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Rockfalls on slopes and structures

Abstract:

Natural hazards are major concerns in Switzerland. Many natural hazards result from excessive rain, snowmelt or freezing and thawing processes, sometimes together with man made influences e.g. deforesting. Many are linked directly to geotechnical hazards by causing loss of stability of soil slopes or rock faces, leading to downhill movement of large masses of material. These moving masses can severely damage local structures and infrastructure such as roads, railway tracks or electrical lifelines. Typical examples of natural hazards are: - Rock falls - Creeping slopes and landslides - Debris flows - Avalanches. Local infrastructure and buildings must be protected against these hazards, and this can be achieved only partially by selective placement. Key infrastructure systems must be guided through narrow gaps, even where the risk of damage from these particular natural hazards is high. When possible, the infrastructure must be protected by means of galleries, nets, walls or dams. This research project deals with the influence of a significant rock boulder falling onto slopes, and the infrastructure that is located at the foot of these slopes. Contrary to most of the past research projects, which dealt with the trajectories (velocities) of the falling boulder, this proposal focuses specifically on the load deriving from the impact events and ideas for dissipating this impact energy by using novel cushioning materials. Multiple rock falls are excluded, as are the judgements of the stability of a rock face itself. Thus it links strongly into other research projects at both ETH's dealing with permafrost, creeping slopes, numerical simulations of rockfalls, laboratory scale testing due to vertical rock fall, and the application of nets as protection measures to shield infrastructure from rockfalls. Most of the experimental research work will be conducted in the newly established ETHZ Geotechnical Drum Centrifuge. This will permit existing gaps to be closed, for example between laboratory scale tests with limitations on mass of boulder, fall height, and hence the input energy levels (up to 100 kJ events), and the less frequent full scale investigations with high dependability on the specific site boundary conditions. Prototype energy scenarios can be simulated in the Drum Centrifuge using small-scale models, which may be exposed to enhanced gravity to replicate a full-scale event, by taking advantage of well-proven scaling laws. Impact energy conditions will be varied (up to 3000 kJ) and detailed impact stress distributions on top of a gallery can be measured under defined boundary conditions. A systematic variation in the system parameters: slope form, slope angle and slope roughness as well as boulder/block shape and mass allow the determining equations to be proven. Sufficient data will also be obtained for specific impact cases to carry out a probabilistic evaluation of impact loads and their likely standard deviation. New ideas for cushioning materials for damping the impact on top of the galleries will also be investigated. The physical modelling will be supported by analytical and numerical work on the overall behaviour of the system, and the local behaviour at the point of impact. Laboratory investigations will supplement the modelling, to determine the damping parameters of slope and cushioning materials as well as the integrity and response of the boulder and the associated parameters. Various scenarios will be treated in depth. The first phase starts with the modelling of one of the EPFL test series for the vertical freefall of a block onto a concrete slab (single rebound). The energy input levels will then be increased. In addition to the energies and forces estimated from e.g. accelerometer data, the stress distribution on a gallery will be measured using 2000 pixel pressure pads, allowing observation of the boulder footprint on the gallery roof. These results can be used to validate the centrifuge model data against earlier tests and to calibrate existing design and analysis codes. In the second phase, more complex slope-gallery systems will be modelled, by varying boulder impact locations, slope angles and initial energy inputs, to permit a more detailed analysis of prototype site conditions. Research on suitable cushioning materials will be carried out in the centrifuge (phase 3) to examine the energy absorption characteristics of soils or common recycled or composite materials, due to crushing or densification.

Authors:

Chikatamarla, Ravikiran and Laue, Jan and Springman, Sarah M.

Index Terms:

Rockfall; SoilGroup; Protection Structures; slope stability; rock sheds; micro mechanics; damping materials

Remarks:

The Project is funded by Swiss National Foundation.

Further Information: