Identification of influence region on ground subsidence based on in-situ NSWS testing

The Swedish weight sounding test machine (SWS test machine) used in the in-situ ground investigation method, is easy to move, shortens test time, is simple to use for testing, and less expensive compared to mechanical boring test. However, this method cannot be used in dense sand layers and stones/cobblestones layers. An in-situ ground investigation machine, NSWS (Nippon Screw Weight System) that enables identification of super soft zones and has the functions for controlling hydraulics and pneumatics of loading, and for segmentation of measuring interval was developed by utilizing the measuring system (using load and rotational penetration resistance) of the SWS test machine. In this paper, we report a case in which the NSWS test (machine) was used for identifying the subsidence locations on the river bank made up mainly of cobble-mixed gravels. Specifically, the NSWS test (machine) was used for cobble-mixed gravel layers in the initial investigation stage with the purposes of identifying the area loosened due to ground subsidence and assessing its impact to the dam's body. In addition to the measurement results of the amount of load, the penetration rate and the number of rotations, the area loosened due to ground subsidence was identified by paying attention to the scuttle frequency considering subsidence frequency specific to a NSWS test (machine) as a parameter. The test result was verified by boring measurement, and the loosened area was relatively small at the subsidence locations. Moreover, at an early stage, it was determined that the impact on the dam's body was not devastating.