Search

Orthogonal matching pursuit (OMP) is a widely used greedy algorithm for sparse signal recovery in compressed sensing (CS). Prior work on OMP, however, has only provided reconstruction guarantees under the assumption that the columns of the CS matrix have equal norms, which is unrealistic in many practical CS applications due to hardware constraints. In this paper, we derive sparse recovery guarantees with OMP, when the CS matrix has unequal column norms. Finally, we show that CS matrices whose column norms are comparable achieve tight guarantees for the successful recovery of the support of a sparse signal and a low mean squared error in the estimate., Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Team Nitin Myers, Team Michel Verhaegen

Fast millimeter wave (mmWave) channel estimation techniques based on compressed sensing (CS) suffer from low signal-to-noise ratio (SNR) in the channel measurements, due to the use of wide beams. To address this problem, we develop an in-sector CS-based mmWave channel estimation technique that focuses energy on a sector in the angle domain. Specifically, we construct a new class of structured CS matrices to estimate the channel within the sector of interest. To this end, we first determine an optimal sampling pattern when the number of measurements is equal to the sector dimension and then use its subsampled version in the sub-Nyquist regime. Our approach results in low aliasing artifacts in the sector of interest and better channel estimates than benchmark algorithms., Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Team Nitin Myers, Signal Processing Systems, Team Sander Wahls, Team Michel Verhaegen

Discussion is presented of a model to characterise the mechanical behaviour of cable bolts under axial loading and subjected to different boundary conditions (including constant confining pressure and constant normal stiffness). The model is based on the modified Mohr-Coulomb failure criterion as well as a newly proposed non-linear dilation formulae for the development of governing equations. The cone shaped failure mode of grout due to the axial movement of cable bolt is considered as a widely observable failure mode at the cable bolt to the grout interface. The load carrying capacity of the cable bolt under axial loading is significantly affected by both the progressive reduction in the volume of failed cone zone of grout annulus and the continuous degradation of dilation angle at the cable bolt to the grout interface. The full load-displacement performance of different cable bolts under axial loading subjected to both constant confining pressure and constant normal stiffness are simulated. Experimental data from Garfold bulbed, Nutcased, Australian Megabolt and Birdcaged cable bolts were used to calibrate the proposed model under constant confining pressure. An acceptable agreement between the simulations and experimental data confirms the proposed model is suitable for predicting the mechanical behaviour of different cable bolts under axial loading., Discussion is presented of a model to characterise the mechanical behaviour of cable bolts under axial loading and subjected to different boundary conditions (including constant confining pressure and constant normal stiffness). The model is based on the modified Mohr-Coulomb failure criterion as well as a newly proposed non-linear dilation formulae for the development of governing equations. The cone shaped failure mode of grout due to the axial movement of cable bolt is considered as a widely observable failure mode at the cable bolt to the grout interface. The load carrying capacity of the cable bolt under axial loading is significantly affected by both the progressive reduction in the volume of failed cone zone of grout annulus and the continuous degradation of dilation angle at the cable bolt to the grout interface. The full load-displacement performance of different cable bolts under axial loading subjected to both constant confining pressure and constant normal stiffness are simulated. Experimental data from Garfold bulbed, Nutcased, Australian Megabolt and Birdcaged cable bolts were used to calibrate the proposed model under constant confining pressure. An acceptable agreement between the simulations and experimental data confirms the proposed model is suitable for predicting the mechanical behaviour of different cable bolts under axial loading.

Discussion is presented of the influences of a range of parameters on the mechanical performance of cable bolts. The Taguchi and Response Surface Methodology (RSM) statistical techniques are used to demonstrate their effectiveness for the proper design of experiments leading to a minimum number of tests without compromising the output quality. Data reported in the literature was used in the study and quantitative post-data processing was carried out using Analysis of Variance (ANOVA) to quantify the contribution of different independent parameters (such as confining medium and grout strengths as well as borehole diameter) on the dependent ones (such as peak and residual loads as well as the initial stiffness) individually and interactively. RSM was also used for the post-data multivariate statistical analysis. It is concluded that with the aid of design of experiments techniques such as Taguchi it is feasible to develop a minimum number of experiments to obtain the most representative tests results without compromising the quality, along with some quantitative post-data processing using ANOVA. It was also demonstrated that one of the most important specifications of RSM is to provide the quantitative and visual interactive effects of independent parameters on the mechanical performance of different cable bolts using multiple regression modelling., Discussion is presented of the influences of a range of parameters on the mechanical performance of cable bolts. The Taguchi and Response Surface Methodology (RSM) statistical techniques are used to demonstrate their effectiveness for the proper design of experiments leading to a minimum number of tests without compromising the output quality. Data reported in the literature was used in the study and quantitative post-data processing was carried out using Analysis of Variance (ANOVA) to quantify the contribution of different independent parameters (such as confining medium and grout strengths as well as borehole diameter) on the dependent ones (such as peak and residual loads as well as the initial stiffness) individually and interactively. RSM was also used for the post-data multivariate statistical analysis. It is concluded that with the aid of design of experiments techniques such as Taguchi it is feasible to develop a minimum number of experiments to obtain the most representative tests results without compromising the quality, along with some quantitative post-data processing using ANOVA. It was also demonstrated that one of the most important specifications of RSM is to provide the quantitative and visual interactive effects of independent parameters on the mechanical performance of different cable bolts using multiple regression modelling.

The Coal Mine Floor Rating (CMFR) system is based on the underlying failure mechanisms and quantifies the stability of floor strata by considering uniaxial compressive strength and discontinuity spacing of the floor units. The CMFR is incorporated into an empirical method, the floor heave index, developed by statistical analysis of a database obtained from actual floor failure cases. The floor heave and non-floor heave cases are determined depending on the CMFR and the horizontal stress rating, a proxy that represents the magnitude of horizontal stress. The new floor classification system and the empirical method help assess the potential of significant floor heave on development in underground coal mine roadways for new mining projects or future workings., The Coal Mine Floor Rating (CMFR) system is based on the underlying failure mechanisms and quantifies the stability of floor strata by considering uniaxial compressive strength and discontinuity spacing of the floor units. The CMFR is incorporated into an empirical method, the floor heave index, developed by statistical analysis of a database obtained from actual floor failure cases. The floor heave and non-floor heave cases are determined depending on the CMFR and the horizontal stress rating, a proxy that represents the magnitude of horizontal stress. The new floor classification system and the empirical method help assess the potential of significant floor heave on development in underground coal mine roadways for new mining projects or future workings.

Several underground coal mines have reported floor heave associated with the buckling mechanism. However, studies on the coal mine floor, especially the buckling failure mechanism in the floor and relevant analysis techniques, are still insufficient. This paper introduces a new floor classification system, the Coal Mine Floor Rating, based on the underlying failure mechanisms. The new floor mass rating system quantifies the stability of floor strata by considering two main factors: uniaxial compressive strength and discontinuity spacing of the floor units. The Coal Mine Floor Rating is incorporated into an empirical method, the Floor Heave Index, developed by statistical analysis of a database obtained from actual floor failure cases. The floor heave and non-floor heave cases are determined depending on the Coal Mine Floor Rating and the Horizontal Stress Rating, a proxy that represents the magnitude of horizontal stress. While obtaining underground floor data is challenging, the new floor classification system and the empirical method help assess the potential of significant floor heave on development in underground coal mine roadways for new mining projects or future workings.

© 2018 Elsevier Ltd Changes in effective stress around a borehole due to pressure drop can cause formation instabilities leading to issues such as sand/solid production and wellbore stability. Analytical studies have been conducted in the past to gain insights into physical mechanisms involved in this complex process. The developed analytical solutions for such hydro-mechanical instability often consider perfectly plastic behaviour for the rock, whereas the field observations suggest strong strain softening as a result of pore pressure drop. In addition, these analytical solutions are not verified with experimental data in many instances. In this study, we developed a series of closed form solutions to predict the stress and strain changes around a borehole within poro-elasto-softening-plastic concept using Mohr Coulomb failure criterion. For the purpose of describing the pore pressure induced softening behaviour; an exponential function was used to relate the evolution of yield surface to plastic shear strain. In a further step, a series of hollow cylinder experiments were conducted on sandstone samples where different hydrostatic confinements at elevated pore pressures were applied. The hollow cylindrical samples were imaged before and after the experiments using 3D X-ray micro Computed Tomography technique (micro-CT) to precisely identify the collapsed-damaged and plastic zones formed around the borehole. Finally, the results from the experiments and micro-CT analysis were compared with the model prediction and a good agreement was observed.

This study was focused on the effect of hydrothermal treatment in buffer solutions to improve mechanical properties of oil palm wood (OPW). To control the destructive effects of the released acids caused by the degradation of acetyl groups of the hemicelluloses, the hydrothermal treatment in buffered media was conducted using surface methodology (RSM). Central composite design (CCD) is a useful empirical design in RSM. The CCD and RSM were also applied to optimise the hydrothermal treatment variables (buffer solutions, temperature and time) as effective actual variables with 20 experiments. The results showed that the samples treated in neutral media and low temperatures displayed higher modulus of rupture (MOR) (MPa) and modulus of elasticity (MOE) (MPa) due to control of the medium acidity with low degradation of hemicelluloses. Furthermore, the treatment in buffered media significantly affected on mechanical properties of hydrothermally treated OPW. The effect of the temperature on bending strength were more notable than buffer solutions and time. The actual response according to predicted treatment conditions (pH = 7.12, temperature = 110 °C and time = 120 min) explained closest agreement with the predicted value with residual standard error (RSE) of less than 5%.

Understanding the mechanical properties of shaly sandstone is of great importance in reservoir geomechanics. Because of the lack of core data, measurements based on acoustic wave velocities are often conducted. In such measurements, the mechanical properties such as uniaxial compressive strength (UCS) and Young's modulus (E) are related to the porosity of sandstone and shaly sandstone. However, in many instances, the porosity fails to directly correlate to mechanical properties especially when dispersed clays are present. Therefore, in this study, a set of uniaxial compressive tests on a number of different shaly sandstones was performed. The X-ray computed tomography was conducted on the representative samples from each type of sandstone to assess the porosity, rock composition, and texture. Independent compositional analysis was also carried out on all representative samples to link the samples' composition to their microstructure. Interestingly, it was found that the mechanical properties of these shaly sandstones are well-correlated to the combined clay volume and effective porosity in a multivariate analysis. It was concluded that the univariate analysis solely performed on porosity cannot always correlate well to the mechanical properties of sandstone containing clays. Therefore, a combination of clay volume and effective porosity is more accurately correlated to mechanical properties.

© 2017 American Rock Mechanics Association (ARMA). All rights reserved. While rock strength is an important input parameter in geotechnical engineering, the size effect of intact rock samples on strength has long been recognised as an issue. Conventionally, it has been believed that increase in size leads to decrease in strength. A few investigations have reported different trend to that consider conventionally where the uniaxial compressive strength (UCS) follows ascending and descending behaviour. Masoumi et al. [1] investigated this behaviour from analytical and experimental viewpoints that resulted in the formulation of the unified size effect law (USEL) which accounts for the reverse size effect behaviour of intact rocks. A limitation of the current USEL is that at very large sizes, the UCS prediction by USEL tends to zero. This paper outlines an improvement to USEL to overcome this limitation based on the multifractal theory. Attributes of the improved USEL model were highlighted and verified using UCS data from different rock types. Finally, it was demonstrated that there is a good agreement between the improved USEL prediction and the experimental results.

The design of structures on or within a rock mass requires an estimation of the strength of the intact rock blocks. These blocks can be many orders of magnitude greater in scale than the core samples typically tested. Thus, including the scale effect into a failure criterion is required for a more realistic estimation of rock strength with various scales. In this paper, a modified multiaxial failure criterion with scale effect parameters was developed based on the model proposed by Christensen. The modification process was carried out according to the proposed scale effect equations for different stress paths, such as uniaxial compression, point loading, uniaxial tension and pure shearing. Furthermore, the scale dependency of the proposed parameters for the modified failure criterion was assessed using the results from the uniaxial compressive and point load tests. Finally, it was confirmed that one of the modified failure criterion parameters is scale independent when the results from uniaxial compressive and point load tests are included.

Total rock porosity is a key parameter in a wide range of disciplines from petroleum to civil and mining engineering. Porosity is particularly important in petroleum engineering applications, e.g., from estimation of hydrocarbon in place to prediction of geomechanical properties. Conventional techniques used to measure the total porosity, i.e., mercury intrusion, nitrogen physisorption, focused ion beam–scanning electron microscopy (FIB–SEM), nuclear magnetic resonance (NMR) spectroscopy, gas porosimetry, and X-ray micro-/nano-computed tomography (micro-CT), have yielded inconsistent results for unconventional shale gas samples. A new robust yet practical method is thus required for measuring total porosity in tight formations to be added to the toolbox of the porosity measurement. We propose and develop here a new technique using the concept of heat transfer in porous media. This new heat technique route (HTR) was tested on a highly homogeneous Gosford sandstone benchmark to evaluate its reliability and repeatability in estimation of the total porosity. An excellent agreement was found with the conventional mercury intrusion, gas porosimetry, and micro-CT imaging techniques. Subsequently, the total porosity of an organic-rich shale sample was measured using the HRT method and compared to the conventional techniques of nitrogen physisorption–mercury intrusion and FIB–SEM techniques. Finally, a Monte Carlo analysis was performed on heat transfer measurements, proving its robustness for total porosity measurements.

The complete uniaxial or triaxial stressâ€"strain behavior of a rock sample can be achieved in compressive testing using a servo-controlled testing system. Initially, the load-deformation data is recorded during an experiment, and then to eliminate the scale dependency of force, this data are converted to the stressâ€"strain curve for final reporting. It has been found that because of the particular design of the current sample instrumentation system used to measure the circumferential or lateral deformation during the uniaxial or triaxial compressive tests, a modification is required to be included in the conventional method of radial strain calculation. As a result, a new analytical solution has been developed to improve the accuracy of the radial strain calculation. The modification procedure differs depending on whether the test-sample diameter is greater or less than 50 mm. A comparative study was conducted based on visual observation and quantitative analysis to show the variation in a number of rock parameters including elastic modulus, Poisson’s ratio, peak stress, critical strain, residual stress strain, softening fracture energy, and residual stress when this modification was implemented. For a 50-mm-diameter sample, which is the suggested size for rock testing according to the International Society for Rock Mechanics, the effect of this modification on the parameters obtained from the post-peak region, such as critical strain, residual stress strain, and softening fracture energy varied from 5 % to 9 %. It was concluded that the impact of this modification procedure is more significant with smaller-diameter samples used in rock testing.

This paper presents the results of in situ measurements during dynamic pile testing at a construction site in Louvain-la-Neuve. Main objectives are the investigation of the pile response and the free field vibrations due to low strain dynamic loading on a single cast in situ pile with a 5.5 kg hammer impact on the pile head. Whereas low strain testing is usually performed to assess the integrity of the pile as a structural member, this study focuses on both pile and ground vibrations. The pile head response and ground motions are measured with accelerometers during different blows with the impact hammer. The dynamic characteristics of the soil are determined with a SASW test. Experimental results are compared with predictions obtained with a coupled finite element-boundary element model. The computed pile head and free field response show a good correspondence with the measured response. In addition, the static stiffness of the pile estimated by means of the mobility function shows a very good agreement with the value calculated by an analytical formulation. (C) 2008 Elsevier Ltd. All rights reserved.