Estimating Resilient Modulus for Pavement Design
Choosing the right method for your project
Resilient modulus is recognized internationally as the fundamental property for characterizing the materials for use in pavement design. It is a measure of the elastic response of pavement materials subjected to impulse loading, taking into account certain nonlinear characteristics.
GeoDesign offers several ways to measure or estimate the resilient modulus of pavement materials. In most cases, it is our preference to conduct in situ deflection testing coupled with back-calculation analysis, but we also offer laboratory testing and other forms of in situ testing. We’ll discuss below some of the pros and cons of the tests we perform for estimating resilient moduli.
Method 1: Laboratory Testing
Laboratory testing offers a direct measure of the resilient modulus of pavement materials and is the most accurate evaluation. It is the most appropriate method for designing new pavement but
some clients also request it for rehabilitation design.
AASHTO T 307 is the industry-standard laboratory test for resilient modulus testing. The test is appropriate for evaluating “undisturbed” subgrade samples and reconstituted materials. Tests are conducted to measure the response of materials under conditions simulating moving wheel loads.
Figure 1: Resilient Modulus versus Bulk Stress shows an example of test results for a dense-graded base course (coarse-grained) aggregate material indicating that the resilient modulus increases with increasing bulk stress (i.e., sum of principal stresses). The regression equation shown on Figure 1 is used to estimate the resilient modulus of the aggregate at various depths. We can also develop similar models for fine-grained subgrade soils, except with resilient modulus as a function of deviator stress (i.e., impulse stress imparted by a passing wheel load).
When conducted on “undisturbed” samples, laboratory testing provides the most accurate estimate of subgrade soil resilient modulus. However, the results apply only to the locations where materials are obtained. In situations where soil type changes appreciably along a stretch of roadway, comprehensive evaluation of all or the majority of soil types can require a substantial number of tests, particularly for projects of considerable length. As discussed below, deflection testing and back-calculation analysis provides considerably more data at less cost and results in a more comprehensive estimation of subgrade resilient modulus at the expense of the accuracy inherent in the laboratory test.
Method 2: Deflection Testing and Back-calculation Analysis
Deflection testing offers a rapid estimate of overall pavement response to a passing wheel load. As a result, dozens to hundreds of locations are tested for a typical project. We routinely use deflection testing on a full range of pavements, small-scale (e.g., half-street improvement) projects, on typical pavement preservation projects of up to a few lane-miles, and on larger-scale projects involving evaluation of many miles of multi-lane highways.
AASHTO T 256 and ASTM D 4695 provide details regarding devices and procedures for conducting pavement deflection tests. Our discussion is focused on use of a Falling Weight Deflectometer (FWD). This device drops a weight onto a load plate resting on the pavement surface, creating a pulse load simulating a passing wheel load. This generates a bowl-shaped deflection basin centered about the load as illustrated in Figure 2: FWD Deflection Basin. A load cell measures the load and several deformation sensors measure the surface deflection at various distances from the center of load impact. Dropping the weight from various heights allows imparting loads of various magnitudes to simulate truck and aircraft wheel loading. A typical testing sequence can be performed in about a minute. We perform back-calculation analysis to estimate the resilient modulus of each pavement layer using multi-layered elastic theory.
Given the short test duration at each location, we can typically conduct up to 250 tests in a given work shift as opposed to a few laboratory tests per shift, depending on soil type. Accordingly, extensive evaluation of a project can be accomplished in a relatively short period.
For this reason and others, use of an FWD is the preferred choice of many clients. However, use of an FWD is restricted to evaluating existing pavement structures and supporting subgrade, and is not used directly on deeper native soils. Accordingly, the data and analysis results are more directly applicable for rehabilitation (e.g., grind/inlay) or reconstruction projects with minimal grade changes. The data are not as applicable for designing pavements for new alignments; although, there are exceptions. While the results from FWD testing can help identify pavement management savings, the fees can be considered cost-prohibitive for some clients, leaving other forms of testing and reliance on correlations.
Method 3: Correlations with Other Tests
Several researchers have developed correlations with other tests: the Resistance Value (R-value), the California Bearing Ratio (CBR) value, and the Dynamic Cone Penetrometer (DCP) value, to
name a few. Resilient modulus has also been correlated with soil classification. R-value testing is not commonly requested by our clients in Oregon or Washington, nor is CBR testing. Correlations with soil classification generally provide wide ranges for resilient modulus values, leaving selection of a design value to engineering judgment. Correlation with DCP results, on the other hand, provides estimation based on measurement of the shear resistance properties of the subgrade soil.
ASTM D 6951 describes the DCP apparatus and test methodology. It can be used to evaluate undisturbed soil and/or compacted, unbound layers within existing pavements. Accordingly, we can use DCP testing to estimate the subgrade resilient modulus for new pavement design and base course and subgrade moduli for rehabilitation design.
The test involves driving a cone-tipped rod into the unbound layer(s) by dropping a sliding hammer from a prescribed height onto an anvil attached to the rod as illustrated in Figure 3: In Situ DCP Testing. The total penetration for a given number of blows is measured and used to calculate the rate of penetration in millimeters per blow. The device is relatively inexpensive and testing can be performed in a short period. Accordingly, our fees for DCP testing are much less than those for FWD testing. One disadvantage of DCP testing, however, is that evaluations are typically conducted at a limited number of locations, and typically at the location of pavement cores which also includes the cost for drilling.
Figure 4: Reduced DCP Test Results shows the results of a DCP test conducted in a pavement not far from our Portland office. The data are reduced
to display cumulative blows plotted against cumulative penetration in millimeters. As indicated, there are two distinct regions of relatively constant slope with the change in slope occurring at about 300 millimeters; data to the left of this point represent the rate of penetration through the base course whereas the data to the right represent the rate of penetration into the subgrade. ASTM D 6951 provides correlations of the slopes to the CBR value, which has been correlated to resilient modulus by a number of researchers.
The Oregon Department of Transportation (ODOT) adopted the relationship developed by the Kansas Department of Transportation as one means of
estimating base course and subgrade resilient moduli and has published the correlation in the ODOT Pavement Design Guide. Applying the relationship to the data shown on Figure 4 gives moduli very much in line with back-calculated moduli derived from FWD testing on similar materials.
Which method is best?
The answer to this question is: That depends. Each method has advantages and disadvantages and the fees charged for each method are diverse.
Laboratory testing provides the most accurate assessment of resilient modulus, but is the most expensive on a per-test basis. Given that dozens to hundreds of FWD tests can be performed along a typical project, FWD testing can provide the most comprehensive assessment of subgrade and pavement moduli and is the least expensive on a per-test basis. Even so, the costs can be a deterrent for some clients. DCP testing also provides accurate assessment of resilient modulus and is less expensive than laboratory testing on a per-test basis, but tests are typically conducted at core locations or in test pits, thereby providing assessment of moduli at limited locations.
The most appropriate method often depends on client preferences, which are sometimes driven by costs. Some clients prefer or require FWD testing while others prefer DCP testing, and occasionally we receive requests to conduct laboratory testing. In any case, our preference is to apply our extensive expertise to work with our clients to develop a scope of services that addresses their needs, yet minimizes their risks and fits within their budget.
Article by: Todd Scholz, PhD, PE
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