ONCE the vibro rig is above where the column is to be installed it begins to push its vibrating flot or poker through the ashy ground.On the Wolverhampton site this made ground was discovered to be at least 10 m deep in places.
Using the results of the ground investigation the rig operator knows roughly the depth at which he should meet denser competent soils.
While the flot will easily displace the made ground it will struggle to penetrate any further, reaching a point known as refusal.Once this occurs the flot is lifted slightly and stone stored in a hopper at the top of the flot is allowed to flow down a tremmie tube running the length of the flot to fill the space created.
The flot then repenetrates the stone to compact it until it again reaches refusal.This process is repeated in cycles until a column is formed up to ground level.
The vibrating action of the flot also shakes up the granular material surrounding the column and increases its competency, effectively offering two treatments for the price of one.
This method, known as dry bottom-feed, is a slight change on the more recognised method of building stone columns as it allows for the stone to be delivered straight to the bottom of the bore.Under more conventional top-feed vibro the flot would be removed fully from the borehole, with stone then pushed down the hole for compaction. But, given the poor nature of the made ground, there was a danger that the borehole would collapse if the flot was removed, hence the requirement for the dry bottom feed.