António Viana da Fonseca

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.

Fundamental Parameters for the Stiffness and Strength Control of Artificially Cemented Sand

The treatment of soils with cement is an attractive technique when the project requires improvement of the local soil for the construction of subgrades for rail tracks, as a support layer for shallow foundations and to prevent sand liquefaction. As reported by Consoli et al. in 2007, a unique dosage methodology has been established based on rational criteria where the voids/cement ratio plays a fundamental role in the assessment of the target unconfined compressive strength. The present study broadened the research carried out by Consoli et al. in 2007 through quantifying quantifies the influence of voids/cement ratio on the initial shear modulus ( G0 ) and Mohr-Coulomb effective strength parameters ( c′ , ϕ′ ) of an artificially cemented sand. A number of unconfined compression and triaxial compression tests with bender elements measurements were carried out. It was shown that the void/cement ratio defined as the ratio between the volume of voids of the compacted mixture and the volume of cement is an ap...

Influence of Cement-Voids Ratio on Stress-Dilatancy Behavior of Artificially Cemented Sand

The addition of cement is an interesting remediation technique when the project requires improvement of the local soil for the construction of pavement base layers, in slope protection of earth dams and canal linings, as a support layer for shallow foundations and to prevent sand liquefaction. The present study was carried out to quantify the influence of the amount of cement and the porosity in a cement- voids ratio, defined as the ratio between the volume of cement and the volume of voids of a mixture, on the stress-dilatancy behavior of an artificially cemented sand. A program of triaxial compression tests considering three distinct cement-voids ratios was carried out with two combinations of volumes of voids and volumes of cement at each cement-voids ratio. Results showed that the stress-dilatancy relationship is alike for a given cement-voids ratio and that the stress-strain behavior is also similar. The cement-voids ratio is therefore an appropriate parameter to assess stress-dilatancy of the sand-cement mixture studied. DOI: 10.1061/(ASCE)GT.1943-5606.0000565.

Effect of the Porosity/Cement Ratio on the Compression of Cemented Soil

AbstractThe compression behavior of an artificially cemented soil was analyzed by the adjusted porosity/cement index using a correlation established in the recent literature. It was found that for each value of this index, defined as the ratio of porosity to the volumetric cement content, there was a unique normal compression line (NCL). The NCLs of the cemented specimens for each adjusted porosity/cement index did not converge with the NCL of the uncemented silty sand at large stresses, but reached a line parallel to it. The NCL of the cemented sand plotted further from the NCL of the uncemented sand as the porosity/cement index decreased.

Voids/Cement Ratio Controlling Tensile Strength of Cement-Treated Soils

The improvement of locally available soils with cement can provide great advantages, including avoiding the need to borrow volumes of appropriate material and disposing of the local soil in deposits. This research aims to quantify the influence of the amount of cement, the porosity, and the voids/cement ratio in the assessment of splitting tensile strength (qt), also known as indirect diametrical tensile (IDT) strength, of three distinct soils from Brazil and Portugal. From Brazil, clayey sand derived from Botucatu sandstone and uniform Osorio sand were considered; from Portugal, silty sand derived from weathered Porto granite was studied. A number of splitting tensile strength tests were carried out. The results show that qt increased with the amount of cement (C) and decreases in porosity (η) for the three soil-cement mixtures. A power function was well-adapted to fit both qt-C and qt-η. Finally, the tensile strength was plotted against the porosity/volumetric cement content relationship (η/Civ), in whi...

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...

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...

Mechanical and durability properties of a soil stabilised with an alkali-activated cement

Alkali-activated cements (AAC) have been extensively studied for different applications as an alternative to Portland cement (which has a high carbon footprint) and due to the possibility of including waste materials such fly ash or slags. However, few works have addressed the topic of stabilised soils with AAC for unpaved roads, with curing at ambient temperature, where the resistance to wetting and drying (WD) as well as the mechanical properties evolution over time is particularly relevant. In this paper, silty sand was stabilised with an AAC synthesised from low calcium fly ash and an alkaline solution made from sodium silicate and sodium hydroxide. The evolution of stiffness and strength up to 360 days, the tensile strength, and the performance during WD cycles were some of the characteristics analysed. Strength and stiffness results show a significant evolution far beyond the 28th curing day, but still with a reasonable short-term strength. Strength parameters deduced from triaxial tests were found to be very high with stress–strain behaviour typical of cemented soils. Durability properties related to resistance to immersion and WD cycles were found to comply with existing specifications for soil–cement, giving validity for its use as soil–cement replacement.

Experimental and Numerical Observations of the Frequency-Domain Method in Bender-Element Testing

AbstractIn bender-element (BE) testing, shear-wave velocities are measured using two main methods: time-domain analysis of vibration data from pulse excitation and frequency-domain analysis of vibration from sine-sweep excitation. The frequency-domain analysis can be performed automatically. However, its results show high variability because they are affected by different variables such as the frequency content of the excitation, the resonant frequency of the BE system, and the resolution of the frequency-domain analysis. The reliability of the frequency-domain method is studied in this paper using simplified mathematical models and experimental results from BEs, miniature accelerometers, and resonant-column (RC) tests. The mathematical models are used to understand the effects of different variables in the frequency-domain method. Both laboratory and numerical results show that amplification peaks in the transfer function of the BE system can significantly affect the estimation of the shear-wave velocity...