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Correlating soil compaction measuring devices to soil densities and moisture conditions

Daniel J. Vetter, P.E.
Principal Engineer
Gas Technology Institute
Des Plaines, Illinois
Presenter, 2005 APWA Congress

Summary
Research was conducted to evaluate and rank soil compaction measuring devices that are currently available in the market for use in compaction control of utility trenches, bellholes, and keyholes. The research correlated the output of the devices with the measurements of the Nuclear Density Gauge. The report from this research recommends modifications to selected soil compaction devices to enhance their performance and gain their acceptability as a replacement of the Nuclear Density Gauge.

This research was initiated by the USEPA, FHWA and several local natural gas distribution companies across the country. The main reason was to find a viable replacement for the Nuclear Density Gauge due to stringent controls on possession of a nuclear source and to allow crew personnel to have the capability to control compaction during the backfill process without the expense of having a Nuclear Density Gauge (cost of gauge, licensing, training, testing, etc.).

The soil compaction device measurements were correlated to the density and moisture measurements of the Nuclear Density Gauge and the Sand Cone tests (the Sand Cone was used only as a control to verify the Nuclear Density Gauge readings). Measurements were performed in full-scale 3-foot by 3-foot bellholes, 2-foot wide trenches, and 18-inch diameter keyholes. The test sections varied in depth from 14 to 36 inches. Fifteen sets of tests were performed in these sections in order to evaluate the devices with various backfill types, lift heights, compactor types, soil relative compactions, and moisture contents. The results were utilized to establish the criteria for selecting the most applicable devices for use in compaction control of trenches and bellholes.

Soil compaction and soil compaction measuring devices
Soil compaction is one of the most critical components in the construction or repair of roads. In generic terms, soil compaction is simply the process of increasing the density of the soil by applying a compactive force to remove the air present in the soil. The durability and stability of a road following its construction or repair are directly related to the ability to correctly compact the soil underneath the road. The structural failure of a road can often be traced back to the failure to achieve correct soil compaction.

Two of the main parameters that control the performance of soil compaction are the density of the soil and the soil's moisture content. Unfortunately, many soil compaction measuring devices (other than the Nuclear Density Gauge) do not provide direct readouts of soil density or soil moisture values. Most soil compaction measuring devices are strength or stiffness measuring devices. These monitor the change of the soil's engineering properties due to the application of impact or vibrating force on the soil. These devices, however, provide a measure of soil densification and can provide acceptable results when they are calibrated with the soil type, compaction efforts, and moisture conditions.

The use of these devices instead of the commonly used Nuclear Density Gauge for field compaction control requires replacing the density requirements by strength or stiffness values as a measure of compaction acceptance. This approach is supported by the concept that the performance requirements (e.g., maximum soil strength, minimum permeability, or minimum compressibility) may not correspond to the maximum soil dry density at its optimum moisture content. However, soil type, its moisture condition, and compaction effort are essential parameters that need to be quantified whether the compaction measuring device reads soil density or stiffness values.

In order to establish the relationship between the readings of these devices and soil dry densities at various soil moistures and compaction efforts, there is a need to evaluate the sensitivity of the measured strength and stiffness parameters to the changes of soil types and properties. A comprehensive experimental testing program was performed in an effort to correlate the readings of soil compaction devices to soil densities and moisture conditions. The program evaluated the devices during the compaction of confined backfills in trenches and bellhole utility cuts in roads and highways. The backfills commonly used in these applications are the reusable silty-clay spoil that is excavated from the subgrade, granular soils, and stone base materials.

The compaction measuring devices were categorized into two types: Quality Control (QC) and Quality Assurance (QA) measuring devices. The QC measuring devices are used in layer-by-layer measurements during the compaction process. The QA devices are used in measuring the post-compaction profile of the whole section.

The QC devices evaluated in the testing program were the Soil Compaction Supervisor, the 10-Kg Clegg Hammer, the Humboldt's GeoGauge, and the Utility Dynamic Cone Penetrometer. The post-compaction QA devices were the Dynamic Cone Penetrometer (DCP) and the Panda Meter. The measurements of these devices were correlated to the density and moisture measurements of the Nuclear Density Gauge (NDG) and the Sand Cone tests.

Testing program
Compaction tests were performed in the field to determine the correlation between the QA and QC devices and the soil densities measured by the Nuclear Density Gauge and Sand Cone tests. The tests were performed in bellhole and trench cuts in pavement. The soils were compacted at various layers of thicknesses, relative densities, and moisture contents. Sandy soils and gravels were compacted using a vibrating plate while the silty-clay soils were compacted using a rammer compactor.

The devices were used to measure compaction at the center of the cuts and near the sidewalls. Measurements were taken when the compacted soil reached 90 percent of its maximum dry density based on the modified proctor test results. Some lifts were purposely compacted to lower densities. In some tests, the measurements were carried out after each pass of the compactor.

Conclusions on the results of correlation tests
The results of the research program "Evaluation of Soil Compaction Measuring Devices" have shown the applicability, and limitations, of the tested compaction measuring devices in restoring utility trenches, keyholes and bellholes. The final report (available at GTI's website, www.gastechnology.org) established:

  • Correlation curves between the output of these devices and soil densities and moisture contents. These correlations can be used to set the pass/fail compaction criteria for each of the devices.

  • Ranking the devices according to their performance. The ranking criteria included their ease of operation, output accuracy, consistency of results, correlation in various soils, boundary effects, ease of use and cost.

The three highest-ranking devices were selected for further modifications and improvements in order to gain acceptance by the utility and regulators as Q/C devices. These devices are the 10-Kg Clegg Hammer, the Utility Dynamic Cone Penetrometer, and the Soil Compaction Supervisor. The suggested modifications relate to the operation of the devices, moisture measuring capabilities, output display, adding and improving their data storage, and improving their field durability. Most agencies have been open to starting a pilot program using all three devices. A few examples of the acceptance by agencies of an alternative compaction measuring device include some agencies in California which have accepted the use of the DCP, an area in Tennessee which is allowing the use of the Soil Compaction Supervisor, and New York City which has a program with the local gas company to try all three recommended devices.

Furthermore, the participation of federal agencies (FHWA and EPA) in the previous study has shown their interest in the development of devices that replace the Nuclear Density Gauge in the compaction of backfill soil and asphalt pavement.

Daniel J. Vetter, P.E., will give a presentation at the 2005 APWA Congress in Minneapolis entitled "A New Paradigm for Managing the Right-of-Way: Urban Micro Surgery." The session takes place on Tuesday, September 13, at 3:30 p.m. He can be reached at (847) 768-0623 or Dan.Vetter@gastechnology.org.