Investigation by Cone Penetration Tests of Piled Foundations in Frozen Soil Maintained by Thermosyphons

Abstract

Infrastructure built on permafrost may experience differential settlement and deformation due to permafrost degradation caused by many factors such as climate change, man-made impact, redistribution of snow cover, alteration of subsurface water flow, etc. Even a small change in the temperature of frozen soil can significantly alter (increase or decrease) the mechanical properties of the soil and hence the bearing capacity of piles. Modern Cone Penetration Testing (CPT) provides direct measurement of soil resistance and temperature in permafrost, thereby permitting the validation of pile capacity in permafrost conditions. An example application of CPT tests to diagnose the condition of piled foundations at the Salekhard College, Western Siberia, Russia, is described. The frozen soil is maintained by thermosyphons that are exposed in the crawl space below the building.  CPT tests were completed from within the crawl space to measure cone resistance q_c [MPa], sleeve friction f_s [kPa] and temperature T [°C] at several locations close to the thermosyphons and piles.  The data was obtained both within the zone of influence of the thermosyphons and at distance from the thermosyphons and piles, to provide “baseline” measurements. Based on this data; 1) the cooling effect of the thermosyphons was directly measured (soil temperature decrease), 2) the pile bearing capacity could be estimated and 3) pile capacity could be compared at different locations beneath the building.  In future projects it would be useful for reference during subsequent verification of foundation conditions to obtain data on the baseline soil properties and temperature conditions before installation of the piles and thermosyphons.