This paper deals with the interpretation of the results of large flat jack(LFJ) tests performed in weak rock masses. The influence of the in-situ state of stress on the results,as well as the determination of the deformability modulus and some special aspects in the testing technique are discussed.

Results already obtained in several test localities (with values of the deformability modulus lying between118 and 2170MPa) and tables which allow the calculation of the deformability modulus for different cases of LFJ association are presented.


The method is based on the measurement of the relative displacements of the walls of a slot (opened in the rock mass by means of a diamond disk with a diameter of 1 m) that undergoes a uniform pressure applied by a flat jack with thin, deformable walls and a great area. The flat jack contains four deformeters, with electric strain gauges, which measure the variation of the slot opening,fig. 1.

(Figure in full paper)

These jacks can be placed in slots opened side by side and tested either simultaneously or one by one, thus allowing the test to interest a volume big enough to be representative of the rods mass,fig. 2.


The operations of opening the slot and the central hole(through which passes the support column of the diamond disk that permits the progression of the slot down to the desired depth)are performed by a set composed by a support structure and an oil motor which moves the drill or the diamond disk. The oil motor is fed by oil alder pressure coming from an oil pump which in turn is moved by an electric motor, to which it is joined by an elastic coupling.


The state of stress induced by the test near the border of the slots is always a traction, which often leads to the opening of a crack in the slot plane. The interpretation of the test results is rendered difficult by the lack of knowledge on the depth attained by that crack, due to the fact that it is difficult to evaluate the crack dimensions in the field even when the crack is visible in the test chamber. If the crack propagated infinitely, and if we may neglect the dimension normal to the test plane, the study would reduce itself to the study of two half-spaces submitted to a uniformly distributed load in the area defined by the contour of the flat jacks. But as the crack is always finite, the relative displacement of the two half-spaces must be zero in the zone outside the crack, thus provoking at the crack smaller displacements than the ones due to the deformation of the half-infinite space.

Formulation of the problem

According to the hypothesis formulated above, the determination of the state of stress and strain of the rock mass may be done starting from the study of a half-infinite space submitted to a pressure p,applied by the flat jacks in the area defined by its contour,

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