Engineering geology of British rocks and soils Mudstones of the ...

Reports Boreholes
Figure 6.2 Table structure for the geotechnical database.
arithmetic to be checked within and between data records,
standard codes to be applied to appropriate data fields and
additional geological and key fields added (a significant
amount of error was found in all digital source data). The
data were only then transferred to the database, which
enforces a strict referential integrity between the data tables.
The total data set for the Mercia Mudstone comprised
values for approximately 6500 samples and 3600 SPT tests
from over 1400 boreholes and test pits, abstracted from the
reports for 32 investigations.
6.4 DATA QUALITY
The data sources comprised recent contracts carried out by
four of the leading ground investigation contractors for the
Highways Agency or its predecessors with the exception of
the Teesside area in which the identification, description
and logging of the Mercia Mudstone appeared to be of a
high standard. Source data in digital format were used
wherever possible and subjected to extensive checking
before entry, otherwise data were abstracted and entered
directly from hard-copy by one person. The extreme values
in each data field were examined, and those that appeared
to be gross errors were deleted. Nevertheless, it has to be
recognised that much of the data, as in other geoscience
fields, was potentially 'dirty' in the statistical sense. The
values in the database are often the final result from a succession
of field, laboratory and transcription procedures,
during which one must expect errors to occur, however
small or infrequent. Some data, such as stratigraphy and
weathering zone, are interpretative and hence subjective.
30
Geology
Core
Fracture
Samples
SPT
In situ
permeability
Index
Particle size
Chemistry
Triaxial
Consolidation
Compaction/
CBR
Rock
Fracture indices were initially entered into the
database. However, it soon became apparent that there
was a wide disparity in the manner in which these indices
had been determined. Some were recorded as fractures
per metre, and others, in the opposite sense, as the
average length between fractures. Whilst the text of the
original definition (Franklin et al., 1971) could have been
stated with more precision, the accompanying figure
showed the index was to be used in the latter sense, and
expressed in millimetre units. Where the indices have
been recorded as fractures per metre, they can readily be
converted to the correct form. However, the basis for
determining the indices does need to be consistent. The
index, as defined, should be given for a core interval in
which the frequency of natural fractures is generally
constant, and should not in any way be related to individual
core runs. The limits of these intervals would
normally be coincident with the changes in stratum, and
where there is a clear change of fracture frequency within
a stratum. This issue was most evident in the data from
one major investigation, in which the core logging had
been undertaken by two people. It appeared that one had
recorded fracture indices much in accordance with the
definition. However, the other recorded values for each
metre interval within each core run (and without regard to
stratum boundaries). The level of the values, furthermore,
suggested that there had been a distinct difference in the
identification of fractures, as ‘natural’ or ‘drillinginduced’.
Fracture logging of the Mercia Mudstone is
clearly difficult to achieve with the consistency and
objectivity needed to justify statistical analysis, and entry
of this parameter was discontinued.