Evaluation of the Humboldt Stiffness Gauge

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CAIT project no.: 71a RU904

Fiscal Year: 1999/2000

Status: Final

Rutgers-CAIT Author(s): Ali Maher, Thomas Bennert, Nenad Gucunski

External Author(s): Anthony Chmiel



This report pertains to the evaluation of a non-nuclear means of determining the dry density of soils. The device is called the Humboldt Stiffness Gauge (HSG). The device is essentially a small-scale plate load test and takes approximately 1 minute. The test parameter from the test is a stiffness value that is average over approximately 6 to 14 inches beneath the HSG. The manufacturer of the HSG then uses a method to determine a regression equation to use the stiffness parameter to determine the dry density of the soil. However, to establish the regression equation, prior knowledge of the soil’s moisture content and dry density is needed.

Research was conducted both in the laboratory and in the field to evaluate the HSG. The laboratory testing was used to establish the depth extent of the measurement of the device, as well a repeatability of the device. This was done by conducting controlled tests within constructed “soil bins”. Laboratory testing was also conducted to evaluate how certain objects located beneath the device affected the measurements, as well as to determine if the device may be used as a locator for buried objects. Both a steel pipe and a PVC pipe were selected as objects that the device may encounter under field conditions. The objects were buried at various depths below the device with HSG testing conducted directly over the object, as well as in the immediate vicinity of the object.

All “soil bin” tests were accompanied by density balloon testing to verify the soil’s dry density. Results of the laboratory testing program show that the device is very repeatable and that it typical has a measurement depth of approximately 6 to 10 inches beneath the device. The laboratory testing also indicated that the device does not have a potential for future use locating buried objects in a homogeneous soil, meanwhile, measurements do not seem to be affected due to small objects located below the device.

The field-testing consisted of utilizing the device on a full-scale research project involving the beneficial reuse of Portland cement amended dredge material as an embankment. Over 400 hundred tests were conducted during the placement of the embankment material, with nuclear density gauge and Clegg Impact Hammer (CIG) tests. Results indicate that the device can be used as an alternative means of estimating the dry density of the soil, as long as there is a way of determining the moisture content of the soil.