Civil Engineering Reference
In-Depth Information
necessary adjunct to good quality fieldwork if these circumstances are to be properly
recognized.
2.6 Backfilling site investigation boreholes
On sites likely to be piled, the backfilling of site investigation boreholes should be
carried out with special care. This is particularly important in the case of bored piles
where the loss of a pile shaft due to accidental connection to waterbearing strata via
a badly backfilled borehole may occur. This may be accompanied by loss of ground
in granular deposits. In an under-reamed pile base, the likelihood of striking a site
investigation borehole is increased and the consequences of a collapse can be more
serious.
Where the risks associated with striking an open loosely backfilled borehole are high
it may be necessary to employ a grouted weak cement backfill or, if suitable material is
available, heavily compacted clay plugs may be formed at frequent intervals in a bore
otherwise filled with arisings. For future reference, an accurate survey of the boreholes
should be carried out on completion of the work.
Special requirements relate to the boring/drilling of boreholes where contamination
of underlying aquifers can occur. In the United Kingdom, the Environment Agency
will need to be aware of any work where such an occurrence could take place, and
special precautions may well be necessary, such as boring through a bentonite seal and
backfilling with cement/bentonite.
2.7 Piling working platforms
In the United Kingdom it is generally necessary that piling rigs (and indeed other
tracked plant) operate from a granular platform of over-site fill that is compacted
into place in order to provide a safe working area. There are exceptions when a rig is
operating from an existing hard-standing or in certain situations 'navvi' mats can be
employed i.e. thick timber decking units that can be moved around the site. Such mats
are not as effective as a properly designed and installed working platform and tend
to slow down plant movement. They are only employed where a working platform
is impracticable. The platform design is carried out using the recommendations of
the Federation of Piling Specialists (FPS) and Building Research Establishment (BRE),
Working platforms for tracked plant (2004).
In order to design a platform, it is necessary to know in detail the properties of the
site soils at shallow depth. Therefore considerably more effort needs to be expended
at the site investigation stage to provide an adequate coverage of shallow exploratory
locations (e.g. trial pits) and to extend the sampling and testing (including as appro-
priate, in situ testing) to provide proper and representative information, particularly
the undrained shear strength or relative density profile and any variation across a site.
As the upper site surface is quite likely to include made ground this may not be a
straight-forward task but every effort needs to be made to satisfy these requirements.
It is of course essential to thoroughly compact the backfill to any trial pits such they
do not create a new hazard.
It should be appreciated that together with the exploratory work, a thorough desk-
study needs to be carried out to establish the location of potential backfilled
excavations for services or voids from basements, buried tanks, etc.
