Dynamic Strain of Reinforced Concrete Piles at the Head in Driving

Abstract

This study has been carried out to decide the reinforcement method of the impacted concrete pile by the pile driving.
From the analysis of measured values of the dynamic strain on the pile head, the following conclusions have been obtained.
Moreover, the author wishes to promote this study and to find out the behaviours of the pile driven in the ground.
(1) The relation between the dynamic energy of the hammer and the dynamic strain in the pile.
ε_V.100=F(1-k)/EA(S+R/2) (1)
ε_V.50=F(1-k)/EA(S+R) (2)
where ε_V.100=the dynamic strain at the point of 100cm from the top of the pile
ε_V.50=dynamic strain at the point of 50cm from the top of the pile
F=dynamic energy of hammer
A=sectional area of the pile
E=Young's modulus of concrete
S=penetration
R=rebound
(1-k)=efficiency of the dynamic energy of hammer
W=hammer weight
H=hammer stroke
For F, (1-k) and E in Eqs. (1) and (2), the following values in Table I and Table II is available.
Table I
Table II
F_c=ultimate compressive strength of concrete
(2) The relation between the sectional area of the pile and the dynamic strain of the head of the pile.
When the PC pipe pile (350mmφ or 300mmφ) is driven in the ground with the Type 22 diesel hammer, the value of ε_V.50 is assumed by Eq. (3)
ε_V.50=(987-0.57A)×10^-6 (3)
where, A=equivalent sectional area of the pile (cm²)
(3) The distribution of the dynamic strain of pile head
The axial dynamic strain ε_V.x at the point of distance x from the top of pile is expressed as the following general Eq. (4).
ε_V.x=ax^-b (4)
where, a and b=constant characteristic
Then the dynamic strain of PC pile, which is driven with diesel hammer 22, is expressed in Eqs. (5) or (6).
For PC pile (D=300mmφ, t=70mm)
ε_V.x=9.02×10⁴×x^-0.0897 (5)
For PC pile (D=350mmφ, t=80mm)
ε_V.x=10.92×10⁴×x^-0.152 (6)
(4) The relationship between the vertical (axial) dynamic strain ε_V and horizontal dynamic strain ε_H.
The value of ε_H/ε_V at the point (such as x=10cm), near the top of pile, shows the variable value as ε_H/ε_V=0.1∼0.4.
But ε_H/ε_V values at 35cm point and at 70cm point from the top of pile become the constant values as follows:
For RC pile (ε_H/ε_V)=0.2 (7)
For PC pile (ε_H/ε_V)=0.24 (8)
(5) The suggestion for the reinfocement range of the head of pile
By assuming the ultimate compressive strain of concrete as ε_cu=2500×10^-6 and the ultimate tensile strain as ε_tu=125×10^-6 and applying ε_tu and ε_cu to the Fqs. (5), (6), (7) and (8), the author has suggested the following idea.
1) The minimum width of the iron band to reinforce the pile head is necessary to be in the range of 20∼30cm.
2) The pile must be specially reinforced by spiral hoop at least 100cm in length from the top of the pile head.