United States Department of Transportation
i
Falling weight deflectometer for estimating subgrade resilient moduli.
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2003-12-01
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Edition:Final report; Jan. 2002-Dec. 2003.
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Abstract:Subgrade soil characterization expressed in terms of resilient modulus, MR, has become crucial for pavement design. For
new pavement design, MR values are generally obtained by conducting repeated load triaxial tests on reconstituted/undisturbed
cylindrical specimens, employing TP46 protocol. Because of the complexities encountered with the test, in situ tests would be
desirable if reliable correlation can be established. In evaluating existing pavements for rehabilitation selection, subgrade
characterization is even more complex. The focus of this study is to investigate the viability of Falling Weight Deflectometer (FWD)
for direct testing of subgrade with the object of deriving resilient modulus, via a correlation between FWD modulus and MR. In
support of this research, side-by-side Automated Dynamic Cone Penetrometer (ADCP) tests were also conducted.
Ten as-built subgrade sections reflecting typical subgrade soil materials of Mississippi were selected and tested with FWD.
Both fine- and coarse-grain soils were included in the program. Undisturbed samples were extracted using a Shelby tube and tested in
a repeated load triaxial machine for MR, employing TP46 protocol. Other routine laboratory tests are conducted to determine physical
properties, and, in turn, classify the soil being tested.
Employing seven FWD sensor deflections, elastic moduli, E1 to E7, are calculated employing forward equations (assuming
static half-space). E1 and E3-5 (average of E3, E4, and E5) are regressed against MR, advancing two models for MR prediction.
Employing E1 and E3-5, two distinct resilient moduli are derived, with the lesser of the two serving as the design resilient modulus. A
feature of the model is that both center sensor modulus and offset sensor moduli enter in the process, yielding a representative, but
conservative, resilient modulus for design. Having been derived from multiple sensor moduli, this procedure promises to be a viable
method for subgrade characterization, considering significant nonhomogenity expected of built-up subgrades. Also suggested is a
short-cut procedure for predicting resilient modulus which employs an E3-5 section average for a low moduli range, that is, E1<9000
psi (62 MPa), and lesser of E1 and E3-5 for E1>9000 psi (62 MPa).
An exclusive program, FWDSUBGRADE, is developed to analyze FWD deflection data from subgrade tests, extracting first
sensor modulus E1, and average of three offset sensor moduli, E3-5, from which only design resilient modulus is derived. The
program, in addition to calculating station-by-station resilient modulus, relying on what is known as “cumulative difference”
technique, delineates homogenous units of the subgrade, outputting mean and standard deviation of the resilient modulus for each
homogenous section. A graphical plot of resilient modulus of each station is another output of the program.
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