Crust Raft - Design.

1 Introduction
The crust raft is discussed  previously where it is explained that it is a stiffer and stronger version of the
nominal crust raft. In this chapter it is intended to take this a stage further through the design procedure and to an actual example.

2 Design decisions
The crust raft is used where normal ground bearing substrata is relatively poor, where the depth to good load-bearing soils is excessive, but where by dispersing the  loads differential settlements can be controlled. It is more attractive where these conditions exist on a relatively level site, i.e. where few steps or changes in level exist.

The considerations for thickening layout and profile are as for the nominal crust (see section 13.2).

The design of the crust raft and its element cross-sections tends to be based on a similar simplified analysis to that of the nominal crust raft but adopting a slightly more analytical approach. However, in order to arrive at a suitable span and depression diameter a more detailed analysis of the ground conditions would be carried out. For example, if the raft was to span over possible swallow-holes or shallow
mine workings a detailed study of borehole information particularly with regard to the existence of voids below ground, combined with historical evidence of previous  collapses/depressions would be carried out. Excessively large voids or voids which were creating particular prob- lems in the design of the raft could be considered for grouting in order to reduce the risk of collapse and reduce the diameter of design depressions. If grouting was to be adopted then this would be carried out prior to construction of the raft foundation.

When studying old shallow mine workings reference should be made to Chapter 6. Historical details of pillar and stall workings may be used in the anticipation of the maximum diameter of collapse at cross-over positions, etc. (see Fig. 13.16).

Fig. 13.16 Subsidence prediction for shallow
mine working.

A word of caution should be given at this stage with regard to the reliance on mining records since, as was emphasized previouly, while shaft locations are often quite accurate any records of pillar and stall workings tend to be less reliable. This is due to disintegration during oxidation of the pillars, and/or the practice of robbing pillars at the end  of the workings’ normal life. These actions result in a tendency for larger depressions to occur but usually within  a shorter period after completion of the mine workings.

The earlier completion of subsidence is an advantage. In some cases, however, if pit props have remained in position un-rotted, early subsidence is prevented. These sorts of conditions will be taken into account by the experienced engineer in assessing the borehole records and other records of the possible collapse mechanism and type and size of void or depression to be spanned or grouted.