Need To Estimate Undrained Shear Strength Fast?

The determination of the undrained shear strength of soil is a key point during the design process in civil engineering. The most reliable and traditional method of determining this parameter is to perform Triaxial testing: this implies the use of triaxial equipment necessitating significant investment, laboratory space, and longer testing time.

However, there are times when such parameters must be obtained in a rush, for example when testing offshore.

Is There An Alternative Out There?

Fortunately, the answer is Yes! It’s called a Laboratory Vane Apparatus, specially designed for the direct and rapid measurement of the undrained shear strength of soil.

The Laboratory Vane Apparatus was created from an original design by the UK Transport Research Laboratory. It measures the undrained shear strength of cohesive soils and is particularly useful for soils with low shear strength, both in undisturbed and remolded conditions. However, bear in mind that such Vane apparatus is not recommended for sandy, silty clay soils and fissured clay as the nature and composition of these types of soils produce specific failure surfaces that underestimate the undrained shear strength, causing unreliable results.

Laboratory vane apparatus 27-WF17D30
                              Laboratory vane apparatus 27-WF17D30

Let’s see how this apparatus works

The vane is held in a rotating socket positioned in the lower part of the vane head assembly. This socket rotates in a ball seat. A calibrated spring allows the dimension of the applied necklace. The vane head is handed with two different graduated scales the inner scale gives the applied necklace( T) whilst the external scale gives the vane gyration. still, the inner indirect graduated scale rotates through the same angle as the upper end of the spring, and a necklace( T) is applied to the spring, If the vane is averted from rotating.

The applied cargo( T) can be determined by multiplying the angular deviation( Δ) from the graduated scale and the applicable spring estimation constant( pitch of the wind in the Spring estimation map); in volition, the applied necklace( T) can be attained directly reading the angular deviation always using the applicable spring estimation map still, the quantum of deviation of the vane can be seen on the external fixed scale. If the vane is permitted to rotate. According to the anticipated soil strength, different types of estimation springs and vanes should be used generally, the lower the soil strength the larger should be the vanes.

Chart laboratory vane apparatus
                                      Figure 2: example of calibration chart

What happens at the end of the test?

At the end of the test, the Undrained Shear strength is calculated from the equation:

su = T/K

su = Undrained shear strength in kPa.

T = Torque to shear the soil in Newton per Meter.

K = Constant depending on the dimensions and shape of the vane.

The main advantages of the Vane Shear Test

Quick and economical: it only takes a few minutes to obtain undrained shear strength on undisturbed and remolded soil clay samples.

Versatile: it is suitable for testing soil in molds, sample containers, sample jars, and also in sampling tubes.

Safety-conscious: Soil Sensitivity, an important parameter that estimates the loss of strength and structure in the soil body under the effect of static or seismic loading, can be determined using vane shear test results from undisturbed and remolded soil samples.

Reliable: results are comparable with those coming from unconfined compression tests.

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