United States Department of Transportation
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Performance-based quality assurance/quality control (QA/QC) acceptance procedures for in-place soil testing phase 3.
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2015-01-01
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Abstract:One of the objectives of this study was to evaluate soil testing equipment based on its capability of measuring in-place stiffness or modulus values.
As design criteria transition from empirical to mechanistic-empirical, soil test methods and equipment that measure properties such as stiffness and
modulus and how they relate to Florida materials are needed. Requirements for the selected equipment are that they be portable, cost effective,
reliable, accurate, and repeatable. A second objective is that the selected equipment measures soil properties without the use of nuclear materials.
The current device used to measure soil compaction is the nuclear density gauge (NDG). Equipment evaluated in this research included lightweight
deflectometers (LWD) from different manufacturers, a dynamic cone penetrometer (DCP), a GeoGauge, a Clegg impact soil tester (CIST), a Briaud
compaction device (BCD), and a seismic pavement analyzer (SPA). Evaluations were conducted over ranges of measured densities and moistures.
Testing (Phases I and II) was conducted in a test box and test pits. Phase III testing was conducted on materials found on five construction projects
located in the Jacksonville, Florida, area.
Phase I analyses determined that the GeoGauge had the lowest overall coefficient of variance (COV). In ascending order of COV were the
accelerometer-type LWD, the geophone-type LWD, the DCP, the BCD, and the SPA which had the highest overall COV. As a result, the BCD and
the SPA were excluded from Phase II testing.
In Phase II, measurements obtained from the selected equipment were compared to the modulus values obtained by the static plate load test (PLT),
the resilient modulus (MR) from laboratory testing, and the NDG measurements. To minimize soil and moisture content variability, the single spot
testing sequence was developed. At each location, test results obtained from the portable equipment under evaluation were compared to the values
from adjacent NDG, PLT, and laboratory MR measurements. Correlations were developed through statistical analysis. Target values were
developed for various soils for verification on similar soils that were field tested in Phase III.
The single spot testing sequence also was employed in Phase III, field testing performed on A-3 and A-2-4 embankments, limerock-stabilized
subgrade, limerock base, and graded aggregate base found on Florida Department of Transportation construction projects.
The Phase II and Phase III results provided potential trend information for future research—specifically, data collection for in-depth statistical
analysis for correlations with the laboratory MR for specific soil types under specific moisture conditions. With the collection of enough data,
stronger relationships could be expected between measurements from the portable equipment and the MR values.
Based on the statistical analyses and the experience gained from extensive use of the equipment, the combination of the DCP and the LWD was
selected for in-place soil testing for compaction control acceptance. Test methods and developmental specifications were written for the DCP and
the LWD. The developmental specifications include target values for the compaction control of embankment, subgrade, and base materials.
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