Abstract
The thick sandstone with an upper layer of clay (2 m to 3 m) was identified at the transmission line projects in western China. Tower foundations are affected by extreme weather such as strong wind and snow all the year round, so the research of foundation uplift is very significant. Belled piles are widely used in the area due to its high uplift bearing capacity, however, further research studies regarding the uplift bearing capacity of the belled piles in this special scenario still need to be conducted. The uplift bearing characteristics of five rock-socketed belled piles were studied by field testing method. The load-displacement curves, fitting analysis of load-displacement curves, change rate of pullout stiffness, distribution of axial force alone the pile, and pile side friction were analyzed. Observation results show that the load-displacement curves of rock-socketed belled piles are steeply changed. Moreover, we find that the hyperbolic model can provide a good fitting result for load-displacement curves, and the ultimate uplift bearing capacity is suggested to be predicted by using a reduction parameter of 0.64–0.85. Both uplift bearing capacity coefficient Nu and utilization ratio of pile material η increase with the increase of L/d under a same diameter, while the growth rate decreases. In this paper, the significant size effect is observed with increasing of pile diameter. The change rate of pullout stiffness-displacement curves are consistent with the trend of the safety factor-displacement curves, and can be fitted by power function model. Additionally, the change rate of pullout stiffness at the limit state will decrease to around 154 in this study, which can be used as the instability threshold for the rock-socketed belled piles in a similar soil foundation.
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Abbreviations
- A :
-
Bottom area of enlarged head
- A′ :
-
Cross-sectional area of the pile
- a, b :
-
Hyperbolic model parameter
- c i :
-
Cohesion of pile-rock interface
- D :
-
Enlarged head diameter of the test piles
- d :
-
Diameter of the test piles
- d i−1 :
-
Diameter of pile segment i − 1
- E :
-
Elastic modulus of the pile
- F :
-
Safety factor of pullout
- fsi, Ni :
-
Pile side friction of layer i in belled pile toe and normal pressure of layer i in belled pile toe
- H :
-
Enlarged head length of the test piles
- L :
-
Length of pile
- m, n :
-
Parameter of power function
- N u :
-
Uplift bearing capacity coefficient
- Q, δ :
-
Uplift load and corresponding displacement of pile top
- Q i :
-
Axial force of pile body of section i
- Q u :
-
Ultimate uplift bearing capacity
- q si :
-
Average value of pile side friction resistance of layer i
- UCS:
-
Uniaxial compressive strength of intact rock
- V :
-
Volume of pile
- γ :
-
Natural unit weight
- ΔK :
-
Change rate of pullout stiffness
- Δl :
-
Length of pile segment between the i section and the i−1 section
- ΔQ, Δδ :
-
Change of the load on the top of the pile and the displacement variation at pile top
- δ u :
-
Displacement corresponding to ultimate uplift bearing capacity
- ε i :
-
Corresponding measured strain of section i by the strain gauges
- η :
-
Material utilization ratio of piles
- ϕ, c :
-
Internal friction angle of rock and cohesion of rock
- μ :
-
Friction coefficient
- σ i :
-
Axial stress of the pile of section i
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Acknowledgements
The research was supported by the National Key R & D Program of China (Grant No.2016YFC0802203) and Science and Technology Research and Development Program of China Railway Corporation (Grant No.2013G001-A-2).
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Wang, Q., Ma, J., Xiao, Z. et al. Field Test on Uplift Bearing Capacity of Rock-Socketed Belled Piles. KSCE J Civ Eng 24, 2353–2363 (2020). https://doi.org/10.1007/s12205-020-2011-0
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DOI: https://doi.org/10.1007/s12205-020-2011-0