Snowpack effects induced by blasts: experimental measurements vs theoretical formulas.

Size: px

Start display at page:

Download "Snowpack effects induced by blasts: experimental measurements vs theoretical formulas."

Rodney Cain
6 years ago
Views:

1 Snowpack effects induced by blasts: experimental measurements vs theoretical formulas. B. Frigo *, B. Chiaia, M. Cardu 2. Department of Structural, Buildings and Geotechnical Engineering, Politecnico di Torino, Italy. 2 Land, Environment and Geo-Engineering Department, Politecnico di Torino, Italy. ABSTRACT: To analyse the response of snowpack to explosives and the induced artificial triggering of avalanches, an experimental campaign has been realized during the Winter 200 in MonterosaSki resort Gressoney La Trinité (AO- Italy). Two different explosives have been tested (dynamite and emulsion); 24 charges were separately detonated at different elevations from the snowpack (on the snow surface, at 0.5 m, m, and 0.5 m below the surface). The measurements of the craters dimensions induced by the different blasts in the snowpack have been performed, coupled with the pre and post snowpack survey. In this paper, the correlation between geometry of the craters (maximum and minimum diameter and depth) and blasts (type of explosive, change and elevation) parameters is analysed and the empirical formulas usually used to design the shots on the snowpack are tested. From the energy point of view, a comparison between dynamic criteria given by different kinds of explosive, charges and elevation and static one is done to understand the influence of artificial triggering in the snowpack stability. The aim of the paper is to define the critical explosive charge: the best agreement between mass of explosive and detonation height to induce an artificial trigger of avalanches. KEYWORDS: Craters, Snowpack, Explosives. INTRODUCTION The unknown artificial avalanche release is a very complicate mechanism in which many technical and physical variables play fundamental parts: from the energy of the explosion to the charge position related to the air-ground interface; from the snowpack and soil properties; etc... Also the mechanisms that induce the artificial release are not so identified. Obviously the impulsive overload given by explosion on the snowpack is indispensable, but the fracturing and the collapse of the snowpack can be plays an important role, especially to artificially release cornice and wet avalanches (Takashi et al., 2008). So, from the scientific and technical point of view, the cratering phenomenon of the snowpack induced by explosive is very interesting. Moreover, cratering tests give the opportunity to study the characteristics of blast-induced shock waves transmitted through earth medium and, indirectly, study the physical-mechanical properties of the snow (Ingram et al, 960). Corresponding author : Barbara Frigo. Dep. of Structural and Geotechnical Engineering Politecnico di Torino Corso Duca degli Abruzzi, Torino, Italy barbara.frigo@polito.it Within the Operational programme Italy - France (Alps - ALCOTRA), Project DynAval - Dynamique des avalanches: départ et interactions écoulement/obstacles, the response of snowpack to explosions that induces artificial triggering of snow avalanche is analysed. To try to evaluate the effects of the induced overpressure on snowpack, an experimental snowfield has been realized during the Winter 200 in MonterosaSki resort Gressoney La Trinité (AO- Italy) testing n.24 blasts caused by two types of explosive employing different charges and detonation height. Thanks to this campaign, few data on cratering of the snowpack are available. This paper presents the working progress on the research to identify practical formulas to estimate the size of the crater producing by explosions referred by a charge position at Politecnico di Torino. 2 EXPERIMENTAL SNOWFIELD The experimental test site at lake Gabiet - Gressoney La Trinité (AO- Italy) is described in Frigo et al. (200) together with field procedure, weather conditions and related measurements. Snowpack properties In the snowfield, the snowpack presented a total height from 60cm to 50cm. The two snow- 943

Explosives properties The two explosives employed in the test had been chosen between the commercial and the ones used for artificial release of avalanches: the dynamite and the emulsion (Table ).

surface of the snowpack. The sequence of the shots was based on the height of the explosion and the mass of charge, respectively: for example, the first set of charges was dynamite at -0.

2 Explosives properties The two explosives employed in the test had been chosen between the commercial and the ones used for artificial release of avalanches: the dynamite and the emulsion (Table ). Explosion tests 24 different charges have been tested (2 for dynamite and 2 for emulsion) with, 2 and 3 kg of explosive, respectively located at detonation height 0 m, +0,5 m, + m and -0,5 m from the surface of the snowpack. The sequence of the shots was based on the height of the explosion and the mass of charge, respectively: for example, the first set of charges was dynamite at -0.5 m of the surface of snowpack (inside the snowpack) with a sequence of 3 kg, 2 kg (Fig. 2), kg, on the first shot line, and so on (Frigo et al., 200). Figure. Snowpack profile close to the explosion line. -pack profiles (Fig. ) characterised the snowpack with total height about m and a meltfreeze crust after 30 cm of the fresh snow on the surface (given by the snowfall with 70 kg/m3 of density). At -50 cm, a sandwich crust with a maximum thickness of 5 cm was presented. The presence of the sandwich crust was confirmed over the entire snowfield from measurements with the steel probe. The rest of the snowpack was characterised by uniform snow with density between 220 and 290 kg/m 3. The bottom of the snowpack was composed by a strong layer of ice. Table Main properties of the explosives employed to perform the experimental blasts. Figure. 2. Explosion inside the snowpack (Photo: L. Pitet, 200). 4 CRATERS SURVEY For each point of shot of the explosion lines, the measure of the height of the snowpack and the depth of the sandwich crust were noticed. Type Emulsion Dynamite Commercial name Premex 3300 Goma 2 Eco Density [kg/m 3 ] Energy release [kj/kg] VOD in free air [m/s] Gas Volume [dm 3 /kg] Charge Mass [kg] Figure 3. Crater dimensions. 944

3 For each crater (with the exclusion of the craters generated by a 2 kg charges), the survey provided the measure of: the dimensions of the crater (maximum and minimum diameter and depth, figure 3), section shape, snow density and temperature, snow samples for chemical analysis. Fig. 4 reports the crater nomenclature used in the paper: - apparent crater: excavation that appears immediately after a blast; - true crater: excavation that appears after removing material lifted from its original position by blast and fallen back into the excavation. Comparisons (Corps of Engineers, 96) between the dimension of true and apparent craters indicated that the true crater radius is roughly 0 to 20% larger than the apparent one, depending on the charge position and physical properties of the medium; - relapsed material: material lifted from its original position by blast and fallen back into the crater and on the surface; - depth of crater (H): depth of the apparent crater; - radius (or diameter) of crater (R or D): radius (or diameter) of the apparent crater. snowpack and the variability in size and shape of snow craters. Based on the charge position, three regimes can be identified to classify the shots and related interaction with earth medium: - the below-ground regime; - the air-ground interface; - the above-ground regime, tested in the Gabiet Lake experimental campaign with charge position height, respectively - 0,5; 0; +0,5 and m. To analyse the cratering phenomenon in a variety of earth media, the Corps of Engineers of U.S. Army Engineer Waterways Experiment Station (96) referred sets of cratering capability curves with reduced crater position, λ c, defined as: Z ft c in 3 W 3 lb () where Z and W are the position of the charge above (+) or below (-) air-ground interface (in ft) and the weight of the charge (TNT equivalent) (in lb). Table 2 reports λ c calculated the shots of the Gabiet Lake experimental campaign. Table 2 Reduced crater position λ c of the blasts related to a kg explosive for the Gabiet Lake experimental campaign (Frigo et al, 200). Z [m] λ c Group -0,5 -,26 below-ground regime 0 0 below-ground regime 0,5,26 air-ground interface 2,52 above-ground regime Table 3 Reduced crater position λc of the blasts related to a -0,5m position of the charge for the Gabiet Lake experimental campaign (Frigo et al, 200). Figure 4. Crater nomenclature. In each earth medium, the position of the charge related to the air-ground interface, is the most important factor influencing the crater size and shape, since the charge position relative to the interface fixes the amount of energy partitioned between the two media (Corps of Engineers, 96). RESULTS AND DISCUSSION The aim of the paper is to study the effect of charge position in crater formation into the W [kg] λ c Group -,26 below-ground regime 2 -,00 below-ground regime 3-0,87 below-ground regime Regarding the size of the induced craters, it seems that the crater dimension (width and depth) grows with the increase of the mass charge and decreases with the shot height. To understand the correlation between crater size and charge position with different explosives, Figs. 5 and 6 report the measured diameters and the depth of craters. 945

presents the a shape (Fig. 7) formed when the value of the reduces charge position λ c lies between +0,5 and -2,0 (Corps of Engineers, 96). Figure 5.

4 the snowpack, but not to the excavation of the medium. About the craters generated by charges placed in the below-ground regime and airground interface (charge position equal to -0,5m and 0m, respectively), observation show that all craters correctly presents the a shape (Fig. 7) formed when the value of the reduces charge position λ c lies between +0,5 and -2,0 (Corps of Engineers, 96). Figure 5. The diameters of the crater measured during the test reported for dynamite (GD) and emulsion (EM). Figure 6. The diameters of the crater measured during the test reported to dynamite (GD) and emulsion (EM). Note that craters have formed only for the charges placed in and, dully, on snowpack that is in above-ground regime and air-ground interface (Corps of Engineers, 96). In fact, from Figs. 5 and 6, the dimensions are referred to the track of compression induced by explosions on Figure 7. Typical crater shapes in belowground regime (Fig. from Corps of Engineers, 96). Figure 8. Charge of explosive vs (a) minimum diameter, (b) maximum diameter and (c) depth of crater reported for dynamite and emulsion in below-ground regime (charge position equal to -0,5m). 946

5 Craters generated by explosions inside the snowpack show a constant growth of the minimum diameter increasing the weight of explosive charge (Fig. 8.a). The growing of the geometrical size of the crater can be approximate with a power law: in particular, for the minimum diameter, the approximated power law are: - for dynamite: 2,7 W 0,2 Dmin (2) - for emulsion: 2,7 min W 0,085 D (3) Craters produced by shots on snowpack (at Z = 0 m) show a very interesting approximated power law about the depth of the crater: - for dynamite: 3,5 W 3,76 H (4) - for emulsion: 3,5 W 2,7 H (5). Trying to compare theoretical formula and measurements, the Machida practical formula (Morisue et al., 997; Machida et al., 2006; Takashi et al., 2008) (6) has been applied to compare the measured and calculated radius of crater in the belowground regime, where: - R is the radius of the destruction hollow; - F is the explosive force; - L is the charge of explosive; - V is the detonation velocity; - H is the snow depth above the detonation point; - d is a constant equal to,2. It resulted that the Machida formula can be applied only for craters in below-ground regime with λ c < -2,0 (Figs. 7.b and 7.c). 6 CONCLUSIONS The paper presents the working progress on the effect on explosions on snowpack related to the artificial avalanche release at Politecnico di Torino. In particular, the aim of the study is to identify practical formulas to estimate the size of the crater producing by explosions referred by a charge position. Few available data, technical/practical problems and no-homogeneous procedures and measurements make this research very hard. More experimental campaigns will do in next winters with more methodical procedures and innovative surveys. 7 ACKNOWLEDGEMENT We acknowledge financial support by the Operational programme Italy - France (Alps ALCOTRA) - Project DynAval Dynamique des Avalanches: départ et interactions écoulement/obstacles, Regione Piemonte, Regione Autonoma Valle d Aosta and Politecnico di Torino. We are very grateful to our colleague ing. Alessandro Giraudi for the precious help and fruitful discussions. 8 REFERENCES Binger C., Nelsen J., Olson K.A., Explosive shock wave compression in snow: effects of explosive orientation and snowpack compression. In Proceedings of the International Snow Science Workshop, Telluride, Co, October Cardu M., Chiaia B., Chiaravallotti L., Cornetti P., Frigo B., Modello meccanico per l innesco delle valanghe di neve. GEAM - Geoingegneria Ambientale e Mineraria. XLIV. : Frigo B., Chiaia B., Cardu M., Giraudi A., Godio A. and Rege R., 200. Experimental analysis of snowpack effects induced by blasts. International Snow Science Workshop, Lake Tahoe CA U.S.A., pp Takashi M., Lu M., Isao K and Atsushi S., A basic study on technology of inducing artificial avalanche by explosive detonation inside the snowpack. International Snow Science Workshop, Whistler BC Canada, pp Corps of Engineers U.S. Army Engineer Waterways Experiment Station, 96. Cratering from high explosive charges: analysis of crater data. Technical Report n , Vicksburg, Mississippi, pagg. 08. Livingston C. L., 968. Explosions in snow. Technical Report n. 86 Cold Regions Research & Engineering Laboratory, Hanover, New Hamshire (U.S.A.), pagg. 24. Machida M., Hayakawa N., Machida T., Sakaue S., The Technology to induce an Artificial Avalanche, Journal of the Japanese Society of Snow and Ice, Vol. 69, pag Morisue H., Takeuchi N., Hayakawa N., 997. Effect of Explosive Detonation in the Snow Layer, Journal of the Japanese Society of Snow and Ice, Vol.59, pag Ingram L.F. et al, 960. Measurement of explosion induced shock waves in ice and snow, Greenland, 957 and 958. Corps of Engineers U.S. Army Engineer Waterways Experiment Station, Miscellaneous paper n , Vicksburg, Mississippi, pagg

A simple method to study snow erosion and deposition processes in small avalanches: the straw test

A simple method to study snow erosion and deposition processes in small avalanches: the straw test E. Bovet 1,2, * M. Maggioni 3, L. Pitet 4, B. Chiaia 1, M. Freppaz 3, V. Segor 4 and P. Dellavedova 2