The task was to critically evaluate protection measures in the near vicinity where we (the students) live. Therefore, a literature research with past and present events were asked and the assessment of the effectiveness of the protection measure chosen. Finally, our opinion was required about eventual reviews of the existing protection measures in view of climate change and risks in future.

Location

Slope Stability Measures in the Mülenen Gully, Saint Gall City

The Mülenen is situated in a gully formed by the river Steinach on the south slope of the valley where Saint Gall is situated in.

Map (1:1829) of the Mülenen gully with the river Steinach. The upper level at the crossing Falkenburgstrasse/Gottfried-Keller-Strasse (green star) is at 740 m above sea level and the end (green star) where the Steinach river disappears in a covered channel (green star) is at 680 m. The red letter “n” indicates net locations and “b” the site of the path fixed with the nails.

The water of the Steinach river formed the gully into the upper marine molasse [1] of the sedimentary deposits. The formation of the Alps and the conglomerate “Nagelfluh” is discribed here including some sketches. Nagelfluh consists of rounded stones and pebbles backed together by concrete. Although the conglomerate is stable, erosion occurs by freeze and thaw cycles, which loosen the material and provoke a breaking off of the rock.

The steep walls of the Mülenen gully are formed by conglomerate. At the Swiss foothills of the Alps it is called Nagelfluh. It consist of round stones and pebbles backed together by concrete.

Protection measure by a net prevents people from rockfall hazards.

Natural hazards
The synoptic hazard map of the Mülenen gully area indicates falling, sliding and water involving processes. Choosing St. Gallen Stadt at www.geoportal.ch and inserting “Mühlenweg” at the same link will provide all information about different risks at the Mülenen gully.

Hazard map of the Mülenen gully where red indicates high hazard, blue moderate and yellow residual danger. Falling, sliding and water based hazards are indicated. If different hazard areas overlay, the highest danger is relevant. The blue area is originating from water driven hazards, the yellow from land slides and the red from rock falls.

To get an overview of the hazards in the Mülenen gully, not ony the hazard map but also the event cadastre is relevant. The geoportal also provides information about avalanches, floods, rockfalls, and landslides from 1974 – 1999. For the area investigated here, no event was recorded. The only source of threat is at the ”Untere Mühlenentreppe” (see map above) in the information of the board decision from 1980 [2]. Desintegrated pieces of the already sprayed Nagelfluh wall with a concrete layer in 1950 put pedestrians at the Mühlenstrasse at risk. Some pieces got mobilized in thaw weather in spring and fell 13 m down on the roof of house 17 (see map above) and on the Mühlenstrasse.

Protection measures
The protection measures for the natural hazards in the Mülenen gully are indicated in the map. The city council installed nets (see photos above), nails and left forest in order to prevent pedestrians from rockfalls and landslides. Also the Steinach river is partly put in an open channel to ensure a save passage of the Mülenen gully and to preserve the houses and infrastructure.

The nails inserted into the wall in 1980 have a distance of two meters between each row and are four meters long. The nails directly below the pavement are not visible any more. The protruding ends of these nails might have been cut and covered by concrete. Below the concrete wall, a net is installed. Originally, two beams with nails in a row were planned immediately below and about four meters the path. Between the two beams humus should be sprayed to the wall It was also planned to spray some humus to the wall after some excavation to plant some bushes and shrubs. Obviously, the planned bushes and shrubs could not grow.

Concrete wall fixed with nails below the path in order to stabilize the pavement. To preserve the construction from erosion drainages were installed below the wall.

Installed net below the nailed wall to protect pedestrians and house 17 from rockfall. Note the concrete sprayed on the Nagelfluh to the right of the wall. Some water left a round peeled area where erosion can occur.

Effectiveness of the protection measures
I think the protection measures (nailed wall and nets) in the different locations (see plan above) are appropriate and effective. The wall effectively prevented the path from sliding down because the tarred pavement is still intact. However, it is possible the the pavement could have been replaced in these more than thirty years after construction. The nails below the path are short but long enough to hold the wall at the Nagelfluh. They have the purpose to fix and strengthen the wall and are not a retaining wall nor a protection measure against sliding processes because there is only one layer or rather bed-rock, the Nagelfluh. The hard and stable Nagelfluh material is appropriate to install nails. As pointed out above the groundwater is controlled by the drainages under the wall.

In contrast to anchors, the bearing capacity of nails can not be controlled. As deformations of the nails mostly occur during and after construction, it was probably controlled during installation 30 years ago. The resilience of the beam seems to be sufficient, because twelve nails are installed in three levels and are still intact. Nevertheless, the nails are probably not a permanent construction. If so, they would have a corrosion protection which is obviously not the case. The protruding ends of the nails are totally covered with rust.

The channel below the nets (see picture with the rockfall sign), where the decelerated pieces fall in, are constantly emptied by the street workers of the city. The nets are made of high-tensile steel wire with a high tear resistance of 1770 N/mm2. The curtains are installed steel width of 3.5 x 30 m. One width is carefully fixed with the next one by a stainless steel rope woven through the overlapping ends (see picture with the rockfall sign). Some nets are almost invisible if the space between slope and net is planted. I consider the wall and the planted nets an equal appropriate and effective protection measure as the net. Plants between net and Nagelfluh only grow, if the slope is not vertical. Thus, plantation, though enhancing the attractiveness of the rural city, is not possible at any slope.

View downhill: The plantation of the slope makes the net almost invisible. The slope looks naturally
planted from the remote view.

View uphill: The net is only visible at view places and only for the attentive
observer.

Influence of climate change
The climate change could have an accelerating influence on the loosening of the stones out of the Nagelfluh because temperatures in summer and winter might reach extremes. If climate change provokes an increase of rain events the installed drainages at the beam might be too small for the plus of water. The wall might crack and not be stable any more and more of the sprayed concrete could peel off the Nagelfluh. A constant observation of the protection measures, not only due to the climate change but also due to the age of the nailed wall by the responsible persons of the municipal works service will be necessary to assure save streets and paths in and around the Mülenen gully. Maybe the nailed wall must give way to a stronger protection measure in future as for example an anchored retaining wall including some excavations. Or the “untere Mühlentreppe” path must be closed. Similar to the nails which might be replaced by a stronger protection measure, nets might be replaced be retaining walls as for example bore pile walls or gravity masonry. In addition, the forest in the Mülenen gully might be renewed especially in the light of climate change where storm events, droughts, extreme temperatures and increased carbon dioxide might choke trees.

Gravity masonry which has the same function as a net. The authorities from the Civil Engineering Office of the city of Saint Gall decided that a wall would better fit into the picture of the residential area than a net.

Acknowledgments
I thank Mr Schmid from the municipal works service for his time for the interview, the little excursion in the city to different protection measures and for going into the archives of the municipal works service for the plans. I also thank Mr Brändle (municipal works service) for the hazard maps because geoportal was not yet existing in February 2011. Further I thank Mr Dietschi from the comittee for natural hazards for the discussion on the phone.

References
[1] Szönyi, M., 2007. Geoland Schweiz; Landschaften entdecken – Natur erfahren. vdf Hochschulverlag AG an der ETH Zürich, p 336, ISBN-13:978-3-7281-3077-8
[2] Beschluss Nr. 3701 vom 18. März 1980 des Stadtrats St. Gallen: Sanierung der Nagelfluhwand an der Mühlenstrasse