Conference Paper

Impact loads of falling rocks on granular material

Authors:
  • Pfeifer Isofer
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Abstract

This contribution presents results of 54 tests with two different boulders made of concrete, which dropped on two different ground layers. The deceleration of the boulder during the impact was measured with accelerometers. The analysis of data delivered useful results such as maximum deceleration, penetration depth and braking time. This allows the description of the correlation between maximum deceleration, penetration depth and impact velocity. The characteristic of the deceleration of each test was analyzed specially. The normalization of the decelerations shows an independence of the course of deceleration from the drop height respectively from the impact velocity. The results are a useful basis to design earth dam against rockfall or help to calculate the rockfall process in simulations.

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... Forced based (FB) models are conventionally used in design practices which essentially provide estimates of forces that are built up at the point of contact between the impactor and the cushion material placed in front of the barrier (or on top of the shelter). Substantial experimental investigations have also been carried out to ascertain this peak value of the highly transient contact force as well as the force transmitted to the concrete surface through the cushion layer [11,[13][14][15][16][17][22][23][24][25][26][27][28]. Those investigations added useful intuitions into the use of different cushion materials to control the peak contact force thereby minimising the damage to the surface of the barrier. ...
... A special experimental arrangement was used to determine the radial stress and force distribution through the cushion layer and later extended that into the cracking of the slab panel. The impact force generated by a rock vertically impacting on the ground was also studied experimentally [27] When it comes to cushioning a rockfall shed or similar impact resting structure laying a granular material on top of the structure is common. Soil has been used as the most common cushioning material in rockfall shed structures because of higher availability and lower cost [22,23,27]. ...
... The impact force generated by a rock vertically impacting on the ground was also studied experimentally [27] When it comes to cushioning a rockfall shed or similar impact resting structure laying a granular material on top of the structure is common. Soil has been used as the most common cushioning material in rockfall shed structures because of higher availability and lower cost [22,23,27]. Coarse granular sand [11,16,25,53,54] expanded clay aggregate [11], Sand [28], Cellular glass (Misapor) [12,16,17], Gravel [16,53] are some of other cushion materials that are experimentally studied. ...
Thesis
Protection against rockfalls occurring alongside landslides contribute to the major part of the disaster management budget in many counties like Switzerland, Japan and Hongkong. Protective structures are usually built over disaster trajectories to safeguard lives and properties. Reinforced concrete barriers that are fitted with gabions are one common form of installations to provide the protection. Few experimental investigations involving impact testings of a rigid reinforced concrete barrier which was fitted with a gabion cushion cover have been reported in the literature. But these investigations were limited to studying the localised actions of impact. The change of structural response behaviour of the barrier as a whole by the presence of a cushion layer is typically not within the scope of the reported investigations. Design methodologies that have been developed are typically limited to overly simplified calculations based on applying an equivalent static force to the barrier. To fill this knowledge gap full-scale pendulum tests have been conducted by the authors on a barrier that was fitted with a gabion cushion layer. The structural response behaviour of the barrier, contact force and tensile strains in the longitudinal reinforcement were of interests. Results recorded from the tests were compared with results from control experiments which were without the protection of any cushion materials. The introduction of a layer of cushion is shown to be able to have the deflection demand on the structure reduced by more than 70% when the amount of energy delivered by the impact is kept constant. An analytical procedure employing the Hunt and Crossley contact model, Swiss code model and two-degrees-of-freedom (2DOF) system modelling technique is presented for evaluating the flexural response demand behaviour of the cushioned barrier. The proposed analytical procedure is shown to be able to predict the reduced deflection demand with a reasonable degree of conservatism. At the end of the thesis, a simple hand calculation procedure featuring the use of design charts is presented for engineering applications. The procedure is illustrated by a worked example which is based on a realistic rockfall scenario.
... The penetration depth of an impacting mass has been studied by falling-weight impact tests on granular material (Gerber & Volkwein 2010). Fifty-four impact tests were carried out, using two blunt boulders with masses of 800 and 4000 kg, dropped from heights up to 15 m on two different ground layers. ...
... Falling-weight impact test on soil layer with hard underlying bedrock(Gerber et al. 2010) ...
... Celle-ci est obtenue en multipliant la masse de l'impactant par son accélération. Pour les essais d'impacts en général, les accéléromètres ont largement été employés ((Montani Stoel, 1998), (Gerber et Volkwein, 2010), (Lambert, 2007), ), testés et ont montré leur pertinence. Lambert (2007) ayant observé un problème de retour à zéro sur des accéléromètres piézoélectriques, trois technologies d'accéléromètres ont été testées lors des expérimentations à échelle réduite : piézoélectrique, piézorésistif et capacitif. ...
Thesis
Ce travail de thèse s'intéresse au comportement mécanique des merlons pare-blocs à technologie cellulaire. Ces ouvrages de génie civil sont construits en surélévation et placés sur la trajectoire potentielle des blocs rocheux, en amont des zones à protéger. Le caractère cellulaire est offert par l'utilisation de gabions (géocellule) alliant une enveloppe grillagée et un matériau de remplissage granulaire de type pierres concassées, mélange de sable et de déchiquetas de pneus ou encore ballast. L'absence de méthodes de dimensionnement relatives à ce type de structure a conduit à la création du projet de recherche ANR REMPARe alliant expérimentations et modélisations numériques multi-échelles. Dans le cadre de ce projet et dans la continuité des précédents travaux, deux dispositifs expérimentaux ont été développés, l'un permettant d'étudier le comportement d'un structure à échelle réduite constituée d'un assemblage de cellules, l'autre permettant d'étudier le comportement de merlons à échelle réelle. Il apparait que le comportement des structures est conditionné par les matériaux mis en ½uvre et par les conditions aux limites. L'efficacité des structures est évaluée à travers leur capacité à dissiper l'énergie d'impact en concentrant les dégradations au parement et en réduisant les déformations et les contraintes transmises à la partie arrière. Les résultats des expérimentations fournissent une base données pour le calage et la validation de modèles numérique développés en parallèle. Le retour d'expérience permet d'élaborer des prescriptions pour la construction, le suivi et la maintenance de ces ouvrages.
...  concrete slab with 40 cm compacted gravel cushion  concrete slab with 120 cm cellular glass cushion (see Schellenberg, 2008 andSchellenberg et a., 2008)  50 cm of non-compacted soil on top of bedrock  130 cm of non-compacted soil (see Gerber and Volkwein, 2010) Every test has been recorded by 6 co-linear deceleration sensors. The capacities of the sensors were 100 g and 500 g where g is the acceleration due to gravity. ...
Article
Full-text available
I n t e r d i s c i p l i n a r y R o c k f a l l W o r k s h o p 2 0 1 1 – I n n s b r u c k -I g l s R o c e x s : R o c k f a l l E x p e r t N e t w o r k INTRODUCTION Braking forces acting on moving masses can become very large, especially due to rockfall on bare concrete. In order to avoid such high forces, rockfall protection galleries are covered by gravel or some other damping material. An increased cushion depth reduces the impact forces further. However, additional factors influence the forces, such as the gravel's degree of compaction or its behaviour when rupture occurs. This paper shows the results of free fall tests on concrete slabs compared to those from identical tests except with a gravel cushion of different layer thicknesses. Figure 1: Concrete slab with gravel layer during compaction and 800 kg test block The measured boulder accelerations show interesting correlations between maximum deceleration, penetration depth (braking distance) and braking time. TEST SETUP Blocks of different weight were dropped from heights varying from 8 -15 m on different cushion materials. This contribution shows a comparison of results using an 800 kg block (see Fig. 1). The following series of free fall tests has been executed:  concrete slab with 40 cm compacted gravel cushion  concrete slab with 120 cm cellular glass cushion (see Schellenberg, 2008 and Schellenberg et a., 2008)  50 cm of non-compacted soil on top of bedrock  130 cm of non-compacted soil (see Gerber and Volkwein, 2010) Every test has been recorded by 6 co-linear deceleration sensors. The capacities of the sensors were 100 g and 500 g where g is the acceleration due to gravity. The measurement interval was 3 s at a sample rate of 5 kHz. Some of the tests have been recorded using a high-speed video camera at 250 frames per second. RESULTS The maximum values of the decelerations are plotted against the falling height (or impact velocity, respectively). The different damping materials on the concrete slab exhibit different deceleration rates of the falling boulder. Compacted gravel results in a maximum deceleration of 700 – 4200 m/s 2 , whereas a cushion layer consisting of cellular glass only results in 400 – 700 m/s 2 (Fig. 2). The maximum decelerations obtained for natural soil were 300 – 1600 m/s 2 (Fig.3).
Conference Paper
ZUSAMMENFASSUNG In der Versuchsanlage Walenstadt sind 96 Fallversuche mit würfelförmigen Betonkörpern durchgeführt worden. Unterschiedliche Massen sind auf verschieden starke, geschüttete Bodenschichten aus unterschiedlichen Höhen fallen gelassen worden. Dabei umfasste die kinetische Energie der Fallkörper einen Bereich von 20-1'200 kJ. Während den Versuchen sind die Beschleunigung resp. die Verzögerung der Fallmassen mit Sensoren gemessen worden. Aus diesen Messdaten konnten die maximale Verzögerung, die Eindringtiefe und die Eindringzeit ermittelt werden. Aus den Parametern Masse, Fallhöhe und Bodenstärke ist mit einer multiplen, linearen Regression die mittlere Eindringtiefe berechnet worden. Die entsprechende Formel wird im Beitrag dargestellt. Zusätzlich wird in diesem Beitrag auch ein Modell vorgestellt, mit dem der Abbremsvorgang in Abhängigkeit der Zeit erklärt werden kann. Dabei stimmen die Werte der Verzögerungen, der Geschwindigkeiten und der Eindringtiefen physikalisch überein. ABSTRACT 96 falling weight tests with cube-like bodies made of reinforced concrete have been performed at the WSL test site in Walenstadt. Different masses fell on dumped soil with varying thicknesses. The kinetic energy of the falling bodies ranged 20-1'200 kJ. During the experiments the deceleration and re-acceleration of the test specimen have been measured using fix attached acceleration sensors. From the measured data the maximum deceleration, the penetration depth and the braking time could be retrieved. The parameters mass, falling height and soil thickness were used to evaluate an averaged penetration depth through multiple linear regressions. This resulted in a suggested formula for practical application proposed in this contribution. Furthermore, this article presents a model to explain the braking process in dependency to the time. Decelerations, velocities and penetrations physically match.
Article
In the framework of rockfall trajectory modelling, the bouncing phenomenon occurring when a rock block impacts with the slope surface is the most difficult to predict, owing to its complexity and its very limited understanding. Up to now, the rebound is commonly quantified by means of (one or) two coefficients of restitution expressing the amount of energy dissipated during impact. These restitution coefficients generally are evaluated from a rough description of the ground material, whereas other parameters likely to influence the rebound phenomenon as the characteristics of the block itself and the kinematics are often neglected. In the framework of this thesis, two experimental campaigns have been performed in laboratory to acquire a better knowledge of the impact mechanisms governing the rebound phenomenon of rock blocks on granular (sandy) slopes and to quantify the discovered dependencies. About 200 impact tests on a small scale have helped to identify first the most significant impact parameters and to qualify their influence. Further, a half-scale testing campaign has been performed to quantify these influences. The impact of a rock block on a granular material is modelled for varying impact parameters, concerning: the ground material (internal friction angle, compaction) the block (weight, radius, shape) and the kinematics (slope angle, impact direction (vertical or inclined), impact velocity). The impact process has been filmed by a high-speed camera. The analysis of the block movement before, during and after the shock allowed to gather information concerning the impact process itself (velocity and acceleration of the block, penetration into the ground material, duration of impact etc.) and to determine a criterion for which the impact process is completed. By means of this criterion, the normal (Rn), tangential (Rt) and energetic (RTE) coefficients of restitution have been evaluated for the mass centre of the block according to the most common formulations (ratio of the normal or tangential velocities respectively the total energies before and after impact). The qualitative analysis of the small and half-scale tests proves that the rebound of rock blocks as well as the coefficients of restitution commonly used to characterise the rebound depend not only on the ground characteristics (material, slope inclination), but also on parameters related to the block (weight, geometry) and the kinematics (impact velocity and angle). A thorough observation of the impacts has shown that the block motion during impact is governed by three mechanisms (penetration, sliding, rotation), acting partly antagonistically. For different impact conditions, one or another of these mechanisms is privileged, governing on his part the block motion after impact. The quantitative interpretation of the half-scale tests leads first to a proposition of formulations expressing the maximal penetration of the block into the ground material, the maximal contact force and the rotation of the block acquired during impact. Parting from these formulations and inspired by the principle of the conservation of linear momentum, expressions for the normal and tangential component of the coefficients of restitution are developed. The implementation of coefficients of restitution defined by similar formulations as the proposed ones in rockfall trajectory codes should lead to a better prediction capacity of the latter and finally to a better delineation of areas at risk by hazard maps. Bei der Modellierung von Steinschlagsturzbahnen stellt der Abprall eines Gesteinblockes von der Geländeoberfläche den am schwierigsten vorherzusagenden Teil der Sturzbahn dar. Gründe hierfür sind sowohl die Komplexität des Abprallvorgangs als auch dessen bisher sehr begrenztes Verständnis. Die meisten Programme zur Berechnung von Sturzbahnen (Trajektorien) modellieren den Abprall mittels (ein oder) zwei Restitutionskoeffizienten, die den Betrag der während des Stoßes absorbierten Energie ausdrücken. Diese Restitutionskoeffizienten werden generell anhand einer groben Beschreibung des Hangmaterials abgeschätzt. Andere den Abprall beeinflussende Parameter, wie die Eigenschaften des stürzenden Blocks und die kinematischen Randbedingungen, werden dabei im allgemeinen außer Acht gelassen. Im Rahmen der vorliegenden Arbeit wurden zwei Versuchskampagnen unter Laborbedingungen durchgeführt, um den Stossvorgang und den Abprall von Gesteinsblöcken von granulären (sandigen) Hängen besser verstehen und mathematisch erfassen zu können. Mit Hilfe von mehr als 200 kleinmaßstäblichen Versuchen konnten die wichtigsten Stossparameter identifiziert und deren Einfluss qualitativ ermittelt werden. Für die quantitative Auswertung der Einflüsse wurde des weiteren eine Kampagne mit halbmaßstäblichen Versuchen durchgeführt. Der Zusammenprall zwischen Felsblock und granulärem Bodenmaterial wurde dabei für folgende variable Stossparameter modelliert: Bodenmaterial (innerer Reibungswinkel, Lagerungsdichte) Block (Gewicht, Radius, Form) und Kinematik (Hangneigung, Stossrichtung (vertikal oder schräg), Stossgeschwindigkeit) Der Stossvorgang wurde mittels einer digitalen Hochgeschwindigkeitskamera gefilmt. Die Untersuchung der Blockbewegung vor, während und nach dem Stoss ermöglichte es, Informationen über den Stossvorgang zu gewinnen (Geschwindigkeit und Beschleunigung des Blocks, Eindringtiefe in das Bodenmaterial, Dauer des Stoßes usw.) und ein Kriterium zu bestimmen, für das der Stoss als beendet erklärt werden kann. Mittels dieses Kriteriums wurden die normale (Rn) und tangentielle (Rt) sowie die energetische (RTE) Komponente des Restitutionskoeffizienten für den Schwerpunkt des Blocks nach den gängigen Formulierungen ermittelt. Dafür werden die normalen oder tangentiellen Geschwindigkeitskomponenten bzw. die Gesamtenergie vor und nach dem Stoss ins Verhältnis gesetzt. Die qualitative Analyse der kleinmaßstäblichen Versuche beweist, dass der Abprall sowie die Restitutionskoeffizienten keineswegs ausschließlich von der Bodenbeschaffenheit (Material, Hangneigung), sondern auch von Blockeigenschaft (Gewicht, Geometrie) und Kinematik (Stossgeschwindigkeit und –richtung) abhängen. Eine eingehende Untersuchung der Daten hat gezeigt, dass die Blockbewegung während des Zusammenpralls von drei teils antagonistisch wirkenden Mechanismen (Eindringen, Rutschen, Rollen) gesteuert wird. Je nach Stossbedingung überwiegt einer der Mechanismen und bestimmt so die Art der Blockbewegung nach dem Stoss. Bei der quantitativen Interpretation der halbmaßstäblichen Versuche wurden zunächst Formelvorschläge zur Berechnung der maximalen Eindringtiefe des Blocks in den Boden, der maximal wirkenden Kontaktkraft und der durch den Stoss erzeugten Rotation entwickelt. Darauf aufbauend und inspiriert vom Prinzip der Impulserhaltung wurden mathematische Formulierungen für die normale und tangentielle Komponente des Restitutionskoeffizienten entwickelt. Die Implementierung von ähnlichen Formulierungen in Steinschlag- Simulationsprogramme kann in Zukunft zu einer besseren Vorhersage der Sturzbahnen führen und in der Folge zu einer besseren Demarkation von Gefahrenzonen mittels Gefahrenkarten. Lorsqu'on analyse la propagation de blocs rocheux dans des versants montagneux, on constate que le phénomène de rebond qui se produit lorsqu'un bloc percute la surface du versant est très complexe et mal connu. Bon nombre de logiciels de trajectographie quantifient le rebond à l'aide d'un ou deux coefficient(s) de restitution fonction uniquement de la couverture du terrain, négligeant de la sorte l'influence de caractéristiques du bloc et de la cinématique. Dans le cadre de la thèse, deux campagnes expérimentales ont été réalisées en laboratoire afin de mieux comprendre et quantifier le phénomène d'impact et de rebond de blocs rocheux sur des pentes sableuses. Quelque 200 essais à petite échelle ont d'abord permis d'identifier les paramètres d'impact les plus significatifs et d'analyser qualitativement leur influence. Une campagne d'essais en semi-grandeur a ensuite été menée pour quantifier ces influences. L'impact d'un bloc rocheux sur une pente granulaire est modélisé en variant les conditions d'impact suivantes: le matériau de la pente (angle de frottement interne, compacité) le bloc (poids, radius, géométrie) et la cinématique (angle de la pente, direction d'impact (verticale ou inclinée), vitesse d'impact). L'impact est filmé par une caméra digitale d'acquisition rapide. L'analyse du mouvement du bloc avant, pendant et après le choc a apporté des informations sur le processus d'impact (vitesse et accélération du bloc, pénétration dans le terrain, durée de l'impact etc.). Un critère de fin de choc a été déterminé à l'aide duquel les coefficients de restitution normale, tangentielle et énergétique ont été évalués pour le centre de masse du bloc selon les définitions utilisées habituellement (rapport entre les vitesses normales ou tangentielles, voir les énergies totales avant et après l'impact). L'interprétation qualitative des essais à petite échelle et en semi-grandeur confirme que le rebond de blocs rocheux et les coefficients de restitution, qui traditionnellement le caractérisent, dépendent non seulement de caractéristiques du terrain (compacité), mais aussi de paramètres relatifs au bloc (poids et géométrie) et à la cinématique (vitesse et angle d'impact). Une observation attentive des impacts a mis en évidence que le mouvement des blocs est fonction de trois mécanismes (pénétration, glissement, rotation) partiellement antagonistes. Les conditions d'impact privilégient l'un ou l'autre de ces mécanismes qui à son tour conditionne le mouvement du bloc après impact. L'interprétation quantitative des essais en semi-grandeur conduit d'abord à la proposition de formules pour évaluer l'enfoncement maximum du bloc dans le versant, la force de contact maximale ainsi que la rotation acquise par le bloc au cours de l'impact. Ensuite, s'inspirant du principe de conservation de quantité de mouvement, elle aboutit à des expressions pour les composantes normale et tangentielle des coefficients de restitution. A terme, l'implémentation de telles formules dans des logiciels de trajectographie devrait se traduire par un accroissement de la capacité de prédiction des trajectoires de blocs rocheux et dès lors par une meilleure délimitation des zones à risques.
Wegleitung Objektschutz gegen Naturgefahren Vereinigung kantonaler Feuerversicherungen (Hrsg.), Bern. Heidenreich, B. (2004) Small-and half-scale experimental studies of rockfall impacts on sandy slopes Large-scale impact tests on rockfall galleries
  • Astra T Vogel
References ASTRA (1998) Richtlinie Einwirkungen auf Steinschlagschutzgalerien. Eidg. Departement für Umwelt, Verkehr, Energie und Kommunikation, Bundesamt für Strassen, Bern. Egli, T. (2005) Wegleitung Objektschutz gegen Naturgefahren. Vereinigung kantonaler Feuerversicherungen (Hrsg.), Bern. Heidenreich, B. (2004) Small-and half-scale experimental studies of rockfall impacts on sandy slopes. Ph. D. thesis no. 3059, Ecole Polytechnique Fédéral, Lausanne. Schellenberg, K., Volkwein, A., Roth, A. and Vogel, T. (2007) Large-scale impact tests on rockfall galleries. Proc. of the 7 th Int. conf. on shock & impact loads on structures. Beijing, China. pp. 497-504.
On the design of rockfall protection galleries. Dissertation Nr. 17924, ETH Zürich. SN 670 008a: Identifikation der Lockergesteine
  • K Schellenberg
Schellenberg, K. (2008) On the design of rockfall protection galleries. Dissertation Nr. 17924, ETH Zürich. SN 670 008a: Identifikation der Lockergesteine. Eingetragene Norm der Schweizerischen Nor-menvereinigung. Vereinigung Schweizerischer Strassenfachleute VSS, Zürich.
Large-scale impact tests on rockfall galleries
  • K Schellenberg
  • A Volkwein
  • A Roth
  • T Vogel
Schellenberg, K., Volkwein, A., Roth, A. and Vogel, T. (2007) Large-scale impact tests on rockfall galleries. Proc. of the 7 th Int. conf. on shock & impact loads on structures. Beijing, China. pp. 497-504.
Wegleitung Objektschutz gegen Naturgefahren
  • T Egli
Egli, T. (2005) Wegleitung Objektschutz gegen Naturgefahren. Vereinigung kantonaler Feuerversicherungen (Hrsg.), Bern.
Richtlinie Einwirkungen auf Steinschlagschutzgalerien
ASTRA (1998) Richtlinie Einwirkungen auf Steinschlagschutzgalerien. Eidg. Departement für Umwelt, Verkehr, Energie und Kommunikation, Bundesamt für Strassen, Bern.