FHWA-HRT-23-089 JUNE 2023

IN-SITU SCOUR TESTING DEVICE (ISTD), 

STATE DEMONSTRATIONS OF FIELD 

SOIL TESTS, BOSTON, KY

INTRODUCTION

The ISTD is an advanced system designed by the hydraulics 

research team at the Turner-Fairbank Highway Research 

Center to measure the erosion resistance of fine-grained, 

cohesive soils directly in the field. The system features an 

innovative erosion head that, when inserted into a standard 

drill casing, can direct a horizontal radial water flow across 

the surface of the soil, resulting in erosion. The erosion 

resistance is measured in terms of critical shear stress, which, 

when coupled with the decay of hydraulic shear forces 

(water loads) with scour depth, is the basis of the Federal 

Highway Administration’s (FHWA’s) NextScour research 

initiative for improving the accuracy of future bridge 

scour estimates.

BACKGROUND

The Kentucky Transportation Cabinet (KYTC) hosted 

the 11th ISTD field demonstration on State Route 62 at 

the bridge over Rolling Fork, KY, located 2 mi west of 

Boston, KY. The demonstration was held east of the river in 

the floodplain under the bridge. KYTC planned to replace 

the 13-span bridge, originally built in 1938, with a shorter, 

nine-span structure by lengthening the embankment on 

the eastern side.

While KYTC’s original 1938 bridge plan contained the 

historical subsurface soil profile at the site, KYTC conducted 

a new geotechnical investigation in March and April 2019 

to get updated soil information. KYTC took 13 borings 

located at the proposed piers and abutments of the new 

bridge. These borings revealed layers of medium-stiff to 

stiff, brown, and gray clay down to 30 ft. Near the surface, 

some borings revealed a layer of sandy clay. One day 

before the demonstration, KYTC conducted a cone 

Emerging ISTD technology uses an innovative erosion head that more accurately 

measures soil erosion resistance, resulting in more cost-effective foundation designs 

and greater reliability and resiliency in bridge performance.

penetration test (CPT) at the site to obtain more detailed 

information about the soil profile. The CPT confirmed that 

beneath an initial 2-ft layer of sandy silt and clay were 

alternating layers of clay and silty clay from 3 to 24 ft. 

The N-values jumped quickly to 30 at 3-ft depth but then 

dropped to around 10 by 6 ft and then ranged between 

5–10 down to 24 ft. The layer starting around 7 ft was 

chosen as the targeted testing layer for the ISTD.

TEST PROCEDURE

The demonstration took place on May 30, 2019, but the 

drill crew and the hydraulics team arrived a day earlier 

to conduct as much ISTD field testing as possible in the 

two-day span. To access the floodplain area underneath 

the bridge deck, the crews used an access road entrance 

to a farm about 1,000 ft east of the bridge. When they 

arrived at the proposed drilling location, it was inaccessible 

because of brush overgrowth, so both groups agreed 

to move the test two span lengths closer to the east 

abutment. At this point, the weather took an unfortunate 

turn. Heavy rains fell at the site and continued over the 

two days of testing. The drill crew proposed positioning 

the rig under the bridge deck, but there was not enough 

clearance to raise the rig mast. Fortunately, they were able 

to position the rig adjacent to the deck to offer everyone 

some relief from the rainfall. The drillers ended up augering 

to a depth of 12 ft, a 5-ft segment past the targeted 

depth. The hydraulics team then assembled the remaining 

equipment, including the water tank, pump, piping, hoses, 

linear drive, and laptop to prepare for the first test.

RESULTS

Over the course of the testing, the hydraulics team collected 

almost 3 h of erosion data, captured in eight test runs ranging 

The demonstration group observes the ISTD field test. 

Source: FHWA

The ISTD equipment assembled in front of the drill rig.

Source: FHWA
from 6–50 min in length. They tested about 5 ft of soil with eight 

different flow rates ranging from 0.144–0.271 ft3

/s.

Despite some early difficulties with a stuck sensor and an 

instance where 10 inches of clay washed out quickly from 

the Shelby tube, the testing was very successful, obtaining 

a wide range of data points. With this data, FHWA identified 

nine different segments and extracted erosion rates using 

a best-fit line through each set of data. The corresponding 

mean flow rates were also calculated for each segment. The 

nine data points are detailed in the Summary of Results table. 

The erosion rates are plotted against flow rates, showing the 

correlation between the two values. With more data points, 

a nonlinear power curve can be fit to the data to extract the 

critical flow rate.

Due to the presence of some low erosion rates during 

testing, this ISTD demonstration revealed that this location 

could potentially have a clay layer with erosion resistance. 

However, additional testing is needed to confirm this result 

and produce more consistent data.

Summary of Results

Depth

(ft)

Duration

(min)

Flow Rate 

(ft3/s)

Erosion Rate 

(inch/min)

13.35 25:35 0.196 0.243

13.90 23:55 0.271 0.155

15.22 1:25 0.231 0.180

11.87 13:40 0.168 0.029

11.91 10:15 0.240 0.849

13.16 24:50 0.144 0.143

13.53 14:30 0.178 0.490

14.17 19:35 0.167 0.106

14.43 27:50 0.202 0.175

For additional 

information,

please contact:

Daniel Alzamora

Senior Geotechnical Engineer 

FHWA Resource Center

720-963-3214

daniel.alzamora@dot.gov

James Pagenkopf

Research Hydraulic Engineer 

FHWA Hydraulics Laboratory

202-493-7080

james.pagenkopf@dot.gov

https://highways.dot.gov/laboratories/hydraulics-research-laboratory/hydraulics-research-laboratory-overview

Recommended citation: Federal Highway Administration, In-Situ Scour Testing Device (ISTD), State Demonstrations of Field Soil Tests, Boston, KY

(Washington, DC: 2023) https://doi.org/10.21949/1521393

Erosion rate versus flow rate for the Boston ISTD demonstration. 

With more data points, a nonlinear fitted power curve could 

be used to extract the critical flow rate where erosion begins.

Soil Properties

Parameter Value

Depth (ft) 12.4–14.4

Water content (%) 33.6

Liquid limit (%) 51.0

Plasticity index (%) 22.0

Clay fraction (%) 47.6

Percent fines (%) 96.8

Soil classification (USCS) MH

Soil classification (AASHTO) A-7-6(26)

Unconfined compressive strength (psi) 11.77

USCS = Unified Soil Classification System; AASHTO = American Association 

of State Highway and Transportation Officials.

ADDITIONAL RESOURCES

ISTD Field Demonstration Webinar:

https://connectdot.connectsolutions.com/ph8wgrf8erz7

AASHTO Hydrolink Newsletter:

https://transportation.org/design/wp-content/uploads/

sites/31/2023/05/Hydrolink-Issue-16.pdf

NextScour Journal Paper:

https://doi.org/10.1680/jfoen.20.00017

Notice—This document is disseminated under the sponsorship of the 

U.S. Department of Transportation (USDOT) in the interest of information 

exchange. The U.S. Government assumes no liability for the use of the 

information contained in this document. The U.S. Government does 

not endorse products or manufacturers. Trademarks or manufacturers’ 

names appear in this fact sheet only because they are considered 

essential to the objective of the document.

Quality Assurance Statement—The Federal Highway Administration 

(FHWA) provides high-quality information to serve Government, 

industry, and the public in a manner that promotes public 

understanding. Standards and policies are used to ensure and 

maximize the quality, objectivity, utility, and integrity of its information. 

FHWA periodically reviews quality issues and adjusts its programs and 

processes to ensure continuous quality improvement.

Source: FHWA

Soil layer’s erosion rate (e) calculated from the slope of the 

best-fit line.

Source: FHWA