Pile test campaign to monitor thawing permafrost
Frozen marine deposits which are widely distributed in the fjord-valleys in Svalbard are particularly vulnerable to thawing under present and future climate conditions. This spring several piles were set up on the plain east of UNIS. These will contribute to more knowledge about permafrost thawing in Svalbard.
26 June 2020
Text: Chuangxin Lyu, PhD candidate (UNIS and NTNU).
The pile test campaign at the UNIS east test-site in Longyearbyen is a part of the Nunataryuk project, funded by the European Union’s Horizon 2020 Research and Innovation Program. The main goal of Nunataryuk is to determine the impacts of thawing land, coast and subsea permafrost on the global climate and on humans in the Arctic and to develop targeted and co-designed adaptation and mitigation strategies.
The warmer climate threatens the stability and durability of structures installed on permafrost in Svalbard. Annual average temperature has increased between 3°C and 5°C during the last 40 years. Thawing of frozen soil weakens the soil’s mechanical strength and leads to larger time-dependent deformations. Frozen marine deposits (saline clay and silt) which are widely distributed in the fjord-valleys in Svalbard are particularly susceptible and vulnerable to thawing under present and future climate conditions. In order to investigate this and to provide accurate soil test data that can be used to benchmark analytical and numerical models of frozen soil, test piles are inserted in the ground and loaded with large weights, and carefully monitored with respect to the development of vertical displacements over time. The piles will be loaded stepwise to failure, where failure is defined by pile sinking in the ground, i.e. unacceptable pile settlement rate. The piles are designed to resist the external loading only at the pile tip (footing) deep in the ground, to eliminate disturbance from frost jacking in the surface soil active layer.
- Three φ320 mm holes have been bored through the active layer and sandy permafrost down to the clay permafrost.
- Piles are standing at circular footing plates 4,2 m down in the ground, carrying load only at their tip.
- Steel casing pipes support the soil around the boreholes, provides an air gap along the shaft of the piles and prevents leakage of surface water to the piles.
- Load cells at the top of each pile measures the actual weight on each pile.
- Vertical displacements are monitored by a wire displacement sensor between each pile and the steel pipe casing frozen stuck in the soil. An unloaded steel bar is fixed 5,5 m down in the ground for reference and checking vertical movements. Movements are also checked against the UNIS building.
- Temperature sensors are placed at each footing plate.
- Thermal insulation around the pile at the surface prevents unintended heat leak into the soil, aiming to maintain the natural temperature conditions in the soil and at an undisturbed site.
- Annual temperature at 4,2 m depth varies from -2.5⁰C to -3.2⁰C through the year. As summer heat takes time to penetrate the froze soil, the warmest month is February and the coldest month is May at the site.
- As loading blocks concrete slabs with weight of 1400 kg each are used.
- Load, temperature and settlement are three key test parameters. In addition, laboratory tests with strength measurements, soil grain characterization, density, pore water salinity, frozen/unfrozen pore water, etc. are performed in the UNIS labs.
The illustrations below show some details of the pile installation and loading frame:
The bearing strength of a pile, when it resists loading only at its tip, is commonly determined from a theory denoted “cavity expansion theory”. In principle this is a mechanism in the soil in a conical zone under the footing. Figure 2a sketches the model volume. The failure is a wedge indentation mechanism. It was recognized (Ladanyi & Johnston, 1974) for theoretical calculation of settlement or foundation resistance deep in frozen soil where punch penetration tests revealed fracture because of appearance of cracks instead of soil surface lateral heaving observed around more shallow foundations. To determine the strength at a pile tip we need accurate description of soil shear strength, soil strength at confined (three-dimensional) conditions, and time-dependent properties.
Figure 2b shows a finite element model of the pile loading test. Only scarce laboratory and field test results have validated the application of this theory. Besides, saline permafrost can undergo large yield strain and deviate from the assumption of the cavity expansion theory.
The test results will give valuable data for saline permafrost and hopefully contribute to validating current bearing and settlement theories and develop better ones. Besides, a thermo-hydro-mechanical (THM) model has been developed in the Geotechnics group at NTNU and UNIS, and the field-tests will be used for back-calculation of this model.
Most important, a robust and cost-saving pile design framework can be developed as a result of the tests and analysis, which would benefit future design for sustainable infrastructure and building in the Arctic.
Figure 2: Cavity expansion theory and FE simulation result
This investigation is located at the permafrost test field established as part of the Norwegian GeoTest Sites (NGTS) project.
Project name: Settlement of foundation on marine clay in Svalbard – thawing coastal and subsea permafrost
Test duration: March 2020 – December 2022
Chuangxin Lyu, PhD student, UNIS and NTNU
Arne Aalberg, Professor, NTNU and UNIS
Gustav Grimstad, Professor, NTNU
Gudmund Reidar Eiksund, Professor, NTNU
Knut Vilhelm Høyland, Professor, NTNU
Aleksey Shestov, Associate professor, UNIS
Contact person at UNIS: Aleksey Shestov.