U.S. Department of Transportation
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
This study develops a scour testing field device, the in situ scour testing device (ISTD), to determine the erodibility of soils around bridge foundations. An effective in situ scour testing device could more accurately define the scour potential for a given set of hydraulic design conditions. The ISTD will support the development and implementation of the next generation of scour evaluation guidelines.
|The prototype in situ scour testing device (ISTD) is tested in the laboratory where the river bed is simulated using the sump of the laboratory. The cylindrical erosion head of the ISTD produces shear stresses induced by radial flow. The erosion head is specially designed to ensure uniform distribution of shear stresses across the soil surface. The advancement rate of the erosion head is recorded representing the erosion rate.|
|The Lab-ISTD is a laboratory version of the ISTD concept. The cylindrical erosion head of the Lab-ISTD produces shear stresses induced by radial flow. The erosion head is specially designed to ensure uniform distribution of shear stresses across the soil surface. The erosion head stays stationary and the erosion rate is determined by advancing a piston on which the soil specimen is mounted.|
|The portable Demo-ISTD was designed to demonstrate the concept of the in situ scour testing device at conferences and exhibitions.|
The study addresses the incipient erosion and erosion rate of cohesive soils. Scour on cohesive soils is a very complex phenomenon that is not completely understood. The ex situ scour testing device (ESTD), a special erosion apparatus, was developed to apply hydraulic loading on cohesive soil samples. The study will develop new design procedures for scour prediction in cohesive soils.
|The ex situ scour testing device (ESTD) is designed to measure the erodibility of cohesive soils under flow conditions with log-law velocity profiles. The ESTD uses a pump and a moving belt to propel water in a 92-centimeter-long, 12-centimeter-wide, and 2-centimeter-deep channel. The velocity profiles are measured using Particle Image Velocimetry (PIV) and simulated by computational fluid dynamics (CFD). In this picture, the moving belt is lifted up to be visible. A direct force gauge accommodates a soil specimen (a diameter of 63.5 millimeter and a height of 20 millimeter) on its sensor plate. The gauge can measure instantaneous forces acting on the soil specimen. The specimen can be elevated up and down to keep flush with the channel bottom. The mass loss during a period of erosion can calculate the erosion rate under a certain flow condition. The ESTD and the data acquisition are automatically controlled.|
|Preparation of cohesive soils and the soil sample in the erosion channel. The cohesive soils are prepared by a pugger mixer. Different percentages of clays, silts, and sands can be mixed and vacuumed in the pugger mixer. The mixer then pugs soil specimens with a diameter of 63.5 millimeters. The top photograph shows the pugging process. The bottom is a photograph of the erosion recording of a soil specimen. Synchronizing with the instantaneous force recordings, a better understanding of the relationship between the soil erosion and forces acting on soil can be obtained.|
The new multifunction flume system (MFS) is designed to support a variety of hydraulic and sediment transport modeling. Its capability of high bed shear simulation and sediment recirculation is unique to the United States. The new flume can accommodate a large range of tilting, channel width, channel/pipe geometry, and clear-water/live-bed capability. Computational fluid dynamics simulations are performed to assist the design of main components, such as headwork and sediment infeed of the MFS. Proposed modular channel sections can be rescaled to different cross sections or rearranged to test other hydraulic structures. Construction is expected to start in January 2015.
|Three-dimensional rendering of the assembled new multifunction flume system (MFS) fitted with a 6-foot-wide testing channel.|
|Computational fluid dynamics (CFD) simulation on the inlet performance of the multifunction flume system (MFS).|
Local scour at bridge piers is a potential safety hazard of major concern to transportation agencies. If it is determined that scour at bridge piers can adversely affect the stability of a bridge, scour countermeasures to protect the pier should be considered. Riprap is one of many countermeasures to prevent scour and to secure the pier from failure. Current design methodologies and scour evaluation procedures do not provide a clear means to analyze when the rocks might become displaced. Further advanced computational mechanics techniques are required to assess rock stability and, thereby, ascertain the current scour vulnerability of the bridge. This research study focuses on a new methodology for coupling computational fluid dynamics (CFD) software and computational structural mechanics (CSM) software applied to assess the effectiveness of a rock riprap mattress used as a scour countermeasure around pier 3 at the Middle Fork Feather River Bridge (Bridge Number 09 0063). The bathymetry of the riprap mattress and the riverbed in the Middle Fork Feather River was obtained from a multibeam 3D sonar scan of the bridge site.
|Underwater multibeam three-dimensional (3D) sonar scan of the rock riprap mattress installation around pier 3 at the Middle Fork Feather River Bridge.|
|Coupled computational fluid dynamics (CFD) and computational structural mechanics (CSM) simulation of the rock riprap mattress around pier 3 at the Middle Fork Feather River Bridge.|
One of the hazards of placing a structure in a river or channel is the potential for scour around the foundations. Scour around a shallow foundation, or undermining, can cause excessive deformation or structure collapse. The objective of this research project is to study the performance of shallow foundations using a scaled model of a geosynthetic reinforced soil (GRS) vertical-wall bridge abutment. The GRS-abutment is seated on a shallow reinforced soil foundation (RSF) composed of granular fill material compacted and encapsulated in geotextile. The settlement and external stability (deformation) of the GRS-abutment is monitored with a laser distance sensor mounted on a scanning robotic carriage. The second phase of this research study focuses on clear-water abutment scour experiments on erodible uniform bed material. Computational fluid dynamics (CFD) is used to investigate distribution of flow velocity, unit discharge, and bed shear stress associated with different riprap installations recommended in Hydraulic Engineering Circular 23 (HEC-23).
|Hydraulic performance testing of geosynthetic reinforced soil (GRS) vertical wall abutment.|
|Two examples of as-built riprap installation layouts around the scaled model of a geosynthetic reinforced soil (GRS) vertical wall bridge abutment tested in the flume. The performance of these two layouts was compared for clearwater scour conditions.|
Due to high flow that occurred in March 2011, a massive scour hole developed around pier 22 of the Feather River Bridge (Br. No. 18-0009) on Route 20 in Sutter County, California. The severity of the scour prompted an emergency structural retrofit of pier 22 that was completed in December 2011. To estimate the potential scour of the new retrofitted pier, this research study uses sonar bathymetric data taken in 2007 and after the 2011 flood to construct full-scale CFD models and identify the hydraulic erosion/scouring force distribution. Furthermore, laboratory experiments using 1:60 scaled pier models of both the scoured complex pier 22 and the new retrofitted design are being conducted, along with CFD modeling to study the change of the hydraulic as the scour forms.
|Bathymetry scan (top) of the scour hole around a 1:60 scaled model of the original pier 22. Computational fluid dynamics (CFD) results (bottom) of the associated bed shear stresses.|
|3D rendering (top) of the scoured bathymetry around pier 22 of the Feather River Bridge based on sonar data after the March 2011 flood event. Full-scale computational fluid dynamics (CFD) results (bottom) of the associated bed shear stresses.|
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