On this page go to Principle Method Bore locations Depth Drilling methods Logging Interpretation of subsurface profiles Related pages

	Soil and geological profile classification: summary
	Soil and geological profile classification: principle, method
	Soil and geological profile classification: applicability, practical implementation, experience from the trials, indicative costs

Method

• As part of site characterisation.
• To help define seepage mechanisms.
• To assign seepage rates to soil types and hence determine seepage through changes in soil type. (This can be conducted at a regional scale, using available soil maps and published data on seepage rates, or at a local scale involving local soil mapping and seepage testing.)

• Bores drilled adjacent to the outside toe of the channel banks (or as close as practical to the banks) reflect subsurface channel conditions more accurately. While drilling in the channel may also be of assistance to determine the stratigraphy directly beneath the channel, this is generally not practical due to the considerable expense of drilling from a barge or boat, except in dry-channel conditions. Interpolation between drilling results on either side of the channel can be used to determine conditions directly beneath the channel.
• Bores should be located so as to sufficiently define significant changes in soil conditions and stratigraphy. Some indication of the variability can be obtained from geophysical surveys. The greater variability at the site, the greater the number of bores required to characterise the site.
• Near-channel drilling should coincide with the typical location of the EM31 and EM34 surveys adjacent to the channel bank and provides supporting data for geophysical interpretations. The selection of drilling sites along the channel will depend on the study objectives and the extent of other works being conducted:
• If the investigation is to rely on sub-surface profiling as the main technique to identify seepage paths, it is recommended that bores be placed at regular intervals along the length of channel.
• If additional information is available, particularly geophysical survey data, bores should be located at sites to cover the range of geophysical response at the site, as well as in locations representing changes in geophysical response, which potentially represent changes in geology.

• Channel seepage is effected by the upper profile, particularly the top 2-3m, and as a minimum drilling should be conducted to these depths. In the trials drilling was typically to 4-5m, which coincides with the approximate penetration depth of EM31. When resistivity surveys were used in the final year of the trials, the drilling was increased to 10m, due to the deeper penetration of the resistivity surveying. Drilling should be at least 2m into the watertable.
• The depth of drilling may be limited by the type of drill rig employed, which in turn may be controlled by cost constraints. Even drilling to several metres provides valuable information regarding potential seepage paths and mechanisms.
• The depth of drilling is governed by project objectives:
  • For projects where soil profiles are likely to be the sole basis for identifying potential seepage paths, or to support pondage test data alone, extensive drilling to 4-5m is likely to provide the most useful information.
  • For projects where groundwater monitoring is a key seepage measurement tool, the minimum depth of drilling will depend on the depth to watertable.
  • For projects where geological and soil data is used to support geophysical interpretation, drilling will need to cover the penetration depth of the geophysical tool. For EM31 this is 4-5m and for EM34 and resistivity this is around 10m. (depending on coil / array separation, refer geophysical surveys).

Drilling methods

• 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.
• Most drilling methods cause disturbance of the sample and care must be taken in logging the strata profile. While an undisturbed sampling technique such as split spoon sampling offers greater accuracy, this method would be too costly for most channel investigations.

Logging

• Ideally a geologist or soil scientist should be available to log the bores. One of the most important aspects of logging is consistency, which is best achieved by logging to recognised standards. The Unified Soil Classification System is considered to be the most appropriate logging system. Numerous systems exist, including the Northcote system, which is often used by soil and agricultural scientists.

Interpretation of subsurface profiles

• The raw data is best used when incorporated into cross-sections along the length of the channel (Figure 1). A geologist should undertake geological interpretations of the extent and level of uncertainty in correlations between bores.
• As shown on the example of a geological profile (Figure 13), it is useful to plot the location (elevation) of the centreline so that a comparison of the materials in the walls and the base of the channel can be made. If pondage tests are conducted, showing the location of barriers and ponds assists when comparing lithology with measured seepage rates.

Figure -1 Geological cross-section at Toolondo channel Enlarge - view in a new window

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