Physical models of dolomitic marbles from greek quarries with a pair of twin pre-existing cylindrical holes subjected to uniaxial compression are studied in the laboratory and then, they are simulated numerically with a Bonded Particles Model by employing the two-dimensional version of the Particle Flow Code and the Flat-Joint model. The micro-cracking, the fracturing process and the sequence of their appearance during the numerical tests of the hollow plates are in good agreement with the laboratory tested physical models. Each numerical fracture pattern is similar to the one of the respective physical models. Also, the regions of micro-cracking in the numerical models are similar to the regions of intense deformation observed from Digital Image Correlation analysis on the respective physical models. A comparison between the required applied axial stress for the primary fracture and sidewall spalling initiation of the physical and the numerical models is presented. Then, these stress values are compared to the ones of previously published hollow plates with a single cylindrical opening of the same diameter in compression of the same materials.

The purpose of this research is the calculation of the stress field as well as the displacements occurring around tunnels of unlined rectangular shape, in a linear elastic ground due to SV seismic excitation. The numerical analyses are carried out with the finite-differences Code FLAC_3D. Then, the results are compared to those obtained from an analytical method. The analytical model is based on infinite series expansion of Bessel & Hankel type for the estimation of the wave potentials of incident, reflected and scattered waves, in combination with conformal mapping. This mapping transforms the area between the free surface and a tunnel of depth H in plane w onto two unit circles in the plane ζ, the centers of which are at vertical distance b the one from the other. The analytical calculation of the stress field as well as the deformations of the sections of underground structures, subjected to seismic excitation of shear SV waves, is achieved with the use of MATLAB. A series of dynamic analyses are performed the thickness of the soil layer above the structure in a half space. The model is subjected to seismic excitation simulated by a harmonic vertically propagating shear wave in terms of stress history. SV waves cause racking motion of the cross section of the tunnel. The results of the analyses showed that the sectional stresses and deformations depend on the structure depth, i.e. the deeper the structure the smaller the effect. To sum up, the comparison between the analytical and the numerical method in the elasticity range provided satisfactory results.

Hollow plates of plaster and calcitic and dolomitic marbles, extracted from greek quarries, with a single pre-existing cylindrical hole of various diameters are subjected in uniaxial compression. The influence of the material and the hole’s diameter on the fracturing process are studied and compared to the literature. Full-field digital image correlation technique and acoustic emission analysis are employed for the monitoring of the fracturing process, the initiation of the main phenomena, their interaction, and the failure mode of the physical models. New precise techniques on the determination of the true time of the primary fracture and spalling initiation by digital image correlation are implemented and presented. The study highlights a new behavioural type of such physical models, with different fracture patterns than those presented in the literature. A comparison between the required applied axial stress for the primary fracture and spalling initiation of the studied materials and the previously published results is presented, showing an exponential relation to the hole’s diameter.

This paper presents the statistical processing of data obtained from multistage triaxial tests of selected rock types from East Attica Prefecture of Greece. Two strength models for the peak and residual strength were fitted to the experimental data, i.e., the Mohr - Coulomb criterion and the Hoek - Brown criterion. The derived strength parameters for these distinct states of stress are compared with findings of other researchers and their dramatic change during the residual state of the rock samples is discussed.

Analytical Solutions are proposed for the calculation of the stress and deformation fields around cavities, of various shapes, in the linear elastic Half Plane Space, due to seismic excitation of SV Waves. The analytical model, is based both on the expansion of infinite series of Bessel & Hankel type, for the estimation of the wave potentials, both from the cavity and the ground surface of incident, reflected and scattered, combined with the Complex Variable Method. The close form Solution, is developed in two stages. In the first, by applying the Complex Mapping Theory, a mapping is proposed so as the region in the plane w consisting of the half space with a cavity of various shapes (circular, square, hoop) is mapped conformally onto two vertical circles in the plane zeta. With the application of this mapping the problem of the free stress boundary conditions at the surface is solved. In the second stage, the complex Wave Potentials are calculated in the transformed plane zeta and represented by their Bessel and Hankel series expansion. Thus the calculation of the stress field as well as the deformations of the sections of underground structures, subjected to seismic excitation of shear wave, of SV type is achieved with the implementation of the boundary conditions both in the surface of the cavity and the upper boundary of the half space assuming t(rr) = t(r theta) = 0. The language of technical computing, MATLAB, is applied for the composing of the relevant computing codes. Finally in order to compare the results with previous approximations, an analysis without taken into account the scattered wave potentials from the free surface (Relaxed Boundaries) has been developed, for various cavity shapes. This assumption can be considered because the values of these potentials are small. By omitting these wave potentials this problem becomes a full space case, under incident and reflected SV waves. Comparing the proposed analytical methodology with that with Relaxed Boundaries the former calculated stresses and displacements are larger than those of the latter. Although the values of the scattered wave potentials, from the free surface are small, their contribution to the stress field is significant. This proposed analytical methodology solved the problem of the calculation of the stress field around cavities in case of half space under seismic loading of SV waves.

The exploitation of marble in Greece is extended lastly underground. Its productivity is largely dependant on the ability to excavate large stable openings. The prediction of the mechanical response of such an exploitation is endeavoured with the observational method. Numerical models and closed form solutions provided by the voussoir beam theory are used to analyse the structural behaviour of the excavation. The necessary initial estimates for the deformability data of the in-situ surrounding rock are evaluated by back- analysing the response of a pilot tunnel. These data are used then to calculate at a first stage the stability of the enlarged opening. This observational method may be repeated then contiguously at the various subsequent stages of the excavation enlargement, which is performed in slices, by using the updated data evaluated at each stage of the underground structure, to the analysis of the following ones.

An arched roof can support a much greater load than a horizontal beam. This increase in the carrying capacity stems from the fact that the downward movement due to gravity creates a compressive arch. The behavior of multi-jointed hard rock roof under gravity load is hereby investigated under the novel view of the compressive arch forming a catenary curve. Analytical formulae are derived, that evaluate, for any given geometry, the loading and mechanical parameters of the multi-jointed roof and its deflection and strain, in terms of the extreme and mean arch thickness. Relations between the applied load, opening and sag are provided for the three basic modes of failure: buckling, crushing and slip. These relations are validated in two ways; though experimental data and by comparison to a real case of an underground magnesite mine. Roof support design to prevent the above modes of failure is provided.

The aim of this study is to investigate the statistical correlations between the point load strength index and certain physical properties, e.g. the dry density and the dry longitudinal ultrasonic wave velocity of prasinites (metabasites). Statistically significant correlations established between the physical properties as well as between each physical quantity and the point load strength index. According to bibliography, this is one of the first efforts to develop relations between physical and mechanical properties for this particular petrological type, and therefore the derived equations can be a useful tool to the investigation of these petrological types, either in the study area or in other sites, where prasinites of similar structural characteristics, are examined for the foundation of various constructions.

In this paper, a numerical investigation of uniaxial compressive tests on prismatic specimens with single cylindrical pre-existing cavities in brittle rock is performed. To investigate the rock fracture around cavities and to assess the potential of the numerical model to simulate this behaviour, published laboratorial physical models on granite are simulated numerically with a Bonded-Particles Model (BPM) by using a distinct elements code. The numerical models are presented and the calibration of the BPM micro-parameters is described. Then, the calibrated numerical models are used to investigate the potential of the BPMs to simulate the fracture initiation and propagation of the physical specimens. It is concluded that the laboratory and the numerical observations are in good agreement. (C) 2017 The Authors. Published by Elsevier Ltd.

The mechanical behaviour of single and hybrid FRC mixtures, with one type of steel and two types of polypropylene fibres at 0.5% and 1.0% volume fractions, is investigated through the double punch Barcelona test. Indirect tensile strength and toughness characteristics are evaluated at ambient temperature and after heating of the specimens at 280°C. It is shown that the modified toughness estimated through Barcelona test may be a suitable index to classify FRC in terms of ductility, both for the unheated and for the heated specimens. It was also found that hybrid FRC, containing an appropriate amount of different fibre types, is superior in terms of ductility compared to the single FRC. Finally, equations are proposed for the calculation of the Total Circumferential Opening Displacement (TCOD) values by using the axial displacement measurements, considering basic characteristics of the fibres and of FRC mixture, such as fibre’s length, rupture strength and volume fraction, respectively.

Fire resistant geopolymers are developed and the performance under thermal loading is examined and compared in this paper.The geopolymers were prepared by mixing the solid phase, metallurgical slag and metakaolin with a highly alkaline potassium hydroxide aqueous phase in order to create a paste that was subsequently cured at 70˚C for a certain period of time.The developed materials were tested for the mechanical, physical and thermal properties.The behaviour of the geopolymers upon exposure on fire was studied following the EFNARC guidelines for testing of passive fire protection for concrete tunnels linings.The geopolymers were subjected to the most severe fire scenario, the Rijks Water Staat (RWS) temperature-time curve.Both geopolymers appeared great behaviour after the test reaching temperature lower than the RWS test requirement, proving the ability of both materials to work successfully as an efficient thermal barrier.Thus, the concrete slab protected by the geopolymers did not appear any form of spalling or degradation of its compressive strength.

The paper focuses on the post-cracking behavior of FRC and discusses the effectiveness of two different testing methods. Three different fibre types (steel and polypropylene) were added in concrete at two percentages (0.5 and 1.0%) and by their type combination (hybridization), ten concrete mixtures were produced. Two testing methods have been employed in order to characterize the mechanical behavior of FRC after the fracture; the bending test and the double-punch test (Barcelona test). The fibre reinforcement of concrete, especially in the hybrid mixtures, resulted in the development of residual tensile strength and of improved toughness. Also it has been validated the main advantages of the Barcelona test, that it is a convenient compressive test which offers rapid and compact results.

An essential element of design of concrete structures is to ensure that the structural elements will not fail due to the high temperatures which may be developed during a fire. For this problem, several passive fire protection methods have been developed. However there is always a need for developing a new material with improved fire resistance properties and low cost. Geopolymerization technology seems to be very attractive in developing effective fire resistant materials. The developed materials were tested for their resistance under high temperatures according to E.F.N.A.R.C specifications and guidelines. From the test results it is concluded that the materials perform well under various fire scenarios without yielding or spalling, according to the standard fire temperature curves employed in international norms, and that may occur during a real fire incident. Also as it was shown that they have similar properties to the commercially available materials.

In this paper, a distinct elements code is used to perform a numerical investigation for the size and stress gradient effects on the fracture initiation and propagation around single or pairs of pre-existing cavities in brittle rock. To investigate the rock fracture around cavities and to assess the potential of the numerical model to simulate this behavior, published laboratory physical model on granite is simulated numerically with a Bonded Particles Model (BPM). The numerical model is presented and the calibration of the BPM micro-parameters is described. Then, the calibrated BPMs are used to investigate the effect of the size of the cavity on the primary, secondary and side wall fracturing, as well as on the fracturing modes. Moreover, BPMs with two circular cavities were used to study the interaction of these holes of the same diameter and to investigate the importance of their relative distance. Finally, the simulated material was studied by biaxial tests on BPMs with a pre-existing hole. A R T I C L E I N F O Article history: Received 26 August 2016 Accepted 1 November 2016

The paper focuses on studying experimentally the potential benefits (in terms of stiffness, strength, ductility) stemming from the use of different fibres on the flexural behaviour of concrete beam specimens. Different types of steel and polypropylene fibres were added into the concrete mix at volumetric contents of 0.5% and 1.0% either individually or in different combinations. Three-point bending tests were carried out to establish the behaviour exhibited by the fibre reinforced concrete specimens produced up to failure and determine the associated mechanical properties of the subject material. The residual mechanical properties of the same specimens were also established after exposing them to temperatures of 280 °C. From the test data obtained it was observed that even though the use of hybrid fibres can result in the development of higher residual tensile strength and improved toughness, at elevated temperatures, the use of steel fibres appears to provide optimum post-cracking behaviour.