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Bearing and uplift capacities of under-reamed piles in soft clay underlaid by stiff clay using lower-bound finite element limit analysis

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Abstract

Ensuring a safe foundation design in soft clay is always a challenging task to engineers. In the present study, the effectiveness of under-reamed piles in soft clay underlaid by stiff clay is numerically studied using the lower-bound finite element limit analysis (LB FELA). The bearing and uplift capacities of under-reamed piles are estimated through non-dimensional factors Ncul and Fcul, respectively. These factors increased remarkably and marginally compared to Ncul and Fcul of the piles without bulbs when the bulb is placed in stiff and soft clay, respectively. For a given ratio of undrained cohesion of stiff to soft clay (c2/c1), the factors Ncul and Fcul moderately increased with the increase in the length-to-shaft-diameter ratio (Lu/D) and adhesion factors in soft clay (αs1) and stiff clay (αs2). The variation of radial stress along the pile-soil interface, distribution of axial force in the under-reamed piles, and state of plastic shear failure in the soil are also studied under axial compression and tension. The results of this study are expected to be useful for the estimation of the bearing and uplift capacities of under-reamed piles in uniform clay and soft clay underlaid by stiff clay.

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Abbreviations

a :

a component of the equation representing the Mohr-Coulomb yield criteria

A s :

area of the cross-section of the shaft

c :

undrained cohesion of soil

c 1 :

undrained cohesion of soft clay

c 2 :

undrained cohesion of stiff clay

d :

a component of the equation representing the Mohr-Coulomb yield criteria

dr i, dz i :

radial and vertical distances between the lower and upper nodes of the ith edge along the shaft and surfaceof bulbs, respectively

D :

shaft diameter

D u :

bulb diameter

e base :

number of edges at the pile base

e s,1, e s,2 :

total edges along the shaft below and above the bulb, respectively

e bs,1 e ts,1 :

total edges along the bottom and top surfaces of the bulb, respectively

F cul :

non-dimensional uplift capacity factor for the layered clay

h b :

distance of the centre of the bulb from the pile base

LB FELA :

lower-bound finite element limit analysis

L 1 :

depth of the soft clay (top) layer

L 2 :

depth of the stiff clay (bottom) layer

L h :

horizontal domain extent from the pile surface

L u :

depth of embedment of the under-reamed pile from the ground level

L v :

vertical domain extent below the pile base

N cul :

non-dimensional bearing capacity factor for the layered clay

Q cl :

ultimate collapse load under axial compression for the layered clay

Q cl,base :

resistance from the base under axial compression

Q cl,bulb :

resistance from the bulb under axial compression

Q cl,shaft :

resistance from the shaft under axial compression

Q ul :

ultimate collapse load under axial tension for the layered clay

Q ul,base :

resistance from the base under axial tension

Q ul,bulb :

resistance from the bulb under axial tension

Q ul,shaft :

resistance from the shaft under axial tension

r i, r i+1 :

radial distance of the ith and (i + 1)th nodes along the pile base from the pile center

r 1,i, r u,i :

radial distance of the lower and upper nodes of the ith edge along the bottom and top surfaces of the bulb

α b :

adhesion factor at the pile base

α s :

pile-soil adhesion factor along shaft

α s1, α s2 :

pile-soil adhesion factor of soft and stiff clay, respectively

β :

angle of the bulb surface with respect to the horizontal plane (under-ream angle)

γ :

unit weight of soil

σ r :

normal stress in the r-direction

σ z :

normal stress in the z-direction

τ nt :

tangential stress along the surface of the bulb

τ rz :

shear stress in the r-z plane

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Acknowledgements

The authors gratefully acknowledge the financial support from ISIRD, SRIC, Indian Institute of Technology Kharagpur (No. IIT/SRIC/CE/PPL/2015-16/108).

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  1. Department of Civil Engineering, Indian Institute of Technology Kharagpur, Kharagpur, 721302, India

    Mantu Majumder & Debarghya Chakraborty

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Correspondence to Debarghya Chakraborty.

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Majumder, M., Chakraborty, D. Bearing and uplift capacities of under-reamed piles in soft clay underlaid by stiff clay using lower-bound finite element limit analysis. Front. Struct. Civ. Eng. 15, 537–551 (2021). https://doi.org/10.1007/s11709-021-0708-x

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