In-Situ Scour Testing Device (ISTD), 

State Demonstrations of Field Soil Tests,

Grifton, NC 

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. 

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. It 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 a 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 North Carolina Department of Transportation (NCDOT) 

hosted the eighth ISTD field demonstration on State Route 11 

at the bridge over Contentnea Creek, located 1 mi west of 

the town of Grifton, NC. The demonstration was held in the 

median, north of the creek. The bridge was not scheduled for 

replacement, but NCDOT was interested in the technology, 

and the site was easily accessible. 

The subsurface soil profile was initially determined from 

boring logs taken in 1993 for a scour investigation that 

showed a layer of clay among mostly silty sand at the site. 

A week before the demonstration, a cone penetration test 

(CPT) was conducted at the site to obtain detailed 

information about the soil profile. The CPT confirmed that 

beneath the sand was a layer of clay silt and silty clays 

beginning around 24 ft and continuing to 37 ft. The day 

before the demonstration, the drillers performed a 

continuous standard penetration test (SPT) between 23 

and 34.5 ft and found a continuous layer of gray, silty, 

micaceous clay with blow counts (i.e. N-values) ranging 

from 9 to 13. The layer starting around 23 ft was chosen 

as the targeted testing layer for the ISTD. 

TEST PROCEDURE

The demonstration took place on March 20, 2019, but the 

drill crew and the hydraulics team arrived a day in advance 

to perform the SPTs and conduct as much ISTD field testing 

as possible in the two-day span. Unfortunately, unforeseen 

events, including equipment malfunctioning on the drill rig 

and issues with the commercial pump delivery, caused 

lengthy delays on the first day. The drill team was able to pull 

some samples to return to the laboratory, but most of the 

day was lost. After an early start on the second day, the 

drillers were quickly able to auger to a depth of 25 ft, then 

lowered in the casing and Shelby tube. Despite the stiff clay, 

the drillers successfully pushed the Shelby tube 14 inches 

with full recovery. The hydraulics team then assembled the 

remaining equipment, including the linear drive, water tank, 

piping, hoses, and laptop, and began the first test. 

RESULTS

Over the course of the testing, the hydraulics team collected 

almost 4 h of erosion data, captured in a single test run. They 

tested almost 1 ft of soil with eight different flow rates ranging 

from 0.174 to 0.424 ft3/s. 

The clay at this site was very stiff compared to the previous 

sites, which resulted in extremely slow erosion. The Shelby 

tube was only pushed once, and in 4 h of testing, less than 

FHWA-HRT-23-008 NOVEMBER 2022 

The demonstration group observes the ISTD field test. 

Source: FHWA. 

The ISTD equipment assembled in front of the drill rig. 

Source: FHWA. 
Summary of Results

Depth 

(ft) 

Duration 

(min) 

Flow Rate 

(ft3/s) 

Erosion Rate 

(inch/min) 

24.79 19:10 0.174 0.006 

24.80 29:30 0.296 0.015 

24.84 48:20 0.347 0.048 

25.09 36:50 0.318 0.060 

25.25 36:40 0.389 0.058 

25.41 21:20 0.265 0.000 

25.41 26:25 0.332 0.003 

25.42 14:30 0.424 0.099 

ADDITIONAL RESOURCES 

ISTD Field Demonstration Webinar: 

https://connectdot.connectsolutions.com/ph8wgrf8erz7/

AASHTO Hydrolink Newsletter: 

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

sites/21/2018/02/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.

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

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 

For additional 

information, 

please contact: 

Soil Properties 

Parameter Value 

Depth (ft) 26.1–28.7 

Water content (%) 33

Liquid limit (%) 76

Plasticity index (%) 50

Clay fraction (%) 57

Percent fines (%) 91

Soil classification (USCS) CH

Soil classification (AASHTO) A-7-6(52) 

Unconfined compressive strength (psi) 60.44 

USCS = Unified Soil Classification System; AASHTO = American Association 

of State Highway and Transportation Officials. 

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

of the best-fit line. 

Source: FHWA. 

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

(Washington, DC: 2022) https://doi.org/10.21949/1521951. 

Erosion rate versus flow rate for the Grifton ISTD demonstration. 

With more data points, a nonlinear fitted power curve could 

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

9 inches of soil was eroded from the 14-inch sample. Due 

to deteriorating weather conditions, more testing was not 

possible. From the data, eight different segments were 

identified, and erosion rates were extracted by using a 

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

mean flow rates were also calculated for each segment. 

The eight data points are detailed in the Summary of 

Results table. The erosion rates are plotted against flow 

rates, which show the correlation between the two 

values. With more data points, a nonlinear power curve 

can be fitted to the data to extract the critical flow rate. 

Due to the presence of low erosion rates during testing, 

this ISTD demonstration revealed that this location could 

potentially have a clay layer with significant erosion 

resistance. However, additional testing would be needed 

to confirm that result and produce more consistent data.