Geotechnical Report

Vane Shear Test: The vane shear test is used to measure the                 GROUNDWATER
undrained shear strength (Cu) of typically very soft to firm fine
                                                                            Where groundwater levels are measured in boreholes, there are
grained cohesive soils. The vane shear is normally performed in the
                                                                            several potential problems:
bottom of a borehole, but can be completed from surface level, the
bottom and sides of test pits, and on recovered undisturbed tube                  Although groundwater may be present, in low permeability soils
samples (when using a hand vane).                                                  it may enter the hole slowly or perhaps not at all during the time
                                                                                   it is left open.
The vane comprises four rectangular blades arranged in the form of
                                                                                  A localised perched water table may lead to an erroneous
a cross on the end of a thin rod, which is coupled to the bottom of a
                                                                                   indication of the true water table.
drill rod string when used in a borehole. The size of the vane is
dependent on the strength of the fine grained cohesive soils; that is,            Water table levels will vary from time to time with seasons or
larger vanes are normally used for very low strength soils. For                    recent weather changes and may not be the same at the time of
borehole testing, the size of the vane can be limited by the size of the           construction.
casing that is used.                                                              The use of water or mud as a drilling fluid will mask any
For testing inside a borehole, a device is used at the top of the casing,          groundwater inflow. Water has to be blown out of the hole and
which suspends the vane and rods so that they do not sink under self-              drilling mud must be washed out of the hole or ‘reverted’
weight into the ‘soft’ soils beyond the depth at which the test is to              chemically if reliable water observations are to be made.
be carried out. A calibrated torque head is used to rotate the rods         More reliable measurements can be made by installing standpipes
and vane and to measure the resistance of the vane to rotation.             which are read after the groundwater level has stabilised at intervals
With the vane in position, torque is applied to cause rotation of           ranging from several days to perhaps weeks for low permeability
the vane at a constant rate. A rate of 6° per minute is the                 soils. Piezometers, sealed in a particular stratum, may be advisable
common rotation rate. Rotation is continued until the soil is               in low permeability soils or where there may be interference from
sheared and the maximum torque has been recorded. This value                perched water tables or surface water.
is then used to calculate the undrained shear strength. The vane
is then rotated rapidly a number of times and the operation                 FILL
repeated until a constant torque reading is obtained. This torque           The presence of fill materials can often be determined only by the
value is used to calculate the remoulded shear strength. Where              inclusion of foreign objects (eg. bricks, steel, etc) or by distinctly
appropriate, friction on the vane rods is measured and taken into           unusual colour, texture or fabric. Identification of the extent of fill
account in the shear strength calculation.                                  materials will also depend on investigation methods and frequency.
                                                                            Where natural soils similar to those at the site are used for fill, it may
LOGS                                                                        be difficult with limited testing and sampling to reliably assess the
The borehole or test pit logs presented herein are an engineering           extent of the fill.
and/or geological interpretation of the subsurface conditions, and          The presence of fill materials is usually regarded with caution as the
their reliability will depend to some extent on the frequency of            possible variation in density, strength and material type is much
sampling and the method of drilling or excavation. Ideally,                 greater than with natural soil deposits. Consequently, there is an
continuous undisturbed sampling or core drilling will enable the            increased risk of adverse engineering characteristics or behaviour. If
most reliable assessment, but is not always practicable or possible to      the volume and quality of fill is of importance to a project, then
justify on economic grounds. In any case, the boreholes or test pits        frequent test pit excavations are preferable to boreholes.
represent only a very small sample of the total subsurface conditions.
The terms and symbols used in preparation of the logs are defined in        LABORATORY TESTING
the following pages.                                                        Laboratory testing is normally carried out in accordance with
Interpretation of the information shown on the logs, and its                Australian Standard 1289 ‘Methods of Testing Soils for Engineering
application to design and construction, should therefore take into          Purposes’ or appropriate NSW Government Roads & Maritime
account the spacing of boreholes or test pits, the method of drilling       Services (RMS) test methods. Details of the test procedure used are
or excavation, the frequency of sampling and testing and the                given on the individual report forms.
possibility of other than ‘straight line’ variations between the
boreholes or test pits. Subsurface conditions between boreholes or          ENGINEERING REPORTS
test pits may vary significantly from conditions encountered at the         Engineering reports are prepared by qualified personnel and are
borehole or test pit locations.                                             based on the information obtained and on current engineering
                                                                            standards of interpretation and analysis. Where the report has been
                                                                            prepared for a specific design proposal (eg. a three storey building)
                                                                            the information and interpretation may not be relevant if the design
                                                                            proposal is changed (eg. to a twenty storey building). If this happens,
                                                                            the Company will be pleased to review the report and the sufficiency
                                                                            of the investigation work.




February 2019                                                         4