Creep of snow-supporting structures in alpine permafrost
Embed Size (px)
Transcript of Creep of snow-supporting structures in alpine permafrost
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 a1 (BUWAL/WSL 2000). Anchors are28 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
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 3050slopes. 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 28 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 a1, 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 29302980 NW 38 1.8 m (gneiss)2 29502970 NE 39 2.7 m (dolomite)3 30103140 ENE 39 2.5 m (gneiss,
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 46 m long on the test sites), and plates. Frost
resistant mortar, with a compressive strength of at least35 N mm2, 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.
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 (510 m) wereequipped with inclinometer tubes at each site. Theupper 23 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 m1. 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 0.02C.
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.
Figure 2. Snow-bridge (left) and snow-nets (right).
supporting beam crossbeams
Figure 3. Snow bridge with micropiles and a plate foundation.
wire cable net
micropile /waisted tube
Figure 4. Snow net with cable anchors and micropiles/tubes.
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 a1, in20002001 (Table 2, Fig. 5A). At Site 2, the structuresunderwent a maximum displacement in 199899,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 19992000 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).
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 m2 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 MNo. of anchors 17 3 6 2 12 2 4 4 31 20Mean a1 (cm) 1.6 0.9 1.7 1.7 1.7 3.4 1 0.75 4.9 6.4Max (cm) 19971998 3.5 0.7 2.2 2.5 3.2 0.5 1 1.2 Max (cm) 19981999 5.6 3.7 4 2.3 14.9 7.8 1.7 1.1 Max (cm) 19992000 6.5 7.4 1.5 0.9 9.3 15.5Max (cm) 20002001 5.7 0.9 2.1 1.6 3.2 0.7 1 1.3 9.9 19.6