The falling weight deflectometer (FWD) is a useful tool for measuring the resilient modulus of the subgrade at a project site. A test procedure developed by the LTPP program makes data from FWD testing suitable for use in pavement design equations.

The resilient modulus, or stiffness, of the soil and rock in the subgrade at a paving project is a critical factor in determining how thick a pavement should be (see sidebar below). Many highway agencies use a falling weight deflectometer (FWD) to measure the resilient modulus of subgrade materials. The FWD is towed to the project site, where it measures how the subgrade reacts to a heavy weight dropped on the ground above.

The FWD field test quickly and accurately simulates the stresses caused by traffic loads on large areas of subgrade. But the equations in the pavement design guidelines* call for the resilient modulus to be measured using a carefully controlled laboratory test on a small sample of the subgrade. The results of the laboratory test are then used to calculate a correction factor for the FWD test data, which will make the FWD test results suitable for use in the pavement design equations.

To overcome this resistance, FHWA's long-term pavement performance (LTPP) program developed a standardized laboratory procedure to measure the resilient modulus of subgrade materials. The LTPP program also developed a related laboratory startup and calibration verification procedure. The procedure carefully lays out the steps involved in conducting the resilient modulus laboratory test. As a result, highway agency staff can obtain consistent results, and they no longer have to worry whether they are performing the test properly. The resilient modulus test procedure has since been adopted as an AASHTO provisional standard, "Standard Test Method for Determining the Resilient Modulus of Soils and Aggregate Materials" (TP46-94).

To help State highway agency laboratories make the decision whether to adopt the laboratory test method for measuring resilient modulus, three new videotapes have been produced by FHWA through a cooperative agreement with the Minnesota Department of Transportation.

• Laboratory Resilient Modulus Testing: Is This the Right Time? This 8-minute videotape explains resilient modulus and why it is important to adopt the resilient modulus test procedure. It also explains the developments that have made resilient modulus testing more consistent and easier to adopt. The videotape is intended for administrators and engineers.
  
• Laboratory Resilient Modulus Testing: Startup and Quality Control Procedure. This 15-minute videotape for laboratory managers and technicians begins with a detailed definition of resilient modulus. It then explains the procedure developed under the LTPP program to ensure that a laboratory is set up properly to conduct the resilient modulus test procedure and to collect accurate test results. The procedure ensures that test results are comparable regardless of where or when they were obtained.
  
• Laboratory Resilient Modulus Testing: Sample Preparation and Test Procedure. This 13-minute videotape for managers and technicians describes each of the steps in the resilient modulus test procedure, including how to prepare soil and aggregate samples.

To order the videotapes, or for more information, contact Max Grogg at FHWA (phone: 518-431-4224, x223; fax: 518-431-4208; email: max.grogg@fhwa.dot.gov).

* 1993 AASHTO Guide for Design of Pavement Structures (Publication No. C97-GDPS-4)

To understand what the resilient modulus of the subgrade means for a pavement, it helps to think about the shock absorbers and springs in your car's suspension system. When your car hits a bump, the shock absorbers and springs compress and then quickly spring back to keep your car's wheels in contact with the road. Each time you hit a bump, your shock absorbers wear out a little bit. This wear adds up, and eventually your shocks will need replacing.

The subgrade beneath a pavement works something like a suspension system. When a vehicle travels over a pavement, the weight of the vehicle compresses the subgrade slightly, causing it to bulge out to the sides. After the vehicle passes, the subgrade springs back to its original shape. But each vehicle wears the subgrade out a little. These loads add up, and eventually the subgrade will no longer spring back to its original shape. At that point, the subgrade can no longer adequately support the pavement. Cracks, ruts, and other forms of distress will start to appear in the pavement.

A subgrade with a high resilient modulus—that is, a stiff subgrade—can carry more loads before it stops providing support to the pavement than can a subgrade with a low resilient modulus.

Knowing the resilient modulus of the subgrade at the project site allows the pavement designer to determine how thick the pavement should be. If the subgrade has a low resilient modulus, the pavement must be relatively thick to make up for the lack of support. If the resilient modulus of the subgrade is high, the pavement can be relatively thin, which saves money on labor and materials.