Large diameter coring is a technique whereby a core greater than 150 mm diameter is cut out of the bound layers of the road surface in one piece and stored for re-use when reinstating the road. Excavation of the underlying unbound material is typically accomplished by vacuum extraction. After work on the apparatus is complete, the unbound material is replaced and compacted, and the extracted core is re-fitted to the road in its original orientation and bonded to the surrounding surface. Reinstatement by re-using the extracted core is a specialist process and is described in S6.7.17.
Large diameter coring is particularly useful for gaining access to individual pieces of apparatus such as buried valves, and it is considerably quicker than conventional excavation and reinstatement methods. The technique should only be considered where the location of buried apparatus is precisely known. It is not permitted where there is a risk of cutting through services located within the bound layers.
The method of reinstatement by core re-use is only permitted for single cores, two overlapping cores, or multiple cores with a minimum clear separation between core perimeters of 300 mm. The reason for these limitations is that three or more overlapping cores present too many bonded cuts to guarantee the integrity of the reinstated surface while very closely spaced cores create compaction problems in the unbound layers.
As such, where there are three or more overlapping cores or where the separation between multiple cores is less than 300 mm, core re-use is not permitted unless agreed to by the authority (see Figure S6.2). Where this is the case, reinstatement must be carried out in accordance with S6.2, S6.3 and S6.4 or S6.5, as appropriate. Excavations resulting from aborted coring works must be considered as large diameter cores within the above.