ABSTRACT
Measuring the displacement of a sloping ground using contact sensors such as potentiometers or LVDTs is problematic because the direction of movement is not known, and the contacting forces can either reinforce the soil or affect the measurements. This is especially true if there is a possibility that sensor attachments might move in liquefied soil. Others have used image-based methods to determine displacements but capturing clear photos of an underwater soil surface has proved challenging. This paper describes the development of a new wave suppressing window and camera setup for recording displacements of a submerged slope during earthquake-induced liquefaction. The bottom of the wave suppressing window was located beneath the water surface and acted like a glass bottom boat. Five GoPro cameras recorded movement of surface markers located on the slope. The videos were converted to displacement time histories using GEOPIV and the process described herein. The displacement time histories from the cameras is consistent with relative displacements calculated by double integration of accelerometer data and with residual displacements from before-and-after hand measurements of the surface markers. Results from this analysis have shown this method for tracking displacements is extremely accurate and can be used to better understand how liquefied slopes displace during strong shaking. The camera data in turn, lend credence to a proposed method to estimate relative displacement time histories from a hybrid of accelerometer measurements, Integrated Positive Relative Velocity (IPRV) and independently measured permanent displacements.
