Estimation of long-term set-up of driven piles in sand

ABSTRACT

Observations derived from updated field test databases indicate that total axial pile capacity of driven piles in sand continues to increase with time long after complete dissipation of excess pore pressure generated by pile driving. The increase in pile capacity with time is attributed to time-dependent increase in shaft capacity which is termed in the literature pile ‘set up’. Although, many hypotheses have been proposed in literature, the exact mechanism that governs the observed set-up is not yet fully understood and the estimation of the magnitude and rate of driven piles set-up remains relying on empirical relationships derived from widely scattered data.

In this paper, a new hypothesis is introduced which attributes the observed set-up to the recovery of the soil surrounding the pile shaft from the friction fatigue processes induced during pile driving and a new methodology is formulated for the estimation of the magnitude and rate of driven pile set-up in cohessionless soil. Comparison of predicted and measured shaft friction capacities of some well-documented field cases comprising previously untested piles indicates acceptable results and suggests that the proposed approach can be used to estimate long time set-up of driven piles in sand.

KEYWORDS:

• Pile
• set up
• pile aging
• friction fatigue

List of SymbolsEnglish Symbols

A	=	a parameter representing the percent increase in total pile capacity per log cycle of time.
As	=	is the surface area of the pile shaft.
B	=	a parameter representing the percent increase in shaft friction capacity per log cycle of time.
c	=	a parameter representing the rate of gain in shaft friction with the logarithm time.
$\bar{c}$	=	a parameter representing the average rate of capacity gain with the logarithm of time.
$D$	=	pile diameter.
$D_r$	=	Sand relative density.
$D_{50}$	=	mean diameter of sand particles.
$f_1$	=	correction factor to account for the effect of the direction of loading.
$f_2$	=	correction factor to account for the pile pint condition.
$k_o$	=	at-rest earth pressure coefficient.
$k_a$	=	Rankine’s active earth pressure coefficient.
$k_a$	=	Rankine’s passive earth pressure coefficient.
$k_{max}$	=	maximum earth pressure coefficient.
$k_{\mathrm{m}\mathrm{a}\mathrm{x}\left(tip\right)}$	=	maximum value for $K\left(z\right)$ at the pile tip.
$k_{min}$	=	minimum value for aaaaa suitable for very long pile.
$k_t$	=	earth pressure coefficient at time (t).
$k_{10DAD}$	=	earth pressure coefficient 10 days after pile driving.
$L/D$	=	the slenderness ratio of the pile.
$OCR$	=	over-consolidation ratio.
$P_a$	=	atmospheric pressure (101.3 kPa).
${\left(Q_s\right)}_{max}$	=	the maximum shaft friction capacity a driven pile in sand can reach long time following pile driving.
${\left(Q_s\right)}_t$	=	the ultimate shaft friction capacity at any time after pile driving.
Qo	=	pile capacity at time t = to.
Qt	=	pile capacity at time t.
$q_p$	=	pile end bearing pressure.
$R_{cal}$	=	center line average roughness.
$t_1$	=	a reference time (equal to 10 day).
$t_{75}$	=	the time required for the pile to reach 75% of its maximum shaft capacity after driving.
$t_m$	=	The time at which the pile reaches its maximum shaft friction capacity.
$t_{°}$	=	a reference time to ($t_{°}=1$) day.

Greek Symbols

${\gamma }^{\prime }$	=	effective unit weight of sand.
$\delta$	=	pile-sand friction angle.
$\mu$	=	rate of exponential decay of shaft friction.
${\sigma }_h^{\prime }$	=	horizontal effective stress.
${\sigma }_{ho}^{\prime }$	=	original horizontal effective stress.
${\sigma }_{hm}^{\prime }$	=	maximum horizontal effective stress.
${\sigma }_v^{\prime }$	=	vertical effective stress.
${\sigma }_{v,tip}^{\prime }$	=	vertical effective stress at the pile tip.
$\tau \left(z\right)$	=	unit shaft friction at depth z.
${\tau }_t$	=	unit shaft friction at any time, t.
${\bar{\tau }}_{EOD}$	=	the average unit shaft friction along the pile at the end of pile driving (1 day after driving).
${\bar{\tau }}_t$	=	average unit shaft friction at time t ≤ 10 days.
${\bar{\tau }}_{max}$	=	the average maximum unit shaft friction along the pile.
${\tau }_{10DAD}$	=	unit shaft friction along the pile after 10 days of pile driving.
${\bar{\tau }}_{10DAD}$	=	the average unit shaft friction along the pile after 10 days of pile driving.
${\bar{\tau }}_{1DAY}$	=	average unit shaft friction one day after driving.
${\phi }^{\prime }$	=	friction angle of sand.

Disclosure statement

No potential conflict of interest was reported by the authors.