure cooker. The results, however, were not repeatable due to slight deviations in the uniformity of the applied pressure. The potential difference (mV) readings from the SPS were plotted against the weight readings from the scale.
These discrepancies detected during calibration mean that this design may be sensitive to pressure uniformity. During a second calibration attempt it was decided that a known pressure could be applied to the sensor using an inner tube of a tire, a window was cut into a tire and. This implies that the inner tube exerted a more uniform pressure on the SPS, compared to the ladder technique.
Testing
Although liquid-filled sensors have been used for some time to measure soil pressure, the use of liquid-filled bulbs is to the author's knowledge a relatively new development, and although this method shows potential, it needs to be more definitively investigated. . An advantage of the liquid-filled SPS bulb over the SPS direct strain gauge is that they can be inflated once inserted, ensuring they make positive contact with the surrounding soil. However, some questions have been raised as to whether the SPS fluid-filled bulb measures the maximum applied pressure or the horizontal soil force.
A brainstorming exercise was conducted to explore materials that could be used to make a liquid-filled SPS bulb. This resulted in the decision to use fmgertips gloves in the design and development of a liquid-filled SPS lamp (Figure 4.10). Bulbs filled with liquid were inserted into a hole drilled by a post at an angle of 45° to the ground surface and perpendicular to the direction of travel.
Effect of bulb deformation on accuracy
The above theory can be summarized as; a sphere filled with fluid exerts a pressure in all directions equal to the maximum applied pressure. Because the horizontal ground tension is generally lower than the vertical ground tension, the liquid-filled sphere will flatten. To test the above theory, a pressure pot was constructed similar to that used by van den Akker (1989), described in Section 3.1.4 (p. 23) and illustrated in Figure 3.5.
Two of these ring earth pressure sensors and two standard liquid-filled bulb SPS were placed at the same depth and equidistant from the center of the pressure pot. The average pressure readings for the ring and standard sensors at seven different load ranges were plotted in Figure 4.15. A null hypothesis was used to check whether the ring sensors can measure higher pressures than the standard liquid-filled bulb SPS.
Testing
The liquid-filled bulb received a rating of five, while the SPS direct voltage meter, even with the insertion technique used, only received a rating of three. Both the direct voltmeter and the liquid-filled SPS bulb were relatively small and therefore received a rating of four. The liquid-filled SPS bulb was found to perform better in all respects and received higher ratings than the direct voltmeter SPS.
The transducers used for the SPS liquid filled bulb were calibrated and found to be very accurate. The insertion technique of the direct strain gauge and the liquid-filled SPS bulb was felt to cause minimal discomfort and both were assigned a rating of four. Insertion of the direct strain gauge and liquid-filled SPS bulb was found to be time consuming, but insertion of the direct SPS strain gauge was more physically demanding and was therefore only assigned a rating of two.
5 FIELD TESTING OF FLUID-FILLED BULB SENSOJlS
Methods
The band did not pass the center of the nest and therefore the x-positions of the sensors were adjusted by subtracting 40 mm from their original position. Tire position was determined by drawing a vertical line from the lowest point of the rim to determine the center of the wheel (Figure 5.4). co 0 N C">. After the position of the wheel relative to the sensors was calculated, the pressure recorded by each sensor was plotted against the relative wheel position.
Negative wheel position values represent the pressure detected before the center of the wheel passed the sensors. It should be noted that the wheel did not pass over the center of the nest of sensors, requiring their x-positions to be adjusted. Another cause could be the distance between the sensors, which was exacerbated by the wheel not passing over the center of the nest.
No Data
Discussion
More research to determine if there are any correlations between a sensor's position and when it first begins to register pressure or when it reaches its peak value could be significant if a deeper understanding of soil compaction is to be gained. This trial demonstrated the effectiveness of the liquid-filled bulb SPS to measure the pressure in the ground at various depths and numerous positions perpendicular to the wheel's direction of travel. The lack of data between the soil surface and the sensors at the 100 mm depth, as shown in Figure 5.10, makes the understanding of the soil compaction process difficult.
This would require the development of a model which makes it possible to estimate a pressure at any given point in the ground. Although the value of such an equation will contribute to the understanding of the soil compaction process, it should be recognized that extrapolation of measured results can in many cases generate misleading information. However, demonstrating the future potential of such a model could help in planning the layout of future trials to validate this model.
6 DISCUSSION, CONCLUSIONS AND RECOMMENDATIONS
Discussion and Conclusions
The fabrication and data collection of the liquid-filled SPS bulb was found to be cost-effective compared to the direct SPS voltmeter. The design of the SPS direct voltage meter presented a major problem in terms of their calibration. Both insertion techniques developed during this study were thought to cause minimal soil disturbance, but insertion of the direct SPS strain gauge was time consuming and labor intensive.
Force can also be transferred to the side walls of the cylinder by the friction between the walls of the cylinder and the ground. If measuring the residual earth pressure is considered important, the durability of the SPS must be considered. An alternative sensor layout may be needed during future trials to statistically verify the use of the model.
7 REFERENCES
Interaction between soil stability and pore water pressure as a function of loading time during the compaction test. Two-dimensional prediction of the spatial variation in the compaction of the topsoil of a sandy loam field, based on the measured horizontal force of the compaction sensor, cutting depth and moisture content. Soil & Tillage Research 74: 91-102. Effects of agricultural machinery with high axle loads on soil properties of normally managed fields.Soil & Tillage Research 75: 75-86.
Methods for simultaneous recovery of stress and displacements in soil under wheels of agricultural machinery. A sensitive method for measuring and visualizing deformation and compaction of the subsoil with a photographed point grid. Interactions between axle load, soil water regime and soil texture on long-term subsoil compaction and crop yield in North America.
APPENDIX A
AN INTRODUCTION TO NUMERICAL INTERPOLATION AND EXTRAPOLATION OF OBSERVED PRESSURE DATA
A Gaussian function was preferred because of its symmetry and provided the best fit to the data. An example from y-position equal to zero can be seen in figure A.I below and the equation of the Gaussian function is shown in equation A.I. Second-order polynomial trend lines were fitted to all figures giving the constants shown in Table A.I, and the example ofkais equation shown in Equation AA.
The model seems to realistically extrapolate and interpolate measured pressure data to the Earth's surface. In Figure A5, isobars are elongated in the direction of rolling of the wheel and have a center point in front of the center of the wheel. There appears to be a concentration of pressure at the point where the rut forms, rather than in the center of the wheel, as would be expected if it were lowered onto a level ground surface.
