Petrographic and Mechanical Analyses of Selected Granitic Rock Deposits in Ado-Ekiti, South-West Nigeria.

In this study, the petrographic and mineralogy of selected granitic rocks on their physical and mechanical strength were evaluated. Granitic rock samples sourced from seven locations were analyzed to investigate their mineral micro-texture and compositions using petrographic examination and X-ray diffraction (XRD) analysis respectively. Mechanical properties including compressive strength and hardness of the examined rocks were performed following ASTM D7012–10 and EN 1534 standards. Their physical properties in term of specific gravity, porosity and water absorption were examined following ASTM C97/C97M. Results show that the predominant minerals in rock samples are quartz and kaolinite. Stress-strain curve displays plastic fragmentation after initial fracture in most granitic rock samples with rock samples from Olorunda Zone 9 and Olorunda Zone 2 exhibiting superior plastic deformation over a wide strain elongation. Their physical properties were within the acceptable range for construction applications.

Numerical Modeling of Temperature Effect on Tensile Strength of Granitic Rock

The aim of this paper is to numerically predict the temperature effect on the tensile strength of granitic rock. To this end, a numerical approach based on the embedded discontinuity finite elements is developed. The underlying thermo-mechanical problem is solved with a staggered method marching explicitly in time while using extreme mass scaling, allowed by the quasi-static nature of the slow heating of a rock sample to a uniform target temperature, to increase the critical time step. Linear triangle elements are used to implement the embedded discontinuity kinematics with two intersecting cracks in a single element. It is assumed that the quartz mineral, with its strong and anomalous temperature dependence upon approaching the α-β transition at the Curie point (~573 °C), in granitic rock is the major factor resulting in thermal cracking and the consequent degradation of tensile strength. Accordingly, only the thermal expansion coefficient of quartz depends on temperature in the present approach. Moreover, numerically, the rock is taken as isotropic except for the tensile strength, which is unique for each mineral in a rock. In the numerical simulations mimicking the experimental setup on granitic numerical rock samples consisting of quartz, feldspar and biotite minerals, the sample is first heated slowly to a target temperature below the Curie point. Then, a uniaxial tension test is numerically performed on the cooled down sample. The simulations demonstrate the validity of the proposed approach as the experimental deterioration of the tensile strength of the rock is predicted with agreeable accuracy.

Relaxation damage control via fatigue-hydraulic fracturing in granitic rock as inferred from laboratory-, mine-, and field-scale experiments

The ability to control induced seismicity in energy technologies such as geothermal heat and shale gas is an important factor in improving the safety and reducing the seismic hazard of reservoirs. As fracture propagation can be unavoidable during energy extraction, we propose a new approach that optimises the radiated seismicity and hydraulic energy during fluid injection by using cyclic- and pulse-pumping schemes. We use data from laboratory-, mine-, and field-scale injection experiments performed in granitic rock and observe that both the seismic energy and the permeability-enhancement process strongly depend on the injection style and rock type. Replacing constant-flow-rate schemes with cyclic pulse injections with variable flow rates (1) lowers the breakdown pressure, (2) modifies the magnitude-frequency distribution of seismic events, and (3) has a fundamental impact on the resulting fracture pattern. The concept of fatigue hydraulic fracturing serves as a possible explanation for such rock behaviour by making use of depressurisation phases to relax crack-tip stresses. During hydraulic fatigue, a significant portion of the hydraulic energy is converted into rock damage and fracturing. This finding may have significant implications for managing the economic and physical risks posed to communities affected by fluid-injection-induced seismicity.

Menir de Fontelo (Armamar, Viseu) Primeira notícia

We bring to light the discovery of a new tall menhir, currently laid on the ground, away from its primitive erection site, in Vale de Naçarães, southwest Fontelo village (Armamar, Viseu) allowing some considerations. It is a monolith sculpted with local granitic rock, subcylindrically shaped, but presenting pointed extremities, measuring 5.32 m long and 1.18 m in maximum width. The exposed surface shows three engraved cup-marks, one near the top and two paired on the mesial volume, as well as at the central area four large pecked concentric circles, containing on its inside a fifth circle, smaller and not centred with the larger ones. Other cup-marks were detected, but all the engravings are much eroded and were possibly made when the menhir was already down. This kind of monuments was probably built in the 5th millennium B.C., when the first passage graves appear in the region. The large dimensions of this monolith, when erected allowed his viewing from a far distance thus ordaining the surrounding space. The authors propose that this menhir to be re-erected and signalised through explanatory board on his historical and archaeological importance.