Vincent.yunansan

The Vane Shear Test (also known as the Insitu Vane Shear Test) is a method of testing soil characteristics on site. Vane Shear Test is a very popular type of insitu shear test among Geo-technical engineers, geologists, and other instances that benefit from soil data. It was originally used in Sweden in 1919 and has been employed widely on a worldwide basis in the 1940s following further studies by Carlsson, Skempton , and Cadling and Odenstad.

The Vane Shear Test is mainly used to determine the in situ undrained shear strength and the sensitivity of a saturated cohesive soil. It is limited to fine grained soils which can retain water content during the testing, in other words, sample soil has to stay saturated throughout the test.The test works by inserting a vane and rotating it until the soil fails. The undrained shear strength and sensitivity can then be calculated by analyzing the obtained torque and the diameter of the vane used.

The data acquired from vane shear test provide a simple and convenient index of shear strength and guidelines for foundation construction.Unlike other types of shear tests, vane shear test does not require sample taken or additional lab equipment to analyze the soil. This is beneficial for initial site testing since it generally requires less time and cost to operate.

Equipment

A set of equipment is needed to conduct the Vane Shear Test

vane

It has been universally agreed that the vane used in Vane Shear Test should consist of 4 plates fixed at 90' angles to each other and the height width ratio of each plates should be h:d = 2:1. The 4 blades are welded onto a rod that can be attached to extension rods to reach deep soil strata. Blades have to be parallel with extension rods and no distortion is allowed to ensure equipment safety and the accuracy of soil data. The diameter of rod shaft and welding on rod has to be as small as possible for the same reasons. The shaft also has to be of such rigidity to withstand full load conditions such that it does not twist or bend significantly. Slip coupling can also be added to prevent torque to carry over between the vane and the extension rods through the connection.

extension rods

The extension rods shall have a diameter and torsional stiffness large enough to transmit generated torque by the housing during the test to the vane yet small enough to acquire data without significant disturbance. The extension rods has to be straight and thread eccentricity in relation to the outer diameter shall be less than .11 millimeters for rod joints. The diameter of extension rods must be at least 20 millimeters , based on field experience and testing.If bending shall occur, as shown in many cases, it must not exceed 2 millimeters over each 1 meter of length, measured from the arch. Casing tubes can also be used to prevent buckling of rods as long as the inner diameter of tubes is large enough to minimize friction between the tubes and rods. The friction between tubes and rods have to be taken into account when calculating data acquired.

Measuring meter

The measuring meter is designed so that it is possible to read the actual value of torque. The measuring meter has to be calibrated at least once every six months or when it is damaged, overloaded, or repaired. This is to ensure accurate reading of torque. There are mainly 2 types of measuring meters: digital measuring meter and analog measuring meter. The digital measuring meter has an electronic display and is very practical to use. The analog measuring meter is a scaled gauge with a rotating vane that is connected to a spring.

Process

To conduct the insitu vane shear test, several steps are taken to ensure accurate data collection:

The shear vane is pushed into the soil through a pre-existing outer casing into the soil. The pre-rexisting outer casing is usually placed by drilling into the soil carefully without disturbing the soil to much.
The shear vane is inserted while rotating it at a very slow speed, around 60 degrees/ minute. This is to minimize lateral friction between the shear vane and the soil which can affect data accuracy.
During the insertion, field data on torque is recorded in real time. Torque data is recorded in a computer and is measured in every 15-30s intervals, depending on the level of accuracy needed.
The shear vane can be rotated until soil failure occurs. At this point, maximum torque can be calculated. The maximum torque is then related to the maximum shear strength of the soil using mathematical expressions.
To calculate soil sensitivity, shear vane is rotated 10x its normal speed to remold the soil after failure occurs.

Calculation

According to the {American Society for Testing and Materials}, the undrained shear strength can be calculated using these formulas:

(Si)fv = 6Τmax/ΊπD^3 ; for rectangular vane of H:D = 2:1
(Si)fv = 12 Τmax/(πD^2 x (D/cos iτ+D/cos ib +6H)) ; for tapered and other vanes

Where,

(Su)fv = undrained shear strength
Tmax= max.torque corrected
Bulleted list item
D= vane diameter
H= height of vane
iτ= angle of taper at vane top
ib= angle of taper at vane bottom

From the above equation, Peak shear strength can then be determined. To calculate the sensitivity of the soil, soil is remolded by rotating the vane quickly at approximately 10x the normal speed. Once soil is remolded, the remolded shear strength can be determined. Sensitivity can be calculated using the equation:

(Sτ)fv = Su(undisturbed)/Su(remolded)

where,

(Sτ)fv= sensitivity
Su(undisturbed)= peak undrained shear strength
Su(remolded)= remolded undrained shear strength

Laboratory Vane Shear Test

Vane Shear Test can also conducted within the confinement of a laboratory. The laboratory Vane Shear Test measures the shear strength of cohesive soils and it is useful for soils of low shear strength for which triaxial or unconfined test cannot be performed. Just like the insitu vane shear test, the laboratory vane shear test measures the undrained strength of the soil. The undisturbed and remoulded strength obtained are useful for evaluating the sensitivity of soil at higher accuracy than the insitu vane shear test.

Conditions

Several working conditions has to be fulfilled in order for the Vane Shear Test to work:

The Vane Shear Test is not applicable to sands, gravels, or other high permeable soils. This is due to the lack of saturation in the soil strata which affects soil movement.
Appropriate size of vane to be used. Small= stiff soil. Large = soft soil. As mentioned above, The diameter of extension rods must be at least 20 millimeters and if bending shall occur, it must not exceed 2 millimeters over each 1 meter of length, measured from the arch.

Correction

data acquired from the vane shear test may be off due to friction and other disturbances on the soil by the vane. Several factors have to be taken into account to generate accurate results. Correction for friction between the vane and the soil, sensitivity of soil, dan factor of safety have to be factored in into the equation to generate final data.

Report

A report has to be organized after every vane shear test. Information included in the report include administrative data such as location, authorized personnel conducting the experiment (drilling foreman and supervising engineer), and experiment location. Soil data such as soil conditions, elevations, drilling method, and boring number has to be included in the report. The report needs to take into consideration the geometry and description of the vane such as its size and shape, depth of the vane tip, drilling method, and equipment has to be recorded. Special conditions such as soil resistance pushing back the vane while drilling, as well as the calibrations of device also need to be included.

Most commonly, data described above are written in the report in form of the depth of vane penetration, time elapsed from the beginning to the end of penetration, soil friction, method of applying and measuring torque, number of vane revolutions, maximum and intermediate torque, soil failure, and the rate of remolding.

The data acquired from vane shear test can be used to determined:

Undrained sheer strength.
Evaluate rapid loading strength for total stress analysis
Sensitivity of soil to disturbance.
Analysis of stability problems with embankment on soft ground.

Limitations

Vane shear strength is limited to insitu, undrained, fine grained clays and silts and other fine geomaterials. It cannot be used on unsaturated or highly permeable soils. Vane shear test is highly dependent on competence of personnel using the instruments. Personnel using the instruments have to understand basic data reading and common soil characteristics. Condition of equipment also affects accuracy of data acquired.