This paper presents the characteristic deformation behaviour under static stress condition obtained from experiments, and the consideration on the common and different properties owing to the origin. The experiments performed mainly consist of consolidation test and conventional drained triaxial test on undisturbed rocks.


As the term of soft rock is named after the nature of mechanical properties of rocks, it is interesting to consider the difference and situation between each rock using foundamental properties characterizing mechanical properties of soft rocks. Then, these relations were investigated using saturated water content or porosity as an index of compaction or density of rock, and unconfined compressive strength qu or tensile strength σt as an index of the bonding or cementation strength.

Figs. 1(Yoshinaka & Onodera 1978) and 2 show the relation of wsat to qu of sedimentary rocks, qu and σt to n of weathered granite respectively. Fig. 1 appears to have no relation at glance, however the systematic relation can be distinguished from point of view of mineral and grain compositions, geological system of each sediment. For example, the sedimentary rocks belong to same geological system have a linear relation between w-log qu, and the angles of lines of linear arrangement reflect the consistency of rock forming materials, the geological environment and history, etc. From Fig. 1 it seems that sedimentary soft rock can be classified into the density dependent and cementation dependent types. Porosity of granite increases from 0.5% to 50% according with weathering. Mechanical properties of granite decrease remarkably, but almost quantity of intergranular bonding strength losses by increasing of n up to about 10–20 % as shown in Fig. 2. Therefore it may conclude that mechanical properties of highly weathered granite as soft rock mainly depend on the intergranular fabric and mechanical properties of grains. The w-qu relation obtained from Fig. 2 is shown by a dotted line in Fig. 1, in comparison.


To clarify the compression behaviour of soft rocks, the relation of void ratio to consolidation pressure (0.1–20 MPa) of many kinds of rocks are compiled in Fig. 3. The data used are in Table 1. To compare with soft rock to soil, the consolidation curves of sand and clay are also shown in Fig. 3.

From Fig. 3, the following properties are obtained:

  1. The volumetric deformation curves of soft rocks under isotropic consolidation can be classified,into 3 patterns, A,B, and C,as shown in Fig. 4.

(Figure in full paper)

If these patterns are compared with those of soils, A and B may correspond to over consolidated clay or dense sand, and C may correspond to normally consolidated clay. But the compression behaviour of soft rock seems to contain fundamentally different elements from soils.

A distinct yield point Py arises in case of A type which have relatively large void ratio, and the value of Py generally corresponds to qu. The compressibility suddenly changes at Py as boundary, and in low pressure under Py seems to depend on bonding strength of cementation.

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