Geology Reference
In-Depth Information
hard they were hit. This was proved by driving some piles to a target
depth next to boreholes, 1.5m into the sand rather than to set or
to carpet level. Piles that had not reached set on initial drive were
then re-driven (they did not advance) and some were tested without
re-driving to 1.5 times working load, which was carried perfectly
satisfactorily. In hindsight, the original GI had been too simple; the
standard clack valve-type sampling of the sand horizon, typical of
UK practice, was not adequate for establishing the true nature of the
soil. The variability across the site was due to the outwash fan origin
of the sand with local pools of clay and silt-rich soils in the glacial
landscape. These dif
culties added perhaps 10% to the cost of pile
production (Hencher & Mallard, 1989). The problem had not been
anticipated and was related entirely to the method of construction. If
bored piles had been used instead of driven piles for the Drax founda-
tions, which would have been feasible, then the problem would not
have arisen. What was particularly galling was when a senior consul-
tant, who had been involved in the construction of the
first phase of the
power station, visited the site and said, when informed of what was
happening,
'
Oh you have found that again, have you?
'
Wales, UK
Fookes (1997) discusses a number of examples of unexpected ground
conditions where he believes that the situation could have been antici-
pated prior to tender, and includes several dealing with karstic lime-
stone. One that is described is for a river crossing inWales, and Fookes
argues that karstic conditions could readily have been identi
ed from
local evidence and boreholes speci
c to the project. The case is rather
more complicated than that. The karst was indeed recognised prior to
tender by all parties and, as is good practice (Cole, 1988), a ground
improvement contract was let to wash out and grout voids in the
limestone around pile locations prior to the piling contract. That
being so, the conditions anticipated by the piling contractor were as
shown in
Figure 7.21.
In the event, for various reasons, the grouting
was not comprehensive, in particular because some arbitrary rockhead
was adopted at some depth below true rockhead (see
Chapter 3),
which rather negated the point of the grouting.
During the piling contract, where the rock mass was of good quality
(either naturally or due to successful grouting), piles were installed
without dif
culty. Elsewhere, the contractor encountered soil over-
lying a very irregular rock pro
le containing voids, some of which
were open, others in
lled with soil and some grouted. Cased holes
collapsed and there was loss of concrete from the holes once formed,
with a risk of pollution to the adjacent river. The karstic conditions
were not unforeseen, as suggested by Fookes, but the lack of ground
