In-Situ Scour Testing Device (ISTD), 

State Demonstrations of Field Soil Tests,

Franklin, VA 

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 Virginia Department of Transportation (VDOT) hosted 

the ninth ISTD field demonstration at the bridge carrying 

Route 671 over the Nottoway River, located 4 mi southwest 

of the town of Franklin, VA. VDOT planned to replace the 

aging bridge structure and had questions about the scour 

depths calculated assuming a sand subsurface profile. 

The calculated scour depth based on these assumptions 

appeared to be greater than expected based on 

observations of the performance of the existing structure. 

VDOT believed that including the clay layers encountered at 

the site could potentially reduce the calculated scour depths. 

Prior ISTD field demonstrations were typically performed at 

an abutment or in the floodplain, but for this site, the test 

was conducted adjacent to the proposed pier in the main 

channel, which required conducting the ISTD test on a barge 

on the river. This setup required careful coordination with 

VDOT and its subcontractors and more advanced planning 

than at previous sites. 

The subsurface soil profile was initially determined from 

boring logs taken from across the site in 2012. Those borings 

showed a 10-ft-thick layer of gray, soft, sandy lean-to-fat 

clay. The clay layer was located beneath a 14-ft-thick layer 

of sand that started at the channel bed. On the morning of 

the demonstration, the drillers performed a continuous 

standard penetration test (SPT) from the barge to confirm the 

soil properties. They found that the clay layer was about 8 ft 

beneath the sand layer, and it was about 8-ft thick with SPT blow 

counts (i.e. N-values) ranging from 2–4. The clay layer starting 

at 8 ft was selected as the targeted testing layer for the ISTD. 

TEST PROCEDURE

The demonstration took place on March 27, 2019. One of the 

major challenges with conducting an ISTD test on the barge 

was the placement of the pump because of the limited 

space on the barge platform. To save room, FHWA and VDOT 

decided to position the pump in a closed lane of the bridge 

deck and run the hoses down to the barge. In preliminary 

meetings, the distance from the deck to the water surface 

was estimated to be only 10 ft, but it was between 15–20 ft. 

The researchers were uncertain if the pump would be 

powerful enough to supply the range of desired flows with 

that height differential. 

A second major challenge was the use of mud rotary drilling 

instead of hollow stem augers because mud rotary is more 

adept at drilling in water and sand. The drillers utilized a 

bentonite slurry as the drilling fluid. When the hydraulics team 

tried inserting the erosion head and polyvinyl chloride (PVC) 

pipes into the borehole, they encountered excessive 

resistance as the bentonite mixture struggled to flow through 

the narrow channels of the erosion head. When the pump 

was started, it was completely unable to circulate the 

bentonite mixture through the system. The erosion head 

was removed, and the drillers then flushed the bentonite 

mixture from the casing with a tri-cone bit. When the erosion 

head was reinserted, the pump was finally able to circulate 

water through the system, but the flow meter indicated the 

flow was oscillating at amplitudes up to 0.03 ft3/s. The 

hydraulics team relocated the hoses, gauges, and flow 

meter to the bridge deck, which helped steady the pulsating 

flow, along with operating the pump at higher flow rates. 

The Virginia ISTD field demonstration was performed on 

a barge in the Nottoway River. 

FHWA-HRT-23-007 NOVEMBER 2022 Source: FHWA. 

The ISTD equipment assembled in front of the drill rig on the barge. 

Source: FHWA. 
At this point, the hydraulics team was able to successfully 

run several ISTD erosion tests and collect data.

RESULTS

Over the course of the testing, the hydraulics team 

collected about 2 h of erosion data, captured in two test 

runs. They tested 1.5 ft of soil with 4 different flow rates 

ranging from 0.121 to 0.270 ft3/s. 

The two test runs produced varied results. It is unknown 

how the pulsating flow affected the erosion. At a few 

points, suspended sand in the water might have 

interfered with the sensors. Despite these uncertainties, 

four 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 four 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 the clay layer 

could potentially provide the required erosion resistance. 

However, additional testing would be needed to confirm 

that result and produce more consistent data. 

Summary of Results 

Depth 

(ft) 

Duration 

(min) 

Flow Rate 

(ft3/s) 

Erosion Rate 

(inch/min) 

10.07 21:30 0.121 0.040 

10.15 23:10 0.219 0.016 

10.61 14:05 0.241 0.035 

10.67 26:35 0.270 0.488 

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) 11–11.5 

Water content (%) 29 

Liquid limit (%) 32

Plasticity index (%) 12

Clay fraction (%) 19

Percent fines (%) 56

Soil classification (USCS) CL

Soil classification (AASHTO) A-6(4) 

Unconfined compressive strength (psi) 5.05 

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. 

Erosion rate versus flow rate for the Franklin 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. 

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

(Washington, DC: 2022) https://doi.org/10.21949/1521950.