Civil Engineering ETDs

Publication Date

6-3-1968

Abstract

The static evaluation of a recently developed soil-stress gage, primarily designed for laboratory use in dry cohesionless soils, is presented. Two variations of the same basic soil stress gage design are discussed. Tests were performed to evaluate the gage linearity, temperature sensitivity, and electrical response to applied soil pressure. The gage responded linearly to pressure and the calibration curves remained stable within ± 5 percent over a period of two years. With a 10 ma bridge current, the gage had an output sensitivity of 0.040 mv per psi. Temperature changes produced signals which could not be distinguished from pressure signals. This amounted to a pressure change of 3.33 psi per degree centigrade. Temperature also affected calibration signals by ±0.47 percent per degree centigrade. During loading, the response of the gage in Ottawa sand was linear and could be reproduced on subsequent cycles of loading with negligible deviation from the first cycle loading curve. During unloading the gage was hysteretic by about 4 percent of the maximum load. The nature of this hysteresis appeared to depend upon the soil type. The response of the gage in a plaster sand was linear, however the gage output increased by about 12 percent over the output in Ottawa sand. The gage could be placed as close as 1/4 inch (horizontally) to other objects with no adverse affect on its response. A statistical sample of free-fields stress measurements in Ottawa sand indicated that the two basic designs examined had mean over-registration ratios of 1.63 and 1.40. The over-registration ratio was unaffected by confinement for at least the two confining conditions of the soil used in these tests. Seating was observed to be the most critical fact determining the response of the gage to soil pressure. The over-registration ratios could change by as much as 40 percent with extremes in placement in Ottawa sand. When a consistent placement technique was used, the coefficient of variation for free-field over-registration ratios could be kept to about ± 6 percent. There is a possibility that the over­-registration ratio for each gage was unaffected by the direction of principal stresses, but absolute verification of this was not possible. However, there was a trend for the registration ratios to follow theoretically computed stresses with some factor greater than unity.

Document Type

Thesis

Language

English

Degree Name

Civil Engineering

Level of Degree

Masters

Department Name

Civil Engineering

First Committee Member (Chair)

George Emmanuel Triandafilidis

Second Committee Member

John Bryan Carney Jr.

Third Committee Member

William Walter Hakala

Fourth Committee Member

J. Lewis

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