Mechanics of shallow landslides (Hangmuren) and interaction with flexible barriers
Abstract
It is the aim of this project to develop new flexible barrier technology for use as a mitigation measure against shallow landslides, an important natural hazard worldwide Shallow landslides are mixtures of water, soil and debris that initiate on steep slopes during periods of intense rainfall. Infrastructure, buildings, roads and railways are thereby threatened with destruction or closure. In the past, flexible barriers have been used to mitigate rockfall hazard and in recently they have been modified to stop channelized debris flows. Presently, there is considerable interest to apply flexible barriers to efficiently stop shallow landslide flows, especially after the recent extreme precipitation events in Switzerland and elsewhere in the world. However limited data indicate that the loading produced by shallow landslides differs significantly from previous applications, suggesting that additional research is necessary as a basis for optimal design. The project requires (1) quantifying the loads that a shallow landslide exerts on a flexible barrier (2) designing the barriers to withstand these forces on open (non-channelized) hillslopes and (3) optimal energy dissipation mechanics. The research project scientifically addresses these questions using a combination of full-scale field tests, laboratory experiments and numerical modelling, thus opening the door to a new market.
Scientific goals
In the past, the corresponding research of the WSL has concentrated on understanding the mechanics of extreme (large-sized, say 100’000 m3) rapid mass movements. Large mass movements can travel kilometers from their origin. In this project, we plan to initiate a new research direction. We propose to investigate the interaction of small (say 100 m3) sized rapid mass movements of snow and debris avalanches with protective nets. This requires a basic understanding of their flow mechanics, primarily near the source of their initiation. Although small, these dangerous events have fall heights of only a few 10s of meters, yet have the potential of destroying infrastructure and blocking transportation ways. For example, in August 2005, at least 5000 shallow landslides occurred principally in the regions Zentralschweiz and Berner Oberland. Average flow volumes were less than 200 m3 and fall heights less than 100 m.
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Figure 1: Examples for small landslide stopped by ringnet barriers which were dimensioned for rockfall protection. Landslides produce an aerially-distributed load rather than a point load as with rockfall, so a new dimensioning procedure is necessary.
The scientific deliverables of this project are:
a. Full-scale landslide dynamic tests. We plan to conduct full scale field tests with 100 m3 mixtures of water and debris. The tests will be instrumented with basal shear and normal force plates and pore water pressure sensors, similar to those installed at the Illgraben test site for debris flows. In addition flow and deposition heights will be monitored using ultrasonic sensors. Front velocities will be captured using video-grammetry. At the end of the acceleration zone, nets will be placed and cable and support forces induced be the shallow slide will be measured. We plan to perform 10-20 full scale experiments, depending of the results from the initial tests and the need for additional experiments to fine-tune the ringnet barrier structure for optimum performance.
b. Shallow landslide flow dynamics. Using the experimental results, a runout model for shallow landslides shall be developed and implemented into the existing software package RAMMS (http://www.wsl.ch/hazards/ramms). Energy dissipation in the body of the landslide will be investigated using the force and velocity measurements. In particular, the role of random particle fluctuation energy at the basal shear plane will be studied as a possible mechanism explaining pore water pressure fluctuations and the frictional behavior of the sliding material. Validated with the experimental data, the numerical models will enable the calculation of velocities and flow heights of the flow over time. Such a model would have a wide application area, outside the scope of this project.
c. Load model / Physics of fluid debris interaction with flexible and porous structures. Considerable research has been invested in finding appropriate load models for rigid and solid structures (masts, pylons, walls, dams) under the action of rapid mass movements, primarily snow avalanches. In these investigations the granular properties of the flow (granule hardness, size, free path length, velocity, continuum density) are needed to understand the interaction. However, the interaction with flexible barriers poses additional questions. The interaction of fluid-granular systems with flexible barriers has been the object of last KTI project between the WSL and GEOBRUGG. The impact between the barrier and debris flow front, the in-filling process as well as the dewatering phase was experimentally documented. Appropriate engineering models for the net and anchor forces have been developed and are currently under internal review and further testing for the final project report expected during summer 2008. The nets can effectively stop the flow material when they can be anchored in the channel. Shallow landslides, because they often occur on only weakly channelized or open slopes have different loading characteristics, both in magnitude as well as in spatial and temporal dimensions. Although smaller in mass, flow velocities might be larger than in typical debris flows (the maximum debris flow velocities measured in Illgraben are typically up to 5 m/s). Furthermore, the loads are expected to be distributed over a wider area, due to the typically-observed unconfined spreading of the landslide. It is also not clear whether the nets should be designed for a single and intense surge, or for multiple surges or surges, including surges from neighboring hillslope segments. A “design” scenario, consisting of different load cases, based extensively on the fundamental research deliverable must be developed. Creating a load model for shallow landslides requires a fundamental understanding of shallow landslide dynamics and their geomorphology. The WSL has extensive shallow landslide databases which will be used to in the design of the multiple-surge design scenario.
Innovations brought into the project
The protection from shallow landslides has been accomplished mostly by so-called active protection systems. This means that structures are inserted on the hillslope to prevent landslide release. This is the reason why existing research on shallow landslides concentrates mostly on the triggering process.
If active measures are not possible, so-called passive ones such as flexible fences that prevent the material from flowing over streets or railway tracks are required. However, there is a lack of knowledge on how to dimension such barriers, especially regarding the interaction of the loads with a flexible barrier that guarantees an effective soft stop of the material.
The new aspect of this project is therefore to consider a passive protection system, thus stopping a triggered shallow landslide This requires (a) the modelling of the sliding process itself including parameters such as sliding mass, volume, maximum velocity, release-zone thickness, slope geometry, run-out distance, water content, etc. (such parameters are useful for describing a landslide for natural hazard zoning) and (b) to model the interaction between a moving landslide and the modelled barrier.
Flexible barriers were originally developed to restrain falling rocks. Previous research has extended their energy absorbing capacity by the factor 15 in the last 20 years. Actual research additionally finds design models for flexible barriers used against debris flows (KTI-Project 7493). However, despite the existing knowledge of the barriers against above natural hazards the proposed project is about to investigate a completely different situation:
- Rockfall: Flexible barriers against rockfalls are mostly built like a fence along a traffic route (30 - 1000 m) supported by posts. The falling rock loads is a point load meaning that the designer can activate the energy absorbing elements from along the whole barrier to stop the rock.
- Debris flows: The barriers are mostly installed within a rather narrow river bed (10 – 30 m) anchored to the river banks. The debris flow acts on the complete net area defining the acting forces and energies that have to be absorbed.
- Shallow landslides: Compared to the two above processes barriers against shallow landslides are constructed as fences similar to rockfall nets. However, during an event the barrier is filled completely as it would be the case for a debris flow barrier. But due to the length of the barriers, the impact energy that has to be absorbed is much higher, requiring new design procedures. The other novelty of using flexible barriers against shallow landslides is that we cannot use the existing models for back-filling of the barriers found in the debris flow project. Debris flows usually occur in strongly channelized settings (e.g. gullies or torrent channels) with a comparatively small slope. In comparison, shallow landslides are typically unchannelized and occur on very steep slopes resulting in a completely different flow and filling processes.
Previous research with debris flows and avalanches at the WSL/SLF have allowed us to develop computational procedures to describe the stopping process and distribution of forces in the interaction between barriers and channelized debris flows The resulting load definitions and barrier response models are in a final testing stage within the KTI-Project 7493.2. Models used in hazard process simulation software developed at the WSL/SLF (e.g. DBF-1D, Aval-1D, -2D) have already been tested and allow for physically-plausible modelling of an unconstrained landslide. An additional in-house software package allows for a detailed analysis of the distribution of forces in flexible barriers (special developed Finite Element software FARO). Combining these existing modules will therefore provide the basis for a new simulation program that enables the modelling of the interaction between a landslide and a barrier. This net result will be a physically-based model of the loading and filling process and the coupled deformation of the barrier: This is the basis for proper engineering design. Additionally, new knowledge will be obtained on the dynamics of a landslide delivering a better understanding of dissipation processes that are responsible for the friction properties of the mass movement.
Laboratory experiments are difficult to properly upscale to the field, but nevertheless are very useful to study details of some processes with a degree of control that is impossible in a limited number of more expensive field experiments. The test facilities at WSL/SLF in Birmensdorf and Davos are well-suited to this task and will be modified to conduct such experiments. The tests planned now will deliver additional insights and results on important parameters such as the water pore pressure, shear and normal forces, the significant particle size, the friction angles of the materials.
Finally, the planned full-scale experiments facilitate model corroboration and validation during the development process. Experiments at this scale have not been performed, providing both a unique scientific challenge and an opportunity to test the models at a physically-realistic scale. The test site guarantees a reproducibility of the experiments and its general design and location enable snow avalanches experiments in the winter time.
Motivation of/for industrial partner
Strong rainfall events saturate hillslopes and can trigger hazardous landslides. Meteorologists assume that extreme precipitation events are likely to increase in the future (Global Climate Change) in Switzerland and elsewhere in the world. Protection authorities now believe that the number and severity of shallow landslides will increase, requiring suitable protection measures for roads and railway lines, analogous to the similar barriers are applied against rockfall. Protection against shallow landsliding is also required for existing buildings. National and international clients and governmental institutions (e.g. BAFU or SBB) are now investigating the use of flexible barriers. However, these organizations are not willing to apply these cost-efficient systems without field testing and design guidelines. Therefore, there is a dire need to develop cost-efficient systems and design procedures that address the concerns of potential customers. Research in hillslope debris flows has, in the past, centered on finding triggering thresholds. The problem of event size, motion and impact pressures has largely been ignored. The goal of this project is to address these questions.
Possible goals of the CTI project
- Develop and design flexible slope failure protection systems with high-tensile spiral rope net SPIDER and/or TECCO mesh (impact range from 10 up to 100 kN/m2 for areal loads and 5’000 kJ for punctual loads; width at least 30-100 m; system working height up to 7.5 m)
- Finite element design model for the construction and dimensioning of flexible shallow landslide barriers including validation for areal impact loads and barrier functioning based on 1:1 field tests. This model shall also include the finite element design model for SPIDER net and TECCO mesh.
- Calibration based on quasi-static element tests for determination of new net types and new brake element parameters.
- Validation based on full scale field tests with 50 m3 and 100 m3 impact loads.
- Determination of required system height and retention capacity for expected slope failure volumes.
- Concept for cleaning and maintenance of slope failure barriers in order to determine life cycle costs.
Difference between Debris Flow Barriers and Slope Failure Protection Systems In contrast to a debris flow barrier, the shallow landslide barrier will be applied on unchannelized hillslope surfaces against landslides, not against channelized debris flows. In many applications it is anticipated that it will not be necessary to have a strong barrier (using ring nets as in the rockfall and debris flow applications) Landslide mitigations nets must be light in weight and easy to install over longer distances (in lines). The shallow landslide barrier will also have many posts, special ground plates and lighter mesh or net. It is intended to use the SPIDER spiral wire rope net which was developed by Geobrugg for rock slope stabilization measures. But in order to develop such a barrier and to prove that this net or mesh is suitable, it is inevitable to know the interaction between shallow landslide and flexible construction and also be able to model the system with a FE-software (especially a code for the SPIDER net and TECCO mesh). With this and the dimensioning knowledge, it is possible to create a further business unit as done with the debris flow barrier (and the KTI-project 7493.2 EPRP-IW).
There are various fundamental differences between those two protection systems:
- different impact processes and load transfers,
- protection functioning and concepts,
- completely different layout of protective structure,
- other foundation and anchoring methods (e.g. pressure plates for loose soil, eventually new anchor types),
- introduction of new and more cost-effective protection nets: SPIDER net and TECCO high tensile strength mesh,
- dimensioning as well as modelling of impact forces and load transformation processes (including new net and mesh types)
Samples of different net meshes
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ROCCO ringnet
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High tensile strength TECCO wire mesh
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Spiral wire rope SPIDER ne
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