Giovanni Cascante

A New Quantitative Procedure to Determine the Location and Embedment Depth of a Void Using Surface Waves

Detecting underground cavities beneath construction sites and urban areas is a crucial task for many engineering projects. Each year, subsidence and surface soil failure due to underground voids cause substantial damage around the world. Most of the seismic methods currently used for cavity detection can successfully locate a void but not its embedment depth. In spite of successful case histories, void detection is still a challenging problem because of the lack of a standard, quantitative void-detection technique. In addition, existing non-destructive techniques do not consider the effect of lateral inhomogeneities, i.e., cavities, in the wave propagation. Thus, the detection of underground cavities needs further study. This paper presents the results of numerical simulations of the multi-channel analysis of surface waves (MASW) in a laterally non-homogeneous medium. First, the Lamb solution is used to calibrate a homogeneous model, subsequently, voids with different dimensions and embedment depths are i...

Risk considerations for crown pillar stability assessment for mine closure planning

Evaluation of the long-term surface stability of crown pillars overlying underground mines is an important component of mine closure planning. The definition of a crown pillar, as well as a brief discussion of the assessment of the probability and consequence of crown pillar failure are given in this paper. Techniques for stability assessment using mechanistic, empirical and numerical simulation techniques are discussed. Consequence assessment is discussed, but is still subjective and difficult to quantify. Where crown pillars are suspected to be marginally stable or unstable either at the time of the investigation or over the long term, and where the consequence of failure is medium to high, the closure plan for the site must include proposed rehabilitation alternatives. Selection of the optimum solution depends largely upon financial considerations, but also upon the common public expectation that the result of mine closure planning be a ‘permanent’ solution that does not restrict public access or future land use on the site.

Low Strain Measurements Using Random Noise Excitation

Low-strain wave propagation velocity and attenuation are effective measures of state in particulate media. The standard resonant column test procedure is modified to facilitate the study of wave propagation at low strains. The system uses band-limited random noise excitation in combination with signal averaging to control the signal-to-noise ratio. This procedure is efficiently implemented by replacing typical peripheral devices with a signal analyzer and computer control. The methodology permits testing at very low strains. The effect of non-linear system response on computed transfer functions is addressed. Other results include the analytical treatment of coupling between torsional and transverse modes, the evaluation of local low-strain shear parameters from solid specimens tested in torsion, and the use of multi-mode testing for the evaluation of field parameters.

Resonant Column Testing: The Inherent Counter EMF Effect

The standard magnet-coil driving system in resonant column devices provides the required cyclic excitation; however, it inherently produces a counter electromotive force that opposes the motion. In this study, the resonant column is modeled as an electro-mechanical system to quantitatively examine the counter electromotive effect and to explore its effect on resonant frequency and damping ratio computed from voltage-based measurements. The model is verified with two independent sets of experiments. Experimental and analytical results show that the measurement bias is more pronounced on the damping ratio than on the resonant frequency, the damping bias is not a device constant but varies with frequency, and the error is particularly relevant in low-loss and low-stiffness specimens (such as dry sands at low confinement). The electro-mechanical model permits developing device-specific correction charts that can be used to reexamine previously published damping ratio data gathered with voltage-based resonant column procedures.

Frequency dependent dynamic properties from resonant column and cyclic triaxial tests

The resonant column (RC) and cyclic triaxial (CT) devices are commonly used for the measurement of soils’ dynamic properties. The results of these tests do not agree when extrapolated to similar strain levels. The main objectives of this paper are to evaluate the effect of excitation frequency on the dynamic properties of soils, and to provide a methodology to reconcile shear modulus values obtained from RC and CT tests. The effect of frequency on the dynamic properties is evaluated using the new non-resonance (NR) method in the RC device and CT tests. Sand specimens with varying percentages of bentonite–water mixture and a clay specimen are tested. The results obtained from RC tests utilizing the NR method indicate significant change in shear modulus with frequency. The extrapolation of shear modulus from the conventional RC results to shear strains used in CT is significantly overestimated. The extrapolations improved when the results were corrected for frequency effect inferred from the NR method. & 2010 The Franklin Institute. Published by Elsevier Ltd. All rights reserved.

New Methodology for Source Characterization in Pulse Velocity Testing

The pulse velocity test (PVT) is an ASTM standard for the measurement of compressional wave velocities in geomaterials. The PVT is based only on the first arrival of the wave. Full-waveform analysis can be used to measure the variation in geomaterial properties with frequency but requires the dynamic characteristics of the transducers. This paper presents a new methodology for the dynamic characterization of ultrasonic transmitters based on experimental and numerical results. Different types of excitation pulses (input signals) are used, and their theoretical Fourier spectra are computed. The methodology is demonstrated using a piezoelectric accelerometer to measure the frequency response function of an ultrasonic transmitter (UT) (50 kHz). The complex exponential method is used to extract the dynamic properties of the transmitter from transient time signals. Experimental results show that this methodology can be used for the dynamic characterization of ultrasonic transmitters. Results from finite element numerical simulations of wave propagation agree with laboratory results.

Evaluation of the First Mode of Vibration and Base Fixidity in Resonant-Column Testing

In resonant column testing (ASTM standard), the shear strain distribution along the height of a specimen is assumed linear and fixed at the base. To investigate these assumptions, specimens of dry sand, mine tailings, and cemented sand are tested at different confinement and shear strain levels. The measured mode shapes for dry sands and mine tailings are linear at low and high strain levels; however, for a stiff cemented-sand specimen the first mode shape presents slippage at the end platens. Resonant frequencies decrease up to 50 % while the damping ratios increase up to 200 % because of this slippage. The coupling between the specimen and end-platens is enhanced using three different agents: gypsum cement, portland cement, and epoxy resin. The epoxy resin produces the best coupling, whereas portland and gypsum cements are effective only at low confinements and strain levels. Even after eliminating the slippage at the end-platens, the shear wave velocity of aluminum and PVC probes decreases with the increase in specimen stiffness because of the lack of base fixidity. To correct this apparent reduction, a new model and calibration procedure based on a two-degree-of-freedom system are proposed.

Combining Portable Falling Weight Deflectometer and Surface Wave Measurements for Evaluation of Longitudinal Joints in Asphalt Pavements

Longitudinal joints in asphalt pavements typically have lower densities than the interior portion of the mat. Therefore, they tend to exhibit surface distresses such as cracking and raveling more rapidly. The objective of this study is to evaluate the ability of nondestructive testing (NDT) for assessing the relative condition of a longitudinal joint. NDT has been significantly developed for pavement evaluation during the past decade. Deflection and surface wave methods are the most commonly used for pavement evaluation. The portable falling weight deflectometer (PFWD) is increasingly used in quality control and quality assurance to provide rapid determination of the equivalent surface elastic modulus. However, the multichannel analysis of the surface waves (MASW) method consists of using ultrasonic transducers to measure surface waves traveling through the pavement and invert for the elastic modulus of different layers. This study presents results from both deflection and seismic methods to assess the quality of longitudinal joints. Both methods are performed at the same locations of the centerline of a test track. Pavement deflection is also measured on the wheelpath for comparison with previous data. Across the joint, changes smaller than 7% were observed in deflection values. These preliminary results cannot be directly related to joint quality because the contribution of sublayers to the modulus measured with the PFWD needs to be estimated. MASW measurements showed promising results for evaluating the attenuation of surface waves due to the joint. Further work is required to improve the coupling between the transducers and asphalt surface.

New Method for the Evaluation of Material Damping Using the Wavelet Transform

Material damping is a fundamental parameter required for dynamic analysis of geotechnical and civil infrastructure. The material damping ratio is very difficult to measure in situ. A new methodology for in situ measuring of material damping using surface waves is presented in this work. This methodology is successfully evaluated on laboratory scale models and numerical simulations. Ultrasonic waves are used in this work because of the size of the laboratory models. The output force of an ultrasonic piezoelectric transmitter is modeled by using a Morlet function. The wave attenuation and phase variation of propagating surface waves with distance are analyzed using the wavelet transform. Numerical results show that the material damping ratio calculated using the wavelet transform gives a global value that represents an average damping ratio for the frequency bandwidth imposed by the seismic or ultrasonic source. Experimental results, from tests on a cemented sand and a concrete plate, show good agreement wi...

Condition Assessment of Cementitious Materials Using Surface Waves in Ultrasonic Frequency Range

Surface waves propagating in a medium provide information about the mechanical properties and condition of the material. Variations in the material condition can be inferred from changes in the surface wave characteristics. Multichannel analysis of surface waves (MASW) is a well-established surface wave method used for determination of the shear-wave profile of the soil layers near the surface. The MASW test configuration is also applicable to assess the condition of construction materials using appropriate frequency range. Previous studies on the detection of surface-breaking cracks in concrete elements, using the dispersion and attenuation of ultrasonic waves, were successful; however, a complete damage assessment of the whole element was not in the scope of these studies. In this study, different wave characteristics, such as Rayleigh wave velocity, wave attenuation, and phase velocity dispersion, are investigated to evaluate their sensitivity to the damage in a medium. The condition of a test specimen, which is a half-space medium made of cement and sand, is evaluated using ultrasonic transducers for different damage cases. The recorded signals are processed using the Fourier and wavelet transforms to determine the surface wave characteristics. A new dispersion index (DI) is introduced, which represents the global correlation between the dispersion of phase velocity and damage level. All features are found to be capable of reflecting the damage in the test medium with different levels of sensitivity. Among the investigated parameters, the proposed dispersion index shows high sensitivity and linear correlation with the damage.