Design parameters for rock sockets in Scottish Carboniferous rocks

摘要

There are a wide variety of methods available which can be used to calculate the shaft capacity of pile rock sockets. In the UK the design approach is often based on equations developed by Rowe & Armitage (1984) and Williams & Pells (1981). These methods, along with many others, calculate shaft capacity from empirical correlations with the unconfined compressive strength of the rock. In Scotland 'presumed' values of side friction may be used which are dependent upon rock type. However in all methods of calculation there remains considerable uncertainty in the reliability of the designs produced. It is recognised that due consideration needs to be given to local geological conditions and load tests need to be carried out if design assumptions are to be verified. Rock sockets in coal-bearing Carboniferous rocks in Scotland are often designed with the assumption of little or no contribution from the base capacity as the coal layers will be generally be much weaker than the surrounding rock. In such cases it is the capacity of the shaft rather than the base which is of greater importance. This paper looks at design methods and back-calculates mobilised shaft capacity from pile tests undertaken on sites located in the Central Belt of Scotland. In one test the pile shaft capacity was determined explicitly by the use of a 'soft toe' at the base of the pile.