smiths lake planning study volume 1: text - Great Lakes Council

SOILS AND GEOTECHNICAL 10
Bore
The results of testing undertaken by Robert Carr & Associates Pty Ltd (RCA) as part of this
study are presented in Table 2-4. This has involved Atterberg limit and linear shrinkage testing
of clay samples as well as Emerson crumb tests to assess soil dispersion.
Table 2-4 Atterberg Limit, Linear Shrinkage and Emerson Dispersion Class (RCA)
Depth
Soil Type
Liquid
Limit
(%)
Plastic
Limit
(%)
Plasticity
Index
(%)
Linear
Shrinkage
(%)
Emerson
Class
(m)
RCA3 0.5-0.6 Clay 63 15 48 4.7 5
RCA6 0.25 Sandy clay 5
RCA6 0.75 Silty clay 82 27 55 6.0 5
RCA7 0.4-0.65 Sandy clay 5
RCA7 1.0-1.1 Clay 14.3 5
RCA10 0.4 Clay 8.1 5
RCA12 0.2 Gravelly clay 10.6 5
RCA15 0.3-0.4 Silty clay 5.1 5
P17 Clay 5
P18 Clay 5
P19 Clay 5
P20 Clay 5
Figure 2-4 shows a plot of the Atterberg limits test results on the plasticity chart for classifying
fine-grained soils by the Unified Soil Classification System. The Atterberg limits test results
indicate that the clays would be generally classified according to the Unified Soil Classification
System as clays of high plasticity (CH). The linear shrinkage results indicate that the clay soils
are susceptible to volume change (shrinkage and heave) with variation in moisture content.
The Emerson class numbers shown in Table 2-4 indicate that the clays and silts tested are nondispersive
in nature. Dispersive soils contain clays that go into suspension in contact with water
and are indicated by an Emerson class of 1 or 2.
2.3.5 Erosion
The magnitude of erosion that can occur at a particular location is dependant on the potential of
erosive agents such as wind, rain and runoff to erode soils and the erodibility of the soil.
Assessment of soil erodibility takes into consideration soil properties such as texture, structure,
dispersion, depth and infiltration and generally provide a general indication of relative
resistance to water erosion.
The Soil Conservation Service of NSW (Dyson, 1985) has noted that the soils in the general
study area have a moderate to high erodibility. The soils are noted to be prone to sheet erosion
once disturbed and the presence of locally dispersive subsoils has been noted in the soils
developed on the Yagon Siltstone (Smiths Lake Village Area). Site specific soil profile
assessment undertaken by the DLWC and shown in Table 2-2, confirms the moderate to high
erodibility of the study area soils.
The Emerson crumb dispersion test results indicate that the clay soils tested are non dispersive.
This indicates that the erosive nature of the clay soils is related to soil structure and texture
SMITHS_LAKE_PLANNING_STUDY.DOC
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