Creep of Snow-Supporting Structures in Alpine Permafrost
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Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 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 aϪ1, 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. 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 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 aϪ1 (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 mmϪ2, 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 Ϯ 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 Ϯ0.15 mm mϪ1. 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 mea- surement and information system” network (IMIS), using a Campbell SR50 Ultrasonic Distance Sensor (precision Ϯ2.5 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 Ϯ0.02°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 aϪ1, 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.