Deep boreholes are essential for applications like oil and gas exploration and monitoring tectonic stresses in rock formations. Breakout failure often occurs in horizontal boreholes, especially at great depths or high-stress conditions, due to stress-induced fractures. Apart from the unsupported borehole wall where a bi-axial stress state exists, the borehole experiences a true triaxial stress state. Thus, understanding borehole behaviour under complex stress conditions is crucial for economic and safety reasons. This study uses a 100 mm artificial cubic sample made from self-compacting cement mortar, with a horizontal cylindrical borehole drilled at its centre, subjected to true triaxial stress states. The experimental results provide insights into borehole shear failure, detailing the sample preparation methodology and assessing its effectiveness in simulating borehole breakout failure in laboratory-scale studies.