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21 CHAPTER 2 2. EXISTING EROSION TESTS This chapter describes some of the most common tests developed over the century to quantify soil erodibility. The drawbacks and advantages of these testing methods are evaluated and identified. Generally, erosion testing is divided into the two following groups: 1) Laboratory erosion testing 2) Field erosion testing The erosion tests presented in this chapter are divided into two sections: laboratory tests and field tests. It must be noted that some of the erosion tests have applications in both the laboratory and the field; however, they are discussed only once in either section 2.1 or 2.2. 2.1. Laboratory Erosion Testing Erosion Function Apparatus (EFA) In the early 1990s, the idea of the Erosion Function Apparatus (EFA) was first developed and established by Briaud at Texas A&M University (Briaud et al., 1999, 2001a, 2001b). Today, the EFA is being manufactured by Humboldt, Inc. and used widely by many engineering organizations. This test was originally developed to evaluate the erodibility of a wide range of both cohesive and non-cohesive soils including gravel, sand, clay, and silt. Figure 4 shows a schematic diagram as well as a photograph of the EFA device. Schematic diagram of the EFA EFA photograph Figure 4. Schematic diagram of the Erosion Function Apparatus and a photograph of the testing apparatus (Texas A&M University). Cross Section of Pipe 76.2 mm Pressure Port 50.8 mm 1016 mm Pressure Port 101.6 mm ï´ 50.8 mm Water Flow Soil PISTON PUSHING AT THE RATE = Z
22 Soil samples are taken using ASTM standard Shelby tubes with an outside diameter of 76.2 mm (ASTM, 1999). Similarly for soft rocks, a rock core sample can be extracted and placed into a Shelby tube for rock erosion testing. Water, as the eroding fluid, is driven using a pump into a 1.2 m long rectangular cross section 101.6 ï´ 50.8 mm as shown in Figure 4. The water flow can be adjusted using a valve, and the average water velocity is measured by a flowmeter in line with the flow. One end of the Shelby tube is placed on the bottom of a circular plate which is connected to a piston. The piston is designed to push the sample up into the flow when necessary. The sample is pushed upward until it becomes flush with the bottom of the rectangular cross section pipe. The test procedure is as follows (Briaud et al., 2001): 1) Place one end of the Shelby tube on the circular plate piston, push it up until it becomes flush with the bottom surface of the rectangular cross section pipe. 2) Fill the rectangular pipe with water and wait for one hour 3) Initiate the flow with a small flow velocity, typically 0.2 m/s. 4) Start recording time. Hold the sample surface flush with the bottom of the rectangular pipe during the induced flow velocity. The test operator needs to make sure that the soil surface is kept flush all the time by pushing the soil with the piston as it is eroded by the water and maintain a level interface. Continue this until 50 mm of the soil is eroded or 30 mins have passed. Read the protrusion height by observing the change in the height of the bottom of the piston. 5) Redo step 3 and 4 for a new and higher flow velocity (i.e. 0.2 m/s, 0.6 m/s, 1 m/s, 1.5 m/s, 2 m/s, 3 m/s, 4.5 m/s, and 6 m/s). The scour rate versus flow velocity is plotted. Figure 2 shows an example of EFA test results as both erosion rate versus velocity and erosion rate versus shear stress. The shear stress on the eroded surface of the soil is calculated by using Moody chart (Moody, 1944). Ï (7) Where, refers to the shear stress (Pa), is the density of water (1000 kg/m3), and is the flow velocity (m/s). is the friction factor obtained using Moody chart. Advantages 1) Minimize the sample disturbance effect, as it takes the sample directly from the field using Shelby tubes. 2) Can be used for natural samples as well as man-made samples 3) Gives the critical velocity and the critical shear stress. Can give the erosion function directly. 4) EFA test results are directly used as input to the TAMU-SCOUR method for bridge scour depth predictions (Chapter 6 of HEC-18). 5) EFA can test the erodibility of the soil at any depth as long as a sample can be recovered. 6) While the EFA test is a surface erosion test, it can be used to evaluate internal erosion as well because the EFA erosion function represents the erodibility of the soil at the element level. 7) Can test very soft to very hard soils. Very broad applications.
23 Drawbacks 1) Shear stress is indirectly measured from the average velocity using Moody charts which might not be accurate. Also, the average flow velocity is used in the calculations instead of the actual velocity profile. 2) In some cases that field samples are required, obtaining samples is difficult and costly. The test needs to be done on the sample before the sample is affected by long periods of storage. 3) Particles larger than 40 mm in size cannot be tested with confidence as the diameter of the sampling tube is 75 mm. 4) The EFA is a fairly expensive device (around $50k to purchase). Several other organizations have utilized the concepts and principles of the EFA and developed similar devices such as the Sediment Erosion Rate Flume (SERF) and the Ex-situ Scour Testing Device (ESTD) which are also presented in this chapter. Sediment Erosion Rate Flume (SERF) This apparatus was developed by Sheppard and his colleagues at the University of Florida to measure the erodibility of cohesive and non-cohesive sediments (Trammel, 2004). Figure 5 shows a photograph of the SERF. Schematic diagram of the SERF SERF photograph Figure 5. SERF apparatus at University of Florida (Trammel, 2004) SERF has a 9 ft. long rectangular channel with dimensions of 5.08 cm ï´ 20.32 cm elevated at 5.5 ft. which is fed by two 500 gpm parallel pumps from a large 1100-gallon water tank. The flow channel is designed to have a 1 ft. straightener in the beginning to reduce the turbulence of the water discharge. The specimen cross section is placed in the center of the rectangular channel at the base of the flume. Flow is driven through a 3 ft. long channel, and right after, reaches the specimenâs cross section. It then proceeds another 4 ft. of the channel and is directed to the
24 reservoir tank. The reason that two pumps are used is to account for harder soil samples, where both pumps can be running. Also, erosion of the Shelby tube size sample is continuously monitored by the control computer using an attached video camera next to the test section. An array of sonic SeaTek Transponders is attached at the top of the flume right above the test section; these transponders give the mean elevation of the sample surface which is used to prompt the computer to advance the piston and keep the sample surface flush with the bottom of the flume. Basically, SERF is controlled and monitored automatically by computer software. The summation of upward movements recorded and steps by the motor for a specific flow velocity (shear stress) divided by that particular time period reflects the erosion rate. SERF includes pressure ports at 2 ft. upstream and downstream from the center of the test section which calculates the pressure drop in the flume using the following equation. Ï (8) Where, refers to the hydraulic shear stress (Pa), Î is the recorded pressure drop (Pa), Area is the cross-section area of the rectangular channel, L is the distance between pressure ports (which is 4 ft.), and (2w+2h) refer to the hydraulic radius in the channel. In addition to similar advantages mentioned for EFA, SERF is independent from the operator and runs with an automated system. Advantages 1) Can be used for natural and man-made samples. 2) Can give the erosion function directly. 3) Runs with an automated system. Drawbacks 1) SERF is no longer being used; however, the device requires a bulky setup. 2) The automation of the process requires the use of very expensive instruments which also require significant expertise when they break down or need to be adjusted. 3) The samples have to be prepared in the cylinder of the test device. This limits the use of SERF to disturbed or man-made samples. Ex-situ Scour Testing Device (ESTD) The Ex-situ Scour Test Device (ESTD) was developed by Kerenyi and his colleagues at the FHWA Turner Fairbanks research center (Shan et al., 2015). The purpose was to simulate the velocity profile for open channel conditions. Figure 6 shows a schematic diagram along with a photograph of this test device.
25 Schematic diagram of the ESTD ESTD photograph Figure 6. Schematic diagram and photograph of Ex-situ Scour Testing Device A cylindrical soil specimen with the diameter of 63.5 mm and height of 15 mm is placed on the top of a direct force gauge. A 580 mm rectangular channel with dimensions of 12 cm ï´ 2 cm connects the inlet tank to the outlet tank. Similar to EFA, a flowmeter is attached to the device to measure the flow velocity in the channel. A moving belt, as shown in Figure 6, is used to entrain the water and reproduce the expected log-law velocity profile in the field. Also, the roughness of the channel is controlled by attaching a wide grit range of sand papers to the bottom of the channel surface. Instead of calculating the hydraulic shear stress at the interface of the eroding fluid and the soil indirectly from velocity or pressure drop, a direct force gauge is used to instantaneously capture both normal forces and shear stress induced on the soil surface. The samples for the ESTD are prepared in the lab typically using a Pugger Mixer which prevents existence of air bubbles in the specimen. Also, the samples are left in water to slake before performing the ESTD test. The advantages and drawbacks of the ESTD are. Advantages 1) ESTD is automated. 2) ESTD is designed to reproduce the open channel flow condition. 3) Existence of sensors to measure the vertical force and shear stress directly, are very helpful. 4) Effect of turbulence can be more precisely studied using the results of the vertical force on the interface. Drawbacks 1) The ESTD set-up is time consuming (up to couple of days). 2) It cannot reflect the actual field conditions, as the soil specimens are all hand-made in the lab. In other words, the intact sample from the field cannot be tested directly in the ESTD. Sediment Erosion at Depth Flume (Sedflume) This apparatus was originally developed at University of Santa Barbara for the purpose of measuring the sediment erosion at high shear stress and with shallow depth (McNeil et al., 1996). Inlet Tank Outlet Tank
26 Sedflume has been used by researchers for coastal applications and by the US Army Corps of Engineers. The primary application of this device is to study sediment transport and suspension rate during high stress floods. Figure 7 shows a schematic diagram of Sedflume along with a photograph of the testing device. Schematic diagram of the Sedflume (McNeil, 1996) Sedflume photograph (USACE) Figure 7. Schematic diagram of Sedflume with a Sedflume photograph The channel of the flume is 120 cm long and the rectangular cross section is 10 cm ï´ 2 cm as shown in Figure 7. The Sedflume also exists as a portable lab which can be moved to a site. At the end of the straight channel is a 15 cm long section where the top of the soil sample is exposed. The rectangular soil sample is 1 m long and has cross section dimensions of 15 cm long ï´ 10 cm wide. Test samples are collected in two ways: directly from the site, or recreated in the lab using a sediment slurry. In deeper waters, divers might be needed to put the coring tube and extract sample from the site. In soft soils, the sampler can be pushed into the soil while in stiffer soils, a Vibracoring head can be used. The test sample is then placed on a piston with a hydraulic jack which is used to manually adjust the height of the sample. As in the EFA, the operator needs to keep the sample flush with the flume surface. The flow also is adjusted by a valve, and a flowmeter is attached to the flume to measure the flow rate of the eroding fluid. Water powered by a pump flows into the flume and testing proceeds in the following steps (McNeil et al., 1996): 1) Place one end of the rectangular coring tube on a piston located at the bottom of the test section. 2) Use the piston to extrude the sample upward until it becomes flush with the bottom of flume surface. 3) Make sure that as the soil surface erodes, you maintain a level interface between the sample and the bottom of flume surface. 4) Record the amount of eroded sediments by recording the upward movement of piston. 5) Repeat the steps 2 to 4 for higher flow velocities and thus higher shear stress values. As for the EFA, the shear stress is calculated using Eq. 7. Advantages and drawbacks of the Sedflume are presented below:
27 Advantages 1) The field version helps minimize the sample disturbance effect. Also, with the field version, the existing water in the field can be used in the test. 2) Directly measures the erosion rate versus shear stress curve from the field, and determines the critical shear stress Drawbacks 1) The shear stress calculation is based on the mean flow velocity and the use of Moody chart instead of direct measurements. 2) The lab apparatus is very bulky and costly (more than $100k). The sample preparation is time consuming and the test setup in the field is also time consuming. 3) Only disturbed or reconstructed samples can be tested. 4) There are several limitations in sample collection especially for offshore conditions. Roberts et al. (2003) developed a similar device to the Sedflume called the Adjustable Shear Stress Erosion and Transport (ASSET) Flume. ASSET Flume was designed to be larger than Sedflume to overcome a common problem with all flume tests, the channel wall effects on the flow. The other difference was that the eroded sediments were collected and then dried to obtain the bed-load and suspended fractions in the flume. Some other flume tests In addition to the EFA and similar tests, several researchers have performed flume tests to study the erodibility potential of different soils. Gibbs (1962) conducted some large-scaled flume tests in the laboratory on intact soil samples (mostly CL and ML) from canal banks. The purpose of his tests was to investigate the influence of different plasticity properties and in situ density of the soils on the erosion resistance of these soils. His findings will be summarized in the next chapter of this report. A few years later, Lyle and Smerdon (1965), from Texas A&M University, constructed a laboratory flume test to study the effect of soil properties (especially compaction) on erosion resistance of soils. Lyle and Smerdon (1965) conducted erosion tests on seven Texas soils in a 22 m long hydraulic flume with a 76 cm ï´ 40 cm cross sectional area. The slope of the flume was 0.2%. The 5.5 m long sample soil was placed in the center cross section of the flume. The velocity of the water flow was measured using six pitot tubes installed in 6 different points of the flow. The depth of flow was also measured accurately using 17 piezometers along the flow channel. The shear stress induced on the eroded surface was calculated as the product of hydraulic gradient (s) and water unit weight. Lyle and Smerdon then defined the critical shear stress as the shear stress which initiates the erosion of the soil. The samples that were replicated using the suspended samplers during each increase in the flow were tested in the lab and the plasticity index and void ratio (compaction) were recorded. The critical shear stress was then plotted against the compaction and plasticity properties of the soil. The findings of Lyle and Smerdon (1965) will further be discussed in the next chapter of the report.
28 Kandiah and Arulanandan (1974) also used flume tests to study the erodibility of Yolo lam clay. The main purpose of their research was to compare the erodibility results obtained from the flume test and the rotating cylinder test. Also, the effect of compaction and water content on the critical shear stress was investigated. Some of their findings will be discussed in the next chapter of this report. Arulanandan and Perry (1983) studied the erodibility potential of dam core materials used for better representing the common dam filter design method which was being practiced during that time. In order to quantify the critical shear stress imparted on the cracks within the core of the dam, Arulanandan and Perry used both flume tests and rotating cylinder tests. The research on evaluating the soil properties were continued and different researchers used different approaches to find the erodibility parameters. Shaikh et al. (1988) performed flume tests to evaluate the influence of clay material and compaction of soil on the erosion resistance of soils. To do so, they constructed a 250 cm long rectangular channel with a 15.5 cm ï´ 11 cm cross-section (see Figure 8). The slope of the flume was adjustable. As shown in Figure 8, three samples which were 15.2 cm long with a 10.5 cm ï´ 2.25 cm cross section, could be tested at the same time with the same slope. The flow depth could be adjusted between 80 cm and 210 cm. The flow velocity was also measured using a pitot tube as shown in Figure 8. Figure 8. A schematic diagram of the flume test used by Shaikh et al. (1988) Chow (1959) calculated the shear stress induced on the surface of the sample by using the smooth channel flow equation: â 5.5 5.75 log â (9) Where, â is a parameter called the shear velocity ( ), V refers to the velocity of flow at a depth y in the turbulent zone, is the water density, and is the viscosity of water. Shaikh et al.
29 (1988) defined the erosion rate as the rate of weight removal in a given time. Then, they could plot the erosion rate (N/m2/min) versus hydraulic shear stress (Pa). Six years later, Ghebreiyessus et al., (1994) developed a new enclosed flume test to study the soil resistance to erosion as well as the influence of geotechnical parameters in soil erodibility. For this purpose, a 250 cm long rectangular flume with a 20.3 cm ï´ 2.5 cm cross section was constructed (Figure 9). The flume dimensions were selected to generate a steady flow condition according to Chow (1959) equations. Figure 9. A schematic diagram of the constructed enclosed flume (Ghebreiyessus et al., 1994) The test samples were cylindrical with a 10.2 cm inner diameter and were mounted on a mechanical piston to maintain a level interface with the bottom surface of the flume. Erosion rates were calculated as the rate of dried mass removal in a given time. The shear stress on the soil sample was predicted as: Ï (10) Where, (Pa) is the hydraulic shear stress on the soil surface, hL (m) refers to the head loss measured using two stand pipes at both sides of the sample, R (m) is the hydraulic radius of the flume, L (m) is the length of flume, and (N/m3) refers to the unit weight of water. Several other attempts were also made to develop a flume erosion testing apparatus. Some examples are the attempts made by Navaroo (2004), Hobson (2008), and Wang (2008) all at Georgia Tech University to modify the EFA method. Jet Erosion Test (JET) The JET is an erosion test which can be credited to Hanson and developed at the USDA-ARS (Hanson, 1990). Hanson (1990) first developed this testing device for the purpose of measuring the soil erodibility in situ. The JET test was standardized as ASTM D5852 in 1995 and includes both the in situ and the lab version of the JET test. It included a nozzle with a diameter of 13 mm,
30 which was held 22 cm away from the center of soil surface. Figure 10 shows a schematic diagram of the in situ version of the JET apparatus. To read the change in the depth of the hole made in the soil by the jet, a pin profiler is used after each jet sequence (Hanson, 1990). The JET test has been modified since its inception. Hanson and Hunt (2007) developed a new laboratory version of the JET apparatus. The circular jet submergence tank in this version has a diameter of 305 mm, and its height is 305 mm. The scour readings are made using a point gauge which is aligned with the orifice, and measures the scour in the center of the specimen. The soil specimen is compacted in a 4â standard compaction mold which is centered in the jet submergence tank and placed below the jet nozzle. The distance between the nozzle and the soil surface in the standard compaction mold is 33 mm. JET is currently used by some Department of Transportations (DOTs) and engineering firms. Schematic diagram of the JET (ASTM D5852- 95) JET photographs of lab version (Hanson & Hunt, 2007), In situ (Hanson & Cook, 2004) Figure 10. Schematic diagram of submerged JET apparatus for field testing (ASTM D5852-95) along with the photographs of lab version and in situ version of JET (Hanson and Hunt, 2007) The step by step procedure of a JET test in the laboratory is (Hanson and Hunt, 2007): 1) Compact the sample in the 4â standard compaction mold, and trim the top surface. 2) Center the specimen in the submergence tank right below the jet orifice. Fill the tank with water. 3) Adjust the pressure head at the jet orifice to be 775 mm. 4) Direct the water jet at a given velocity perpendicularly to the soil surface and record the depth of the hole made by the jet as a function of time (not more than 2 hours), while holding the jet in a stationary position. The last version of the JET test is a miniature of the original JET apparatus, called the mini JET. It was first used in the field by Simon et al. (2010) at 35 sites in Oregon (Al-Madhhachi et al., 2013). Compared to the previous versions, it is easily portable, and can be used both in the field and the lab on the 4â standard compaction mold sample. The submergence tank in the mini JET has a dimension of 101.6 mm, and height of 70 mm. The adjustable mini JET nozzle is 3.18 mm in diameter, and the head pressure at the nozzle is typically 450 to 610 mm.
31 Figure 11 shows the stress distribution at the soil surface proposed by Hanson and Cook (2004). The erosion rate is calculated as the slope of the curve linking the depth of the hole to the time of jetting. The shear stress associated with the jetting process is calculated as a function of the maximum stress due to the jet velocity at the nozzle using the following equation: Ï C (11) Where, is the coefficient of friction (typically 0.00416), is the velocity of the jet at the origin ( 2 ), is the fluid density, is the initial jet orifice height from the soil surface, and is called the potential core length (6.3 nozzle diameter). The critical shear stress, , is defined as the stress which exists when the hole is deep enough that the jet is no longer adequate to cause additional downward erosion (Hanson and Cook, 2004). To describe the relationship between the JET erosion rate and the jet velocity or calculated shear stress (erosion function), Hanson used a linear relationship and called the slope of the line the erosion coefficient KD (Hanson, 1991 and 1992; Hanson and Cook, 2004): ï¨ ï©D cz K ï´ ï´ ï· ï½ ï (12) Figure 11. Stress distribution at the soil surface in Jet Erosion Test Based on many JETs performed over time, Hanson classified the erodibility of soils according to their KD value as shown in Figure 12.
32 Figure 12. Jet Erosion Test: Hansonâs classification according to the erosion coefficient (Hanson and Simon, 2001; Chedid et al., 2018) Advantages 1) The JET can be run both in the field and in the lab. 2) The latest version of the JET is simple, quick, and inexpensive (around $15k to purchase) compared to other types of erosion test. 3) The JET can be performed on any surface vertical, horizontal, and inclined (Hanson et al., 2002). 4) The JET is good as an index erodibility test. Drawbacks 1) Particles larger than 30 mm in size cannot be tested with confidence because of the small size of the sample. 2) Coarse grained soils (i.e. non-cohesive sand and gravel) tend to fall back into the open hole during the jet erosion process thereby making the readings dubious. 3) Very small-scale test application. 4) Typically used for man-made samples. Natural are more difficult to test 5) The flow within the eroded hole and at the soil boundary is complex and difficult to analyze. 6) Only gives three of the erodibility parameters ( , , and ) out of the five possible parameters. 7) The elements of erosion are inferred rather than measured directly. 8) There are multiple interpretation techniques to predict the critical shear stress which give significantly different results.
33 Prior to Hansen (1990), a few scholars had conducted some studies on erodibility of soils by shooting jet into the surface of the soil. Here is summary of some of that work. Jet apparatus to measure the tractive resistance of cohesive channel beds Dunn (1959) used a jet test to calculate the critical shear stress of cohesive channel beds. This test contained a vertical submerged impinging jet perpendicular to the soil surface. Dunn observed that the location of the maximum shear stress is the same for different pressure heads at the nozzle. In order to measure the induced shear stress, Dunn used a device that included a steel plate which was almost fully covered with soil particles, except for a one square inch area at the location of maximum shear stress that was not covered with any soil. Dunn also measured the vane shear strength of the tested samples. Using this approach, he was able to observe the change in maximum shear stress with change in vane shear strength for each soil sample. Dunn found that the vane shear strength was proportional to the maximum shear stress at the start of the erosion process. Dunn proposed that the most important soil properties affecting resistance to erosion were the percent of clay and silt, the soil plasticity and the grain size distribution. A summary of his findings in correlating erodibility parameters to soil properties is presented in the next chapter. This jet apparatus was conceptually similar to the JET developed later by Hanson (1990), and therefore, has similar advantages and drawbacks that were discussed earlier. Submerged jet test at the University of Texas Moore and Masch (1962) developed a submerged jet test at the University of Texas. In the proposed test, the change in the scour depth of the sample was obtained for different jet velocities but the hydraulic shear stress at the soil surface was not calculated. Moore and Masch used a cylindrical sample with a diameter of 127 mm (5 inches) and height of 101.6 mm (4 inches). The jet velocity was kept constant for more than one hour; meanwhile, the eroded weight of the sample was recorded every 10 minutes. Using the volume of the soil removed, the change in depth of the hole was calculated. The same procedure was repeated for higher velocities, and data were compared. It was inferred that the depth of the hole in the sample can be affected by the following parameters: velocity of submerged jet, diameter of the impinging jet, head pressure at the jet nozzle, eroding fluidâs viscosity, and the âscour resistance of the sedimentsâ. Figure 13 shows a schematic diagram of the vertical jet scour test. This jet apparatus was conceptually similar to the JET developed later by Hanson (1990), and therefore, has similar advantages and drawbacks that were discussed earlier.
34 Figure 13. Schematic diagram of the vertical jet scour test developed by Moore and Masch (1962) Moore and Masch defined a variable called the Scour Rate Index, Ks. This parameter was the slope in the , where Save was the depth of the scour hole for a specific jet velocity, h0 was the distance from the jet orifice to the soil surface, t was the time of scour for a specific velocity, refers to the viscosity of the eroding fluid, is the eroding fluid density, and d is the nozzle diameter. Moore and Masch could observe that measured Ks was linearly correlated with the Reynolds number. The work of Moore and Masch was studied and used by Hanson for his JET analysis. Rotating cylinder apparatus developed in University of Texas In addition to trying a submerged jet test, Moore and Masch (1962) developed a new scour testing apparatus which worked by subjecting the sample to a rotating flow around the side of the cylinder. The main purpose to implement this device, was that by that time, other available tests for predicting the erodibility of soil (i.e. jet apparatus developed by Dunn (1959)) could not accurately measure the hydraulic shear stress on the surface of the soil. A 76.2 mm (3 inches) diameter cylinder of cohesive soil with a length of 76.2 mm (3 inches) was used as the test specimen. The testing apparatus included a larger translucent cylinder which had the option to rotate around the vertical axis. The maximum rotation speed that the apparatus could handle was 2500 rpm. The test specimen was then placed into the cylinder coaxially, and the residual space between the sample and translucent cylinder was filled with the eroding fluid. The fluid rotation would apply the shear stress onto the soil surface. Figure 14 shows a schematic diagram of the rotating cylinder testing device.
35 Figure 14. Schematic diagram of the rotating cylinder test developed by Moore and Masch (1962) As shown in Figure 14, the soil sample was constructed around a flexure pivot, which could help calculate the torque directly applied to the sides of the sample. The induced torque on the sample was then calculated and thus, the shear stress could be back calculated. One of the challenges that Moore and Masch had, was to choose the right liquid as the shear transmitter in the annular space of the cylinder. They found that the use of some liquids such as glycerin would form a resistive layer on the soil surface. Finally, they decided to use water in the annular space to transmit the shear stress onto the side surface of the sample. The test procedure is explained below: 1- Place the cylindrical 76.2 mm diameter sample in the apparatus. 2- Fill the empty space by water. 3- Increase the rotating speed, until you could observe that the surface scour is happening. 4- Record the reading of the torque. Using the calculated torque, measure the shear stress. It is worth mentioning that in order to calibrate the test, the operator needed to use a dummy sample, and induce a known torque on the sample. Then the rpm required to rotate the sample was recorded. This way, a plot of different rpms and different torques would be obtained. Moore and Masch recommended to choose the rate of mass removal during a particular time period as the erosion rate. The rotating cylinder test apparatus was later used and modified by some researchers. Masch et al., (1965) worked more on the recommendations made by Moore and Masch (1962) and subsequently developed the original guideline for the rotating cylinder test. Arulanandan et al. (1973) slightly modified the previous version of the rotating cylinder and studied the effect of clay mineralogy on the erodibility of the soils by conducting some tests on Yolo loam. Kandiah and Arulanandan also used the rotating cylinder test and compared their results with the results of a flume test on the Yolo loam clay. Arulanandan et al., (1975) used the modified rotating cylinder with the purpose of studying the effect of pore fluid composition and also the concentration of salt in the eroding fluid. The soil samples they used were remolded saturated soils. A summary of their results will be presented in Section 3.1of this report. Some of the advantages and drawbacks of the early version of the rotating cylinder test are:
36 Advantages: 1) Contrary to most erosion tests, a very small amount of water is needed. 2) The shear stress can be directly estimated using the induced torque on the side surface of the specimen. 3) Can generate very high shear stresses. 4) The influence of the physico-chemical properties of the eroding fluid (i.e. pH, salinity) on erosion rate can be easily studied. Drawbacks: 1) Due to the existence of the shaft within the soil sample in the apparatus, the test can only be conducted on remolded samples. 2) The samples need to be cohesive and strong enough to stand under its own weight therefore testing of coarse grain soils and soft clays and silts in not possible. 3) There is no direct measurement of torque, as the induced torque are calibrated based on the results on dummy samples. Improved rotating cylinder test Chapius and Gatian (1986) used the same principles of the rotating cylinder apparatus developed by Moore and Masch (1962), and improved the testing technique in order to be able to test not only remolded samples but also intact samples in the test. Before this, the rotating cylinder test could only run erodibility test on reconstructed clays and recreated mixtures in the lab. As mentioned earlier, the clay samples were reconstructed around a center shaft which made it possible to measure the torque on the sample. In the apparatus developed by Chapius and Gatian (1986) no flexure pivot was in the middle of the sample; therefore, the intact sample could be placed between the upper and bottom end of the device. Figure 15 shows a photograph of the testing apparatus. The other advantage of this version of the rotating cylinder compared to earlier versions was that it could directly measure the torque through a pulley weight system. The weight system included masses ranging from 0 to 40 grams and had a precision of 0.1 g. The device could also produce a maximum 1750 rpm rotational speed. It was observed that the roughness of the side surface of the soil was constantly changing during a test, and therefore was affecting the shear stress measurements.
37 Figure 15. A photograph of the improved rotating cylinder test (Chapuis and Gatian, 1986) The step by step procedure of the improved rotating cylinder test was (Chapuis and Gatian, 1986): 1) Place the 75 mm diameter clay sample with a height of 89 mm into the apparatus cylindrical cell. 2) Depending on the purpose of the test, fill the annular space around the sample with the eroding liquid (it can be water, or other chemical liquids). 3) Induce a stationary torque to the sample using the pulley weight system. 4) On the same rpm, record the induced shear stress for stages of 10 to 30 minutes. 5) After each stage, collect the eroded samples, oven dry and record the eroded mass for that particular time under that particular shear stress. Advantages: 1) Small amount of water is enough compared to most other erosion tests. 2) Can generate high shear stresses. 3) Can be conducted on remolded and intact samples. 4) Directly measures the torque through a pulley weight system. Drawbacks: 1) The samples need to be cohesive and strong enough to stand under its own weight therefore testing of coarse grain soils and soft clays and silts in not possible. 2) Expensive test device to build (more than $30k). Rotating Erosion Test Apparatus (RETA) Kerr (2001) and Sheppard et al. (2006) followed the similar concept and modified the previous versions of the rotating cylinder test. The modified version which is called, Rotating Erosion Test Apparatus (RETA) was constructed at University of Florida. This test was modified to be used
38 only for stiff clays and hard rocks such as sandstone and limestone. It basically holds the same constraint which also exists in previous versions: using a self-supporting sample. The RETA test can be run for several days which is required for more erosion resistant materials such as rock. The samples tested with RETA can be both 61 mm (2.4) inches and 101.6 mm (6 inches) with a height of 101.6 mm (4 inches). The test apparatus is equipped with a torque transducer at its base and a load cell to record the weight of the sample. It is also equipped with water cooling system to reduce the temperature for long tests (more than 72 hrs for rocks). The central shaft still exists; therefore, intact samples are not usable unless a center hole can be drilled through them. After drilling the hole, the sample is oven dried, and placed in the device to saturate. During the saturation, the device applies a very small torque to the sample for at least a day to remove any loose material. Sheppard et al. (2006) believed that the results of shear stress would be unexpectedly large if the loose material is not removed before doing the test. After the sample is saturated, it will be inserted into a sleeve and placed in the RETA cylinder for the test. Figure 16 shows a photograph of the testing machine. Advantages: 1) Can be run for very long time (more than 72 hrs for rocks). 2) Can generate very high shear stress values 3) Equipped with a torque transducer 4) Control the flow temperature during the test Drawbacks: 1) Modified to be used only for stiff clay and hard rocks such as limestone 2) Intact samples are not usable unless a center hole can be drilled through them. 3) Similar to earlier versions, samples need to be self-supporting. 4) Bulky and expensive test device to build (more than $30k). Schematic diagram of the RETA RETA photographs
39 Figure 16. A schematic and a photograph of the Rotating Erosion Testing Apparatus (RETA) at University of Florida (Bloomquist et al., 2012) Pinhole Erosion Test Sherard et al. (1976) developed a laboratory test to measure qualitatively the erodibility of fine- grained soils. In this test, distilled water was passed through a drilled hole under a pressure head of 51 mm in the center of the sample, and the erosion resistance of the soil was observed. The punched hole in the center of the sample has a diameter of 1 mm. The test was particularly designed to study and simulate the leakage effect in both dispersive and non-dispersive fine-grained soils, which was the case in most earth embankments. The pinhole test was later standardized as ASTM D4647. The test consists of compacting a 38 mm long soil sample in a 33 mm inside diameter plastic cylinder. A truncated jet nozzle with a diameter of 1.5 mm directs the water through the punched 1 mm dimeter hole in the center of cylindrical specimen. Figure 17 depicts a schematic diagram of the test apparatus. Sherard et al. (1976) noticed considerable differences in the behavior of dispersive and non- dispersive clays when subjected to water flow by observing the appearance of the flowing water and final size of the hole in the tested sample. With some limitations, the test could be done on intact field samples. Figure 17. A schematic diagram of the Pinhole Erosion Test apparatus (ASTM D4647) The procedure for this test is: 1) Create a 38 mm long sample by compacting the soil in the test cylinder above the coarse sand space which is covered by a wirescreen (Figure 17).
40 2) Make sure that the soil specimen is representative of the field conditions in terms of moisture content and dry unit weight. 3) Push the truncated cone jet nozzle into the center of the cylindrical sample. 4) Punch a 1 mm diameter hole in the center of the sample, using the test wire punch. 5) After removing the wire punch and placing a wire screen on the top of sample, fill the remaining space with coarse sand. 6) Start the test by shooting the jet into the hole with a pressure head of 51 mm. 7) Continue the test up to 5 minutes. Depending on the effluent cloudiness and the measured flow rate decide to continue the test or not for higher pressure heads as described in the ASTM standard. The results of the pinhole erosion test are interpreted using Table 2. Based on the criteria defined in Table 2, the erosion resistance of soil is classified in one of the 9 defined categories. Table 2. Interpretation the results of the pinhole test (ASTM D4647) Advantages: 1) Simulate the leakage in dispersive and non-dispersive fine-grained soils. 2) Stanrdized as ASTM. 3) Directly measures the dispersibility of clay soils. 4) Relatively lower cost to build the device compared to other erosion tests (less than $10k). Drawbacks: 1) Qualitative results. 2) Mostly applicable for remolded samples; however, can be run on intact samples with some limitations. 3) Not applicable for coarse-grained soils (i.e. coarse sand and gravel). Drill Hole Test Lefebvre et al. (1985) developed a new technique to predict the internal erosion resistance of natural clays. The concern was the erodibility of the natural clays in the Eastern part of Canada. The test was inspired by the earlier version of the pinhole erosion test. The testing apparatus uses
41 a 10 cm long cylindrical sample with a diameter if 35.5 mm. At the center of the sample, a 6.35 mm hole is drilled. Schematic diagrams of the test are shown in Figure 18. The test is conducted by circulating water through the bored hole into the sample. The pressure drop through the sample is measured using a differential manometer connected to both sides of the sample. A tank is used to produce a 143 cm pressure head and thereafter direct the flow through the sample. The flow is adjusted using a valve and measured by a flowmeter. Schematic diagram of the Drill Hole Test Schematic diagram of the sample in the Drill Hole Test Figure 18. Schematic diagrams of the whole Drill Hole Test assembly along with the sample setup (Lefebvre et al., 1985) Using the adjusting valve, the flow velocity is increased by 0.5 m/s every 15 minutes. The deposited sediment in the reservoir tank is dried and weighed to measure the mass removal rate for that particular velocity. The average diameter of the hole is recorded after each step. The shear stress is calculated using the pressure drop measured by the manometers. The results are reported as removed mass versus velocity or shear stress. Lefebvre et al. (1985) also observed that the change in roughness of the hole during the test can be interpreted using Moody diagram (Moody, 1944), knowing the friction factor and the Reynolds number. Erosion at the clay particle level is accompanied with an increase in the hole smoothness (decrease in relative roughness), while erosion of lumps of clay particles lead to an increase in hole roughness. Advantages: 1) Has a larger hole diameter compared to pinhole erosion test (6.35 mm vs. 1 mm) which minimizes the soil disturbance. 2) Results in quantitative erodibility results 3) Relatively lower cost to build the device compared to other erosion tests (less than $15k).
42 Drawbacks: 1) Shear stress is indirectly measured using Moody charts which might not be accurate. 2) Mostly applicable for remolded samples; however, can be run on intact samples with some limitations. 3) Not applicable for coarse-grained soils (i.e. coarse sand and gravel). Hole Erosion Test (HET) The HET is a laboratory erosion test which evolved from the older pinhole erosion test and can be credited to Robin Fell in Australia (Wan and Fell, 2002; Wahl, 2009; Benahmed and Bonelli, 2012). The test (Figure 19) consists of drilling a 6 mm diameter hole through a soil sample and forcing water to flow through the hole at a chosen velocity while recording the rate of mass removal per unit area as a function of time to obtain an erosion rate (kg/s/m2). The soil is compacted in a 100 mm (4 in.) diameter standard compaction mold. Similar to drill hole test, the sample is connected to a tank which can maintain a variable head ranging from 50 to 800 mm. The flow is also controlled through a valve. The cost of building the device ranges from $20k to $35k. The rate of mass removal per unit area is calculated as , where is the change in diameter of the hole, is the dry unit weight of the sample, and is the change in time. Since the diameter of the hole cannot be monitored during the test, this value is indirectly predicted using the measured flow rate, the hydraulic gradient, and Eqs. 13 and 14 (in SI units). Eq. 13 refers to laminar flow conditions, while Eq. 14 refers to turbulent flow. Turbulent flow is associated with a Reynoldâs number higher than 5000. , (13) ,, (14) Where, Qt is the flow rate at time t, fLaminar,t and fTurbulant,t are the estimated friction factors at time t, is the water unit weight, g is the ground gravity acceleration, and st is the hydraulic gradient obtained from the manometers at both ends of the sample. In these equations, the friction factors are estimated using the recorded hole diameter before and after the test. The test results link the rate of mass removal per unit area to the net shear stress above critical; the shear stress on the wall of the hole is estimated using Eq. 15. This equation is obtained after considering force equilibrium on the body of the eroding fluid along the pre-formed hole at a particular time, t. (15) Where, is the density of water, is the hydraulic gradient across the hole, and is the diameter at time t. The equation used for the erosion function is linear ï¨ ï©e cm C ï´ ï´ ï· ï½ ï (16) Where, Ce (sec/m) is called the erosion coefficient, and is the rate of mass removal. The erosion rate index is then defined as:
43 log (17) Wan and Fell went on to propose some erosion categories based on IHET (Table 3). Schematic diagram of the HET Photograph of the HET sample setup Figure 19. A schematic diagram of the HET and a photograph of the sample setup Table 3. Hole Erosion Test â Fellâs classification according to the erosion index (Wan and Fell, 2002) Advantages 1) Direct similitude with piping erosion in earth dams 2) Can apply a wide range of pressure heads and therefore wide range of hydraulic shear stress at the soil-water interface. Drawbacks 1) The sample needs to be cohesive and strong enough to stand under its own weight. Therefore, the test cannot be run on cohesion-less samples. 2) Very difficult to run on intact samples in Shelby tubes from field. Better for remolded samples in the lab. 3) Difficult and time-consuming preparation of the test (up to days). 4) No direct monitoring of the erosion process. The erosion rate needs to be extrapolated and inferred. 5) The hydraulic shear stress is inferred, and not directly measured.
44 6) The data reduction process is subjective. 7) The flow within the eroded hole and at the soil boundary is complex and difficult to analyze. Slot Erosion Test (SET) Slot Erosion Test was also developed by Wan and Fell (2002) in Australia. The concept of SET is very similar to what was explained for the HET, except that the sample is different in this test. The test (Figure 20) consists of drilling a 2.2 mm wide, 10 mm deep slot at the surface of a 1 m long rectangular soil sample and having water within the slot at a chosen velocity while recording the rate of mass removal per unit area as a function of time to obtain an erosion rate (kg/s/m2). The cost of building the device ranges from $20k to $35k. Schematic diagram of the SET Photograph of the SET sample setup Figure 20. A schematic diagram of the HET and a photograph of the sample setup As in the HET, the shear stress is calculated using Eq. 15. In the SET, the hydraulic diameter ( ) is used instead of in Eq. 15. Aslot refers to the cross-sectional area of the pre-formed slot and, Pw is the wetted perimeter. ISET is also calculated with the same procedure explained for calculating IHET (Eq. 17). All the tests described to study internal erosion require that the soil be a self-supporting (cohesive) soils. Cohesion-less soils cannot preserve an open hole or slot; therefore, HET, SET, drill hole, or pin hole tests, for cases where non-cohesive soils form a high stress portion of the embankment, cannot appropriately simulate the actual field conditions. In fact, advantages and drawbacks of SET are same as the ones discussed in the previous section for HET. For that reason, new internal erosion test devices are being developed. Some of these devices are described below. Stress-Controlled Erosion Apparatus Chang and Zhang (2011) developed a new test for studying the internal erosion in soils at Hong Kong University. They ran some tests on a man-made cohesion-less soil. A schematic diagram of this test is shown in Figure 21. The test consists a triaxial system which is fed by a water supply
45 system and controlled by a computer. The porous stone used in this apparatus is modified to accommodate the high permeability of the tested soil in this experiment. The soil sample is 10 cm in diameter, 10 cm high and is mounted on a hollow base with a 10 mm thick, 95 mm diameter perforated plate. Water flow seeps through the hollow base and the perforated plate and the soil sample. Before the internal erosion testing starts, a 10 kPa confining pressure is applied to the sample. Then, de-aired water is injected slowly into the specimen from the bottom base to saturate the sample. During the erosion test, the vertical deformation of the sample is measured using a linear variable differential transformer (LVDT), and the radial deformations can be measured using a video camera. The test is controlled by adjusting the hydraulic gradient of the seepage water through the sample. A soil collection system is placed at the bottom of the triaxial system. Each hydraulic gradient is maintained for a 10-minute period and the eroded mass of soil is collected, dried, and weighted. Schematic diagram of the stress-controlled apparatus Photograph of the stress-controlled apparatus Figure 21. A schematic diagram of the stress-controlled erosion apparatus (Chang and Zhang, 2011) Advantages: 1) Direct similitude with internal erosion in earth dams. 2) Works properly for cohesion-less samples. 3) Uses advanced measurement techniques for axial and radial deformations. 4) Can control multiple criteria (i.e. confining pressure, hydraulic gradient) during the test. Drawbacks: 1) Requires costly set-up (more than $30k to build). 2) May not be efficient for very low-permeability clays and rocks. 3) Hard to test on intact samples, due to the size of the sample
46 True Triaxial Piping Test Apparatus (TTPTA) Richards and Reddy (2010) developed a True Triaxial Piping Test Apparatus (TTPTA) at the University of Illinois at Chicago to study the internal erosion in both cohesive and cohesion-less soils. The test consists of applying a wide range of confining pressures, with measurements of pore pressure and hydraulic gradient in a true triaxial cell (Figure 22). The results of this test give the critical hydraulic gradient as well as the critical velocity. Schematic diagram of the TTPTA Photograph of the TTPTA Figure 22. A schematic diagram of the TTPTA (Richards and Reddy, 2010) Advantages: 1) Direct similitude with internal erosion. 2) Works properly for both cohesion-less and cohesive samples. 3) Uses advanced measurement techniques for deformations. 4) Can control multiple criteria (i.e. confining pressure, hydraulic gradient) during the test. Drawbacks: 1) Relatively complicated and costly set-up (more than $30k to build). 2) May not be efficient for very low-permeability clays and rocks. 3) Hard to test on intact samples, due to the size of the sample. Constant Gradient Piping Test Apparatus Fleshman and Rice (2013) developed a new test apparatus to evaluate the hydraulic conditions required for starting piping erosion. The testing apparatus is shown in Figure 23. The sample is held in a sample holder, while a constant hydraulic gradient is imposed throughout the sample. During the test, the differential head is increased, and the pore pressure and the soil behavior are monitored. Fleshman and Rice (2013) used this testing device on various sandy soils with different grain size distribution, specific gravity, and gradation, and recorded the critical hydraulic condition (i.e. critical gradient), in which the piping erosion was initiated. It was observed that the initiation of
47 the erosion occurs in four stages: 1) the first observable movement of particles, 2) progression of heave, 3) boil formation, and 4) final or total heave. Figure 23. A schematic presentation of the constant gradient piping test apparatus (Fleshman and Rice, 2013) Advantages: 1) Direct similitude with piping erosion. 2) Includes an automated saturation instrumentation and advanced measurement of flow and head Drawbacks: 1) Only used to evaluate the initiation of piping rather than giving the entire erosion function. 2) No measurement of soil axial and radial deformations. 3) Efficient only for sandy soils. 4) Relatively difficult sample preparation. The test is not designed for intact samples. 2.2. Field Erosion Testing Pocket Erodometer Test (PET) The Pocket Erodometer Test (PET) was developed by Briaud et al. (2012) at Texas A&M University. The Pocket Erodometer is a regulated mini jet impulse generating device in the form of a plastic water gun. The jet is aimed horizontally at the vertical face of the sample. The jet velocity is calibrated to be always 8 m/s and the nozzle is kept 50 mm from the soil surface. The depth of the hole in the surface of the sample created by 20 impulses of water is recorded. The eroded depth is compared to an erosion chart to determine the erosion category of the soil which helps the geotechnical engineer with preliminary design of erosion projects.
48 Many different options were considered during the development of the Pocket Erodometer including the most appropriate device, velocity range, direction of application, distance from the face of the sample, and repeatability from one person to another. Figure 24 shows the schematic diagram of the PET, along with a photograph from the test (Briaud et al., 2012). The original device selected for the Pocket Erodometer is 105 mm ï´ 77 mm ï´ 18 mm. The diameter of the nozzle is about 0.5 mm (see Figure 24). The jet velocity of 8 m/s was chosen because it eroded most tested specimens. The Pocket Erodometer needs to be calibrated before the test to reproduce the velocity of 8 m/s at the nozzle. The following equation is used to calibrate the velocity of the impinging jet at the nozzle: . (18) Where, x and H are shown in the schematic diagram of PET depicted in Figure 24, and v is the initial horizontal velocity right at the nozzle. Schematic diagram of the PET (Briaud et al., 2012) PET photographs (Briaud et al., 2012) Figure 24. Schematic diagram of Pocket Erodometer Test and a photograph of the test device The height of the erodometer (shown as H in Figure 24) must be kept constant during the calibration process. Also, external forces such as wind should be avoided. The PET can be done with any type of apparatus which can meet the requirements of this test and reproduce 8 m/s velocity at nozzle with Â± 0.5 m/s with impulse time period of near 0.15 sec. Briaud et al. (2012) then conducted PET on many samples from different levees and compared the results with the EFA results obtained. The comparisons resulted in an erosion category chart based on the PET depth ranges (See Figure 25). The step by step process of PET is explained below (Briaud et al., 2012): 1) Place the sample horizontally either on a flat surface or by holding it in your hand. Note: The test must not be run with the jet pointed vertically. 2) Smooth the surface to remove any uneven soil. You want to begin with a smooth and vertical surface, so that it is easy to measure the erosion depth
49 3) Hold the Pocket Erodometer (PE) pointed at the smooth end of the sample, 50 mm away from the face. 4) Keeping the jet of water from the PET aimed horizontally at a constant location, squeeze the trigger 20 times at a rate of 1 squeeze per second, forming an indentation in the surface of the sample. Each squeeze should fully compress the trigger and then the trigger should be fully released before it is re-compressed. 5) Using the end of a digital caliper or an appropriate measuring tool, measure the depth of the hole created. 6) The test should be repeated at least 3 times in different locations across the face of the sample and an average should be used to ensure a good estimate. 7) Determine the erosion category using Figure 25. Figure 25. Erosion depth ranges of Pocket Erodometer Test (PET), depicted on the erosion categories proposed by EFA Advantages 1) Very low price ($25 per test). 2) Very handy and simple to operate both in the field and in the laboratory. 3) Gives a quick and crude estimate of soil erodibility. Drawbacks 1) Very small scale. 2) Only gives the erosion category and no measurement of critical shear stress or critical velocity. 3) Only useful for preliminary field evaluation, and not good for design purposes. In Situ Erosion Evaluation Probe (ISEEP) Gabr et al. (2013) developed an erosion testing device, called the In Situ Erosion Evaluation Probe (ISEEP) at North Carolina State University. The test is conducted by advancing a vertical jet probe into the subsurface soil and measuring the rate of advancement. As discussed earlier, all other in situ tests were limited to evaluate the scour potential of the soil only on the ground surface (i.e. JET, Sedflume, etc.), and EFA was the only test that could evaluate the erodibility of the natural soil associated with a particular depth. The ISEEP flow
50 velocities are normally much less than the imparted velocities in the EFA. ISEEP can investigate the erodibility of any soil at any depth provided the probe can penetrate by erosion. The results of ISEEP are reported based on the concept of âStream Powerâ which was first presented by Annandale and Parkhill (1995) who believed that this concept would better represent the erodibility potential of an eroding fluid compared to velocity or shear stress. Annandale (2006) defined the stream power, P, using Eq. 19 and 20. (19) (20) Where, P is the stream power (Watts per unit area), is the water unit weight, q refers to the flow discharge in unit area, H is the energy head, is the shear stress, and U0 is the velocity. Eq. 20 shows that the stream power is a function of both the shear stress and the velocity. Figure 26 shows a photograph of the ISEEP device which was tested at the NCSU lab before being used in the field. Truncated cone probe ISEEP prototype Figure 26. ISEEP apparatus prototype at NCSU (Gabr et al., 2013) The jet nozzle is a truncated cone probe which is guided into the soil as the soil is being eroded by the impinging jet. The eroded material moves up through the annulus space between the probe and the wall of the hole created by erosion. The water velocity at the nozzle is controlled digitally by a variable speed pump on the ground surface. The rate of advancement into the soil subsurface is measured and represents the erosion rate. The body of the probe is divided into sections so that the length can be adjusted for deep locations in the field. The orifice of the jet nozzle is 19 mm (0.75 inches) long and the nozzle velocity can go up to 12 m/s. During penetration, the advancement is recorded using a video camera.
51 The results of this test are reported as the rate of advancement (penetration) versus the stream power value, P. The bed shear stress is obtained from Eq. 21 based on Julien (1995) study: (21) Where, is the bed shear stress, U is the jet velocity, is the water density, and C is a diffusion coefficient which varies depending on flow condition. Some advantages of the ISEEP are mentioned below: Advantages 1) It can evaluate the erodibility of any soil at any depth with a wide range of jet velocity. 2) There is no need for sample extraction and procurement. Drawbacks 1) The penetration may be limited if the probe fails to erode the soil. Better to be used in sandy soils. 2) The use of the stream power makes it difficult to compare this device with other erosion devices. 3) Fairly expensive test device to build (more than $20k), and difficult to interpret. Borehole Erosion Test (BET) The Borehole Erosion Test (BET) is an in situ test developed by Briaud at Texas A&M University (Briaud et al., 2016). The purpose of this test is to quantify the erodibility of the soil layers as a function depth as follows. 1) Drill a hole into the ground, say 100 mm in diameter, 10 m deep. 2) Remove the drilling rods and measure the initial diameter of the borehole with a borehole caliper. 3) Re-insert the rods to the bottom of the hole and circulate water down the rod and up the outside annulus of the hole for a given time, say 15 minutes. 4) Remove the rods and measure the diameter of the hole with the borehole caliper. 5) The increase in diameter of the borehole at a certain depth given by the calipers divided by the flow time is the erosion rate of the soil at that depth for the flow velocity applied during the test. 6) Profiles of erosion rate for different velocities can be prepared in that fashion. Figure 27 shows the schematic diagram of the BET, and field work photographs.
52 Schematic diagram of BET Photographs from BET in the field Figure 27. A schematic diagram of Borehole Erosion (BET) test and photographs of the test at the Riverside campus at Texas A&M University (Briaud et al., 2016) The advantages and drawbacks for this test are below. Advantages 1) Only typically available field equipment (common drilling rig for wet rotary boring, flow meter in line with the drilling rig pump, and borehole caliper) is used to perform a BET test; therefore, the BET can be performed in many drilling projects. 2) Each test gives the erosion function for all layers traversed since a complete borehole diameter profile is obtained from the caliper. This would require many tests on many samples if laboratory tests were to be conducted. 3) It has two component tests: the lateral erosion test associated with the increase in diameter of the borehole and the bottom erosion test associated with the increase in depth below the bottom of the drilling rods during the flow. The later one is much like an in situ jet erosion test. 4) Can be used in any soil or rock where a hole can be drilled. Drawbacks 1) The shear stress is obtained from Moody chart. 2) Limited by pump flow available on the drill rig. 3) The hole needs to be capable of staying open. 4) In sand boreholes, the addition of Bentonite during drilling needs to be controlled in order not to impact the erosion resistance. 5) Due to gravity, the velocity over a given distance changes and needs to be taken into account. 6) Erosion of layers above the tip of the pipe could be altered by the sediment transport from the lower layers. For example, Sheppard (2002) showed that equilibrium scour depths are reduced by the presence of suspended fine sediments. 7) The borehole measurements with the caliper may not be accurate due to material sloughing off. DIAMETER,Â D WATERÂ FLOWÂ VELOCITY,Â V WATERÂ FLOW
53 In situ Scour Testing Device (ISTD) In situ Scour Testing Device (ISTD) is the most recent field erosion test device that is currently under development by the U.S. Federal Highway Administration (FHWA). Zinner et al. (2016) presented the concept behind the ISTD and its application in pier scour studies. This device has a cylindrical shape and can be used in a boring test rig and fit into the steel casing of hollow stem augers to evaluate the erodibility of soil at any depth. The ISTD generates a horizontal flow at the bottom of the borehole. Figure 28 shows a diagram of the cylindrical ISTD concept. So far, the ISTD is applicable only for soils below the ground water table, and with a maximum N-value of 30. Also, ISTD is generally limited to upper soil layers, as the mechanical parts are limited. The development is on-going by the FHWA. Figure 28. A schematic diagram of the cylindrical ISTD concept (Zinner et al., 2016) A summary of all the erosion tests reviewed is presented in Table 4 in terms of their application in the field or the lab. Table 5 shows some of the most common and important erosion tests with information regarding their ability to measure shear stress, with the soil type that can be tested, and the cost associated with them.
54 Table 4. Summary of all types of erosion tests in terms of their application LABORATORY EROSION TESTS: ï¶ Lab Jet Erosion Test (JET) ï¶ Hole Erosion Test (HET) ï¶ Pinhole Erosion Test ï¶ Drill hole Erosion Test ï¶ Slot Erosion Test (SET) ï¶ Rotating Cylinder Test (RCT) and improved versions ï¶ Rotating Erosion Testing Apparatus (RETA) ï¶ EFA and similar versions of it (e.g.: Sedflume, SERF, ESTD) ï¶ Stress-controlled Erosion Apparatus ï¶ True Triaxial Piping Test Apparatus (TTPTA) ï¶ Constant Gradient Piping Test Apparatus IN SITU EROSION TESTS ï¶ Field Jet Erosion Test (JET) ï¶ In Situ Scour Evaluation Probe from North Carolina State University (ISEEP) ï¶ Borehole Erosion Test (BET) ï¶ Pocket Erodometer Test (PET) ï¶ ASSET ï¶ In Situ Scour Testing Device (ISTD) ï¶ Field Flume Tests Table 5. Some of erosion tests with information about their application Erosion Tests Range of soil types that can be tested Range of shear stress that can be applied Cost of the Device1 Reliability of Results Lab JET Sands to clays < 100 Pa Low Good In situ JET Sands to clays < 500 Pa Medium Good EFA Sands to clays < 165 Pa High Good HET Clayey soils Up tp 800 Pa High Good SET Clayey soils Up to 400 Pa High Medium RETA Clayey soils < 100 Pa High Medium PET Sands to clays < 20 Pa Very Low Medium ISEEP Sands to clays < 650 Pa High Good BET Sands to clays < 600 Pa Medium Good 1 High: equal or greater than $30,000; Medium: between $15,000 to $30,000; Low: between $5,000 and $15,000; Very Low: Less than $5,000