Test De Bender

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Ortiz Ullery

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Aug 3, 2024, 3:26:34 PM8/3/24

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The present study was undertaken to provide data on the Bender-Gestalt test for children aged 5 to 11 in Turkey. Although it is well documented that sociocultural factors are important in cognitive evaluations, the effects of type of school and differing educational opportunities provided by these schools on the Bender-Gestalt test have not been previously investigated. The aim of the present study was to investigate the effect of age, sex, and school type on Bender-Gestalt performance. The test was individually administered to 484 children between the ages of 5 and 11 years. The children were enrolled in either public or private schools. Koppitz's Developmental Scoring System was utilized. The results indicated that older children performed with fewer errors. Girls performed with fewer errors than boys. Finally, as expected, private school children outperformed their public school peers. The results are discussed with respect to the importance of taking into account various educational factors in utilizing commonly used tests.

A Bender Element Test is a non-destructive test performed in soil specimens to determine the small-strain shear modulus (Gmax) of the soil. Gmax is an important soil property which helps us to understand the elastic behaviour of the soil and to evaluate its response to dynamic loading, such as earthquakes, passing vehicles and vibrations.

Figure 1 presents the various tests available to determine the shear modulus, G, of a soil. Shear modulus gets its maximum value (Gmax) at very small values of strain (within the elastic deformation field and usually at shear strains less than 10-3 %) which can be achieved only with a bender element test. This makes them very popular tests in soil mechanics. Moreover, due to its nature, Bender Element testing can be combined with another, main soil test on the same specimen (e.g. Triaxial or Consolidation) and can give estimations of the shear modulus at various stages of the main test. Bender Element testing can be performed multiple times on the same specimen as it does not cause irreversible damage.

A bender element is a piezoelectric transducer which can convert electrical energy into mechanical energy and vice versa. Practically, this means that whenever these elements are supplied with a voltage they are deforming on a specific way. On the other hand, when they are deforming, they produce a small voltage. As such, bender elements are used to examine the propagation of ground waves through a soil specimen and measure their velocity which depends on its elastic properties. To do so, a bender element set consists of two elements which are installed at the opposite sides of a soil specimen. One of the elements acts as a transmitter and the other as a receiver. In simple terms, each element consists of a pair of two ceramic plates which are separated by a thin metallic sheet. There is a specific way the two ceramic plates are wired to allow them to deform crosswise, as shown in Figure 2 (left), when they are supplied with a DC voltage. Usually, the DC voltage is supplied in the form of a sine wave, as shown in Figure 2 (right). The sine wave is causing the bender element to deform on the same way causing the soil particles to move as well. The movement of the particles is producing a shear wave (S) that propagates towards the other side of the specimen and arrives there after some time. When it arrives, the shear wave is causing the deformation of the receiving element (the one at the top cap) and this produces a corresponding electrical signal, many times smaller than the electrical signal than initiated the wave. By synchronising and comparing the two electrical signals, the time of travel is obtained. Therefore, by knowing that the distance of travel is the tip-to-tip distance between the two elements, the speed of the shear wave, Vs, can be determined as:

Bender elements are encapsulated within the base pedestal or top cap of the triaxial cell and are sealed with an epoxy coating that insulates them. In this way, the received signal is not interfering with any electrical noise that might occur during the process. Apart from the two piezometric elements, two devices are needed to perform the Bender Element test.

The anisotropy of the soil structure in the small-strain shear modulus is defined as the ratio between the vertical shear modulus and the horizontal shear modulus. The horizontal small-strain shear modulus is given from Equation (2) if Vs is the shear wave velocity determined by the horizontal Bender Element test.

Apart from shear waves, bender elements can be used for the propagation of compression (P) waves through the specimens. To produce this kind of waves, the elements have to be wired differently than in the shear wave elements in order to deform parallel to the direction of the wave propagation. Compression waves are faster than shear waves and their use can be useful for indirect measurement of the saturation level of the sample. This can be done by comparing the speed of the compression wave Vp that propagates through the specimen with the speed of the compression waves when they propagate through water. Since it is known that Vp=1450 m/s for water, the speed of the compression waves through a saturated specimen should have an approximate value.

To perform a Bender Element test on a triaxial apparatus, the base pedestal and the top cap must have embedded a set of bender elements. Triaxial cells must be specifically designed for bender element testing so that they allow the cables of the elements to pass through them without any water leaks. Usually, pedestals and top caps in these cells are interchangeable and can be replaced any time the user desires to run a Bender Element test.

Before using them, the bender elements must be checked in order to achieve the correct polarity. If this is the case, then the transmitted and received signals will start on the same direction at the time graph, i.e. they will both move initially upwards. This can be checked by bringing the two element tips into contact and firing a signal; the results should be two signals moving on the same direction (Figure 4). If the polarity is reversed, the received signal will first move on the opposite direction from the transmitted signal. The reversed signal should be avoided because it can be mixed up with the reversed first arrival seen in received wave forms caused by the near-field effect (see section 3.3). When the correct polarity is found, the base pedestal and top cap must be marked so that it can be visible the correct orientation of the elements when installing them into the specimen.

Moreover, the bender elements must be checked for possible time delay between the transmitted and received signals. This delay, although it is very low, it could be critical for the correct calculation of the shear wave velocity, therefore it should be eliminated. The best way to determine the delay between the two signals is to bring the two elements in direct contact and firing a square-type signal. If no delay exists, the two signals should almost match. If a delay can be detected, it should be determined and extrapolated from the actual test results.

The user must ensure a very good contact between the element tip and the soil sample so that the deformation of the element produces a nice and clean shear wave. The sample placed on the base pedestal with care to avoid any damage that might be caused during the penetration of the element. Moreover, the size of the element must be small enough to prevent any cracks occurred. The donut-shaped porous stone that is placed around the bender element helps to leave exposed only the necessary length that will penetrate the sample. When the specimen sits on the pedestal it should not be rotated because this can cause the loss of the good contact. After the installation of the sample on the pedestal, the top cap is placed on the top surface of the specimen and is slightly pushed so that the element penetrates. A rubber membrane is placed around the sample to prevent any contact of the soil with the confining fluid. Two O-rings are placed on the pedestal and top cap to seal the membrane against the sample. Normally, the applied confining pressure will offer a better contact between the soil grains and the elements and therefore test results will be better during the consolidation stage.

Another important consideration during the sample installation is the alignment of the bender elements. As discussed in section 3.1, the pedestal and the top cap must be marked at the outside so that they can be aligned during the installation. The two elements, transmitter and receiver, must be parallel in order to achieve a good quality received signal.

In the case of horizontal bender elements, their installation happens after the sample has been formed, placed on the based pedestal and wrapped with the rubber membrane. Two small holes are formed on the membrane and the rubber sleeve with the element are passing through it and pushed against the sample to establish a good contact. When they are in place, the interface between the elastic membrane and the rubber sleeve is sealed with some appropriate substance (e.g. silicone).

From a geotechnical point of view, Bender Element is a very simple test that takes no more than a few seconds to be performed. However, care should be taken to the preparation of the specimen and the correct installation of the elements which will ensure a good response from the receiver element and clear captures of the received wave, as described in the previous sections.