<p><p><strong>References:<br></strong><em>Adam and Brandl (1997)<br>Vennapusa and White (2009)<br>Vennapusa et al. (2011)<br>White and Thompson (2008<br>White et al. (2005, 2006b, 2007a, 2007b, 2008a, 2008b, 2008c, 2009a, 2009b, 2009c, 2010)<br>RVS 8S.02.6 (1999)<br>ISSMGE (2005)<br>Ztve StB/TP BF-StB (1994)<br>Mn/DOT (2007)</em></p><p><strong>Method Summary</strong></p><p>Light Weight Deflectometer (LWD), Falling Weight Deflectometer (FWD), and Clegg hammer tests are included in this category. LWD and FWD tests are performed by obtaining plate deflections under dynamic impulse loading. LWD tests apply lower applied contact stresses (about 0.2 MPa or less) than FWD testing (applied contact stresses up to 1.0 MPa). FWD tests are often performed with an array of deflection sensors spaced away from the loading source to develop deflection basin data to assess the stiffness/modulus of the subsurface layers down to a depth of about 2 meters. Clegg hammer test involves measuring hammer decelerations (g’s) under impulse loading (ASTM D5874).</p><p>FWD equipment is trailer-mounted and pulled with a suitable vehicle. LWD and Clegg hammer devices come in an enclosed box and can be carried in a truck. LWD and FWD are available commercially by several manufacturers. Although the methodology of the test is similar, different manufacturers use different type of measurement sensors to measure deflections (e.g., geophones, accelerometers, or sensiometers). For LWD testing, some devices assume a constant load while some devices use a load cell to measure the applied load. These differences between device configurations affect the modulus value. LWDs are generally setup with 200 and 300 mm diameter plates, while FWDs are generally setup with 300 and 450 mm diameter plates. The modulus values are affected by the plate diameter and applied contact stresses. Additional information about factors affecting the dynamic modulus values is documented in Vennapusa and White (2009). Clegg hammer is available with a 10-kg drop hammer or a 20-kg drop hammer.</p><p>FWD, LWD, and Clegg hammer have all been used in correlation studies with roller-integrated CCC measurements.</p><p><strong>Accuracy and Precision</strong></p><p>Vennapusa and White (2009) indicate that the LWD measurement values may vary depending on the type of the device and deflection sensors used in the test (e.g., sensiometers, geophones, or accelerometers). Repeatability varies with the type of LWD device used (Vennapusa and White 2009). Results from FWD tests have been widely used in the US for direct measurement on the pavement foundation material properties. For the Clegg hammer test method, ASTM D587 indicates that the instrument has a coefficient of variation of 2% or better in good laboratory workshop conditions. Accuracy of the LWD, FWD, and Clegg hammer test methods are not documented in the literature as there is no standard test method for comparison.</p><p><strong> </strong><strong>Adequacy of Coverage</strong></p><p>The number of tests depends on the variability observed, but generally it requires many tests to adequately characterize the spatial variability of soils. These tests are relatively fast to perform (approximately less than 5 minutes per test). LWD and Clegg hammer tests have relatively shallow measurement depth (i.e., less than or equal to 0.5 meters) compared to FWD tests (which provide information up to about 2 meters).</p><p><strong> </strong><strong>Implementation Requirements </strong></p><p>Experience and special equipment are necessary. Use of LWD and Clegg hammer generally requires less training and are more economical than FWD.</p><p><strong> </strong><strong>General Comments</strong></p><p>This test method provides a direct measure of elastic modulus and can serve as a good QA tool. Encouraging results have been reported in the literature with correlations between FWD, LWD, and Clegg hammer results and CCC measurements.</p></p>