Influence of local heterogeneity on Doorstopper stress measurements

When using so-called overcoring techniques to estimate stresses in rock masses for site investigation or in concrete structures for monitoring purposes, stresses are derived from recovered strains using elastic stress–strain relationships for which the media is assumed to be homogeneous at all the scales involved in the measurement. As we know, rock, rock masses and concrete are not homogeneous and this will affect the accuracy of stress measurements. By simulating stress measurements with the modified Doorstopper technique which can be applied to a variety of conditions including measurements in concrete structures, it is shown how heterogeneities found at the measurement scale can be accounted for and how the estimated stresses are influenced by heterogeneities found at different scales. Using a numerical model, a two phase heterogeneous medium resembling concrete is modelled and the complete modified Doorstopper stress measurement operations, including stress relief drilling and recovered core reloading are simulated. The RPR method allowing calculating four far-field stress components from a single measurement had been developed for a homogeneous medium and is tested here for its effectiveness in heterogeneous materials. The results show the error on the estimated stresses is little affected by the scale at which the deformability parameters are determined and the main source of error is linked to the estimation of the stress component parallel to the borehole axis.

► Stress measurements in heterogeneous media are simulated.
► The complete stress relief and core reloading process is modeled numerically.
► The simulated measurements are interpreted using the conventional and RPR methods.
► The heterogeneous media are assumed to behave as a homogeneous equivalent anisotropic continuum.
► It is shown the various hypotheses regarding the media equivalent behaviour has little effect on the results in terms of error and scatter.