Geology Reference
In-Depth Information
Potential Repository Zone
O.D
Fresh Water
Calder and St Bees Sandstones
-500 m
Saline Water
Borrowdale Volcanics
-1,000 m
-1,
500 m
Brockram
flow (modi
ed from Chaplow, 1996).
1996). These values are similar to those measured for the Borrowdale Volcanics
50% measured
over50mlengths,withconductivity<1×10
-
10
m/s, according to Chaplow (1996).
-
Later tests
At a later stage,
in the Brockram, and the earlier
modelling had to be revised. Michie (1996) reports hydraulic conductivity measurements within the
Brockramwith a maximum of 1 × 10
−
5
m/s, i.e. four orders of magnitude higher than adopted for the early
models.
field tests were carried out that yielded
'
signi
cant
ows
'
A surprise?
The changed perception for this important stratum might be considered just part of what is to be
expected in any progressive ground investigation. However, the potential for locally high perme-
ability associated with extremely widely spaced and persistent joints, at spacing such that they will
be rarely sampled in boreholes, could have been anticipated, partly because such joints can be
observed directly at exposures in the Lake District. At Hoff
s Quarry to the east of the Lake
District, the rock can be examined, and at a material scale a low permeability would be anticipated
However, at a larger scale, the rock at Hoff can be seen cut by near-vertical master joints which
would affect the mass permeability in a dramatic way (as evidenced from the Sellafield test).There
were also indications from the literature that the Brockram might be permeable at a scale of
hundreds of metres. For example Trotter et al. (1937) commented on the possibility of pathways
through the Brockram, with reference to the distribution of haematite mines within the
Carboniferous Limestone underlying the Brockram.
Lessons: it is very important not simply to rely on site-speci
'
c data when elucidating parameters
for design. There is a need to consider the geological setting, origins and history
-
with all that
entails
advocated by Fookes et al. (2000). Furthermore,
when looking at data from boreholes, especially ones with a strong directional bias, one should
consider all the
-
as per the
'
total geological approach
'
field evidence that might offer some clues as to the validity of the expert elucidation
process.
