Design of Controlled Pre-Split Blasting in a Hydroelectric Construction Project

Geologic conditions and design requirements around upstream Nachtigal Falls, in Cameroon, for the hydroelectric construction project on the Sanaga River dictated close control of blasting procedures with very precise geometry to obtain safe and economical excavation lines. Various techniques of pre-split blasting were used in the initial stage of all major excavations. Hole diameters for pre-splitting were 89 mm, and hole spacing ranged from 0.8 to 1m. Explosive charges varied from 1 to 7 kg per hole, and the detonating cord linear charge ranged from 12 to 60g. The contour blasting technique is aimed at controlling overbreak and improving remaining slope stability. Over-break or over-excavation needs to be controlled since its occurrence compromises the operations in terms of safety (instability in the remaining slope; loosening rocks that increase the risk for operational people; an irregular free face for subsequent blasting); and costs (need for reinforcement of the remaining rock structure through costly sustainment systems; increase in concrete volumes in civil works). This paper discusses in detail the design and field implementation of pre-split blasts successfully carried out to achieve clean vertical walls in moderately dipping, though complexly sheared and jointed gneiss. Based on the results of the experiments, we were able to design a pre-splitting pattern both experimentally and in a very cost-effective manner. It is felt that the methods developed on this project could have useful applications on other major construction projects. Doi: 10.28991/CEJ-2023-09-03-05 Full Text: PD

Analysis of frequency dependence of complex impedance and electrical characterization of Fe2O3/kaolin ceramics for civil engineering applications

The complex impedance spectroscopy (CIS) method is usually used in order to analyze the electrical response of different semiconducting disordered materials as a function of frequency at different temperatures. The real and imaginary parts of the complex impedance can show different semicircles in the complex plane that give evidence for the presence of both bulk and grain boundary contributions. Many parameters can be deduced from the analysis of CIS data, such as relaxation times and activation energies. There are some literature data concerning electrical properties of clays and (semiconductor, sand, cement,…)/clay mixtures. Most of the published works are related to the AC conductivity of rocks with the effect of water or oil content but there are no similar studies on the characterization of the microstructure of individual clays as ceramic materials by analyzing their temperature and frequency dependence of their electrical conductivities. Hence, this paper presents an analysis of electric complex impedance of the Fe2O3/Kaolin composite in the high temperature range up to 740 °C. Sinusoidal voltage with frequency in the range [100 Hz, 1 MHz] is applied to the material in order to measure the electrical conductivity for various concentrations of Fe2O3 from zero to 100 %. The activation energies for the conduction and for the relaxation processes are determined and their dependence on the density of Fe2O3 analyzed. Furthermore, let’s found that Fe2O3 have the effect to increase the electrical conductivity in our samples. From the Nyquist diagrams, only one semi-circle related to the contribution of the grains to the total electrical conduction is identified for all investigated samples

Electrical characterisation and analysis of dominant contributions in disordered semiconducting systems with an application to the pure bentonite material for civil engineering applications

Semiconductors and clay materials have significant applications in environmental, civil engineering and optoelectronic sectors. The application of an electric field to such systems is subject of many works. However, to understand the behaviour of such materials under the influence of an electric field, the perception of its electrical properties is essential. In the present study, the powerful technique of complex impedance spectroscopy (CIS) is introduced to illustrate the electrical characteristics of two types of disordered semiconducting materials. These are Cu5In9Se16, an ordered defect compound of the I-III-VI2 family and a novel bentonite clay system which is an insulator at room temperature and semiconductor above 400 °C. Na-bentonite has been studied extensively because of its strong adsorption capacity and complexation ability while Cu5In9Se16 is considered for its use in solar and phtovoltaique domain. Some of selenides have turned out to be leading materials for electro-optical devices and the tellurides for thermoelectric power generation. It is very likely that study of bentonite clay and other similar materials may lead to the technology of heterojunction and clay composite. The frequency dependence of conductivity of bentonite was investigated using an impedance analyzer in the frequency range (20 Hz–1 MHz). The experimental data of CIS are analyzed using some analytical methods that take into account the effect of the grains and grain boundaries. The impedance data confirm the non-Debye behavior in these systems. Some important parameters related to the identified dominant contribution such as relaxation time and activation energies are estimated for the studied materials in the considered temperature and frequency range

Geotechnical and thermal analysis and complex impedance spectroscopy characterization of pure Moroccan bentonite material for civil engineering applications

Combined modulus and impedance spectra are employed in the present work to explore electrical inhomogeneity and carriers’ behaviors in a pure bentonite Moroccan clay based on equivalent circuit. It has been clearly observed that the electrical properties change due to the increase of temperature from 300 °C to 700 °C. The frequency-dependent imaginary modulus M" and imaginary impedance Z" curves has only one peak at each temperature indicating the predominance of the contribution of grains to the total electrical conduction in bentonite. The positions of these peaks move to higher frequencies when the temperature increases in relation with the distribution of relaxation time. Moreover, the activation energy for the conduction process in bentonite is determined from the slope of ln(ρdc) versus of 1/T in the order of 700 meV in good agreement with that obtained from the proposed equivalent circuit. On the other hand, let’s present a geotechnical study that show that our material is a swelling clay, very plastic and could be used as a binder. The external stress dependence of the bulk density, Young’s module and maximum stress are analysed. The thermal conductivity determined following the device of Lee's disks where two copper disks of thickness of 15 mm and diameter of 30 mm were use

Etude en laboratoire de la liquéfaction des sols

Le travail realise dans cette these concerne l'etude en laboratoire la liquefaction des sables. Il comprend trois parties : la premiere partie comporte une analyse bibliographique des travaux realises sur la liquefaction des sols. On presente l'influence des differents parametres sur la resistance a la liquefaction comme la granulometrie, la presence d'elements fins, la saturation, l'histoire de chargement et la surconsolidation. La seconde partie presente l'appareillage utilise et le mode operatoire suivi. Les tests de repetabilite effectues et les tests preliminaires ont permis de verifier le bon fonctionnement de cet appareillage et la bonne maitrise du mode operatoire. Des essais monotones realises sur des sables montrent l'influence de la granulometrie sur leurs resistance et dilatance, qui conditionnent la resistance au cisaillement des sables. La troisieme partie presente les resultats des essais de liquefaction sur differents sols en faisant varier le niveau de chargement, le prechargement et le degre de saturation. Les resultats obtenus montrent que la liquefaction est principalement affectee par la granulometrie. La presence des elements fins, dans une fraction inferieure a 15% d'argile (keaolinite), a pour effet d'augmenter le risque de liquefaction a cause de l'augmentation de la contractance. Le risque de liquefaction depend egalement du degre de saturation. En effet la reduction du coefficient de skempton s'est traduit par une nette augmentation de la resistance a la liquefaction des sables. Les essais realises montrent qu'un precisaillement important induit une reduction de la resistance a la liquefaction, notamment, dans le cas d'un precisiallement en compression.LILLE1-BU (590092102) / SudocSudocFranceF

Effects of thermal stratification on natural convection in a symmetrically heated channel: Comparison of characteristic quantities between air and water

We present in this paper a numerical investigation on pure convection flow inside a vertical channel heated at symmetrically uniform heat flux and contained in an enclosure. This study aims to investigate how far we can use water to predict natural convection in an air filled channel especially in presence of thermal stratification. The 2D unsteady Navier–Stokes equations under Boussinesq approximation was solved numerically using Finite Differences Method. The investigation is made for moderate Rayleigh numbers in order to study the effect of stratification at the early stages of the flow development. A comparison in terms of flow and heat transfer characteristics is done between the two fluids. It is seen that natural convection heat transfer in such a configuration is more impacted by thermal stratification when air is used as the working fluid. Traditionally used water models to predict the natural convection of air may give rise to misinterpretations about the real phenomenon especially when stratification occurs. This study should also give some clarifications about the discrepancies raised in the literature between numerical and experimental results when both water and air are used

Mechanical Behavior of Sand Mixed with Rubber Aggregates

The main objective of this study is to compare the mechanical behavior of two sands (Hostun or Dune sands) mixed with crushed rubber obtained from used tires. However, it is essential to ensure that his geotechnical application do not result in long-term negative impacts on the environment. The chemical properties of these two sands are given by energy dispersive analysis X-ray fluorescence spectrometry. The mineral composition of these two sands is performed by X-ray diffractometry. The morphological characteristics of the sand grains are given by the analysis of the images of the two sands given by the scanning electron microscope. This study is based on 120 direct shear tests performed on sand-rubber aggregate mixtures. The results show that the rubber content of the aggregates has a significant effect on the shear strength of sand-rubber mixtures in both cases of sand. In fact, the shear strength of the sand-rubber mixture increases with increasing crushed rubber up to 20% for different normal stresses. The analysis of the test results also shows the effect of the angular shape of the sand grains on the interparticle friction. The contribution of the structure effect in the mobilized friction is analyzed by comparing the shear test results of Hostun and dune sand mixtures