Creep of Snow-Supporting Structures in Alpine Permafrost

Creep of snow-supporting structures in alpine permafrost

M. Phillips, S. Margreth & W.J. Ammann Swiss Federal Institute for Snow and Avalanche Research (SLF), Switzerland

ABSTRACT: Snow-supporting structures are built as defence measures in avalanche starting areas on 30–50° slopes. In the Swiss Alps, over 500 km of structures protect settlements, roads and railways. At high altitudes, ground cover typically consists of scree, and in alpine permafrost terrain, this unstable rocky material can contain ice. Snow-supporting structures are anchored at depths of 2–8 m according to the bearing capacity of the ground and the thickness of the scree. A considerable anchor length can be located in the rock-ice mixture. Construction of snow-supporting structures is not advisable if creep rates exceed 5 cm a1, as the maintenance costs are too high. In order to investigate the behaviour of the structures in creeping alpine permafrost terrain, slope movements and structure displacements have been monitored at three sites since 1997. A lasting efficiency of snow- supporting structures is important for optimal avalanche protection and for economic reasons.

1 INTRODUCTION

The importance of snow-supporting structures as avalanche defence measures was demonstrated when up to 5 m of snow fell in the Swiss Alps in the course of February 1999. Over 500 km of snow-supporting structures successfully protected settlements, roads and railways by retaining the snow in avalanche starting areas. Efficient protection with such large snow loads is only possible if the structures are in good con- dition and well anchored. Damage to snow-supporting structures occurs if creep rates are high on steep slopes and in areas of intense rock fall (Stoffel 1995). High creep rates are particularly associated with steep Figure 1. Map of Switzerland showing the locations of alpine permafrost terrain, where the ground cover typ- test sites 1 (Pontresina), 2 (Arolla) and 3 (Randa). ically consists of a mixture of scree and ice (or water in summer when the ice melts in the active layer). In order to avoid excessive damage, the Swiss Federal Table 1. Characteristics of Sites 1, 2, and 3. Guidelines for the Construction of Snow-supporting Slope Scree thickness, Structures advise against construction if creep rates Site Altitude (m) Orientation angle rock type exceed 5 cm a1 (BUWAL/WSL 2000). Anchors are 2–8 m long, according to the bearing capacity of the 1 2930–2980 NW 38° 1.8 m (gneiss) ground and the thickness of the scree, so a consider- 2 2950–2970 NE 39° 2.7 m (dolomite) able anchor length can be located in the unstable 3 3010–3140 ENE 39° 2.5 m (gneiss, scree-ice mixture (Thalparpan 2000). The base of the quartzite, marble) anchor (at least 2 m) should be in bedrock. In order to monitor the performance of snow- supporting structures in steep permafrost terrain, mea- 2 SITE DESCRIPTIONS surements are effected at three sites around 3000 m ASL in the Swiss Alps. They are located above Pontresina All three sites are equipped with avalanche defence (Muot da Barba Peider, Site 1), above Arolla (Mont structures: Site 1 with both snow-bridges and snow- Dolin, Site 2) and above Randa (Wisse Schijen, Site 3) nets, and Sites 2 and 3 with snow-nets only. The most (Fig. 1). Slope deformation, structural stability, snow important site characteristics are summarized in Table 1. distribution and ground temperature have been moni- The volumetric ice content of the scree at Sites 1 and 2 tored there since 1997. varied between 5 and 10% (Phillips 2000) but has not

891 been determined for Site 3. Scree thickness (determined resistant mortar, with a compressive strength of at least by drilling) is less than 3 m at all sites. 35 N mm2, has to be used to inject the anchor bore- Two types of snow-supporting structure are used: holes. snow-bridges and snow-nets (Fig. 2). The latter are The cross-sections and types of foundations and typically used in areas affected by rock fall, due to their anchoring for the two types of structures are shown flexibility (Margreth 1995) and are particularly well in Figures 3 and 4. adapted to creeping permafrost terrain (Thalparpan 2000). Various types of anchors and foundations are 3 METHODS used, including steel rope anchors, micropiles, tubes (all are 4–6 m long on the test sites), and plates. Frost The positions of 101 foundations and anchor heads are surveyed yearly, with a Wild TC 1610 theodolite (precision 2 mm). They are equipped with fixed pins (type “SBB Bolzen”) onto which reflectors can be mounted (Fig. 7b). Reference points are located in the surrounding rock walls as well as on buildings such as churches in the valleys below. Additional vertical boreholes (5–10 m) were equipped with inclinometer tubes at each site. The Figure 2. Snow-bridge (left) and snow-nets (right). upper 2–3 m of the tubes are in the scree and the remainder of the length is in the bedrock. Deformation measurements are carried out in the tubes once a year supporting beam crossbeams using an inclinometer (Sinco Digitilt) with a precision hinge of 0.15 mm m1. The measurements were made at 1 m depth intervals. An automatic camera and snow gauges were used to telescopic post monitor snow depth at Site 1. Snow depth was mea- micropile sured automatically every 30 minutes 750 m NW of Site 2 by a weather station of the “Intercantonal measurement and information system” network (IMIS), using a Campbell SR50 Ultrasonic Distance Sensor (precision 2.5 cm). Site 3 is inaccessible in winter, so there no data are available. However, there is an micropile IMIS station located 8 km N of Site 3 at the same alti- plate foundation tude and those data give a general indication of the snow conditions in the area. Gaps in the data are due to technical problems or to the fact that the IMIS sta- Figure 3. Snow bridge with micropiles and a plate tions were only built in the course of the measurement foundation. period. Ground temperatures were monitored in boreholes ranging between 5 and 18 m depth at Sites 1 and 2, hinged using thermistors (YSI 46008) with a calibrated preci- support wire cable net sion of 0.02°C. cable anchor span anchor 4 RESULTS guy head micropile The average downslope displacements of the anchor stabilising heads and plate foundations at each site are shown in tube Figure 5A. Table 2 shows the mean annual displace- micropile / ment of the foundations and the maximum values waisted tube measured for individual foundations in each category. cable The cumulative deformation of each borehole at 1 m anchor depth is shown in Figure 5A. Snow depth is illustrated in Figure 5B, and ground temperature at 1 m depth for Figure 4. Snow net with cable anchors and micropiles/ Sites 1 and 2 is shown in Figure 5C. Scree thickness is tubes. shown in Table 1.

892 100 80 Borehole deformations occurred at all three sites structures site 1 A structures site 2 every year (Figs. 5A, 6). The strongest deformations structures site 3 borehole site 1 occurred at Site 3, where the average displacement of 80 borehole site 2 60 the structures was also the highest, with values borehole site 3 exceeding the recommended limit of 5 cm a1, in 60 2000–2001 (Table 2, Fig. 5A). At Site 2, the structures 40 underwent a maximum displacement in 1998–99, possibly as a result of the particularly snow-rich 40 winter in the area (Figs. 5A, B). Ground temperature at 1 m depth is directly influ-

20 Average displacement of 20 enced by snow depth, as can be seen in Figures 5B and snow-supporting structures (mm) 5C. Snow depths of over 2 m have a significant effect Cumulative deformation of borehole (mm) in maintaining warm ground temperatures during the 0 0 B winter. Ground temperature was always warmer at site 2 than at site 1 and in parallel, borehole deformations site 2 were more pronounced at Site 2. However, the dis- 300 placements of the structure foundations were very similar at both sites. The inclinometer measurements in boreholes (Fig. 6) 200 show that the slope deformations were restricted to site 1 the layer of loose scree sediments (see scree thick- Snow depth (cm) nesses in Table 1) and that the bedrock was stable at 100 site 3 all three sites. The deformations are highest near the ground surface, which is where the scree is most loosely packed and where rock fall also occurs. A vis- 0 ible result of this is the denudation of the structure C foundations on their downslope side (Figs. 7a, b).

site 2 Unlike the structure displacements, borehole defor- 4 mations increased on an annual basis in a fairly con- stant manner (with the exception of 1999–2000 at site 2) and so it is difficult to determine to what extent the 0 movements were influenced by factors with a sea- sonal character (such as quantity of snow melt water).

-4 Ground temperature at -50 cm (˚C) site 1 5 DISCUSSION

-8 The snow pressure on snow-supporting structures affects their stability and depends on various factors

4/10/96 4/10/97 4/10/98 4/10/99 3/10/00 3/10/01 such as snow density, snow depth, and creep or glid- Figure 5. A: average displacement of snow-supporting ing of the snowcover (BUWAL/WSL 2000, Margreth structures and borehole deformation at 1 m depth. B: Snow 1995). Snow-nets for example, are designed for a depth. C: Ground temperature at 1 m depth (sites 1 and 2). snow pressure of 15 kN m 2 in the middle of a row of

Table 2. Mean annual and maximum individual displacements (cm) of anchors/foundations (SR steel rope, M micropile, P plate, T tube). Site 1 1 1 1 2 2 2 2 3 3 Anchor/foundation type SR M P T SR M P T SR M No. of anchors 17 3 6 2 12 2 4 4 31 20 Mean a1 (cm) 1.6 0.9 1.7 1.7 1.7 3.4 1 0.75 4.9 6.4 Max (cm) 1997–1998 3.5 0.7 2.2 2.5 3.2 0.5 1 1.2 –– Max (cm) 1998–1999 5.6 3.7 4 2.3 14.9 7.8 1.7 1.1 –– Max (cm) 1999–2000 6.5 7.4 1.5 0.9 9.3 15.5 Max (cm) 2000–2001 5.7 0.9 2.1 1.6 3.2 0.7 1 1.3 9.9 19.6

893 0

200

400

600

Figure 7a. Surface erosion of the scree around the base of 800 a plate foundation (Site 1). Site 3

200

400

600 Borehole depth (cm)

Figure 7b. Surface erosion of the scree below a steel rope 800 anchor and accumulation above. A reflector has been Site 2 mounted on the concrete foundation.

0 nets and 60 kN m2 on the edge. Typical design foundation forces are 200–250 kN for pressure and 250–320 kN for tension. 200 Snow density increases with altitude (approxi- mately 2% 100 m1), but does not normally attain 1997 excessive values on shady permafrost slopes. The 3 400 1998 value used in the Swiss Guidelines is 300 kg m at 1999 3000 m ASL (BUWAL/WSL 2000). The creep of the snowcover depends on snow density and slope angle. 2000 600 Surface roughness and orientation both affect the 2001 gliding factor, which is very low at the experimental sites due to the rough nature of the scree and to their northerly orientation. The displacement of the struc- 800 tures is therefore probably not induced by snow pres- Site 1 sure. The depth of the winter snowcover most likely influences the movements of the snow-supporting 1000 structures in two other ways: at the end of a snow-rich 16 12 8 40winter, large amounts of meltwater infiltrate the scree Borehole deformation (cm) and contribute to reducing the shear strength of the Figure 6. Borehole deformations at Sites 1, 2 and 3. scree on these steep slopes. In addition, the depth of Inclinometer measurements were started one year prior to the snowcover in winter directly influences ground the first year shown for each site with null deformation. temperature: a particularly deep snowcover insulates Depth of deformation corresponds to scree thickness. the ground and causes ground temperatures to remain

894 relatively warm. Increased creep velocities of frozen sediments in alpine permafrost are related to warmer ground temperatures (Hoelzle et al. 1998). As long as slope movement is homogeneous, the structures do not display any visible damage, even if the rates of displacement of the foundations are particularly high, such as at Site 3. However, as the structures have a design life of 100 years and are all less than 10 years old, it is too early to determine how long they will last before displaying damage of any importance. For the time being, the only visible effect of the slope movements is the progressive denudation of the foundations and of the tops of the anchors, which are becoming increasingly exposed due to surface move- Figure 8. General overview of the snow-supporting struc- ments of the scree (Figs. 7a, b). The measurements tures at Site 3, above Randa in February 1999 (catastrophic avalanche period). Note the slab avalanche scar on the right indicate that only the top sections of the anchors are where there are no structures. (S. Margreth). creeping, whereas their bases should be stable in the bedrock. It is not known whether this will eventually lead to deformations, anchor ruptures or extractions. often protect entire villages against avalanches. The Anchor or foundation type does not appear to have stability of the structures is therefore a very important any influence, as can be seen from the mean annual issue. displacements in Table 2. Excessive maximum dis- Factors influencing slope stability on avalanche placements of individual anchors and foundations are slopes in steep permafrost terrain were found to be: displayed by different types at each site. Deformations were highest at Site 3 (Figs. 5A, 6) – the thickness of the scree sediments and lowest at Site 1, with the thinnest scree cover. As – the temporal and spatial distribution of the slope angle is similar at all sites, the discrepancy must snowcover be due to the local geology, hydrology and interpartic- – ground temperature within the sediments ular contacts in the scree. Unfortunately ground tem- Other factors which remain to be investigated are: perature was not measured at Site 3 and so it is not possible to determine its influence on the stability of – moisture/ice content of scree the slope. The volumetric ice content of the scree – presence of snow meltwater (a lysimeter has been slope has not yet been determined either. installed at site 1 to measure the time and amount of Creep pressure was generally higher than anchor water percolating into the scree). In situ inclinome- resistance so it is apparent that the anchors do not ters would allow the determination of the time of have a stabilising effect on the slope. As the anchors year when the principal slope movements occur. are embedded in the bedrock at their base, the slope Snow-supporting structures do not appear to function will either continue to creep past them, or they will as soil anchors and cannot prevent downslope move- eventually be damaged. ment of thick sediments on steep slopes. On avalanche slopes where soil creep movements are too high, causing high maintenance costs, it is necessary 6 CONCLUSIONS to use other forms of avalanche defence such as dams in the avalanche deposition zone. Monitoring should continue at these experimental sites in order to determine how long it will take for damage to the structures to occur under these deter- mining conditions. The results are highly relevant for ACKNOWLEDGEMENTS a future cost-effective use of such structures. The measurement of ground moisture content would be of This project is financed by the Cantons Valais and particular interest in order to understand the fluctua- Graubünden and by the Swiss Confederation. M. tions in the movements from year to year and to deter- Hiller and R. Wetter are thanked for all their technical mine to what extent snowmelt water has an influence support. A. Stoffel kindly produced maps for the on the creep. project. P. Thalparpan is thanked for his significant A lasting efficiency of snow-supporting structures involvement in the initial project. We are grateful to is important for optimal protection and for economic the reviewers of this paper for their helpful sugges- reasons. Structures such as those at site 3 (Fig. 8) must tions and comments.

895 REFERENCES safety with snow, ice and avalanches, Proceedings of the ANENA Symposium, Chamonix: 241–248. BUWAL/WSL. 2000. Richtlinie für den Lawinenverbau im Phillips, M. 2000. Influences of Snow-Supporting Structures Anbruchgebiet. Kapitel V: Richtlinie für Lawinenver- on the Thermal Regime of the Ground in Alpine Per- bauungen im Permafrost: 79–96. Bern: EDMZ. mafrost Terrain. Davos: Eidgenössisches Institut für Hoelzle, M., Wagner, S., Kääb, A., Vonder Mühll, D. 1998. Schnee- und Lawinen Forschung. Surface movement and internal deformation of ice- Stoffel, L. 1995. Bautechnische Grundlagen für das Erstellen rock mixtures within rock glaciers at Pontresina von Lawinenverbauungen im alpinen Permafrost. Schafberg, Upper Engadin, Switzerland. In A.G. SLF Mitteilung 52. Davos: Eidgenössisches Institut für Lewkowicz & M. Allard (eds), Proceedings of the 7th Schnee- und Lawinen Forschung. International Conference on Permafrost, Yellowknife, Thalparpan, P. 2000. Lawinenverbauungen im Permafrost. Canada.: 465–471. Collection Nordicanan. Davos: Eidgenössisches Institut für Schnee- und Margreth, S. 1995. Snow pressure measurements on snow Lawinen Forschung. net systems. The contribution of scientific research to

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