Long-term studies of joint technical and biological measures

Bern

2009

S. 9-32

11 Abb., zahlr. Lit. Ang.

Artikel aus einer ZeitschriftUnselbständiges Werk

200159822

Systems used to stabilise steep slopes and gullies usually consist of technical and biological measures. While the lifespan of engineering structures principally depends on the mechanical properties of the building materials, the long-term behaviour of vegetated structures and biological measures is influenced by additional factors and interactions. Based on previous studies, the mechanical strength of 60-year-old wooden check dams was found to still average within the values defined by the now relevant Swiss timber design code. The two steep scree slopes Schwandrübi and Valzalära-Gazi have been documented and investigated since their stabilisation in the early nineteen eighties. They were stabilised by combinations of gabion walls, timber log crib walls and biological measures. As such combinations are quite often used in modern eco-engineering, the two investigation areas may be considered as fairly representative. Not unexpectedly, the gabions were still fully functional after about 25 years. The timber of the log crib walls, on the other hand, had considerably deteriorated and the future stability of the structures is assumed to depend on the surrounding vegetation and/or on the plants growing in the interstices. The actual gradients of slopes successfully stabilised by biological measures were compared with the decisive angle of internal friction of the soil and with the angle of internal friction of planted soil (laboratory samples). It could be shown that the laboratory results, which rated the increase in the angle of internal friction by vegetation effects at about 5°, corresponded well with the actual slope gradients. Whether it is possible to draw conclusions about the long-term behaviour of eco-engineering measures on the basis of investigations carried out 25 years after installation is open to question. Generally it depends on the specific measures included, but vegetated log crib walls and steep slopes stabilised by biological measures are likely to undergo further change in the future.