8 with the parameters as shown in the table 1. In
this report, the piles and the plate are made of concrete. Since the clay is soft, the strength and
the elasticity of the piles and the plate materials are assumed to be much stiffer than the soil.
Then the behaviour of the soil-foundation system depends on the entire system than the
material of the structures. The pile is 1.5 cm in the diameter and 30 cm of length. The centre to
centre space of piles in group is 7.5 cm which is equal to 5 times the diameter of the piles. The
wooden plate is 15 cm x 5 cm, the concrete plate is 15 cm x 15 cm of the area and 2 cm of the
thicknesses. Table 1. The soil parameters
No. parameter value unit 1 water
content 90.65
2 unit weight
1.46 tonm
3
3 specific gravity
2.69 -
4 plastic index
25.92 5 Cu
0.012 kgcm
2
6 q
c
0.08 kgcm
2
The laboratory tests are carried on using the following procedures. The models of pile are
initially driven into the soil individually. Then the load is incrementally applied on the top of
the pile. The load and the displacement are recorded along the loading. The loading is
terminated when the load value seems to be constant for number of displacement steps. The
load-displacement curves are drawn subsequently. The ultimate capacity of each
single pile can be estimated from load- displacement curves resulting from the tests.
Then, the group of the piles are pushed in the same way in to the clay. The axial load the
applied on the centre of the pile group to obtained the load-displacement curve of the
foundation. The load-displacement curves of the pile group are also drawn subsequently. In the
same way, the raft pile foundation and the separate plate on the clay are tested.
There are two types of raft-pile system tested in the laboratory, the first is 2x1 wooden raft-pile
system sand the other one is 2x2 concrete raft- pile foundation. The procedures used in
laboratory for the both foundations are the same. a. Single Pile and Group of Piles
b. Plate and Raft-Piles System Figure 1. Laboratory models of the foundation
4. Test Results
4.1. Test Results For 1x2 Wooden Pile-Raft System
The results of the tests are presented in the terms of load displacement curves as follows. Figure 2
and 3 show the plot of load-displacement of the 2 single piles. the firs one gives the maximum
load of 25 unit of load and the second one is 22. the unit load is the value shown by the dial
which is the unit equal to about 0.2 kg. the maximum loads are reached at the displacement
of 0.8 mm for pile 1 and 1.2 mm for pile 2.
Universitas Sumatera Utara
Novrial
9 Figure 2. Load-displacement of pile 1
Figure 3. Load-displacement of pile 2 The plot of load-displacement of the group of
the pile 1 and 2 is shown in the figure 4. the maximum load of the group is 42.5 unit reached
at the displacement of 1.1 mm. the maximum load is about 91 of the sum of the pile 1 and 2
capacities. the value may be referred as the efficiency of the group.
Figure 4. Load-displacement of the pile group The test result in the terms of load displacement
curves for the plate and the raft-pile system are shown in the figure 5 and 6. The loading test of
the plate gives the maximum load of 6.9 and the 60.4 for the raft-pile system. the maximum loads
are reached at the displacement of 2.00 mm for the plate. For the raft-pile system, the maximum
load is reached at the displacement of 2.20 mm. Figure 5. Load-displacement of the plate
5 10
15 20
25 30
50 100
150 200
disp. o.o1 mm Load unit
Pmax = 25 Dmax = 80
1 2
3 4
5 6
7 8
100 200
300
disp. o.o1 mm Load
uni t
Pmax = 6.9
Dmax = 200
5 10
15 20
25 30
50 100
150 200
disp. o.o1 mm Load unit
Pmax = 22 Dmax = 120
10 20
30 40
50 60
100 200
300 disp. o.o1 mm
Load unit
Pmax = 42.5 Dmax = 110
1 2
3 4
5 6
7 8
100 200
300 disp. o.o1 mm
Load unit
Pmax = 6.9
Dmax = 200
Universitas Sumatera Utara
10 Figure 6. Load-displacement of the raft-pile
system The maximum load values of the models are
represented in the Table 2. It shows that the maximum load of the raft is higher than the
group of pile. Further more, the maximum value of the raft-pile system is higher that the sum of
the piles and the plate. In the other words, the efficiency of the raft-pile system is more than
100 which is 130 for these tests compared to the sum of piles’ capacity. Even the plate
only gives small contribution to the total load max 15 , the load displacement of the pile is
similar to the plate compared to the piles’ one Figure 7.
Table 2. Resume of the results
Model : pile
1 + 2 group of pile plate
piles + plate
raft-pile P max.
unit 46,5 42,5 6,9
51,5 60,4
Ratio to pile 1+2
100 91 15 111
130 Disp. max
o.o1 mm 100 110 200
170 220
Figure 7. Load-displacement of the models If the efficiency of the raft-pile system is defined
as ratio of the system compared to the sum of the individual pile and the raft, the efficiency of
the raft-pile system , E
rp
, is written as: capacity
raft and
pile individual
the of
sum the
capacity pile
- raft
the E
rp
= Using the above equation, the efficiency of the
raft-pile system is:
117 51.5
60.4 E
rp
= =
which is greater than 100.
4.2. Results For 2x2 Concrete Pile-Raft System
