High Pressure Customised Shear Box: Politecnico di Torino

2009 marked the 150th anniversary of Politecnico di Torino (PoliTo). Their long-standing tradition has been the basis for the reputation they enjoy today, as one of the leading technical universities in Italy and throughout the world. According to the Jiao Tong University's league table, PDT is in 7th place in Europe for engineering studies. In the league tables compiled by Vision and Censis, PoliTo comes first in Italy for internationalization and for technical studies. Success is confirmed by the high number of students that apply every year to Politecnico di Torino from all over Italy and from abroad (33% of students come from regions other than Piedmont, and 12% come from outside Italy). 80% of graduates find employment within one year of graduation, compared with a national average of 62%.

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Built in 1957, the Beauregard Dam in the Aosta Valley in Italy, has a potential capacity of 70 million cubic metres of water. Currently, the maximum allowable capacity is 6.8 million cubic metres of water, the applied restriction being due to concerns in the early 1970's of infiltrations of water in the mountain sides reducing the overall stability of the slope adjacent to one of the dam abutments. This caused the regional administration to order the reduction of the size of the artificial lake to the minimum necessary to operate the hydroelectric power plant and the problem has been taxing engineers ever since.

Shear strength of rocks are usually investigated by means of unconfined and triaxial compression tests, which were deemed unrepresentative in the case of the Beauregard Dam. To fully understand the mechanical behaviour of the dam slopes, it was imperative to determine the shear strength and deformability characteristics, including creep behaviour of the material under the desired effective in-situ state of stress. Such test conditions could not be completely reproduced using traditional devices.

Fig. 1 Aerial View of Beauregard Dam (2005)


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


The custom designed and built equipment by GDS & PoliTo allows direct shear tests to be performed on either soil or soft rock. Total stresses may be applied to a dry specimen or, alternatively, one can apply a pore pressure to a saturated specimen and shear it in truly effective stress conditions. The apparatus allows the user to change the pore pressure conditions during testing, so as to reproduce, at laboratory scale, a change of the piezometric level in the field.

If the interest is on rock joint strength, the apparatus can be used as traditional direct shear equipment. Given the possibility to vary the gap between the lower and the upper part of the shear box, either filled or unfilled joints can be tested. When testing a rock joint, the system allows one to apply the water pressure directly to the joint surface by drilling a small hole in the lower half of the specimen. The sample can be placed within a membrane, so that the back pressure is applied only to the joint while the pressure chamber remains empty.

…“The shear testing apparatus designed by GDS Instruments and PoliTo has many innovative features for a direct shear device. Its advanced characteristics, in particular its capability to apply high back pressures to the sample and apply 100kN both axially and in the shear direction, allow for testing to be conducted in a variety of ways. In particular, the shear behaviour of an intact specimen can be investigated under a change of pore pressure as observed in field conditions. It is shown that the new equipment is working properly and that the accuracy of the measurement system implemented allows one to obtain relevant results during both the shearing and creep stages”, says Barla.

Fig. 3 Shows a cross section of the Shear Box and Pressure Chamber

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