In the rock-masses which consist of hard rocks (shall be called as "hard rocks" hereafter), seepage through the joints and cracks existing in the rock-masses prevails, and its permeability have been evaluated with Lugeon value in Japan.
On the other hand, in the diluvial strata on which homogeneous earth-fill dams have been constructed, the permeability of the foundation is evaluated by the permeability coefficient obtained from the in situ permeability test which is generally used in the field of soil engineering.
In the soft rock foundations, Lugeon value is used in some cases and the permeability coefficient is used in other cases, which often causes confusion. The critical pressure of soft rocks is generally low compared with hard rocks, and thus soft rocks have their peculiar problems such as the measurement of permeability at low pressures. Furtheremore, since the critical pressure is low, grouting for soft rocks is less effective compared with hard rocks and the control of grouting operations for soft rocks are sometimes difficult.
This paper describes the features of permeability and seepage control of soft rocks.
In hard rocks joints and cracks comprise the paths of seepage, because of the rock itself being impermeable. And in hard rocks, the measurement of permeability is generally carried out by the water pressure test in a borehole, and is expressed using the Lugeon Value obtained from it. In the Lugeon test, the permeability of rock is measured by applying the water pressure to the portion of borehole sealed off with a packer, as shown in Fig. 1. Lugeon value is the quantity of water takes of rocks, which is expressed by liters per minute in a meter length of borehole at the pressure of 10 kg/cm2
(Figure in full paper)
If the length of borehole under testing is enough long compared to the diameter of the borehole and it is possible to assume that the seepage flow in rocks is governed by Darcy's law, the permeability coefficient in Darcy's law will be given by the following equation (Bureau)
From equation (2), it can be said that 1 Lu is approximately equall to 1.0× 10−5cm/sec. In the hard rock foundations for dams, this Lugeon value have been used for the evaluation of permeability, and the control of grouting works. Empirical knowledge obtained from many grouting works in hard rocks will be summarized as follows:
The permeability of rocks having the Lugeon value less than ten is comparatively low, and observed seepage in these rocks usually corresponds to the discharge estimated from seepage analysis by using the relation of 1 Lu = l × 10−5 cm/sec.
The permeability of rocks having the Lugeon value more than ten is considerably large. These rocks usually have open joints. The permeability of the rocks having the Lugeon value more than 20 to 30 is very large, and the velocity of sevaral centimeter per second is often observed in the seepage measurements.