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| Titel |
Acid test of joint technical and biological measures in slope stabilisation - Impact analysis of the heavy rainstorm event in August 2005 |
| VerfasserIn |
F. Graf, A. Böll |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250029731
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| Zusammenfassung |
| The persisting and heavy rainstorms from 20th to 22nd August in 2005 resulted in loss of
human lives and tremendous damage on infrastructure all over Switzerland. Many of the
measures taken hitherto to protect against such natural hazards were stressed to their limits or
even beyond due to water saturation of the soils and extreme discharges of the torrents. This
particular configuration offered the possibility to investigate the reliability of technical and
biological measures taken within the scope of slope stabilisation, torrent and gully
control.
In the context of a joint project the ancient sliding area “Schwandrübi” in Dallenwil
(Switzerland) providing joint technical and biological measures was chosen to address
aspects concerning the reliability of technical supporting structures, the development of
biological measures in the course of time and their performance under the extreme impact as
well as the effects of biological measures on the stability of slopes.
During 1981 and 1982 joint technical and biological measures had been taken on a large
scale with minor follow-ups shortly after to stabilise the "Schwandrübi”. The underlying
strategy was based on several pilot surveys as thorough soil analysis, e.g. grain size
distribution and determination of the angle of internal friction (Φ’) related to the porosity (n)
and the dry unit weight (γ), respectively.
Basically, the spatial arrangement of the gabions was in accordance with the theoretical
guidelines. However, based on the angle of internal friction (Φ’) determined on the loose
moraine soil material, it was not possible to meet the soil mechanical criterion of inclination
between the constructions in all cases. Regardless of the extreme impact during the
rainstorm (~100-year event), no serious damage occurred neither on the roughly
25-year old gabions nor on the torrent control structures. The recalculated peak
discharge in the outlet channel was ~60 m3s-1 superimposed by high bed load
transport.
The development of the biological measures that consisted of cuttings of Salix purpurea
and rooted plantlets of Alnus incana combined with a conventional seed mixture
(hydro-seeding) was most satisfactory. The natural re-establishment of Salix appendiculata –
rarely used in eco-engineering due to the low vegetative reproduction – reveals the
underestimated potential of this species. The rainstorm left hardly any destructive marks
neither on the vegetation nor on the soil surface. Root excavations in the field and laboratory
investigations confirm a high rooting degree. Furthermore, plant association analysis yielded
in considerable diversity and a species composition close to natural succession
stages.
The stability of the “Schwandrübi” was not affected during the heavy rainstorm although
some of the sections between the technical structures were too steep compared to the
proposed tolerable inclination. A new approach based on the experience of the
“Schwandrübi-project” offers the first time to explain this phenomenon with biological
effects on soil stability. Previously conducted laboratory experiments were validated with
field based investigations conducted in 2006 and confirmed the higher soil stability due to
mycorrhizal fungi and better rooting; in terms of an increase of the angle of internal friction
Φ’ of up to 5Ë .
The concept of joint technical and biological measures applied has proven suitable to
protect the ski resort above and the subjacent villages. There is evidence that the
eco-engineering approach of WSL shapes up as a successful strategy in the long-term,
although the complex interactions of the processes that control soil stability do not adjust
within 25 years to a steady-state. |
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