The Development of a Free-Field Soil Stress Gage for Static and Dynamic Measurements

Abstract

Many problems are inherent in measuring stresses in granular materials such as soils. The presence of an inclusion such as a gage disrupts the stress field, causing either stress concentration or stress relief depending on whether the inclusion is more or less stiff than the medium. This phenomenon can seriously affect gage registration. Inertial effects must also be taken into consideration in designing a gage for dynamic measurements. This paper describes the work of the Waterways Experiment Station during the past 2 1/2 years in developing a free-field soil stress gage to minimize these problems. Two different gage sizes were developed. Both sizes use piezoresistive strain gages bonded to stiff diaphragms as sensing elements. Fluid calibrations for the gages are linear to at least 500 psi. Dynamic rise times are approximately 6 μsec. Results of laboratory calibrations in dry sand, and of field measurements during a 500-ton TNT test are presented.

The Use of Miniature Soil Stress Measuring Cells in Laboratory Applications Involving Stress Reversals

“…Such errors can often be reduced (at the expense of sensitivity) by adopting devices with Rated Capacities higher than the expected soil stresses. (8) It is possible that still larger errors may accumulate if large numbers of cycles are applied during pile installation. However, examples of the measurements made with the miniature cells along with other results obtained in the calibration chamber tests, indicate that the stresses within the sand mass may be still established to within ¿15z with the systems deployed.…”

“…It is common to treat the soil as a simple elastic continuum, which leads to aˆxed ratio between the measured stress, s m , and the true stress, s, developed in the ground (Ingram, 1965). The latter can only be deˆned under calibration conditions where the applied loads are closely controlled and the ideal`undisturbed' distribution of stresses can be estimated reliably within the soil mass.…”

The Changeable Interaction betweeen Soils and Pressure Cells; Tests and Reviews at the Waterways Experiment Station

“…wide inact;ve rim, '1'he registration ratio of these cello was clooe to 1.00 perct-nt for low Iprriucipal stress ratios but it rose to 153-172 perceriu when thVtreiss con--ditions in the soil upproached the failure conditions, ~ee P'art 1]1, Figure 95. Figure 9 and is described in detail in a report by J. K. Ingram (1968). The active diameter of this cell is 0.75 in., and it has a diaphragm on each side so that the cell is nearly symmetrical with respect to the midheight plane.…”

Development of an NBS polymer gage for dynamic soil stress measurement

“…The calibrated linear relationship is not affected by the gage location, i.e., whether in soil or on a concrete pedestal; the thickness of the protective covering; the frequency of the impact load; and the length of the co-axial cable used in testing. 12…”

“…There have been many studies conducted in the development and evaluation of soil stress gages, including the work by Abbott, et al. [1], Ingram [12], Hamilton [10], Selig [20], Triandaf il idis [25], Krizek, et al [15], Weiler and Kulhawy [26], and others.…”