<p>Failure along rock discontinuities can result in economic losses
as well as loss of life. It is essential to develop methods that monitor the
response of these discontinuities to shear loading to enable prediction of
failure. Laboratory experiments are performed to investigate geophysical
techniques to monitor shear failure of a pre-existing discontinuity to detect
signatures of impending failure. Previous studies have detected precursors to
shear failure in the form of maxima of transmitted waves across a discontinuity
under shear. However, those experiments focused on well-matched
discontinuities. However, in nature, rock discontinuities are not always
perfectly matched because the asperities may be weathered by chemical, physical
or mechanical processes. Further, the specific shear mechanism of mismatched
discontinuities is still poorly understood. In this thesis, the ability
to detect seismic precursors to shear failure for various discontinuity
conditions—well-matched (rough and saw-tooth), mismatched (rough), and
nonplanar (discontinuity profile with a half-cycle sine wave (HCS))—was
assessed. The investigation was carried out through a coupled geophysical and
mechanical experimental program that integrated detailed laboratory
observations at the micro- and meso-scales. Shear experiments on gypsum discontinuities were conducted
to observe changes in compressional (P) and shear (S) waves transmitted across
the discontinuity. Digital
Image Correlation (DIC) was used to quantify the vertical and horizontal
displacements along the discontinuity during shearing to relate the location
and magnitude of slip with the measured wave amplitudes. </p>
<p>Results from the experiments conducted on planar, well-matched rough
discontinuities (grit 36 sandpaper roughness) showed that seismic precursors to
failure took the form of peaks in the normalized transmitted amplitude prior to
the peak shear stress. Seismic wave transmission detected non-uniform
dilation and closure of the discontinuity at a normal stress of 1 MPa. The
results showed that large-scale roughness (presence of a HCS) could mask the
generation of precursors, as it can cause non-uniform closure/dilation along
the fracture plane at low normal stress.
</p>
<p>The experiments on idealized
saw-toothed gypsum discontinuities showed that seismic precursors to failure
appeared as maxima in the transmitted wave amplitude and conversely as minima
in the reflected amplitudes. Converted waves (S to P & P to S) were also
detected, and their amplitudes reached a maximum prior to shear failure. DIC
results showed that slip occurred first at the top of the specimen, where the
load was applied, and then progressed along the joint as the shear stress
increased. This process was consistent with the order of emergence of
precursors, i.e., precursors were first recorded near the top and later at the
center, and finally at the bottom of the specimen. </p>
<p>Direct shear
experiments conducted on specimens with a mismatched discontinuity did not show
any precursors (in the transmitted amplitude) to failure at low normal stresses
(2 MPa), while those precursors appeared at higher normal stresses (5 MPa). The
interplay between wave transmission, the degree of mismatch, and the
discontinuity’s micro-physical, -chemical and -mechanical properties was
assessed through: (1) 3D CT in-situ Xray scans to quantify the degree of
mismatch at various normal stresses; (2) micro-indentation testing, to measure
the micro-strength of the asperities; and (3) Scanning Electron Microscopy
(SEM) and Electron Xray Diffraction (EDX), to study the micro-structure and
chemical composition of the discontinuity. The X-ray results showed that
contact between asperities increased with normal stress, even when the
discontinuity was mismatched. The results indicated that: (1) at 2 MPa, the
void aperture was large, so significant shear displacement was needed to interlock
and damage the asperities; and (2) the micro-hardness of the asperities of the
mismatched discontinuity was larger than that of the well-matched discontinuity,
which points to inducing less damage for the same shear displacement. Both
mechanisms contribute to the need for larger shear displacements to the mismatched
discontinuity asperities to cause damage, which is consistent with the
inability to detect seismic precursors to failure. The experimental results
suggest that monitoring changes in transmitted wave amplitude across a
discontinuity is a promising method for predicting impending failure for well-matched
rock discontinuities. Precursor monitoring for mismatched rock discontinuities
seems only possible when there is sufficient contact between the two rock
surfaces, which occurs at large normal stresses. </p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/15048309 |
| Date | 26 July 2021 |
| Creators | Hala El Fil (11178147) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/Shear_Response_of_Rock_Discontinuities_Through_the_Lens_of_Geophysics/15048309 |
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