Use of Permanent Strand Anchors on permafrost
The Erzherzog Johann Hütte is the highest shelter in the Eastern Alps. It is situated southeast of the Grossglockner on the so-called "Adlersruhe" - 3,454 m above sea level. The cabin is located in the middle of the "Hohe Tauern" national park on the border between the provinces of Carinthia and (Eastern) Tyrol in Austria.
The cabin was built in 1880 using the dry masonry wall method and has been expanded several times since. The more recent building sections were built using the steel and timber bar method. In the northern and eastern structures, which are also the oldest ones, numerous cracks have been discovered in the dry walls. In addition, settlements and wet spots or trickling water are present inside the rooms, rendering some rooms unusable.
Some of the cracks are several centimetres wide, up to 60 cm deep and several meters long. The most significant cracks appear in the structure on the valley side and originate from the window corners.
The visual damage (cracks and settlements) implies vertical and horizontal displacements (down to the valley), in particular in the oldest sections of the building on the valley side. Geologically seen, the subsoil consists of former thick basic volcanites which, after metamorphosis, now exist as greenstone slate and prasinite. The rocks show a pronounced foliation and the mountains several marked chasms.
North and east of the building as well as presumably underneath the old structures situated on the valley side, platelike detritus with rock blocks up to 1 m size is situated. The surface mainly consists of stones and rocks. With increasing depth more and more sandy and pebbly materials can be found. The pore spaces of the detritus are mostly filled with ice. The individual rocks are more or less encased by an ice cover, strongly reducing the grain-to-grain contact. It can be assumed that the entire detritus is permafrost.
The slope inclination of the ground situated towards the East is about 25 to 30 degrees.
The rock line runs approximately 2 m below the top edge of the building in the north-western building area and increases to the East (down to the valley) up to 3 to 4 m. After 2 m of a chasm-rich zone, the rock is not weather-beaten and very stable.
It was unfavourable for the structures to be rehabilitated that the mountain-side section is founded on stable rock, and the middle and valley-side section in the frozen detritus. Due to the creeping with various displacement factors in the frozen detritus the mountain cabin is being torn apart. The project consisted of the stabilization or rehabilitation of the northern section of the shelter cabin.
The most cost-effective solution was the stabilization of the frozen detritus underneath the old structures, without founding the building walls in the portative subsoil using deep foundation elements. Underneath the foundation area the creeping detritus was stabilized by a reinforced concrete beam anchored back.
It is situated on the valley-side exterior wall of the cabin and is founded in the stable rock using 9 GEWI® Piles 40 mm dia., BSt 500 S, with a length of 7 m. In addition four reinforced concrete buttresses were placed on this beam which had been anchored back, as well as the beam, using 5 x 0.6" permanent DYWIDAG Strand Anchors, service load 550 kN. The anchors have a length of 17 or 18.5 m respectively and have a bonded length of 9 m.
Because of the extremely high location the entire site installation and the delivery of all equipment and building materials had to be effected by helicopter. For the construction project a total of 150 tons of material had to be flown in, whereby the heavily loaded helicopters had to fly as high as 3,454 m.
Due to low soil temperatures in the permafrost area, special focus was placed on the grouting of piles and anchors. Soil or rock was "pre-warmed" by hammer drilling. Afterwards temperatures from the bottom of the holes to the surface were taken with a measuring head. Temperatures of +3° to +5° centigrade could be obtained. Subsequently the boreholes were grouted with quickly hardening cement grout. After the cement grout had hardened, each anchor or pile was subject to an extended acceptance test in accordance with ÖNORM B 4455.
After the remedial works are successfully completed, the construction of a domestic water supply facility with corresponding water and waste water treatment is planned in addition to further rehabilitation works.