FHWA-HRT-23-088 JUNE 2023

IN-SITU SCOUR TESTING DEVICE (ISTD), 

STATE DEMONSTRATIONS OF FIELD 

SOIL TESTS, LOVELOCK, NV

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 Nevada Department of Transportation (NDOT) hosted 

the 10th ISTD field demonstration on State Route 396 at 

the bridge over the Humboldt River, located 2 mi north of 

Lovelock, which is about 90 mi northeast of Reno, NV. The 

demonstration was held on the south shoulder of the road 

at the west abutment. NDOT had scheduled the bridge for 

replacement due to its poor condition.

The subsurface soil profile was initially determined from 

boring logs taken in August and October 2018, which NDOT 

used to compile a subsurface fence diagram for the site. 

The geotechnical investigation revealed alternating layers 

of low- and high-plasticity clay down to 35–40 ft. The day 

before the demonstration, the drillers performed a continuous 

standard penetration test (SPT) from 10–20.5 ft and found 

a layer of light brown, moist clay, with trace sands, varying 

between high and low plasticity. The clay was very stiff in the 

initial 3 ft but dropped to an N-value of 10 at 13 ft and further 

weakened to values of 5–6 from 14.5–19 ft. The water table 

was measured at 12.5 ft. To avoid the initial stiff soil layer and 

conduct the test in groundwater, FHWA selected 12.5 ft as the 

targeted starting depth for the ISTD test.

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.

TEST PROCEDURE

The demonstration took place on April 17, 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. Because the test was held 

on the shoulder of an active roadway, NDOT organized 

traffic control to close the adjacent lane of traffic. The lane 

closure increased safety for the demonstration but reduced 

the available time to conduct testing. In addition, the rental 

pump was delivered from California and arrived without the 

proper adapter to connect the suction hose. Fortunately, 

an NDOT engineer found the correct adapter at a local 

construction site on an impromptu scouting trip, which saved 

a half-day of testing. The hydraulics team made a note 

to purchase a spare adapter to prevent future delays in a 

similar scenario. After mounting the adapter to the suction 

hose, the hydraulics team finished assembling the remaining 

equipment, including the linear drive, water tank, hoses, 

piping, and laptop. Having previously augered to a depth 

of 13 ft, the drill team lowered in the casing and pushed the 

Shelby tube 13 inches into the clay. The hydraulics team 

inserted the erosion head, connected all the equipment, 

and began the first test.

RESULTS

Over the course of the testing, the hydraulics team collected 

more than 3 h of erosion data, captured in five separate test 

runs of 20–75 min. The hydraulics team tested almost 4 ft of soil 

in 2 boreholes with 11 flow rates ranging from 0.139–0.281 ft3

/s.

From the data, FHWA identified 13 different segments and 

extracted erosion rates using a best-fit line through each 

dataset. The corresponding mean flow rates were also 

Source: FHWA

Source: FHWA

The ISTD demonstration site on State Route 396.

The ISTD equipment assembled in front of the drill rig.
calculated for each segment. The clay at this site tended to 

erode in jumps, and on two occasions, a large segment of 

clay spanning the entire width of the tube broke off, driving 

all the sensors to nearly zero. These segments eventually 

disintegrated and washed out but did affect the data 

collection. The 13 data points are detailed in the Summary of 

Results table. The erosion rates are plotted against flow rates 

that 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, 

the 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.

Summary of Results

Depth

(ft)

Duration

(min)

Flow Rate 

(ft3

/s)

Erosion Rate 

(inch/min)

12.55 11:42 0.215 0.004

12.80 13:05 0.232 0.766

13.37 0:55 0.281 1.369

12.97 11:40 0.224 0.177

13.40 6:15 0.158 0.001

13.44 28:45 0.155 0.100

13.75 13:05 0.181 0.042

14.06 11:30 0.171 0.673

15.10 12:50 0.139 0.180

15.23 2:15 0.155 0.045

15.24 21:15 0.173 0.108

15.47 12:25 0.199 0.220

15.92 5:10 0.221 0.274

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, Lovelock, NV

(Washington, DC: 2023) https://doi.org/10.21949/1521392

Soil Properties

Parameter Value

Depth (ft) 14.5–17

Water content (%) 35

Liquid limit (%) 43

Plasticity index (%) 19

Clay fraction (%) 52

Percent fines (%) 96

Soil classification (USCS) CL

Soil classification (AASHTO) A-7-6(21)

Unconfined compressive strength (psi) 5.71

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

Source: FHWA

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

best-fit line.

Erosion rate versus flow rate for the Lovelock ISTD demonstration. 

With more data points, a nonlinear fitted power curve could be 

used to extract the critical flow rate where erosion begins.