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FACTORS AFFECTING SOIL DENSITY
The factors that affect compaction are soil type, moisture, and compaction effort. Soil type and moisture are straightforward and well defined. Compactive effort is the compactive energy put into a volume of soil by the compactor tool. Therefore, SCM setting, compaction tool, and lift height effect compactive effort. The test results provide insights to these important factors.
SOILS
Confidence intervals for each soil type and test matrix are plotted in Figure 7. The plot lists the confidence interval in terms of Percent Standard Proctor Density. Comparing the different soils for common test matrices illustrates that the SP soils were the easiest to compact, followed by the SWSM soils and then the CL soils. For this comparison, the 95% confidence interval for the total matrix is used. The average and lower limit values are listed in Tables 3, 4, 5, and 6.
As was pointed out earlier, the CL values may have achieved the maximum density permitted by the saturation level of the soil. Given this consideration, it can be said that the SCM’s performance in predicting an optimal compaction level for a given set of operating conditions is not dependent on soil type. This statement is reinforced by the data represented in Figure 5, in which the data is not separated by soil type.
MOISTURE
Figure 9 is a plot of confidence intervals illustrating moisture effects. Again the results are categorized by soil type test matrix and moisture condition. In general, the confidence intervals for GOOD soil conditions are higher than those identified as dry or wet. Figure 9 illustrates a significant difference between the GOOD moisture condition and the other conditions for a given soil type. These results imply that a moderate amount of water will significantly improve compaction, especially if the soil is dry. The testing also demonstrated the need to redefine the soil ball tests for clay soils.
SCM SETTING
Two SCM settings were tested, Setting 4 and Setting 6. These settings were selected after reviewing data from an earlier generation of the SCM. The earlier tests were conducted at SCM Setting 3 and resulted in approximately thirty percent of the tests just missing the 95% Standard Proctor criteria on some soils. The SCM setting defines the cut-off point at which the SCM’s compaction algorithm decides that the soil is compacted as well as it can be compacted given the conditions at hand. A higher setting adjusts the cut-off point higher, and therefore, results in longer compactive effort and perhaps higher densities.
Figure 10 is a plot of the Percent of Standard Proctor Density categorized by soil type, test matrix and SCM setting. The soil and test conditions are arranged in pairs in that common soils and common test matrices are arranged side by side. Of the six pairs of conditions, three have a SCM 4 setting resulting in the higher density and three have a SCM 6 setting resulting in a higher average density. With the exception of one data pair, the confidence intervals significantly overlap, and hence, are statistically the same. The conclusion is that Setting 4 is quite adequate. This conclusion is explained by Figure 5. Evidently, Setting 4 has achieved a point of diminished returns relative to density. While Setting 6 may increase compactive effort, little was accomplished in terms of increasing density above that achieved at Setting 4.
COMPACTION TOOLS
The four tools most commonly used for compacting bellhole excavations were tested. Figure 11 illustrates the compaction results with respect to the soil type, test matrix and compaction tool. Note these results include all the test conditions for a given tool and test matrix, and are not separated by the other test parameters. Figure 11 shows that there is very little difference between tools in producing compaction results. For CL soils, the pneumatic tamper seems most effective. The rammer and the pneumatic tool seem most effective for the SWSM soil. With the exception of the A-matrix jackhammer result (which are quite high), all tools perform about the same for the SP soils. As pointed out in the SOILS section, the SP soil appears to be the easiest to compact and would, therefore, be least affected by the tools used.
LIFT HEIGHT
The lift height results are paired and presented in Figure 12. Figure 12 is a plot of the compaction results as a function of soil type, test matrix, moisture condition and lift height. Common soil, common matrix and common moisture condition pair the data. Neglecting the C-Matrix Total, which includes the results of the other C-matrix data, eight of the ten remaining pairs has the 6 to 8 inch lift height producing a slightly higher average density. There is a significant overlap between the confidence intervals of the data pairs. This is probably due to the fact that the average difference between lift heights was probably closer to three inches than to six inches. Based on these results, the 9 to 12 inch lift provided acceptable results. However, the 6 to 8 inch lifts did result in higher densities.
SOIL STIFFNESS
The meter is based on sensing a compression wave passing through the soil. When the compaction tool impacts the soil, a compression wave is generated at the surface and is transmitted through the soil. As the compression wave passes the sensor, the sensor generates a millivolt signal, which is measured by the soil compaction meter. As the soil becomes denser and stiffer, the compression wave is transmitted more efficiently and the generated signal becomes larger. As the soil approaches a constant density and stiffness, the generated signal approaches a constant value. The meter’s algorithms calculate when the measured millivolt level is very close to a projected maximum level and signals for compaction to stop. Although the meter is designed and calibrated toward soil density, the underlying principles of transmitting a compression wave through the soil is strongly related to soil stiffness and provides an index measurement of that stiffness.
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Comment from MBW Inc.
It should be noted that a large assortment of tools within each tool category were not tested. Indeed, the tools employed in the test program tended to be on the smaller side relative to general specifications within a tool category. Comfort is taken in the knowledge that acceptable compaction results were achieved using compactors on the smaller side of their respective tool categories.
IMPORTANTLY, all parties reviewing the test summary should bear in mind that SCM verification was performed within the context of guidelines pertaining to the compaction work. Lifts ranged from 6 to 12 inches. Moisture was judged to be within acceptable parameters based on a manual characterization method. Compaction tools were matched properly to soil type. And the SCM was turned on only when the compaction tool was directly over the sensor. Simple, but important procedural guidelines for good soil reinstatement and maximizing the value of the Soil Compaction Meter in field work.