Together Politecnico di Torino (PoliTo) and GDS Instruments (GDS), both with extensive knowledge and experience in shear testing devices, teamed up to design and develop a new direct shear apparatus that had the following design brief:-
- Could provide accurate results whilst working at high loads (up to 100kN axial and shear load).
- Accept either disturbed or remoulded specimens (as well as specimens with discontinuities), while applying a back pressure directly to the specimen or at the interface between the jointed samples (up to 10MPa pore pressures).
- The new apparatus had to allow direct shear tests to be performed under closely controlled conditions in terms of axial load and back pressure.
- Highly accurate displacement measurement for the very small yet long term creep movements to be measured.
The agreed design was a direct shear apparatus, with back pressure capability. The High Pressure Back Pressure Shear box (HPBPS) was constructed in stainless steel and designed to host cylindrical specimens of 50 or 100 mm diameter and 50 mm height, allowing a maximum shear displacement of 18 mm. To conduct tests on either rough or filled discontinuities, different gaps up to a maximum of 10 mm could be set by the user between the upper and the lower halves of the shear box.
The axial load is measured by a load cell integrated into the loading ram. The load cell has a 100 kN full scale and 0.024 kN precision.
The shear load is measured by a second load cell of the same range and precision connected to the lower part of the shear box via a horizontal loading ram. The back pressure is applied by a GDS Advanced Pressure/Volume Controller applying a maximum back pressure of 10 MPa, with a 2.4 kPa precision.
Vertical displacements are measured externally by two different means, horizontal displacements are measured by three. One measurement is given by the rotation of the driving system of the electrical motor that moves the axial and horizontal loading rams. This measurement may be affected by mechanical clearance, therefore two potentiometer transducers rigidly connected to the loading rams are placed in contact with the pressure chamber. These transducers form the second external measuring system.
One additional shear displacement measurement is taken inside the pressure chamber by means of a specially designed LVDT assembly. This can be considered the most reliable measurement of shear displacement available in the apparatus, which is not affected by either the mechanical clearance or the deformability of the loading ram. In reality, this proved to be one of the most novel parts of the design, because a pressure compensating balanced ram for the LVDT armature needed to be designed such that the LVDT measurements were not affected by pressure.
GDSLAB software allows the user to strictly control the test procedure and parameters, allowing critical switches from stress-controlled mode to strain-controlled mode to be possible.
Fig. 2 Photograph of the test apparatus