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14
CHAPTER 3
EROSION FUNCTION APPARATUS (EFA)
3.1 CONCEPT (mm/hr) is simply obtained by dividing the length
sion rate z
of sample eroded by the time required to do so.
The EFA shown in Figures 3.1 and 3.2 (Briaud et al.
1999, 2001, as well as http://www.humboldtmfg.com/pdf2/
h
hm4000ds.pdf and http://tti.tamu.edu/geotech/scour) was =
z (3.1)
conceived in 1991, designed in 1992, and built in 1993. A t
sample of soil, fine-grained or not, is taken in the field using
an ASTM standard Shelby tube with a 76.2-mm outside Where h is the length of soil sample eroded in a time t. The
diameter (ASTM D1587). One end of the Shelby tube full of length h is 1 mm and the time t is the time required for the
soil is placed through a circular opening in the bottom of a sample to be eroded flush with the bottom of the pipe (visual
rectangular cross-section conduit. A snug fit and an O-ring inspection through a Plexiglas window).
establish a leak-proof connection. The cross section of the After several attempts at measuring the shear stress in the
rectangular conduit is 101.6 mm by 50.8 mm. The conduit is apparatus it was found that the best way to obtain was by
1.22-m long and has a flow straightener at one end. The water using the Moody Chart (Moody, 1944) for pipe flows.
is driven through the conduit by a pump. A valve regulates the
flow and a flow meter is used to measure the flow rate. The
range of mean flow velocities is 0.1 m/s to 6 m/s. The end of 1
= f v 2 (3.2)
the Shelby tube is held flush with the bottom of the rectan- 8
gular conduit. A piston at the bottom end of the sampling
tube pushes the soil until it protrudes 1 mm into the rectan- Where is the shear stress on the wall of the pipe; f is the
gular conduit at the other end. This 1-mm protrusion of soil friction factor obtained from the Moody Chart (Figure 3.3);
is eroded by the water flowing over it. is the mass density of water (1,000 kg/m3); and v is the
mean flow velocity in the pipe. The friction factor f is a func-
3.2 EFA TEST PROCEDURE tion of the pipe Reynolds Number Re and the pipe roughness
/D. The Reynolds Number is vD/v where D is the pipe diam-
The procedure for the EFA test is as follows: eter and v is the kinematic viscosity of water (10-6m2/s at
20°C). Since the pipe in the EFA has a rectangular cross sec-
1. Place the sample in the EFA, fill the conduit with water, tion, D is taken as the hydraulic diameter D = 4A/P where A
and wait 1 hour. is the cross-sectional flow area, P is the wetted perimeter, and
2. Set the velocity to 0.3 m/s. the factor 4 is used to ensure that the hydraulic diameter is
3. Push the soil 1 mm into the flow. equal to the diameter for a circular pipe. For a rectangular
4. Record how much time it takes for the 1 mm of soil to
cross-section pipe:
erode (visual inspection through Plexiglas window).
5. When the 1 mm of soil is eroded or after 1 hour of flow,
whichever comes first, increase the velocity to 0.6 m/s D = 2 ab ( a + b) (3.3)
and bring the soil back to a 1-mm protrusion.
6. Repeat Step 4. Where a and b are the dimensions of the sides of the rec-
7. Then repeat Steps 5 and 6 for velocities equal to 1 m/s, tangle. The relative roughness /D is the ratio of the average
1.5 m/s, 2 m/s, 3 m/s, 4.5 m/s, and 6 m/s. height of the roughness elements on the pipe surface over the
pipe diameter D. The average height of the roughness ele-
3.3 EFA TEST DATA REDUCTION ments is taken equal to 0.5D50 where D50 is the mean grain
size for the soil. The factor 0.5 is used because it is assumed
versus shear
The test result consists of the erosion rate z
that the top half of the particle protrudes into the flow while
stress curve (Figure 3.1). For each flow velocity v, the ero- the bottom half is buried in the soil mass.