2181 MH Hillegom
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Electric Vane Tester
Determination of the shear strength of a soil is a commonly used method to determine the structural strength of that soil. The electric vane tester is an excellent field instrument for in-situ measurement and evaluation of the undrained shear strength (su) and the remoulded shear strength. Furthermore it gives a good indication of the over-consolidation ratio, the stress-strain relationship and the sensitivity of cohesive soils.
The vane tester provides really good test results in cohesive or slightly over-consolidated soils, such as (saturated) clay and clayey soils. Medium good test results are obtained in peat and silty soils. The instrument is not suitable for use in rock, gravel, sands and silt. The vane test method is not applicable for sandy soils which may allow drainage during the test.
Traditional Vane Testing
Traditionally the vane test method is used in conjunction with fluid rotary drilling or hollow-stem auger drilling. Soil sampling, such as with thin wall sampling tubes is often combined with vane testing, since knowledge of the nature of the soil in which that vane test is done is of great importance for the assessment of the test. The soil sampling procedure can also be used to provide a hole through which the vane tester can be lowered.
Vane Testing and CPT
The vane protection shoe enables the operator to retract the vane in the protective shoe for advancement and incremental testing. This makes it possible to push the vane into the ground from the ground surface by means of a CPT penetrometer rig. The vane measuring instrument is mounted on the penetrometers push-and-pull yoke.
The Vane Tester Set
The electric field vane tester is available in two versions; one with a vane protection shoe and protection tubes (in accordance with Eurocode ENV 1997-3, part 3) and a system without any of these protections. Both versions feature the same electrical instrument for applying and recording the torque and slip-coupling; a state-of-the-art vane measuring instrument which is the heart of the vane tester set. It is to be mounted on the push-and-pull yoke of a CPT penetrometer rig.
This measuring instrument is equipped with a high precision torque sensor piloted by an electronic control system controlling the testing procedures and automating the acquisition of all data with high accuracy (< 1% of FS). The shear loading curve is recorded digitally on the attached PC computer (not included) by means of the vane data acquisition software.
The down-hole equipment consists of vanes, a slip-coupling, extension rods and casing tubes. The Eurocode electrical vane apparatus comes with casing tubes to avoid most skin friction and a vane protection shoe enabling to push the vane directly into the soil without having to make a borehole or similar. A slip-coupling is mounted between the extension rods and the vane in order to be able to measure the friction between the extension rods and the casing tubes. The standard electrical field vane apparatus comes without casing and protection shoe and can therefore only be used in boreholes. A similar slip-coupling as in the Eurocode model is used for measuring the skin friction along the rods.
The vanes are of high quality nickel-chromium steel and specially designed to penetrate the soil easily with minimum disturbance. Their measuring range is 0-50, 0-100 and 0-200 kPa, see below. The standard model vanes have a tapered lower end to improve penetration while the Eurocode model vanes have rectangular blades. Other vane dimensions can be supplied upon request.
Vane Test Procedure
For all vane tests, the shear loading is recorded with the measured torque as function of the applied rotation angle. During a test, the electric vane test instrument rotates the extension rods from the surface. Down-hole, the torque is absorbed during the first 15 degrees of rotation by the slip-coupling on top of the vane. Thereafter, the torque is transmitted to the vane. The speed of rotation is set manually. The recording of the torque, measured with strain gauges, is done every half degree with the vane data acquisition software, installed on a PC computer.
When editing the test data, the rod friction is deducted from the maximum applied torque and given the dimensions of the vane used, the undrained shear strength is calculated, displayed and stored by the software. The stored shear strength values are then displayed as a function of depth according to the Swedish Geotechnical Society (SGF) format.
With the standard model, between tests in a sounding, the vane is further pressed into the ground with the extension rods which run through the chuck of the vane instrument. The Eurocode model comes with a vane protection shoe and casing tubes. After each test, the vane is pulled back up and locked in its protection shoe. The vane protection shoe is then pushed deeper into the ground by means of the casing tubes.
The rotational speed can be adjusted from 0.1° per second up to 15° per second, while the resulting data are recorded every half degree. According to the chosen vane dimension, the software monitors the actual torsion and limits the applied force when the shear strength reaches 100kN/m2. The shape of the curve provides a control of the correctness of the test and an indication of the nature of the soil.
The torque required to turn the rods is first registered, then the total torque applied to both the rods and the vane until failure is registered. The difference between these values is used to calculate the shear strength of the soil. The figures are stored in an individual file for each test. The measured values are graphically visualised in real-time and plotted against the rotational angle. Simultaneously all measured values, sample number and time are logged in a separate file.
Upon completion of the undrained shear test, the user can easily start a remould test. On a key-stroke the electronic controls will turn the vane 5 revolutions at a speed of 15° per second. To prevent damage the maximum torque during remould is limited to 50%. After these 5 revolutions the system returns to the preset turning speed and will log the residual shear strength. If required, the user can repeat the remould operation, e.g. when the initial 5 revolutions revealed to be insufficient. The results of both undrained and residual shear strength are presented in the same graph.
- Eurocode vane model
- Vane (Ø x h, range): 65 x 130 mm, 200 kPa
- Shape of blade: Rectangular at both ends
- Vane protection shoe: Outer diameter Ø75, length 820 mm
- Casing tube: Outer diameter Ø42
- Extension rods: Ø22 x 1000 mm
- Slip coupling: 15° clockwise
- Standard vane models with tapered lower end: Ø50 x 110 mm (200 kPa) / Ø65 x 130 mm (100 kPa) / Ø80 x 172 mm (50 kPa)
Vane Test Instrument
- Max. torque provided: 100 Nm
- Measuring range: 0-50 kPa, 0-100 kPa and 0-200 kPa
- Measurement accuracy: < 1% of FS
- Chuck : Off centre along length axis, 125 and 85 mm from each side
- Chuck size: Ø22 - 25 mm
- Spindle hole: 25 mm
- Weight: 16 kg
- Power supply: 12 Vdc
- Computer communication: RS-232C
Complete system for vane testing from a CPT penetrometer rig comprises:
- Electric vane test instrument with calibration certificate
- Transport case for the vane tester
- Mounting bracket for fitting the vane tester on to the CPT penetrometer pusher (made to measure for the specific CPT penetrometer pusher)
- Fastening ring for fixing the vane tester on to the mounting bracket
- Power supply cable 12 Vdc
- Serial cable from the vane tester to the computer
- Vane data acquisition software (Windows)
- Vane protection shoe
- Coupling vane / inner rods - incorporated in the vane housing
- Vane housing
- Slip coupling
- Vane - number of blades to be determined by the client (2, 3 or 4)
- Inner rods (22 x 1000 mm)
- Casing tubes for the inner rods (42 x 1000 mm)
- Transport case for the casing tubes and inner rods
- Adapter set of hardened steel and spring to lift the inner rods and to retract the vane into the vane protection shoe
- Pressure piece for pushing the casing tubes and inner rods into the ground by means of the CPT penetrometer pusher (made to measure)
- Pulling clamp for pulling the casing tubes and inner rods out of the ground at the end of a test (made to measure)
- Hammer head (brass) 22 mm for hammering the vane into the ground through the inner rods
- Sledge-hammer (4000 gr) with hickory handle
- Open ended spanner 19 mm – Heavy duty design
- Torque wrench with force indicator. To be used to verify the vane tester. Incl. adapter.
- Inverter from 24 Vdc to 12 Vdc for the supply of power to the vane tester
With a nominal torque of 100 Nm all standard but also customised vanes can be applied.
The rotation speed or time to failure varies in the different vane test standards. The Eurocode 7 says that the vane has to rotate at a constant speed between 0.1 and 0.2 degree per second (i.e. 6 - 12 degrees per minute). The speed can therefore be adjusted with a rotation speed regulator or potentiometer on the panel of the vane instrument and can be monitored on the screen display of the software.
Please consider one of the following standards when doing a field vane test:
- Eurocode ENV 1997-3, part 3
- ASTM STP 1014 on Vane Shear Strength Testing in Soils (1988)
- ASTM D2573-08
- ISO 22476-9