In Situ (Field) Scour and Erosion Testing Device

completed

In conceptual terms, the device consists of a confined column of continuously flowing water directed downward then horizontally across the soils that are to support the bridge pier foundations. The shear strength of the flow, and hence the erosion rate, is reduced with the depth of advancing scour to reflect the natural decay of the scouring mechanism (commonly referred to as the "horseshoe vortex" at bridge piers). Initially, the device will be calibrated through physical model testing at laboratory scales in order to identify the input energy needed to produce the scour depths predicted by equations for sand-bed channels. The input energy would then be scaled up for a prototype device and field tests would be run until equilibrium conditions are reached in the resulting scour hole, or until some maximum period of time has elapsed (such as the expected cumulative time the foundation will be exposed to the design discharge over the life of the bridge). The in situ soils would thereby be exposed to the energy necessary to develop the scour depth predicted by the equations. Any equilibrium or maximum scour depth resulting from a field test that is less than the predicted depth for a sand-bed channel would then be attributed to the erosion-resistant characteristics of the in situ soils. The full-scale field device is envisioned to be a closed, recirculation, and filtering system that will operate in both wet and dry conditions while minimizing environmental impacts. The column would be suspended vertically from an overhead crane. Attached to the top of the column would be a weight of sufficient magnitude to advance the column into the soil, incrementally upon release, as the in situ soils are scoured away by the cutting head. The incremental advance of the cutting head and the reductions in flow rate (and shear) will be coordinated by appropriate sensors in the head and computerized controls. The field device would be used for foundation analysis and design in a manner similar to present-day soil borings: testing would be conducted at proposed foundation locations across the channel and floodplain area to be occupied by a proposed new or replacement bridge. The scour depth information resulting from the field test(s) would be used, in conjunction with the subsurface soil boring information, to adjust the design scour depth predicted by the equations for sand-bed channels and reflect the actual erodibility of the in situ soils at the bridge site. The second generation, lab-scale device currently being tested at Turner-Fairbank Highway Research Center (TFHRC) consists of an outer circular pipe column with a concentric cutting head centered within the column. The outer pipe column will advance slightly ahead of the cutting head to contain the incoming flow, but minimize any disturbance to the in situ soils. The inflow enters the cutting head-soil interface from around the perimeter of the head, flows horizontally inward across the soil, and exits vertically upward through an outlet in the center of the cutting head, carrying the eroded material away with it. The pipe column and cutting head are independent components. The intricate shape of the latest cutting head ensures a uniform horizontal shear and symmetrical pressure distribution. The shape was developed with the assistance of three-dimensional, computational fluid dynamics (CFD) modeling performed by the supercomputer at the Transportation Research Analysis Computing Center (TRACC) of Argonne National Laboratory in Illinois. Initial testing of this cutting head has demonstrated excellent performance.