Introduction

Snow supporting structures are designed to retainsnow on potential avalanche slopes. Two main types ofstructure are currently used: snow bridges and snownets. They are built in rows, (Figure 1), are 2.5-4.5 mhigh, according to maximum snow depths, and dis-tances between the rows are 15-30 m depending onslope angle (BUWAL, SFISAR, 1990). For example,structures built on a 45¡ slope with snow depths attai-ning 3.5 m at an altitude of 3000 m a.s.l, must withstandline loads of approximately 80 kN per metre unitlength.

As the lower limit of discontinuous permafrost in theAlps is between 2500 and 3000 m a.s.l, structures areoccasionally located in permafrost terrain. Steep ice-richslopes tend to creep and creep rates of several centime-tres to decimetres per year have been observed (Stoffel,1995). This can induce damages to the structures andreduce their efficiency in avalanche protection (Figure 1).

The suitability of different types of structure andfoundation methods in permafrost (Figure 2) are beingstudied on two experimental sites in the Swiss Alps: atSchafberg above Pontresina, Graub�nden, at 2980 ma.s.l. and on Mont Dolin above Arolla, Valais, at 2850 ma.s.l. Slightly modified conventional supporting struc-tures with favourable characteristics for unstable terrain

are tested on these sites: snow nets (Figure 2, type 1)have wire rope anchors up- and downslope and thecentral hinged support column has a plate foundation.The structure shows a certain deformation capacity ifcreep occurs, and its flexibility allows adjustments inthe event of significant deformation. Snow bridges(Figure 2, type 2) have articulated joints, which reducesthe risk of element overstress if terrain movementoccurs. The geometry of these structures can be read-justed via a telescopic outrigger if strong deformationoccurs. The new sled type construction developed atthe Swiss Federal Institute for Snow and Avalanche

Abstract

Snow supporting structures are widely used in the Swiss Alps to protect settlements and transport routesfrom avalanches. The structures can be located between 2500-3500 m a.s.l, in permafrost terrain. In order tomonitor permafrost temperatures, boreholes were drilled in two slopes at 3000 m a.s.l and equipped with ther-mistors in 1996. Different types of supporting structures were constructed in 1997 with the aim of investigatingtwo main problems: firstly, interactions between supporting structures and permafrost were studied using tem-perature measurements, to determine whether the presence of the structures induces thawing through heatconduction into the ground, which could increase the intensity of creep deformation or even cause local terraininstability. Secondly, construction techniques were tested and improved in order to avoid damage to structuresbuilt in creeping permafrost terrain. In particular, the setting behaviour of mortar for the construction ofmicropiles in permafrost was investigated.

Patrik Thalparpan, et al. 1049

SNOW SUPPORTING STRUCTURES IN STEEP PERMAFROST TERRAIN IN THE SWISS ALPS

Patrik Thalparpan1, Marcia Phillips2, Walter Amman3

Swiss Federal Institute for Snow and Avalanche Research, Fl�elastr. 11, CH-7260 Davos, Switzerland1. e-mail: thalparpan@slf.ch

2. e-mail: phillips@slf.ch3. e-mail: ammann@slf.ch

Figure 1. Deformed snow supporting structures in creeping permafrost ter-rain at 3000 m a.s.l., Randa, Valais.

Research (SFISAR) (Figure 2, type 3) consists of twogirders lying loosely on a concrete strip foundation,supporting a structure similar to the snow bridge type.The upper ends of the girders are attached to wire ropeanchors. To monitor possible slope or structure move-ments, inclinometer measurements are effected and thestructures as well as their foundations are surveyedwith a theodolite at regular intervals.

Interactions between snow supporting structures and permafrost

Steel supporting structures can attain temperatures of40-50¡C when exposed to direct solar radiation in sum-mer. The steel wire rope anchors and piles, which areup to 6 m long and have a diameter of 25-32 mm mayconduct some of this heat into the ground, which couldeventually lead to gradual thawing of ground ice. Inaddition, the structures modify snow distribution inspace and time, which could also trigger modificationsof ground thermal characteristics as the structures arebuilt to last 50 - 100 years. Snow is retained on the slopethroughout the winter, implying that its insulatingeffect is more pronounced than on a naturally avalan-ching slope, depending on the frequency of recurrenceof the avalanches. Snowmelt in spring can be delayed,and the ground between the structures often remainspartly or completely snow covered for a longer timeduring the months when solar radiation is intense. Allthese possible influences on permafrost temperature arebeing studied.

Methods

In summer 1996, two 18 m deep boreholes weredrilled at the Schafberg site to investigate the propertiesof the ground (Figure 3). They were equipped withthermistors at various depths. One borehole is located

between the experimental supporting structures, whichwere constructed in 1997. Temperature measurementsshould indicate whether a modification of the groundtemperature regime occurs. The second borehole is in asimilar location 20 m outside the area of influence of thestructures and constitutes a reference. It is unlikely thata clear effect of the structures on ground temperaturewill be detected in the next few years, so it is necessaryto simulate the long term effects of supporting struc-tures on permafrost. In future, temperature measure-ment data will be back-calculated using a special pur-pose finite element programme developed to studyheat transfer in phase changing geotechnical mediasuch as snow and permafrost (Bartelt and Wittwer, 1998).

In order to determine the ground thermal regimeusing the finite element method, the specification ofupper (surface) and lower boundary conditions is necessary. The lower boundary condition is derivedfrom borehole temperature data in 17.5 m depth.Surface boundary conditions are best established usingground temperature measurements at shallow depths(Gold and Lachenbruch 1973), as the actual surface tem-perature varies constantly in maintaining a balancebetween heat loss and gain. In addition, it is difficult tospecify where the ground surface actually is on a roughboulder cover (scree). Thermistors have therefore beeninstalled in ten boreholes at 0.5 and 1.0 m depths.Ground surface temperatures are measured in winterbeneath the snow cover which insulates the groundfrom atmospheric influences.

Surface boundary conditions are strongly modified bythe presence or absence of a snow cover, as parameterssuch as albedo and emissivity are very different accor-ding to whether the surface is constituted of roughscree or a uniform snow cover (Keller, 1993). The struc-

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Figure 2. Snow supporting structure types tested in permafrost terrain.

tures greatly modify snow quantities and characteris-tics, especially during the critical early summer periodwhen snow melt occurs later. As mean annual groundtemperatures are highly dependent on the amount ofdirect solar radiation received (Hoelzle, 1992), the factthat snow remains longer on the ground due to sup-porting structures may have an enhancing effect on per-mafrost as the ground is protected from direct solarradiation. To monitor snow distribution characteristics,an automatic camera registers snow accumulation,avalanche activity and snow melt patterns aroundsnow stakes at two day intervals on the Schafberg site.Various snowcover scenarios, using varying quantitiesand densities of snow over shorter or longer periods oftime will need to be simulated to establish the impor-tance of parameters such as snow depth, density, andsnow cover longevity on the ground thermal regime.

Preliminary Results

Temperature measurements in an 18 m deep boreholereveal that on the Schafberg test site seasonal fluctua-tions occur above 16 m depth (whiplash curves), andthat below 16 m temperatures are more or less constant(Figure 3). Despite changes in stratigraphy (Figure 3),the ground between 0.5 and 17.5 m depth appears tohave fairly uniform thermal characteristics as there areno sudden changes in temperature gradient. A radicalchange in temperature gradient occurs between 0.5 m

depth and the ground surface (beneath the winter snowcover), within the top layer of rough talus.

The variability of surface temperature measurementsis illustrated in Figure 4. Winter ground surface tempe-ratures in 1996 warmed in mid-November, following a2 m snowfall, and then cooled gradually throughout thewinter. The cooling process under the new snow coverstarted at -3¡C. A zero curtain due to latent heat releaseis absent, as the top layer of the scree is dry. Groundtemperatures then continued to cool and fluctuatedaround -5¡C in February and March before droppingsignificantly to -8¡C in mid-April beneath a snow coverof 1.6 m. These fluctuations and marked temperaturedrops may have two possible explanations: snow set-tling throughout the winter gradually reduced thethickness of the snow cover and simultaneouslyinduced densification, therefore increasing the thermalconductivity of the snow. The relatively high density ofthe snow cover measured in mid April (350 kg/m3)may have allowed a greater heat flux from the groundupwards. Alternatively, cold air may have penetratedthe snow cover near rock faces or through slots andthen circulated beneath the snow cover, between thelarge blocks of rock, cooling the ground through con-vection. Temperature measurements within the snowcover and on the snow surface are planned for winter1997/98 in order to attempt to answer this question.

Mortar curing in permafrost conditions

Micropiles and wire rope anchors used for snow sup-porting structure foundations have to convey compres-sive or tensile forces of up to 450 kN into the ground.Anchor lengths vary between 3 and 6 m, according toground characteristics and forces to be transferred.Micropiles are profiled reinforcement steel bars (Swiss-Gewi, tensile strength 600N/mm2, diameter 28/32/40mm, according to load). Wire rope anchors consist of avery strong galvanised steel (tensile strength 1770N/mm2). They are placed in boreholes of 95 mm dia-meter which are then injected with grout mortar fromthe base of the borehole to the ground surface. A mini-

Patrik Thalparpan, et al. 1051

Figure 3. Borehole temperatures and stratigraphy in the reference borehole,Schafberg. (Temperatures at the surface of the boulder deposit (scree) weremeasured with mini temperature loggers beneath the snow cover, i.e. fromOctober 1996 to May 1997). The temperature gradient in the top 50 cm ofsurface material is represented by thin lines.

Figure 4. Ground surface temperatures below the winter snow cover (bts :bottom temperature of the snow cover) and air temperatures 3 m above theground surface.

mum mortar compressive strength of 35 N/mm2

(35 MPa) is necessary for sufficient bearing capacity ofpiles and anchors (BUWAL, SFISAR 1993).

The curing behaviour of mortar under permafrostconditions was tested in the Swiss Federal Laboratoriesfor Materials Testing and Research. The experimentsaimed at finding practical solutions for attaining suffi-cient mortar strengths in permafrost applications. Theinvestigations were undertaken in two phases.

In a first phase, a conventional mortar approved forthe construction of snow supporting structures in posi-tive ground temperature conditions was tested. Theproduct name of this mortar is ALIVA-LVM: the cement

is PZ 55 (CEM I 52.5R) with a Blaine value of 4000cm2/g. Cement content is approximately 44% (percentmass) and the water-cement factor is 0.45. For frostdurability, an air-pore former (product Marbos) isadded in a dose of 0.5% of cement content. Sand grainsize is 0-0.8 mm. The aim of the first series of experi-ments (see below, experiments 1 and 2) was to establishthe lower critical temperature limit down to which thisconventional ALIVA-LVM mortar can be used.

In the second phase (currently being undertaken), arapidly curing mortar is being tested in colder per-mafrost conditions. Biggar et al. (1993) successfullyused high alumina cement-based grout with an accele-

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Figure 5. Brine bath with samples in refrigerator. Detail of a sample.

rator. In addition to the development of compressivestrength, the practical applicability of the mortar has tobe studied in detail. Experience on mountain construc-tion sites has shown that the use of accelerators inducescuring within the injection hoses. These can be up to 60 m long between the mixer and the borehole andwhen exposed to direct sunlight, they warm up, whichcan lead to clogging.

For the laboratory mortar tests, permafrost conditionswere simulated in a permanently stirred brine bath(Figure 5). The mortar samples were poured into plasticcylinders insulated at the top and at the base with 3 cmof polystyrene and were fully immersed in a brine bath.Thermistors were installed in the centre and on the lat-eral surface of the samples. Temperatures were continu-ously measured during setting time and recorded by adata logger.

Two sets of experiments were undertaken with differ-ent initial conditions. Compressive strengths were mea-sured after 7, 14, 28 and for some samples, 62 day.

EXPERIMENT NO. 1The initial temperature of the mortar was 5¡C, repre-

senting a typical summer air temperature at 3000 ma.s.l. and the temperature of the brine bath was initiallyat -2.0¡C before being cooled to -2.5¡C. Samples werepoured into plastic cylinders with a steel rod (diameter30 mm, see Figure 6) in the centre to represent theanchor rod.

After about 5 hours, the mortar attained the tempera-ture of the brine bath. Then a marked warmingoccurred, from -2.5 to -0.8¡C. The sudden nature of thistemperature increase suggests that it is due to release oflatent heat during the phase change from water con-tained in the mortar to ice. Heat release due to hydra-tion would have occurred more gradually. The super-cooling of the sample below freezing point to -2.5¡C isprobably due to the rapid cooling process and the pres-ence of salt in the mortar. As soon as an ice grain formswithin the sample, the freezing process rapidly propa-gates throughout the sample, releasing latent heat.After this warming kick, cooling occurs more slowly aswater continues to freeze and release latent heat.Comparison of the cooling curve before and after thesudden warming allows estimation of the latent energyreleased and calculation of the amount of frozen water.It was found that 20-30% of the water freezes between 5and 14 hours. The volume increase due to ice formationdamages the cement stone and lowers the compressivestrength. Thin sections of a 28 day old sample show thepresence of many cracks. The degree of hydration isapproximately 50-60%. In order to determine the com-pressive strengths of the mortar, cylindrical sampleswere drilled out (diameter 25 mm, length 40 mm).These were tested in the frozen state and after 1 day at20¡C (Table 1).

The results were well below the required values of 35 N/mm2 (BUWAL, SFISAR 1993) and were stronglyscattered. The wide scattering of values is due to thesmall size of the samples in which every weakness(cracks) has a decisive influence on the compressivestrength.

Patrik Thalparpan, et al. 1053

Figure 6. Mortar cooling process. Initial temperatures: mortar 5¡C, brine -2.5¡C.

Table 1. Compressive strengths of cylindrical samples (diameter 25 mm, length 40 mm). Mean values of 6 samples

EXPERIMENT NO. 2In this test series, the samples were also cylindrical

(diameter 100 mm, see Figure 5), but without a steelrod. This allowed the determination of the compressivestrength on 100 mm cylinders with a much smaller scat-ter of values.

Dry mortar with a temperature of 3¡C was mixed withwarm water (42¡C), which resulted in a grout mixturetemperature of 20¡C (mixture ratio 7 litres of water to40 kg mortar). On mountain construction sites it is easier to heat only the water with a gas burner or boilerthan to heat the whole mixture. Warming prolongs thecritical time until freezing of the mortar occurs. Thebrine bath was set at -1¡C in this experiment.

It is clearly visible that cooling occurs more slowly inthis case (Figure 7). No sudden warming followingfreezing occurs as did in experiment 1, because themortar water does not freeze. The introduction of thewarm samples into the brine bath slightly warms thelatter, as this heat can not immediately be evacuated. Inthe field, the warm mortar heats the ground in a similarmanner. The mortar is not very exothermal. No clearwarming due to hydration can be observed asdescribed by Biggar et al. (1993) in tests using high alu-

mina cement. Hydration warmth is constantly removedby the cooling process. The compressive strengths ofthe 100 mm cylinders were again determined in thefrozen state and after one day at 20¡C (Table 2).

Compressive strengths are higher than in the firstexperiment. The values after 62 days are about 10%below the required value of 35 N/mm2 and scatter islower. If the sample from day 62 (only one sample) isrepresentative, then it is interesting to note the increasein compressive strength of 16% between day 28 and day62. Conventional concrete setting at room temperatureshows a compressive strength increase of 5-8% in thistime period (Krenkler, 1980). It appears that hydrationoccurs more slowly in a cold environment, and lastslonger. Compressive strengths of samples are lower in amelted state than while frozen, as the free water contentin the frozen state increases strength. No cracks werevisible on the thin sections. The degree of hydrationwas 70-80%. At the moment it is unclear whether thesatisfactory compressive strengths can be attributed tothe warmed mortar or to the warmer brine bath (-1¡C inexperiment no. 2 instead of -2.5¡C in experiment no. 1)or to both. Further experiments must be conducted toanswer this question.

Verification of the validity of the laboratorymethods

In order to verify the validity of the laboratory me-thods, cooling of mortar was measured at 2.8 m depthon a 4 m long micropile in the field. The aim was to seewhether mortar cools at the same rate in the field as inthe laboratory. The borehole temperature was 0¡C,therefore 1¡C warmer than the brine bath in experimentno. 2. The mortar temperature was between 20 and26¡C. It is difficult to regulate mortar temperature toexactly 20¡C on the construction site. As an additionalcomparison with the laboratory temperature measure-ments, the cooling of mortar was calulated using thefinite-element programme SAMCEF-THERNL(Samtech Group, Reutlingen, Germany). Heat released

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Figure 7. Mortar cooling. Initial temperatures: mortar 20¡C, brine -1¡C.

Table 2. Compressive strengths of cylindrical samples (diameter 100 mm)

due to hydration or to phase change was not taken intoaccount. Initial mortar temperature was set at 20¡C, andpermafrost temperature at -1¡C. The material characte-ristics assumed for the simulation are shown in Table 3.

The mortar cooling process measured in the laborato-ry and in the field is represented in Figure 8 and com-pared with the finite element calculation. Comparisonshows that the laboratory samples cool much fasterthan mortar in the field borehole or as calculated usingfinite elements. Heat is removed faster by the perma-nently stirred brine bath. It can therefore be assumedthat compressive strengths in the borehole, where themortar is not cooled as rapidly, are equal to or betterthan those obtained in the laboratory.

Summary

To determine the effect of snow supporting structureson ground temperatures in permafrost terrain, longterm simulations using a finite-element method will benecessary. Field measurements serve to calibrate themodel. In openwork boulder deposits (scree), the upperboundary condition in particular must be carefully

measured as this zone can be influenced by a number ofparameters which are difficult to characterise.

Sufficiently high mortar compressive strengths can beobtained with conventional ALIVA-LVM mortar in per-mafrost terrain with summer ground temperaturesdown to -1¡C using the method of prewarming. Forcolder permafrost temperatures, a rapidly curing mor-tar is necessary for attaining sufficient compressivestrength, but its practical applicability still has to betested.

Acknowledgments

This project would not have been possible without theimportant financial support of the CantonsGraub�nden and Valais, and of the SwissConfederation. The mortar experiments were underta-ken in collaboration with Dr. K. Moser in the SwissFederal Laboratories for Materials Testing andResearch. Numerous people at SFISAR have beeninvolved in different ways in the successful launchingof the project. Finally, we thank the reviewers who readthe manuscript and made helpful suggestions and comments.

Patrik Thalparpan, et al. 1055

Table 2. Compressive strengths of cylindrical samples (diameter 100 mm)

Figure 8. Comparison of the cooling processes (as measured in the field, inthe laboratory and as calculated using the FEM).

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