Shear-wave and Compression-wave velocity as Failure Indicator for uniaxial Compressive Strength Tests with Rock Samples

The evaluation of compressive strength in rock and ground samples is a fundamental com-
ponent of geotechnical testing. It provides critical data, including stress-strain graphs, that

allows for the calculation of various mechanical moduli. When the Federal Waterways
Engineering and Research Institute (Bundesanstalt für Wasserbau, BAW) tests compressive
strength sensors are attached around the rock samples to measure lateral stress. These
sensors have a high risk of damage when the samples are pressured to failure, which is
standard testing protocol following the DIN 18141-1:2014 [1] and DIN EN ISO 17892-7:2018
[2]. Preserving the sensors while following these protocols to ensure accurate mechanical
moduli calculations has become an urgent challenge.
The primary objective of this Bachelor Thesis is to predict sample failure during uniaxial
compressive strength tests using changes in seismic wave velocity under varying pressure
conditions, thereby allowing for the timely cessation of testing to protect both sensors and
samples. The preferred time window for indicator detection was set at 10 to 30 seconds before
the stress peak. ... mehrThe study evaluates indicators, such as the peaks of P- and S-wave velocity,
and the last inflection points before the stress peak of P- and S-wave velocity.
A total of 35 cylindrical samples were subjected to compressive strength tests following

established standards (DIN 18141-1:2014 and DIN EN ISO 17892-7:2018), while measuring P-
and S-wave velocities. Of these, 21 samples were made of Conduro plaster, with different

diameters, and 14 were composed of Cordierite Pearl Gneiss. However, the Gneiss samples
could not be pressured to failure, limiting the meaningfulness of the data.
The research findings suggest that the velocity peaks of both P-wave and S-wave are
unsuitable as indicators for sample failure, as they do not consistently fall within the chosen
10-30 second time window. However, the inflection points of the S-wave velocity show
promise, with consistent time windows for Conduro samples and potential suitability for
certain conditions. To enhance the capabilities of the experimental setup and accommodate
a broader range of testable samples, the study recommends the use of transducers with
expanded capabilities and an adapter to connect these transducers to a 600 kN pressure
plate. These hardware enhancements, coupled with a data analysis workflow that operates
during testing, could lead to the practical application of the derived indicator for sample failure.
While the S-wave velocity inflection point currently holds potential as an indicator for
sample failure, further research with a diverse range of materials is needed to validate
its applicability under varying conditions. In conclusion, this research contributes to the
development of a more efficient and sensor-preserving approach to evaluating the compressive
strength of rock and ground samples, opening doors to improved testing methods and reduced
resource expenditure.