Design of protection structures against debris flows and rock avalanches

Natural hazards such as floods, gravity driven transport of solid matter, debris flows and rock avalanches or rock falls are often encountered hazards in Alpine region. Protective structures can either prevent their trigger or protect settlements and lifelines by distracting or stopping such movements. This requires the absorption of high energies by a protective structure.

A protective structure is first loaded by a dynamic force. After the impact, a static load acts upon the protective structure. One of our research focuses is the determination of the dynamic and static loads on protective structures, as well as the prediction of the run-out areas of mass movements. To this end, model tests are carried out and then compared with numerical predictions. Material and geometry variations of the impact on rigid and flexible barriers are investigated in various experimental settings. The measured pressures, flow heights and flow velocities are used to verify existing design approaches. The results of the model tests can also verify the existing design approaches specified in national and international codes for the design of protective structures. Gravitational mass movements are extraordinary natural phenomena, whose frequency increase with global warming. The course of a mass movement is examined by means of a model experiment.

researchers involved: R. HofmannS. Berger

research center: Alpiner Raum, Geotechnik und Naturgefahren


Ermittlung Schutzbauwerke Massenbewegungen 

Protection structures against rock fall

The foundation of rockfall and avalanche protection structures are normally provided by micropiles loaded in tension and compression. The bearing capacity (ULS ultimate limit state) of a micropile is normally taken from empirical values or from static load tests (field tension tests). Tension tests on micropiles are currently regulated in the standards ONR 24806 (Permanent technical avalanche protection - Design of structures) and in ONR 24810 Technical protection against rockfall - Terms and definitions, effects of actions, design, monitoring and maintenance). In Austria, the basic principles for the behaviour and design of micropiles for control structures against rock fall, torrents and avalanches are based on experience with static loading from structural engineering.

Our research intends to provide a recommendation for the modification of the usual safety level in structural engineering (partial safety factors on the resistance side) for the anchoring of rockfall control nets with micropiles. A detailed knowledge of impact loading on piles from field tests is still lacking.

A further objective of the research project is to survey international experience, damage analysis and the state of the technology in foundations. Field tests (in soil) will be carried out with micropiles of various lengths, with static and impact loading. Selected structural elements will be instrumented. From the behaviour of the micropiles under impact loading, the formulae for the limit state of bearing capacity (ULS) can be adapted. The difference between solid bar systems and hollow bar systems will be investigated from the impact loading carried out in the field.  This would make an optimisation of the test procedure in difficult terrain conceivable. Based on the field tests (in soil) and the research work, recommendations will be worked out for the regulations in the standards applicable to the planning, structural design and testing of micropiles.

researchers involved: R. Hofmann, A. Steinwender

research center: Alpiner Raum, Geotechnik und Naturgefahren


Erosionsproblematik im Dammbau 



Rockfall simulations

A further research project at the Unit of Geotechnical and Tunnel Engineering refers to  rockfall simulations. In cooperation with the WLV (torrent and avalanche control authority) sensitivity analyses are carried out using usual software. The analysis is carried out with different masses or different numbers of elements (stones). Furthermore, the influence of rock shape, angle of impact, rock orientation and slope material properties is examined. The results can then be used for future calculations and investigations in practice.



Actions on torrential barriers

Torrential barriers protect steep alpine valleys from further erosion. We investigate the water pressure acting upon such retaining walls. Evaluation of damages and field investigations show that the retained earth material is in many cases not water-saturated. Consequently, a full water pressure needs not to be taken into account in the static design. The related ambiguity is also mirrored in the various design recommendations. The field observations can be explained by the setup of the retained earth but also from a consideration of the distribution of energy head. We propose an empirical assign of the loads acting upon torrential barriers which takes into account the probability of the several loads and exposures.



New Erosion Testing System

Erosion in the subsoil occurs particularly in dam constructions and seriously affects their stability. In connection with insufficient sealing seepage may set on and   cause a grain rearrangement and subsequent piping, which may lead to catastrophes. In the laboratory, a new test facility is used to examine the erosion stability of various soils. The testing system has been designed to investigate erosion of soil under triaxial compression.  The basis of the apparatus are two fully automatic universal testing machines with load and displacement control. The large test frame with an axial load of up to 250 kN offers the possibility to install specimens with up to Ø 300 mm, the smaller test frame with up to 60 kN is suitable for specimen diameters up to Ø 150 mm. One cell is equipped with a measuring technology for recording moisture and runtime measurements (wave propagation) by an ultrasonic system. Additionally, the deformation can be measured. A second innovative test cell is used to validate the test results by computer tomography, which is made possible by the newly developed x-ray transparency of the test cell and measurements under stress conditions. Normal stresses, pore pressures and hydraulic gradients can be applied to both test cells. Pressure-resistant storage tanks up to 500 l are provided for permeability measurements. A pressure-resistant measuring system is integrated to record the eroded material and determine the grain size. The apparatus is equipped with an automatic pressure control system to generate the cell or pore pressures as well as the hydraulic gradients. All system modules as well as all test procedures are controlled fully automatically via the control and data acquisition software GEOSYS.


Geotechnical-engineering-biological embankment laboratory = digital slope

Road embankments are subject to considerable damage, especially after rainfall events. The high rehabilitation costs are related to the road drainage, the amount of rainwater, the interlocking of the topsoil layer with the embankment material, the embankment slope, the water retention capacity (of the humus layer and the embankment material), the maintenance, the planting, etc. In this context, the biodegradability of natural fibre products for erosion control is of crucial importance. In order to reduce the damage on motorways, protection against water erosion by greening as well as surface protection depending on the dam construction, slope inclination and exposures will be dealt with in more detail in a research project, in the course of which the first geotechnical-engineering-biological embankment laboratory will be designed under natural conditions with a high degree of digitalization. An essential point is that the decisive factor precipitation and the associated water balance of the embankment can be modelled by means of irrigation. All parameters will be recorded automatically and in real time (=digital slope). The aim of the research project is to investigate different construction methods under comparable and repeatable conditions with regard to erosion protection or erosion susceptibility. In a field test, it will be found out which earthwork measures in combination with which engineer-biological safety measures prevent or reduce slope erosion most sustainably.


Digitalization of the pile production

The aim of the research project in cooperation with industry is to develop a correlation between the production times of cased bored piles and the properties of the ground. A consideration of the different drill bits, casing systems as well as drilling equipment is the basis of the evaluations. The different production protocols of piles in different geological conditions will be evaluated. Supplementary geotechnical laboratory tests and model tests are planned. Documentation and sampling at current pile construction sites are useful to improve the interpretation. On the basis of the performance records during the production of cased bored piles, the soil properties and the drilling equipment, a forecast of their behavior is to be made in conjunction with numerical calculations.










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