Cristiana Ferreira

A Framework Interpreting Bender Element Tests, Combining Time-Domain and Frequency-Domain Methods

Bender element (BE) testing is a powerful and increasingly common laboratory technique for determining the shear S-wave velocity of geomaterials. There are several advantages of BE testing, but there is no standard developed for the testing procedures or for the interpretation of the results. This leads to high degree of uncertainty and subjectivity in the interpretation. In this paper, the authors review the most common methods for the interpretation of BE tests, discuss some important technical requirements to minimize errors, and propose a practical framework for BE testing, based on the comparison of different interpretation techniques in order to obtain the most reliable value for the travel time. This new procedure consists of the application of a methodical, systematic, and objective approach for the interpretation of the results, in the time and frequency domains. The use of an automated tool enables unbiased information to be obtained regarding variations in the results to assist in the decision of the travel time. Two natural soils were tested: residual soil from Porto granite, and Toyoura sand. Specimens were subjected to the same isotropic stress conditions and the results obtained provided insights on the effects of soil type and confining stress on the interpretation of BE results; namely, the differences in testing dry versus saturated soils, and in testing uniform versus well-graded soils.

Comparison of Simultaneous Bender Elements and Resonant Column Tests on Porto Residual Soil

Bender elements are a powerful and increasingly common laboratory tool for determining the shear wave velocity hence the small strain shear stiffness (G0) in soil samples. There are several advantages of the bender element technique, namely its simplicity and ease of use; however, there is no standard developed for this technique as the interpretation of the results involves some uncertainty and subjectivity. Different approaches have been proposed to deal with these issues, especially in terms of the interpretation techniques, based on the time and on the frequency domain. In the present work, a modified resonant column, equipped with bender elements, has been used, where shear wave velocities can be measured independently and different interpretation methodologies of the bender element results can be applied. For this study, natural samples of Porto granitic residual soil were tested, since this geomaterial has been object of research and interest for many years in the University of Porto. The paper will focus on the comparison of simultaneous results of shear wave velocities by the resonant column and the bender elements. It is intended to provide some contribution to the routine laboratory practice using bender elements, with further insight in the interpretation of the results.

Structural Performance of Alkali-Activated Soil Ash versus Soil Cement

AbstractAlkaline activation of fly ash (FA) was used to improve the mechanical performance of a silty sand, considering this new material as a replacement for soil-cement applications, namely, bases and subbases, for transportation infrastructures. For that purpose, specimens were molded from mixtures of soil, FA, and an alkaline activator made from sodium hydroxide and sodium silicate. Uniaxial compression tests showed that strength is highly increased by the addition of this new binder. The results described a high stiffness material, with an initial volume reduction followed by significant dilation. All specimens have clearly reached the respective yield surface during shearing, and peak-strength Mohr–Coulomb parameters were defined for each mixture. The evolution of the microstructure during curing, responsible for the mechanical behavior detected in the previous tests, was observed by scanning electron microscopy. These results were compared with soil-cement data obtained previously with the same soi...

Determination of the Small-Strain Stiffness of Hard Soils by Means of Bender Elements and Accelerometers

AbstractDirect determination of seismic wave velocities in the laboratory is becoming common practice worldwide, given its great potential in the definition of the stiffness at very small strains. One of the techniques for seismic wave measurement makes use of piezoelectric transducers, such as bender elements (BE). However, some limitations remain to the applicability of this technique, namely for stiff geomaterials, such as compacted soils, naturally or artificially cemented soils and soft or weak rocks. To overcome this issue, two accelerometers have been used in conjunction with BE. In the present paper, this combined test setup implemented on a stress-path triaxial chamber will be detailed. An application study will be presented for a hard soil, prepared by laboratory compaction and tested in triaxial compression at different isotropic stress levels. The equipments, procedures and interpretation analyses will also be described. The advantages of this setup are twofold: (1) the interpretation of the acceleration measurements is straightforward, since the signals are of the same nature; (2) these measurements can be used to verify the BE signals, and thus minimize the subjectivity of the interpretation of BE results. Additionally, the accelerometers can be used autonomously wherever the interpretation of BE becomes too complex. The results of this research enabled to validate the interpretation methods used for BE testing. Moreover, this combined setup of transducers provided a simple yet powerful tool for eliminating the subjectivity inherent to BE testing, enabling reliable measurements of small-strain stiffness for a wide range of materials.

Continuous Stiffness Monitoring of Cemented Sand through Resonant Frequency

Mixture formulation and in-situ quality control of the stabilized soils often represent difficult and challenging tasks. The present paper addresses the possibility of using a variant to a recently developed non-destructive technique for continuous monitoring of stiffness of hardening materials as a supporting means to the above-mentioned tasks. The material to be tested is placed inside a polycarbonate mold placed in simply supported conditions. The technique is based on the continuous monitoring of the first resonant frequency of this composite beam, which evolves as a consequence of the hardening of the material, and can be correlated with its E-modulus. The usefulness and potential of this experimental methodology for mixture formulation and quality control of stabilized soils is shown through a series of tests conducted on laboratory since the instant of mixing until 7 days. The conducted tests include complementary methodologies of characterization such as E-modulus measured on specimens with strain instrumentation, as well as monitoring with recourse to bender-extender elements.

Stiffness Behavior of Soil Stabilized with Alkali-Activated Fly Ash from Small to Large Strains

AbstractAlkaline activation of fly ash creates a geopolymeric cement that can replace ordinary portland cement in several applications such as soil improvement, with the advantage of much lower carbon dioxide emissions and reusing an industrial by-product otherwise landfilled, which averts several environmental problems. In this paper, the behavior of a silty sand improved by the alkaline activation of fly ash is analyzed from small to large strains by presenting uniaxial and drained triaxial compression test results and seismic wave velocities measured throughout the curing period. The dynamic, cyclic, and static tests showed a significant increase in stiffness with curing time, even beyond the 28-day curing period. On the basis of the nondestructive wave-propagation technique, the increase of the shear and compression wave velocities with time were drawn, giving the evolution of the elastic shear modulus and the Poisson ratio values. The dynamic Young modulus was compared to the correspondent secant You...

DYNAMIC MEASUREMENTS AND POROSITY IN SATURATED TRIAXIAL SPECIMENS

Recent work has shown how soil porosity may be obtained from elastic wave measurements in the field. The procedure is based on Biot poroelastic theory, and employs both S-wave and P-wave measurements. Only a limited amount of laboratory tests has been employed to date to substantiate this procedure. All of them relate to sand samples, in tests where porosity variation was small. In this work we apply the poroelastic procedure to infer porosity during triaxial tests instrumented with piezoelectric transducers. The materials tested, Bothkennar clay and Porto granite residual soil, have large variations in porosity, variations that are independently measured. The results would offer some light on the applicability of the poroelastic procedure to the frequency testing range common in laboratory applications.