Earthwork construction acceptance by the Florida Department of Transportation (FDOT) requires in-place (field) testing conducted with a nuclear density gauge (NDG) to determine dry density, which is the current measure for acceptance. The in-place dry density is compared to the laboratory dry density. Specifications require the in-place density to be at least a certain percentage of the laboratory density.
As design criteria transition from empirical to mechanistic-empirical (M-E), test methods that measure properties such as stiffness and modulus and how they relate to Florida conditions should be investigated.
1.1.1 Problem Statement Number 1
Current Quality Assurance/Quality Control (QA/QC) procedures do not measure mechanistic or performance-based properties as defined by the Mechanistic-Empirical Pavement Design Guide (MEPDG).
M-E design criteria are based on the use of resilient modulus (MR) as the primary input
parameter when characterizing soil and base material stiffness. MR is determined by laboratory testing. Field testing is measured using the plate load test (PLT). Our research focused on the study of the various methods for determining the modulus of the in-place soil and base materials.
1.1.2 Problem Statement Number 2
Alternatives to the current nuclear density test method are being evaluated due to the cost of the associated radiation safety program needed for the operation of nuclear density testing
equipment.
1.2 Statement of Hypothesis
In-place soil stiffness/modulus measurements can be substituted for density specifications for compaction control and verification of M-E pavement design criteria. To prove or disprove the hypothesis, the purpose of this project was to determine whether the selected equipment and test procedures provide equal or better precision when compared to the existing soil density QA/QC acceptance program for soil compaction.
1.3 Research Objectives
On this project we identified testing equipment and test methods to produce a stiffness/modulus-based equivalent (or correlation) measurement and conducted the following protocol:
1. Evaluate in-place soil testing equipment. The selected equipment shall have the capability of measuring soil stiffness/modulus values. The selected equipment shall be portable, cost effective, reliable, accurate, and repeatable. This equipment will be used to evaluate in-place soil properties over ranges of measured density and moisture. Although the dynamic cone penetrometer (DCP) does not measure stiffness/modulus directly, it is included and will be used in conjunction with equipment that measures stiffness/modulus such as the lightweight deflectometer (LWD).
2. Compare the stiffness/modulus values obtained with the selected portable testing equipment to the modulus values obtained by static PLTs, the resilient modulus values (MR) obtained in the soils laboratory, and the densities obtained with the NDG.
1.3.1 Significance, Use, and Implementation
Accurate and reliable soil compaction measurements are important for assessing the operational performance and service life of pavement foundation systems. Performance-related compaction control testing, which has the ability to support traffic loads without undue deflection or creating stresses that damage a pavement structure, is expected to increase compaction uniformity as well as inspector safety and productivity (Kim et al., 2010).
1.3.2 QA/QC Acceptance
Although considerable research has been conducted in an effort to compare measured soil property values to those obtained by the equipment selected for this research, the literature review did not reveal precise correlations necessary to employ these methods in the QA/QC acceptance process. One of the main objectives of this research is to evaluate alternative methods of soil compaction control under controlled conditions.
Numerous studies related to compaction control have been conducted by state DOTs,
universities, and national research organizations. The results of these previous studies have been determined to be acceptable in this research and were used to reduce duplication in the overall testing effort, allowing focus on the most viable test methods for specific materials and
conditions found in Florida.
1.4 Laboratory Testing
Laboratory and field testing designed into this research project builds on the findings from the literature review. Phase I testing was conducted in a soil test box and soil test pits. Phase II testing continued the testing started in Phase I. Phase III field testing utilized the devices that exhibited the best performance characteristics in the previous two phases.
1.4.1 Phase I Test Box and Test Pits
Phase I testing was conducted in order to evaluate in-place soil testing devices and their potential for determining in-place soil moduli. An aluminum test box and soil test pits located at the FDOT’s State Materials Office (SMO) were utilized as part of this testing effort. Several soil types—A-3; low fines A-2-4 (12% passing the 200 mesh sieve); high fines A-2-4 (24% passing the 200 mesh sieve); stabilized subgrade (A-3 soil blended with limerock); and limerock base (calcium carbonate from the Ocala formation)—were examined.
The LWD, GeoGauge (also known as soil stiffness gauge [SSG]), Briaud compaction device (BCD), “dirt” seismic pavement analyzer (DSPA), Clegg impact soil tester (CIST), and DCP were evaluated.
1.4.2 Phase II Test Pits
Phase II also was conducted at the SMO test pit facility. This phase focused on accuracy defined as the difference between true and measured values. The single spot testing sequence (SSTS) approach was used in an attempt to minimize the variability expected from separate testing locations in the test pit. Density, moisture, and particle size are the important variables that this method would help control. The data from each device were compared to those obtained from an adjacent 12-in. static PLT modulus measurement, an NDG test measurement, and a laboratory resilient modulus (MR) test run in the FDOT State Materials Laboratory from a material sample obtained as close as possible to the equipment evaluation test location.
1.5 Phase III Field Testing
Phase III consisted of the use of test sections on FDOT roadway construction projects. Test sections were selected based on their capability of providing a wide range of materials and conditions comparable to those obtained in the pit testing results from Phase II.
1.6 Data Collection and Analysis
Phase I pertained to the precision or repeatability of an individual piece of equipment under
to identify the equipment exhibiting the highest level of precision. Phase II pertained to accuracy when the measurements obtained from the selected equipment were compared to soil test values such as the in-place dry density determined by the NDG, the modulus determined by the static PLT, and the MR test run in the laboratory. The measured soil values were used as the baselines for statistical comparisons. Phase III consisted of equipment testing using LWDs, a DCP, a GeoGauge, and a CIST. Comparisons of the measurements obtained by the equipment being evaluated to the measurements determined by the static PLT, the MR test run in the laboratory, and the in-place dry density determined by the NDG were conducted.
Specifically, the COV was calculated in Phase I to assess relative variation for test results obtained from the different pieces of equipment. Correlation analysis was conducted in Phase II to determine the strength of the linear relationship and to test the significance of such a
relationship between laboratory-tested MR and results from the different pieces of equipment used for the in-place testing. Regression analysis was conducted further to quantify the linear relationship between the laboratory-tested MR and equipment measurements for different soil types under different moisture conditions. The prediction expressions were generated from the regression models and validated using in-place test results in Phase III.