Discussion: Session 2—rocks other than chalk

M. J. Tomlinson

As several types of rock are examined in the six Papers under review it has not been possible to make comparisons as was done in the introduction to the discussion on pile behaviour in chalk. However, it is worth trying to see if a philosophy for the design of piles bearing in weak rocks can be established. Two approaches appear to be possible:

(a) to consider the weak rock to behave as a soil and to relate skin friction and end bearing values to measured shear strength properties;

(b) to consider only the settlement of the pile at the working load in compression. Method (a) appears to have worked satisfactorily for the piles in Cretaceous Mudstone described in Wilson's Paper and in the highly weathered diabase as described by Webb. In both cases the rocks could be classed as very weak. The mudstone had an unconfined compression strength of 1 MN/m2, comparable to that of a hard clay, while the weathered diabase had an unconfined compression strength of only 322 to 560 kN/m2, which is representative of the strength of a very stiff to hard clay. The Authors' approach in calculating the carrying capacity of the bored piles using semi-empirical factors for adhesion and end-bearing and relating these to the undrained shear strength of the clayey weathered rock appears to be quite logical. Although the effects of installation are critical the result of installing a bored pile in a very weak clayey weathered rock will be the same as that of installing a bored pile in a stiff clay. The augering will produce the same configuration of a lightly keyed surface to the smooth cylindrical walls of the pile borehole. It would also seem to be reasonable to adopt established adhesion factors to calculate the skin friction on piles driven into very weak clayey rock. The effect of pile driving would be to produce the same effects as a pile driven into a stiff clay, i.e. the formation of a skin of clay adhering to the pile surface and which is carried down with the pile resulting in a clay/clay failure surface during driving and when loading to failure (Tomlinson, 1971). At some stage, with increased strength of a weathered rock, the effects of pile installation will make it impossible to adopt the design methods used for piles in cohesive and granular soils. In the case of bored piles the stronger rocks will break up under the action of the rotary auger to produce a 'rock socket' with a rough surface to which the concrete will be strongly keyed. The stronger and more heavily jointed the rock, the higher will be the load transfer value from pile to rock. Circumstances will be quite different in the case of driven piles which will shatter the weak rocks to produce a mass of broken material around the pile shaft having the characteristics of a granular soil. For driven piles, the stronger and more heavily jointed the rock the shorter will be the penetration of the pile into the rock mass and the looser the fragmented rock around the pile, and hence the smaller the total skin friction. It is probable that the demarcation between considering the pile to be embedded in material of soil-like consistency or texture, and in a material of rock-like character is when the strength