The rheological behaviour under longterm loading or cyclic loadings is quite important in the problems relating to concrete dam foundations, especially in weak rock masses. In situ measurements to evaluate the rheologic characteristics present considerable problems. Testing technique, set up by ISMES, and shown in Fig. 1, is characterized by the following features:

  • extensive loading areas (ø) =1 m), to affect a large rock volume also in depth.

  • Load application by means of flat jacks, to obtain a uniform pressure distribution.

  • Displacement recording at various depths, along the loading axis, by means of borehole extensometers, so as to detect possibledishomogeneities in the rock, and especially a loosened superficial layer.

  • Oleopneumatic system to keep a constant load for a long time.

  • Automatic data recording system.

(Figure in full paper)


This testing technique was applied to the study of a rock mass consisting of very jointed hard argillite (Fig. 2) having characteristics somewhat variable from one side to another. The tests were carried out inside vertical shaft at increasing load levels up to a max of 1,5 MPa. Many loading cycles were performed at each load level; at each side, the test lasted about two months.

(Figure in full paper)

A typical load-settlement diagram, recorded at the centre of the loading plate is shown in fig. 3. It is evident the downward concavity of the curve, the importance of the delayed deformation at constant load and the influence of loading cycles on the deformations. Settlement diagrams, as a function of time (Fig. 4), show initially a deformation phase at constant load lasting about six days; the rate is progressively decreasing and asymptotic values of displacement were determined through linear extrapolation on the inverse time. In the subsequent unloading phase, which was maintained for about three days, both immediate and delayed recovery of part of the settlements were observed. Also in this case the final (asymptotic) value was estimated. The effect of loading cycle on deformation is quite interesting: in fact these cycles cause a marked increase in the creep rate and total displacement becomes even greater than the asymptotic limit evaluated on the basis of the initial constant loading period.

Acomparison of the results obtained in the four testing areas indicates that the importance of the rheological behaviour, expressed by the ratio between the delayed and immediate settlements, decreases as the rock quantity improves (Fig. 5). The above consideration applies both if rock quality is evaluated upon the unloading immediate modulus or through its seismic in situ velocity. It is also observed that the delayed recovered settlements become an increasingly greater part of the delayed settlements in the loading phase as the rock quality improves (Fig. 6),In addition, the analysis of displacements in depth shows that the rheologic behaviour is more important for the superficial loosened layer (less than 1 m).


In order to gain insight on field rheological behaviour, the plate loading test was analyzed for simple rheological models characterized

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