Foundation characteristics of the Upper Carboniferous rocks
Authors: Meigh Alan Cotton 
Published: November 2007

The paper is primarily concerned with the bearing capacity and settlement characteristics. Mining subsidence is not considered. The location of 'exposed' Upper Carboniferous rocks is briefly outlined in relation to urban and industrial areas, and depths to rockhead are discussed.

The characteristic cyclic pattern of deposition of the Coal Measure rocks is described, and descriptions of the principal rock types, and some overlying soils are given. In the Coal Measure rocks the upper surface is usually weathered. Over mudstones there is typically 10 ft or so of material weathered to a clay, followed by less weathered rock. Over sandstones the fully weathered zone is thinner and usually consists of clayey sand containing weathered sandstone fragments. Depth of weathering may be locally increased near faults and where dips are steep.

Exploration is often by soft ground boring methods in the overlying soils followed by diamond drilling in rock. NX-size core barrels are the minimum size which can be expected to give 100 per cent core recovery, and on important jobs it is recommended that a larger size should be used. Drill core required for laboratory testing must be protected against drying-out and against frost action. The value of trial pits is emphasized.

Some laboratory test values of compression strength and deformation modulus are given in the paper and the limitations of laboratory testing are discussed.

Standard Penetration Tests have been used in the Coal Measure shales and mudstones to give a rough guide to rock strength. A tentative correlation between S.P.T. values and rock description is given. Other
in situ
testing techniques are the Menard Pressuremeter Test, the Plate Loading Test and loading tests on piles. For plate loading tests in pits and at the base of piles it is essential that the material below test level is undisturbed. Seepage forces from ground water must be avoided since they can cause rapid deterioration. The usual pile loading test involves load application at the head of the pile, but valuable information can be obtained from tests in which end bearing resistance is measured separately from side shear within the 'rock socket'.

Data is given for three sites where plate loading tests were carried out, together with laboratory testing. From this data and from laboratory test data presented elsewhere in the paper, characteristic values of compression strength and deformation modulus have been chosen.

It has been customary to assign allowable bearing capacities to Upper Carboniferous rocks on the basis of visual inspection; typical values are quoted. For a calculation of allowable bearing pressure recent practice is to use the results of
in situ
loading tests. Unfortunately there do not appear to be any published data concerning the behaviour of full-scale foundations.

It is suggested that for the Upper Carboniferous rocks, bored cast–
in situ
piles are generally preferable to driven piles. Ranges of working loads, and corresponding penetrations, are discussed and comments are made on pile spacings. Pile loading test data presented for three sites appear to support these empirical criteria.

A more rational estimation of pile bearing capacity would involve considerations of end bearing capacity and of side shear within the rock socket. This is difficult since side shear between concrete and rock depends not only on rock strength and deformation characteristics, but also on the response of the rock to the pile boring processes. Further development must depend on correlation of performance data with careful and detailed description of the rocks involved.