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Soil and geological profile classification:
applicability, practical implementation, experience from the trials,
indicative costs
Pages in this section include:
This page provides a detailed description of the
applicability, practical implementation,
experience from the trials, and indicative
costs for the soil and geological profile
classification channel seepage identification and
measurement technique.
Applicability
Using soil and geological profiles in channel seepage investigations
provides a picture of the conditions under which seepage is more
likely to occur. On its own, a geological profile will not provide
estimates of seepage rates.
Results from detailed subsurface profiles are limited by the separation
of bores. Inevitably there are data gaps and a need for interpretation
between the data points.
Estimating losses based on application of a seepage rate for a
given soil type is a useful method for providing a first cut estimate
of zones of seepage loss from a system. However, for more detailed
assessment or quantification, correlations made between soil type
and seepage potential based on hydraulic conductivity estimates
are likely to be of limited accuracy and value.
| Practical implementation |
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Regional scale maps are available for most areas to obtain
general knowledge of site soil and geological properties. However,
care should be taken if investigations rely only on these maps
because of the limitations imposed by the scale of the maps.
In addition, mapped soil data refers to surface layers, whereas
a channel is usually cut several metres into the profile where
soil types may be different.
The drilling technique needs to be appropriate for the likely
strata to be encountered. Equipment needs to be able to drill
to the required depth through the site materials. Qualified
drillers with knowledge of local conditions and appropriate
equipment should be used. For example, in areas of hard rock
auger rigs suitable for softer materials are unlikely to be
successful. A geologist should carefully log the strata profile.
Selected soil bores, if possible, should also be converted
to groundwater observation wells. This will depend on the nature
of the project, but in most cases it is considered that water-level
information provides significant information. This is a relatively
straightforward part of a subsurface investigation program.
In terms of interpretation, there are limitations on the assumptions
that can be made. In particular:
- Seepage rates can vary significantly within one broad
soil type.
- Multiple measurements are required to obtain
a reliable estimate of mean hydraulic conductivity of
a particular
soil type.
- Some significant factors influencing seepage
are not allowed for (e.g. groundwater levels, clogging
layer
at channel surface)
| Experience from the trials |
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Drilling and identification of the subsurface stratigraphy
generally provided good backup to the interpretation of zones
of greater permeability and the inferred higher seepage rate.
For example, zones of low seepage measured by pondage tests,
(e.g. Tabbita) were dominantly clayey sequences. In contrast
some of the more permeable sections of the Donald Channel
displayed higher than average seepage rates. The drilling
data at Donald also showed differences in the profiles on
either side of the channel, which was detected by different
geophysical profiles (IAL, 2003).
An important observation at Dahwilly demonstrates the need
for caution in interpreting seepage rates based solely on
geological profiles. In this location, a very sandy profile,
which was inferred to seep at high rates was observed to
have low seepage rates because of the presence of a silt
layer in the channel which added an additional stratigraphic
layer of low permeability.
Attempts to calibrate average soil types based on actual
soil surveys of the channel with point or pondage tests can
allow estimates of seepage rates to be inferred, if a sufficiently
strong relationship is developed between soil type and seepage
(e.g. pondage test, point measurement). However, this requires
averaging of the soil materials and interpreted correlations
between bore locations. Attempts to correlate average hydraulic
conductivity based on soil properties with pondage tests
results were not successful in the trial program. It is more
useful to use an intuitive approach to identifying seepage
pathways based on stratigraphic cross-sections.
| Indicative Costs |
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Costs of drilling of soil bores vary considerably
depending on the type of drilling contractor
and drilling rig used, and whether an additional
person is on-site for logging of the bores.
In the project trials the drilling was primarily
conducted by the EM31 surveying contractor.
Relatively low costs of $1,200-1,500/day
were due to the fact that the contractor
was already on-site (for the EM survey).
Due to the type of rig used, drilling was
limited to around 4-6m and shallower in
hard rock. The disadvantage of conducting
drilling at the same time as the EM survey
was that bore placement was determined at
the discretion of the EM contractor. The
preferred methodology is to locate bores
based on the EM results, which involves coming
back at a later time.
Typical drilling costs at one site in the
IAL study (approximately 2km of channel),
were $4,500-6,000. This included:
- Ten soil bores to 7m (including
backfilling)
- Three permanent groundwater
bores installed and cased to 12m
- Drill
rig operator and qualified geologist
- Other
expenses (e.g. meals, accommodation).
For estimating purposes, a rate of $40-50/m
for soil bore drilling can be used. Costs
for groundwater observation bores range
from $70-120/m (excluding mobilisation).
These
costs exclude the time for logging of
the bores by a geologist or similarly suitably
qualified personnel.
| Related
pages |
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For a more detailed description of the soil and geological profile
classification technique see:
Soil and geological profile classification:
summary
Soil and geological profile classification:
principle, method |
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