Creep of snow-supporting structures in alpine permafrost

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1 INTRODUCTION The importance of snow-supporting structures as ava- lanche 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 start- ing 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 alpine permafrost terrain, where the ground cover typ- 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 Guidelines for the Construction of Snow-supporting Structures advise against construction if creep rates exceed 5 cm a 1 (BUWAL/WSL 2000). Anchors are 2–8 m long, according to the bearing capacity of the ground and the thickness of the scree, so a consider- able anchor length can be located in the unstable scree-ice mixture (Thalparpan 2000). The base of the anchor (at least 2 m) should be in bedrock. In order to monitor the performance of snow- supporting structures in steep permafrost terrain, mea- surements are effected at three sites around 3000 m ASL in the Swiss Alps. They are located above Pontresina (Muot da Barba Peider, Site 1), above Arolla (Mont Dolin, Site 2) and above Randa (Wisse Schijen, Site 3) (Fig. 1). Slope deformation, structural stability, snow distribution and ground temperature have been moni- tored there since 1997. 2 SITE DESCRIPTIONS All three sites are equipped with avalanche defence structures: Site 1 with both snow-bridges and snow- nets, and Sites 2 and 3 with snow-nets only. The most important site characteristics are summarized in Table 1. The volumetric ice content of the scree at Sites 1 and 2 varied between 5 and 10% (Phillips 2000) but has not 891 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 500km 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 a 1 , as the maintenance costs are too high. In order to investigate the behaviour of the structures in creeping alpine permafrost terrain, slope move- ments 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. Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 Figure 1. Map of Switzerland showing the locations of test sites 1 (Pontresina), 2 (Arolla) and 3 (Randa). Table 1. Characteristics of Sites 1, 2, and 3. Slope Scree thickness, Site Altitude (m) Orientation angle rock type 1 2930–2980 NW 38° 1.8 m (gneiss) 2 2950–2970 NE 39° 2.7 m (dolomite) 3 3010–3140 ENE 39° 2.5 m (gneiss, quartzite, marble)

Transcript of Creep of snow-supporting structures in alpine permafrost

Page 1: Creep of snow-supporting structures in alpine permafrost

1 INTRODUCTION

The importance of snow-supporting structures as ava-lanche defence measures was demonstrated when upto 5 m of snow fell in the Swiss Alps in the course ofFebruary 1999. Over 500 km of snow-supportingstructures successfully protected settlements, roadsand railways by retaining the snow in avalanche start-ing areas. Efficient protection with such large snowloads is only possible if the structures are in good con-dition and well anchored. Damage to snow-supportingstructures occurs if creep rates are high on steepslopes and in areas of intense rock fall (Stoffel 1995).High creep rates are particularly associated with steepalpine permafrost terrain, where the ground cover typ-ically consists of a mixture of scree and ice (or waterin summer when the ice melts in the active layer). Inorder to avoid excessive damage, the Swiss FederalGuidelines for the Construction of Snow-supportingStructures advise against construction if creep ratesexceed 5 cm a�1 (BUWAL/WSL 2000). Anchors are2–8 m long, according to the bearing capacity of theground and the thickness of the scree, so a consider-able anchor length can be located in the unstablescree-ice mixture (Thalparpan 2000). The base of theanchor (at least 2 m) should be in bedrock.

In order to monitor the performance of snow-supporting structures in steep permafrost terrain, mea-surements are effected at three sites around 3000 m ASLin the Swiss Alps. They are located above Pontresina(Muot da Barba Peider, Site 1), above Arolla (MontDolin, Site 2) and above Randa (Wisse Schijen, Site 3)(Fig. 1). Slope deformation, structural stability, snowdistribution and ground temperature have been moni-tored there since 1997.

2 SITE DESCRIPTIONS

All three sites are equipped with avalanche defencestructures: Site 1 with both snow-bridges and snow-nets, and Sites 2 and 3 with snow-nets only. The mostimportant site characteristics are summarized in Table 1.The volumetric ice content of the scree at Sites 1 and 2varied between 5 and 10% (Phillips 2000) but has not

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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 containice. Snow-supporting structures are anchored at depths of 2–8 m according to the bearing capacity of the groundand the thickness of the scree. A considerable anchor length can be located in the rock-ice mixture. Constructionof snow-supporting structures is not advisable if creep rates exceed 5 cm a�1, as the maintenance costs are toohigh. In order to investigate the behaviour of the structures in creeping alpine permafrost terrain, slope move-ments 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.

Permafrost, Phillips, Springman & Arenson (eds)© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7

Figure 1. Map of Switzerland showing the locations oftest sites 1 (Pontresina), 2 (Arolla) and 3 (Randa).

Table 1. Characteristics of Sites 1, 2, and 3.

Slope Scree thickness, Site Altitude (m) Orientation angle rock type

1 2930–2980 NW 38° 1.8 m (gneiss)2 2950–2970 NE 39° 2.7 m (dolomite)3 3010–3140 ENE 39° 2.5 m (gneiss,

quartzite, marble)

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been determined for Site 3. Scree thickness (determinedby drilling) is less than 3 m at all sites.

Two types of snow-supporting structure are used:snow-bridges and snow-nets (Fig. 2). The latter are typically used in areas affected by rock fall, due to theirflexibility (Margreth 1995) and are particularly welladapted to creeping permafrost terrain (Thalparpan2000). Various types of anchors and foundations areused, including steel rope anchors, micropiles, tubes(all are 4–6 m long on the test sites), and plates. Frost

resistant mortar, with a compressive strength of at least35 N mm�2, has to be used to inject the anchor bore-holes.

The cross-sections and types of foundations andanchoring for the two types of structures are shown in Figures 3 and 4.

3 METHODS

The positions of 101 foundations and anchor headsare surveyed yearly, with a Wild TC 1610 theodolite(precision � 2 mm). They are equipped with fixedpins (type “SBB Bolzen”) onto which reflectors can bemounted (Fig. 7b). Reference points are located in thesurrounding rock walls as well as on buildings such aschurches in the valleys below.

Additional vertical boreholes (5–10 m) wereequipped with inclinometer tubes at each site. Theupper 2–3 m of the tubes are in the scree and theremainder of the length is in the bedrock. Deformationmeasurements are carried out in the tubes once a yearusing an inclinometer (Sinco Digitilt) with a precisionof �0.15 mm m�1. The measurements were made at1 m depth intervals.

An automatic camera and snow gauges were used tomonitor snow depth at Site 1. Snow depth was mea-sured automatically every 30 minutes 750 m NW ofSite 2 by a weather station of the “Intercantonal mea-surement 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 anIMIS station located 8 km N of Site 3 at the same alti-tude and those data give a general indication of thesnow conditions in the area. Gaps in the data are dueto technical problems or to the fact that the IMIS sta-tions were only built in the course of the measurementperiod.

Ground temperatures were monitored in boreholesranging between 5 and 18 m depth at Sites 1 and 2,using thermistors (YSI 46008) with a calibrated preci-sion of �0.02°C.

4 RESULTS

The average downslope displacements of the anchorheads and plate foundations at each site are shown inFigure 5A. Table 2 shows the mean annual displace-ment of the foundations and the maximum valuesmeasured for individual foundations in each category.The cumulative deformation of each borehole at 1 mdepth is shown in Figure 5A. Snow depth is illustratedin Figure 5B, and ground temperature at 1 m depth forSites 1 and 2 is shown in Figure 5C. Scree thickness isshown in Table 1.

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Figure 2. Snow-bridge (left) and snow-nets (right).

supporting beam crossbeams

hinge

micropile

micropile

platefoundation

telescopicpost

Figure 3. Snow bridge with micropiles and a plate foundation.

wire cable net

micropile

cableanchor

stabilisingtube

spanguy

hingedsupport

anchorhead

micropile /waisted tube

cableanchor

Figure 4. Snow net with cable anchors and micropiles/tubes.

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Borehole deformations occurred at all three sitesevery year (Figs. 5A, 6). The strongest deformationsoccurred at Site 3, where the average displacement ofthe structures was also the highest, with valuesexceeding the recommended limit of 5 cm a�1, in2000–2001 (Table 2, Fig. 5A). At Site 2, the structuresunderwent a maximum displacement in 1998–99,possibly as a result of the particularly snow-rich winter in the area (Figs. 5A, B).

Ground temperature at 1 m depth is directly influ-enced by snow depth, as can be seen in Figures 5B and5C. Snow depths of over 2 m have a significant effectin maintaining warm ground temperatures during thewinter. Ground temperature was always warmer at site2 than at site 1 and in parallel, borehole deformationswere more pronounced at Site 2. However, the dis-placements of the structure foundations were verysimilar at both sites.

The inclinometer measurements in boreholes (Fig. 6)show that the slope deformations were restricted tothe layer of loose scree sediments (see scree thick-nesses in Table 1) and that the bedrock was stable atall three sites. The deformations are highest near theground surface, which is where the scree is mostloosely packed and where rock fall also occurs. A vis-ible result of this is the denudation of the structurefoundations on their downslope side (Figs. 7a, b).

Unlike the structure displacements, borehole defor-mations increased on an annual basis in a fairly con-stant manner (with the exception of 1999–2000 at site2) and so it is difficult to determine to what extent themovements were influenced by factors with a sea-sonal character (such as quantity of snow melt water).

5 DISCUSSION

The snow pressure on snow-supporting structuresaffects their stability and depends on various factorssuch as snow density, snow depth, and creep or glid-ing of the snowcover (BUWAL/WSL 2000, Margreth1995). Snow-nets for example, are designed for asnow pressure of 15 kN m�2 in the middle of a row of

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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 MNo. of anchors 17 3 6 2 12 2 4 4 31 20Mean a�1 (cm) 1.6 0.9 1.7 1.7 1.7 3.4 1 0.75 4.9 6.4Max (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 15.5Max (cm) 2000–2001 5.7 0.9 2.1 1.6 3.2 0.7 1 1.3 9.9 19.6

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Figure 5. A: average displacement of snow-supportingstructures and borehole deformation at 1 m depth. B: Snowdepth. C: Ground temperature at 1 m depth (sites 1 and 2).

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nets and 60 kN m�2 on the edge. Typical design foun-dation forces are 200–250 kN for pressure and250–320 kN for tension.

Snow density increases with altitude (approxi-mately 2% 100 m�1), but does not normally attainexcessive values on shady permafrost slopes. Thevalue used in the Swiss Guidelines is 300 kg m�3 at3000 m ASL (BUWAL/WSL 2000). The creep of thesnowcover depends on snow density and slope angle.Surface roughness and orientation both affect thegliding factor, which is very low at the experimentalsites due to the rough nature of the scree and to theirnortherly orientation. The displacement of the struc-tures is therefore probably not induced by snow pres-sure. The depth of the winter snowcover most likelyinfluences the movements of the snow-supportingstructures in two other ways: at the end of a snow-richwinter, large amounts of meltwater infiltrate the screeand contribute to reducing the shear strength of thescree on these steep slopes. In addition, the depth ofthe snowcover in winter directly influences groundtemperature: a particularly deep snowcover insulatesthe ground and causes ground temperatures to remain

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Figure 6. Borehole deformations at Sites 1, 2 and 3.Inclinometer measurements were started one year prior tothe first year shown for each site with null deformation.Depth of deformation corresponds to scree thickness.

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

Figure 7b. Surface erosion of the scree below a steel ropeanchor and accumulation above. A reflector has beenmounted on the concrete foundation.

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relatively warm. Increased creep velocities of frozensediments in alpine permafrost are related to warmerground temperatures (Hoelzle et al. 1998).

As long as slope movement is homogeneous, thestructures do not display any visible damage, even ifthe rates of displacement of the foundations are par-ticularly high, such as at Site 3. However, as the struc-tures have a design life of 100 years and are all lessthan 10 years old, it is too early to determine how longthey will last before displaying damage of any impor-tance. For the time being, the only visible effect of theslope movements is the progressive denudation of thefoundations and of the tops of the anchors, which arebecoming increasingly exposed due to surface move-ments of the scree (Figs. 7a, b). The measurementsindicate that only the top sections of the anchors arecreeping, whereas their bases should be stable in thebedrock. It is not known whether this will eventuallylead to deformations, anchor ruptures or extractions.

Anchor or foundation type does not appear to haveany influence, as can be seen from the mean annualdisplacements in Table 2. Excessive maximum dis-placements of individual anchors and foundations aredisplayed by different types at each site.

Deformations were highest at Site 3 (Figs. 5A, 6)and lowest at Site 1, with the thinnest scree cover. Asslope angle is similar at all sites, the discrepancy mustbe due to the local geology, hydrology and interpartic-ular contacts in the scree. Unfortunately ground tem-perature was not measured at Site 3 and so it is notpossible to determine its influence on the stability ofthe slope. The volumetric ice content of the screeslope has not yet been determined either.

Creep pressure was generally higher than anchorresistance so it is apparent that the anchors do nothave a stabilising effect on the slope. As the anchorsare embedded in the bedrock at their base, the slopewill either continue to creep past them, or they willeventually be damaged.

6 CONCLUSIONS

Monitoring should continue at these experimentalsites in order to determine how long it will take fordamage to the structures to occur under these deter-mining conditions. The results are highly relevant fora future cost-effective use of such structures. Themeasurement of ground moisture content would be ofparticular interest in order to understand the fluctua-tions in the movements from year to year and to deter-mine to what extent snowmelt water has an influenceon the creep.

A lasting efficiency of snow-supporting structuresis important for optimal protection and for economicreasons. Structures such as those at site 3 (Fig. 8) must

often protect entire villages against avalanches. Thestability of the structures is therefore a very importantissue.

Factors influencing slope stability on avalancheslopes in steep permafrost terrain were found to be:

– the thickness of the scree sediments– the temporal and spatial distribution of the

snowcover– ground temperature within the sediments

Other factors which remain to be investigated are:

– moisture/ice content of scree– presence of snow meltwater (a lysimeter has been

installed at site 1 to measure the time and amount ofwater percolating into the scree). In situ inclinome-ters would allow the determination of the time ofyear when the principal slope movements occur.

Snow-supporting structures do not appear to functionas soil anchors and cannot prevent downslope move-ment of thick sediments on steep slopes. On ava-lanche slopes where soil creep movements are toohigh, causing high maintenance costs, it is necessaryto use other forms of avalanche defence such as damsin the avalanche deposition zone.

ACKNOWLEDGEMENTS

This project is financed by the Cantons Valais andGraubünden and by the Swiss Confederation. M.Hiller and R. Wetter are thanked for all their technicalsupport. A. Stoffel kindly produced maps for the project. P. Thalparpan is thanked for his significantinvolvement in the initial project. We are grateful tothe reviewers of this paper for their helpful sugges-tions and comments.

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Figure 8. General overview of the snow-supporting struc-tures at Site 3, above Randa in February 1999 (catastrophicavalanche period). Note the slab avalanche scar on the rightwhere there are no structures. (S. Margreth).

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REFERENCES

BUWAL/WSL. 2000. Richtlinie für den Lawinenverbau imAnbruchgebiet. Kapitel V: Richtlinie für Lawinenver-bauungen im Permafrost: 79–96. Bern: EDMZ.

Hoelzle, M., Wagner, S., Kääb, A., Vonder Mühll, D. 1998.Surface movement and internal deformation of ice-rock mixtures within rock glaciers at PontresinaSchafberg, Upper Engadin, Switzerland. In A.G.Lewkowicz & M. Allard (eds), Proceedings of the 7thInternational Conference on Permafrost, Yellowknife,Canada.: 465–471. Collection Nordicanan.

Margreth, S. 1995. Snow pressure measurements on snownet systems. The contribution of scientific research to

safety with snow, ice and avalanches, Proceedings ofthe ANENA Symposium, Chamonix: 241–248.

Phillips, M. 2000. Influences of Snow-Supporting Structureson the Thermal Regime of the Ground in Alpine Per-mafrost Terrain. Davos: Eidgenössisches Institut fürSchnee- und Lawinen Forschung.

Stoffel, L. 1995. Bautechnische Grundlagen für das Erstellenvon Lawinenverbauungen im alpinen Permafrost. SLF Mitteilung 52. Davos: Eidgenössisches Institut fürSchnee- und Lawinen Forschung.

Thalparpan, P. 2000. Lawinenverbauungen im Permafrost.Davos: Eidgenössisches Institut für Schnee- undLawinen Forschung.

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