657
Technical Committee 103 /
Comité technique 103
5.
RESULTS, ANALYSES, AND DISCUSSION.
The results obtained from the model 1 are plotted in the form of
load settlement response curves and presented in Figure 4. The
load settlement response of 12m pier is presented separately in
figure 5.
For the pier lengths of 12m, 16.8m, and 18mm. At any stage of
settlement, it was found that the load taken by the piled raft was
far higher than the load taken by the unpiled raft for the
corresponding settlement. The results are studied independently
for the three cases analysed, and then they are compared. From
the load settlement response of the unpiled raft and the piled
raft, the load shared by the pier (pile group) is computed at
different settlement levels namely 12mm, 25mm, 50mm, 80mm
and 100mm and has been presented in Table 1.
Table 1 Load Sharing Ratio At Various Settlement Levels
Settlement
Pier
length 12m
m 20mm 50mm 80m
m 100mm
12.0m 0.54 0.53 0.37 0.30 0.25
16.8m 0.64 0.65 0.60 0.58 0.56
18.0m 0.61 0.56 0.64 0.59 0.59
In the case of load settlement response of all the three cases, in
the initial stages upto a settlement level of 25mm,the piled raft
exhibits a higher stiffness, with the very small rate of change in
the stiffness. As seen from the table 1, the loads shared by the
pier in the initial stages are higher and then gradually reduces
with settlement. This indicates that the major part of the applied
load is taken by the pile group or the pier. Beyond this level the
rate of fall in the stiffness increases rapidly indicating that the
full friction has been mobilised and the raft starts taking a
higher load. This stage exists upto a settlement level of
75mm.Beyond this level the rate of fall of stiffness further
increases rapidly even for a small increment in the load. In the
case of piled raft with 16.8m deep pier the load corresponding
to 25mm settlement is higher than the previous case by 100%
indicating that the pier mobilises a higher friction in the linear
elastic stage. At 75mm settlement level the increase in the load
taken by the 16.8 m deep pier is higher by 60%, indicating that
the load shared by the pier reduces gradually. and in the case of
18m deep pier this increase is only 15% when compared to
16.8m deep pier.
6.
EFFECT OF PEAT LAYER
The study of the Table 1 and the Figure 6 which presents the
shaft stress distribution with the depth indicates that the load
sharing ratio and the shaft stress indicate an increase and then a
fall. The shaft stress increase commences at a level of 13m and
extends upto 16m level; and then it reduces. In the case of load
sharing ratio the increase takes place at a settlement level of
20mm in the case of 16.8m deep pier and 50mm level in the
case of 18m deep pier. This trend is absent in the case of 12 m
deep pier which is above the peat layer.
The most probable reason for this behavior is that at a higher
load the peat layer generates a negative skin friction causing a
higher load on the pile group. This results in the sudden increase
in the shaft stress and the load sharing ratio value. Figure 6
presents the mobilisation of shaft friction with depth. It is seen
that the shaft friction increases and then falls down rapidly with
depth confirming the ductile behaviour in the sense that major
part of the load is transferred by friction.
7.
HYPERBOLIC BEHAVIOUR
The curve relating to the 12m deep pier exhibits in a distinct
manner a three phase behaviour; namely OA, which is a linear
elastic stage AB a visco- plastic stage and BC the plastic stage.
In the other two cases the third stage is has not reached mainly
because the piled was still capable of taking higher
load.Typically the piled raft with 12 m pier depth had exhibited
a hyperbolic behaviour and it has been loaded close to failure
Figure 4 Load settlement response
Figure 5. Load settlement response 12m pier
Figure 6 Shaft stress mobilisation (18m deep pile)
Figure 7
Chin’s graph (12m pier)
