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Analysis of measured pore pressure response to atmospheric pressure changes to evaluate small-strain moduli: methodology and case studies

The use of pore pressure responses to fluctuations in total stress (resulting from barometric pressure changes) to calculate moduli and other material properties is a recently developed technique being applied to deep aquitard formations. Initially, the method has relied on visual interpretation of the data from grouted-in piezometers, resulting in a qualitative result with little opportunity to define the quality of measured data; more recently, linear and multiple regression analyses were used to assess the same properties with limited success. Here, a methodology is developed to determine loading efficiency from pore pressure measurements using filtering and numerical regression. The results indicate a near linear relationship between the change in pore pressure and change in stress (barometric pressure), resulting in an estimation of loading efficiency and quantification of the quality of fit. Four to 6 days of data appear to best isolate the barometric fluctuation with the developed filters. The technique is successfully applied to a “simple” site, where groundwater conditions are relatively stable, as well as a complex site, where groundwater conditions are changing due to fluctuating river levels. The successful application to the latter site shows that robust analysis is possible, even for dynamic and complex environments, and that the method represents a viable alternative for estimating material parameters of formations that are historically difficult to characterize.