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Overview | Projects | Laboratory Physical Modeling | Laboratory Numerical (CFD) Modeling | Hydraulics Research and Development |
Ongoing Laboratory Activities | Hydraulics Laboratory Personnel | Research Publications | Office of Bridge Technology: Hydraulics

 

 

J. Sterling Jones Hydraulics Research Laboratory

 

Ongoing Hydraulics Laboratory Activities

 

In Situ Scour Testing Device (ISTD)

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 ISTD is shown in Figure 1. This picture shows the ISTD above a 10-foot-deep sump in the J. Sterling Jones Hydraulics Research Laboratory at Turner-Fairbank Highway Research Center. The sump is filled with sandy soil with the purpose to simulate a river bed condition. An aluminum tube with an inner diameter of 4 inches is driven into the sand bed. Inside that aluminum pipe a centered cylindrical erosion head with an outer diameter of three-and-a-half inches is positioned. The erosion head consists of two spacers and a clear acrylic body between them. The spacers maintain a small gap between aluminum tube and the cylindrical erosion head. The picture shows the erosion head before being lowered into the aluminum pipe by a servo motor and rope assembly. During a test water runs through the gap to the perimeter of bottom of the head. The only drain in the system is a small hole in the middle of the head. The small hole is connected to a vacuum pump through a flexible plastic hose. Between the perimeter and the center of the bottom of the erosion head is an erosion chamber shaped to allow a uniform concentric flow. The water running towards the drain produces an evenly distributed shear stress and entrains the sediment. A certain distance is maintained between the bottom of the erosion head and the soil surface. Four ultrasonic doppler velocimetry (UDV) probes reside in the erosion head. The probes measure the distance between the erosion head and the soil surface. When erosion occurs, the movement rate of the erosion head represents the erosion rate. This ensures an optimal shape for the erosion chamber.
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 Laboratory In situ Scour Testing Device was designed to measure the erodibility of soils in a laboratory. A tested soil is accommodated in a transparent plexiglas tube with an inner diameter of 3 inches. The tube is half filled with sandy soil. A cylindrical erosion head with a diameter of two-and-a-half inches is fixed in the test tube above the soil. The special designed erosion head ensures uniform distribution of shear stress across the soil surface. Four ultrasonic doppler velocimetry (UDV) probes reside in the erosion head. The probes measure the distance between the erosion head and the soil surface. During a test, the distance is kept constant by using a motor to push up the soil in the test tube. Water flows down through the quarter-inch gap between the test tube and the erosion head. Water then flows radially toward the center of the erosion head. These radial flows generate horizontal shear stress to erode soils. In the center of the erosion head, an 8 millimeter diameter hole guides water out of the test tube. The pump power together with the distance between the erosion head and the soil surface can vary the magnitude of the shear stress acting on the tested soil.
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 Demo-ISTD consists is a smaller version of the Lab-ISTD and was designed in order to demonstrate the erosion concept of the in situ scour testing device within a laboratory, classroom, or fair setting. The ITEM frame holds a transparent plexiglas tube with an inner diameter of 50 millimeters. The tube is half filled with cohesive soil and is connected to a piston that moves up and down. A cylindrical aluminum erosion head inside the tube with an outer diameter of 45 millimeters is fixed above the soil and includes a three-dimensional printed plastic erosion chamber at the bottom that is shaped to allow uniform concentric flow. In the center of the erosion head, an 8-millimeter-diameter hole guides water out of the test tube. The specially designed erosion head ensures uniform distribution of shear stress across the soil surface. It does this by directing water flow towards the center of the erosion chamber from the gaps between the test tube and erosion head. A pump station that recirculates water in the system consists of a sucking pump, pressure pump, and a water tank. A control unit is connected with the frame and the pump station and allows the engineer to control the device in a manual or PC-controlled remote view. A certain distance is maintained between the bottom of the erosion head and the soil surface by measuring the pressure difference between the inflow towards the erosion chamber and outflow that runs out of the test tube.
The portable Demo-ISTD was designed to demonstrate the concept of the in situ scour testing device at conferences and exhibitions.

 

Scour in Cohesive Soils

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 photo shows the ESTD and its control station. It uses a pump and a moving belt to propel water in a 92-centimeter-long, 12-centimeter-wide, and 2-centimeter -deep channel. 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 millimeters and a height of 20 millimeters) on its sensor plate.
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.

 

The upper photo shows the pugger that mixes and de-airs the soil sample with a short cylindrical sample pushed out on the left. The lower photo shows a closeup of the sample in the erosion channel.
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.

 

New Multifunction Flume System (MFS)

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.

An isometric view of the multifunction (MFS) flume system design is shown with the 6-foot-wide channel modules. An inlet trumpet is located on the lower left corner, while a sediment catch and an outlet is shown at the upper right corner of the image. Between the inlet and the outlet the MFS is fitted with a 6-foot-wide testing channel to test the hydraulic performance and scour vulnerability of bridge substructure components.
Three-dimensional rendering of the assembled new multifunction flume system (MFS) fitted with a 6-foot-wide testing channel.

 

The computer-generated image shows the velocity distribution in the trumpet and immediate downstream of the trumpet. The velocity is color coded. It shows that the velocity distribution downstream of the trumpet is reasonably uniform and free of adverse pressure conditions.
Computational fluid dynamics (CFD) simulation on the inlet performance of the multifunction flume system (MFS).

 

Smart Scour Countermeasure

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.

This photograph illustrates an underwater multibeam three-dimensional sonar digital image (acoustic image) of the rock riprap mattress installation around pier 3 of the Middle Fork Feather River Bridge. The digital sonar scan also shows the footing of pier 3 and the bathymetry of the river bed around the rock riprap mattress. The areal extent of the riprap placement is depicted in detail.
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 and computational structural mechanics simulation results showing the dynamic pressure of a defined approach flow acting on the rock riprap mattress placed around pier 3 of 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.

 

Hydraulic Performance of Shallow Foundations for Support of Bridge Abutments

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).

This image illustrates a physical scaled model of a geosynthetic reinforced soil- vertical-wall bridge abutment during a test in the tilting flume. The superstructure seats on top on the abutment inducing vertical loading according to the similarity theory. The performance of the abutment is tested against flow velocities similar to flood-flow conditions in the field.
Hydraulic performance testing of geosynthetic reinforced soil (GRS) vertical wall abutment.

 

The image shows two examples of as-built riprap installations around a scaled model of a geosynthetic reinforced soil (GRS) vertical wall bridge abutment that were part of the matrix of tests in this research study. Both riprap installation layouts are based on observed installations in the field and design guidelines from Hydraulic Engineering Circular 23 (HEC23). The top photograph shows a riprap layout that is installed flush with the channel bed.
The bottom photograph shows a riprap layout installed with a revetment slope to the abutment front face. Both riprap installations have the same extent from the abutment front face into the main channel. The performance of these two layouts was compared for clear-water scour conditions.
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.

 

Bridge Pier Scour Research

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.

A photograph on the top shows a bathymetry scan of the scour hole around a scaled model (1:60) of the original pier 22 of the Feather River Bridge on Route 20 in Sutter County, California. The complex pier model has a 45-degree skew angle to the approach flow and is composed of 90 H-piles tied at the bottom by a rectangular footing. The scour hole is at a 50 percent stage of the maximum equilibrium scour and its deepest point is located at the outer downstream edge of the pier footing.
A figure on the bottom shows the computational fluid dynamics (CFD) simulation results of the 3D model described in the figure on the left, showing the bed shear stress distribution in the vicinity of pier 22 induced by simulated approach flow conditions occurred in March 2011. The largest value of the bed shear stress is right beneath and on the outer sides of the separated shear layers where the incoming flow is strongly accelerated.
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.

 

The photograph on top shows a  full-scale 3D model constructed from underwater sonar bathymetry scans and a digital elevation model (DEM) showing the scour hole around the original pier 22 in the main channel of the Feather River Bridge on Route 20 in Sutter County, California. The complex pier model 90 H-piles tied at the bottom by a rectangular footing. The scour hole elevation is below the pier footing exposing 19 feet of the H-piles at the pier upstream corner.
The figure on the bottom shows the computational fluid dynamics (CFD) simulation results of the 3D model described in the figure on the top, showing the bed shear stress distribution induced by simulated approach flow conditions occurred in March 2011. The bed shear stress magnitude in the scour hole around pier 22 is lower than on the river bed upstream and downstream of the bridge opening.
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.

 

 

 

Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000
Turner-Fairbank Highway Research Center | 6300 Georgetown Pike | McLean, VA | 22101