85_HWKTTE 4004c - 75 decrease in the force at time...

75
decrease in the force at time 2L/c (see Figure 7-2). This condition is attributed to piles
with small tip capacity, and piles that, although they possess large tip capacity, the energy
imparted by the hammer is too large to mobilize the tip capacity.
The proposed method makes use of the two concepts mentioned to determine the
tip to skin ratio. The basic assumption of this method is that the tip/skin ratio is equal to
the decrease in the wave up trace from point "b" (time 2L/c minus rise time) to point "c"
(time 2L/c from maximum force) divided by the net increase in the wave up trace from
point "a" to point "b". The mathematical expression for the proposed method is
( 29 ( 29
[ ]
( 29 ( 29 [ ]
a a b b
c c b b
ZV F ZV F
ZV F ZV F
Skin
Tip
- - -
- - -
= (7-5)
The use of wave up at point "a" is to account for any error in the instrumentation
readings. Because a decrease in the wave up is needed to obtain a tip to skin ratio, the
hammer must be able to mobilize the whole pile capacity.