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Publication Number:  FHWA-HRT-15-033    Date:  May 2015
Publication Number: FHWA-HRT-15-033
Date: May 2015


Scour in Cohesive Soils


Because many erosion measurement devices do not duplicate open channel flow conditions or are too large and bulky for cost-effective routine testing, the ESTD was developed. It uses a moving belt and centrifugal pump to generate a log-law velocity profile in a channel where the tested soil is flush with the channel bottom.

The ESTD is designed to measure the erodibility of a cylindrical soil specimen under well-controlled flow conditions. The soil specimen has a diameter of 2.5 inches (63.5 mm) and a height of 0.6 inches (15 mm). The system has a total volume of about 145 gal (550 L). The device includes three tanks: an inlet tank, an ESTD tank, and an outlet tank. Inlet and outlet tanks are connected with a rectangular channel located in the ESTD tank. The channel is 22.8 inches (580 mm) long, 4.7 inches (120 mm) wide, and 0.79 inches (20 mm) deep. The device includes a flow meter and a direct force gauge to measure the force imparted by the flowing water on the soil sample. Figure 11 shows a three-dimensional representation of the ESTD.

The photo shows the inlet tank, test conduit, and outlet tank of the ex situ scour test device from right to left. The test channel is open, showing the belt apparatus. All tank sides are clear to facilitate viewing.
Figure 11. Photo. The ESTD

Two cascaded filter cylinders (Shelco, model: 12FOS3) filter the fluid in the system. Each cylinder has a diameter of 1.4 ft (0.42 m) and a height of 4.1 ft (1.25 m), housing twelve 2.5-ft (0.75-m)-tall wound filter cartridges with a filtration capacity of 0.00002 inches (0.0005 mm). The filtration ensures that the fluid near soil specimens is always clear for observation.

Water is propelled by a moving belt above the test channel and a pump as shown in figure 12. A moving belt rolls above the channel in the ESTD tank. The flow velocity profile is S-shaped when only the moving belt is propelling water.(28) The velocity profile takes the form of a parabola in the rectangular test channel when the belt is not moving as illustrated at section (1) in figure 12. Combining the S-shape profile from the belt alone with the parabolic profile from the pump alone may result in the desired log-law velocity profile as illustrated in figure 12 at section (2).

A range of grades of sandpaper are attached to the bottom of the channel to simulate bed roughness. The sandpaper grades are described later in this report.

Operation of the ex situ scour test device is illustrated by showing the circulation of flow from the inlet tank to the test channel to the outlet tank. Recirculation of flow from the outlet tank back to the inlet tank is also shown. The moving belt is shown located at the top of the test channel, with the location of the clay specimen above the direct force gauge near the end of the test channel. Velocity profiles are indicated upstream of the test channel (section 1) and in the test channel upstream of the clay specimen. The profile at section 2, shaped as a log-law velocity profile, is the combination of the profile at section 1, shaped symmetrically in a conduit, with that created by the belt which increases the velocity near the belt.
Figure 12. Diagram. Schematic of the ESTD


The moving belt has dimensions shown in figure 13. The roughness elements on the belt are 0.197 inches (5 mm) wide and 0.201 inches (5.1 mm) high. The net spacing between two adjacent roughness elements is 1.53 inches (38.8 mm). The distance from the roughness elements bottom to the ESTD tank bottom is 0.73 inches (18.5 mm).

The moving belt is enclosed in an aluminum case to minimize the influence of belt vibration on the flow beneath the belt. The case has a cutout on the bottom to expose the belt to the water in the channel. The width of the cutout is 4.53 inches (115 mm). Gaps exist between the belt and the two cutout boundaries. Total width of these two gaps is 0.59 inches (15 mm). The belt and aluminum case are mounted on the lid of the ESTD tank. This lid is closed during testing.

The diagram has three parts. The belt is shown in profile from the side on the upper left, in profile from the front in the upper right, and from the top in the lower half of the figure. The belt is 3.94 inches (100 mm) wide. Thirty roughness elements (profiles) are mounted on the belt spaced at 1.53 inches (38.9 mm) for a total belt length of 51.77 inches (1,314 mm). The belt specification is PU-100T5-1315.
1 inch = 25.4 mm.
Figure 13. Diagram. Dimensions of the moving belt


The ESTD is capable of instantaneously and precisely measuring horizontal shear and vertical forces on a soil specimen with a direct force gauge. The direct force gauge is specifically designed to measure small forces in a wet environment. A rubber membrane separates it into wet and dry parts. The core is a platform held by a bronze leaf spring. On top of the platform sits the sensor disk whose deflection indicates the magnitude of the shear force. A test soil specimen is fixed to the sensor disk so that the eroding forces acting on a soil specimen from the flow are directly measured.

Underneath the platform, a carrier holds two horizontally mounted permanent magnets that dip into two solenoids: SERVO and CALIBRATION. The magnet movement generates a counter movement to the sensor disk that keeps it in a fixed position with residual deflection of 0.0027 inches (0.068 mm) at 2.1 lb/ft2 (100 Pa). The principles of force measurements in the horizontal and vertical directions are applied as follows.

Principle of Horizontal Measurement

Assume a fluid-induced force, Fw, pushed the platform to right as shown in figure 14. A small horizontal deflection of the sensor disk will occur. This deflection moves the center magnet to the right and generates a positive error-voltage at the HALL Sensor. This voltage is amplified and drives a current through the SERVO-Solenoid. This current generates a magnetic field and pushes the permanent magnet to the left with a magnetic force, Fm. This motion lasts until the error-voltage at the HALL sensor diminishes to zero. A residual deflection of the platform always exists to generate the corresponding counterforce to the shear force.

This diagram shows the balance of horizontal forces. The assembly is attached to the tank bottom with flange connectors. The platform supporting the soil sample is suspended by leaf springs that control deflection from a zero position. Two solenoids convert the mechanical deflection to electrical signals. A servo-amplifier and variable source current regulate the signals.
Figure 14. Diagram. Principles of force measurements in the horizontal direction

The current through the SERVO-Solenoid is proportional to the induced force Fw. The correlation between the solenoid current and the generated magnetic force Fm is highly linear. This correlation indicates that the measurement of current represents the measurement of Fw. As all parameters in this SERVO loop are constant, the current measured with high accuracy is a basis for subsequent signal processing to obtain the shear force.

The accuracy and stability of this type of sensor is enhanced because the platform has virtually no deflection, which offers the following advantages:

  1. The sensor disk does not dive (change elevation) because of vertical deflection.

  2. Mutation of the leaf springs due to corrosion or plaque does not materially affect the accuracy because they simply hold the platform.

  3. A small gap between the sensor disk and the aperture ring is obtainable.

With respect to the gap between the sensor disk and aperture ring, it is conceivable that eroded clay particles in the fluid could be captured at the gap hindering accurate measurement. Therefore, a gap of 0.039 inches (1.0 mm) is preferred.

A built-in calibrator provides a valuable tool to calibrate the device. It utilizes the same principle discussed. For calibration, the induced force and the counterforce Fc are reversed compared with their use during testing.

Principle of Vertical Measurement

The deflection of the horizontally fastened bronze plate spring shown in figure 15 indicates that a vertical force is induced. The front view shows how the spring is mounted on the platform. On the bottom of the plate spring, a magnet is fixed to the end of a magnet holder. The gap between the sensor and the magnet is about 0.039 inches (1.0 mm).

Design drawing showing the balance of vertical forces. The soil specimen sits on a bronze spring plate. Magnetic sensors detect changes in the vertical force on the platform. Through a series of sensors and amplifiers, a signal is sent to a control mechanism that raises the platform keeping the top surface of the sample flush with the bottom of the flow channel.
Figure 15. Diagram. Principles of force measurements in the vertical direction

When a vertical force, dF, is induced to the sensor disk, the bronze plate spring bends with a small deflection, dz. The magnet is also lowered the same dz, which results in the change of magnetic field of the permanent magnet. The HALL sensor converts this change into a voltage signal, dU. The relations between the force, dF, the deflection, dz, and the voltage, dU, are highly linear. As with the horizontal measurement, this linearity allows precise determination of the vertical force. The measurement range of vertical force is 0 to 0.225 lbf (0 to 1 N) corresponding to 0 to 0.225 lb (0 to 102 g) of soil.


An important advantage of the ESTD is that the horizontal and vertical measurements work independently. Any vertical force slightly lowers the magnets Z1 and Z2 a maximum of 0.0039 inches (0.10 mm). However, this change does not influence the horizontal deflection of the platform or the shear stress. Similarly, any horizontal force will slightly push the platform horizontally a maximum of 0.0027 inches (0.068 mm), but this change does not affect the vertical magnetic fields. This independence allows simultaneous and precise measurements of both the horizontal and vertical forces.

One challenge for this type of force gauge is that it requires the test surface to be flush with the surrounding fixed surface when used in air.(29) Any depression or protrusion can result in additional forces resulting from flow disturbances caused by these surface discontinuities. For the ESTD, these forces essentially affect the initial shear stress measurement when defining the entrainment of clay clumps. Once erosion starts, the gauge measures the actual shear force on the soil specimen as in the field conditions when its shape evolves.

At the beginning of the test, a soil specimen is placed flush with the channel bottom. As erosion occurs, the specimen is elevated to maintain the flush condition. As further erosion occurs, the surface and volume of the soil specimen changes. The measured forces reflect those changes. The forces are acting on the soil specimen because it is fixed to the force gauge. To the extent that the eroding surface does not maintain a uniform surface, the measured shear force will include some form drag. Potentially, changes in erosion rates may occur because of the form drag. The form drag force cannot be isolated from the overall shear force.

The ESTD has the following advantages:

The ESTD also has the following limitations:

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