Summary

Loosening of rock masses as a result of blasting at the excavation of foundation pits causes an increase of permeability which is to be prevented especially at construction of dams. The range of loosening investigated in migmatized gneiss, is wider compared with usually given radii. In rock masses cemented by grouting, the loosening is substantially smaller and is practically limited to the zone of blasting. Intensity of vibrations generated by explosions at the surface of rock mass was also determined by measurement. An approximate relation between the weight of the charge and the range of loosening was established.

1 — Introduction

Excavation of a foundation pit in a rock mass causes a decrease of overburden and the unloading produces the loosening of the surface zone at the bottom of the pit. This loosening is manifested by higher permeability, compressibility and by the decrease in the strength of rocks. The use of explosives in excavating induces additional loosening with unfavourable consequences especially for dam constructions. A remarkable loosening was e.g. observed at the tests on a dam site built up of migmatized gneiss- Very low and considerably varying values of deformation modulus (2.5 to 24 Mp/cm2) and elasticity modulus (4.2 to 60 Mp/cm2) were established as well as a greater consumption of grouting mixture, compared with permeability tests performed before the excavation. It is always necessary to take into account unloading of the foundation base but the loosening of the bottom of the pit by blasting can be prevented.

2 — Range of Loosening and its Determination

Diminishing of explosive charge is connected with lower effectivity of work. Therefore, it is necessary to determine the range of the loosening in relation to the weight of the charge. The common expression for the radius R of the loosened zone from a concentrated charge is

(Equation)

where r is the radius of a spheric charge. For 1 kg of common explosives is R ≈ 1.50 m. Usually such a loosening is — ratherunreliably — considered by naked eye and largely depends on the orientation of the charge to the jointing of the rock mass, as mentioned by Müller [1]. Two other observations from the author's experience are drawn in Fig. 1. The one was made on medium-jointed granite with polygonal distribution of division planes. The other concerns solidified clayey shales of conspicuous bedding and cleavage parallel with bedding planes. In the first case the loosening provoked by a 0.5 kg explosive charge reached only to an outstanding plane of jointing at a distance of 1.10 m from the centre of the charge, whereas in the second case the loosening was perceptible up to a distance of 3 m. In both cases the boreholes were vertical with cylindric shape of charges, 1 m long.

Résumé

Relâchement des massifs rocheux comme résultat des sautages pendant les excavation cause un agrandissement de la permeabilité, que doit être prévenu, surtout sur les sites des barrages. L’étendue du relâchement examinée au gneiss migmatisé est plus grande que les limites habituellement données. Aux massif solidifiés par cimentation le relâchement est beaucoup plus petit et est limité à la zone de sautage. On a aussi mésuré l’intensité des vibrations que se sont produits sur la surface du massif. Une relation approximative entre le poids de la charge explosive et l’étendue de la fissuration est présentée.

Zusammenfassung

Eine Auflockerung des Felsmassives als Resultat von Sprengungen bei dem Ausbruche der Baugruben hat eine Vergrösserung der Wasserdurchlässigkeit zufolge. Diese Tatsache soll, besonders bei Talsperren, vermeidet werden. Die Reichweite der Auflockerung, die im migmati-zierten Gneiss untersucht wurde, ist grösser als die aus den üblichen Formeln gefundenen Entfernungen. In einem mit Injektionen verfestigten Felsmassiv ist der Bereich der Auflockerung wesentlich kleiner und ist im grossen und ganzen auf die Sprengungszone beschränkt. Die Intensität der bei den’ Sprengungen entstandenen Vibrationen wurde an der Oberfläche des Felsmassivs gemessen. Eine angenäherte Beziehung zwischen dem Sprengstoffgewicht und der Reichweite der Auflockerung ist angegeben.

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