<p><p><strong>References:<br></strong><em>Adam and Brandl (1997)<br>Adam (1997)<br>Adam and Kopf (2000)<br>Adam and Kopf (2004)<br>Brandl and Adam (1997)<br>Hansbo and Pramborg (1980)<br>NCHRP 21-09 (2010)<br>Newman and White (2008)<br>Peterson et al. (2006)<br>Rahman et al. (2008)<br>Thompson and White (2007)<br>Thompson and White (2008)<br>Vennapusa et al. (2009)<br>Vennapusa et al. (2010)<br>White and Thompson (2008)<br>White et al. (2005, 2006, 2007a, 2007b, 2008a, 2008b, 2008c, 2009a, 2009b, 2009c, 2009d)<br>RVS 8S.02.6 (1999)<br>ISSMGE (2005)<br>Ztve StB/TP BF-StB (1994)<br>Mn/DOT (2007a,b)</em></p><p><strong>Method Summary</strong></p><p>Light weight deflectometer (LWD), falling weight deflectometer (FWD), and Clegg hammer tests fall under this category. LWD and FWD tests are performed by obtaining plate deflections under dynamic impulse loading, while Clegg hammer tests record decelerations of the drop hammer under impulse loading. LWD and Clegg hammer apply relatively lower applied contact stresses (about 0.2 MPa or less) compared to FWD testing (applied contact stresses up to 1.0 MPa). FWD tests can also be 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 m. FWD equipment is trailer-mounted and pulled with a van or a truck. LWD and Clegg hammer devices come in an enclosed box and can be carried in a truck.</p><p>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, sensiometers, etc.). For LWD testing, some devices assume a constant assumed 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. Clegg hammer is available in two setups: (1)10-kg drop hammer, and (2)20-kg drop hammer.</p><p>FWD, LWD, and Clegg hammer has been used in correlation studies with roller-integrated compaction measurements. Correlations between FWD and roller measurements yielded comparatively higher coefficient of determination (R²) values compared to LWD and Clegg hammer.</p><p><strong>Accuracy and Precision</strong></p><p>These tests are generally considered repeatable. The values may vary depending on the type of device and deflection sensors used in the test (e.g., sensiometers, geophones, or accelerometers). Results from FWD tests have been widely used by many state and federal agencies in the US for pavement design.</p><p><strong>Adequacy of Coverage</strong></p><p>Sufficient evaluation requires many tests. These tests are relatively fast to perform (approximately &lt; 5 min per test). LWD and Clegg hammer tests have relatively shallow measurement depth (i.e., ≤ 0.5 m) compared to FWD tests (which provide information up to about 2 m).</p><p><strong>Implementation Requirements</strong></p><p>Experience and special equipment is necessary. Use of LWD and Clegg hammer generally requires less training and experience compared to FWD.</p><p><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 (particularly with FWD) with correlations between FWD, LWD, and Clegg hammer results and roller-integrated compaction measurements. Its applicability to each of the potential application is rated below.</p></p>