Stress distribution under portable falling weight deflectometer tests

Abstract

This paper summarizes the study on stress distributions induced by light weight deflectometer (LWD) tests on typical road subgrade soil masses in Hong Kong and they were subsequently compared with the ones from mechanistic and finite element analyses. The key findings are as follows. The vertical stresses were observed to have a more rapid reduction within the upper region (400 mm approx.) of the soil mass along the centre-line under the applied load. An inflexion point exists of stress change occurs at a depth of approximately 400 mm (1.5 times load plate diameter). Beyond the inflexion point, the rates of deterioration in stresses were found to be considerably lower. Both MePaDs and SIGMA/W analyses showed similar findings. For example within the coarse subgrade material with the applied stress of 132 kPa (i.e. full drop-height), 81% and 90% of stress reductions were resulted from the MePaDs and SIGMA/W analyses respectively in the upper region and the actual measurements yieled about 78% of reduction. Along the centre line under the applied load, the horizontal confining stresses decreased dramatically within a very shallow region of the subgrade material. The measurements exhibited most of the confining stress deterioration occurred at the top 300 mm (1 times load plate diameter) whereas both MePaDs and SIGMA/W analyses showed a distnctive inflexion point of change in stresses, being located at approximately 250 mm and 150 mm respectively. Over 95% of the confining stresses diminished beyond the inflexion point and the levels of stress were observed to marginally increase near the bottom of the container. The LWD moduli yielded against the two classic stress ratios: (1) deviator stress / confining stress ratio, and, (2) bulk stress / deviator stress ratio followed similar patterns of the ones for resilient moduli obtained by RLT tests. This has formed important breakthroughs in explaining the ways which in-situ stresses induced by the LWD tests may cause alterations in ELWD as previously investigated by other researchers in this area.