Characteristics of compressive bearing capacity and resistance to foundation freeze-thaw of the isolation helical pile

Based on the isolation helical piles designed to resist vertical loads and eliminate the effects of freeze-thaw of foundation, the characteristics of force evolution, load transfer, and foundation response of two typical types of helical piles (S/D < S/D_cr and S/D≥S/D_cr) during settlement, as well as the influences and mechanisms of pile geometries on the bearing capacity of the single pile are revealed from the perspective of pile force. Furthermore, the performance of the new piles against freeze-thaw deformation of foundation is examined. The results show that: (1) The bottom helixes and the lower pile bodies of the two typical piles share the same stress characteristics during the settlement, whereas the stress magnitude and evolution characteristics of end resistances of the remaining upper helixes and side friction resistances of inter-helixe piles differ significantly. (2) The end resistance of each helix of piles with S/D≥S/D_cr is essentially equal, and the additional stress that the helixes exert on the underlying soil significantly enhances the side friction of piles. This also determines the top-down near-exponential decay trend of the pile side frictional resistance between the helixes. The end resistances of the upper helixes are only about 1/5 of the bottom ones for the piles with S/D < S/D_cr. (3) The compressive bearing capacities of the two types of piles all grow linearly as the embedment ratio, helix diameter and helix number increase, and linearly increase first and then decrease rapidly with the increment of pile diameters. These pile geometries possess heavier impacts on the bearing capacity of piles with S/D≥S/D_cr. The bearing capacity of piles with S/D < S/D_cr increases significantly with the increase of S/D, whereas that of piles with S/D≥S/D_cr decreases linearly. (4) The helical pile has a significant advantage over the conventional pile in resisting the impact of freeze-thaw deformation of foundation. The impact can be further eliminated by installing an isolation sleeve on the pile within the frozen soil depth.