Interpretation of bi-directional pile load Tests using instrumented gauge data

ABSTRACT

This study presents a method that is conventionally used for interpretation of a head-down pile load test, which is modified and extended for a bi-directional pile load test. A parabolic function is used to simulate the load transfer curve along a depth above or below the load cell. The coefficients of the function are obtained by fitting the measured gauge data. Three bored piles are tested using a bi-directional load cell that is installed in the middle of the pile shaft and the results are interpreted using the method of this study. An equivalent head-down load-displacement curve that is obtained using the presented method produces results that are in good agreement with those that are obtained using the conventional method. The relationship between mobilized unit skin friction and displacement that is obtained using the measurement data and using the presented method shows similar trend.

CO EDITOR-IN-CHIEF:

• Ou, Yu-Chen

ASSOCIATE EDITOR:

• Ou, Yu-Chen

KEYWORDS:

• Bi-directional load test
• head-down load test
• bored pile
• equivalent load-displacement curve

Nomenclature

$A$	=	Cross-sectional area of pile.
$A_c$	=	Concrete cross-sectional area of pile.
$A_s$	=	Steel cross-sectional area of pile.
$C$	=	A centroid factor.
$D_s$	=	Diameter of Pile.
$E_c$	=	Elastic modulus of concrete.
$E_p$	=	Elastic modulus of pile.
$E_s$	=	Elastic modulus of steel.
$e_{PR}$	=	Pile compression for the pile section below the load cell.
$e_{PS}$	=	Pile compression for the pile section above the load cell.
$f_R\left(z\right)$	=	Unit shaft resistance.
$k_0$	=	A constant that is determined by regression analysis of back analyzed curve.
$k_1$	=	A constant that is determined by regression analysis of back analyzed curve.
$L$	=	Total pile length.
$L_R$	=	Pile length for the pile section below the load cell.
$L_s$	=	Pile length for the pile section on the top of the load cell.
$P_b$	=	Mobilized base resistance of pile.
$P_0$	=	Equivalent head-down load.
$P_R$	=	Mobilized shaft resistance for the pile section below load cell.
$P_s$	=	Mobilized shaft resistance for the pile section above load cell.
$P\left(j\right)$	=	Pile axial force at any rebar strain gauge level j.
$P\left(z\right)$	=	Axial force in pile at depth z.
$q_u$	=	Uniaxial compressive strength of rock.
U	=	Initial tangent modulus for the concrete in the pile.
$w\left(z\right)$	=	Pile displacement at depth z.
$W_t$	=	Weight of pile.
${\alpha }_1$	=	Constant coefficient.
${\alpha }_2$	=	Constant coefficient.
${\alpha }_3$	=	Constant coefficient.
$\mathrm{\Delta }$	=	Additional pile compression between the head-down and the bi-directional load.
${\mathrm{\Delta }}_b$	=	Pile toe displacement.
${\mathrm{\Delta }}_d$	=	Pile compression induced by equivalent head-down load of $P_0$.
${\mathrm{\Delta }}_h$	=	Pile compression induced by equivalent head-down load of $P_s$.
${\mathrm{\Delta }}_o$	=	Load cell location displacement.
${\mathrm{\Delta }}_{o1}$	=	Pile compression induced by upward load from load cell.
${\mathrm{\Delta }}_{o2}$	=	Pile compression induced by downward load from load cell.
${\mathrm{\Delta }}_s$	=	Average shaft displacement for the section above load cell.
$\in$	=	Measured strain from rebar gauge.
$\phi$	=	International friction angle of soil.

Acknowledgments

This study is part of a research project funded by the National Science and Technology Council (110-2221-E-019 -016 -), Taiwan. The principal author is grateful for this financial support.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the National Science and Technology Council [110-2221-E-019 -016 -].