The bender elements are used for determining the shear wave velocity (Vs), hence small-strain shear stiffness of soil. Two small piezoelectric elements are inserted into a triaxial specimen, commonly at opposite ends. An electrical pulse causes the transmitting element to bend, generating a shear wave that travels through the soil. The receiving element converts the resulting movement into an electrical signal.

 

The shear-wave velocity (vs), and small-strain shear modulus G0), are calculated as:

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Where,

• L is is travel distance between the two tips of the two bender elements.

• T is measured travel time; and

•           is bulk density of the test specimen.

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Although the equipment is relatively simple, the testing setup and data interpretation require care. The principal difficulties can be named as interpretation of the arrival time, near-field effects which involve compression waves, out-of-phase vs in-phase, electrical interference, ground signal noise, boundary conditions, and the protection of bender elements themselves from damage to the element tips during triaxial tests.

 

Advantages of Laboratory Bender Element Testing Over In-Situ Geophysical Surveys

 

• Because the bender elements can be incorporated into triaxial apparatus, and the technique is non-destructive, the soil stiffness can be measured at different consolidation pressures or at different positions along the stress path.

• Bender elements can be mounted at different polarisations and directions. The different arrangements allow investigation of the soil structural anisotropy.

• Bender element testing in the laboratory allows individual factors, including soil  and stress parameters, governing shear-wave velocity and stiffness to be investigated systematically. All the parameters can be well controlled in the laboratory.

• Soil specimens can be reconstituted in the laboratory using various sample preparation techniques.

• The bender element in the laboratory can be used as a reference/benchmark value to evaluate the interpretation of in-situ geophysical surveys.

 

Bender Element Tests have been conducted at GTC laboratory for several oil and gas and high-rise building projects in Middle East and for tailings storage facility (TSF) in South Africa and some mining sites in Africa.

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In GTC, our reliable measurements require proper test setup, good contact between the elements and soil, reestablish of insitu and stress history, clear waveform, precise travel-distance interpretation at several excitation frequencies, etc. The wave signal obtained from the laboratory at GTC is clear, allowing the travel time to be picked up precisely and consistently.

 

Bender Element tests conducted at GTC are strictly complied to ASTM D8295-19, Standard Test Method for Determination of Shear Wave Velocity and Initial Shear Modulus in Soil Specimens using Bender Elements.

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For more information, please feel free to contact us at admin@gtc-lab.com​​​