Proceedings of Pile 2013, June 2-4th 2013
STUDY OF ‘RAFT AND PILE RAFT FOUNDATION’ FOR ADDITIONAL FLOOR
OF PARKING BUILDING, A STRUCTURAL AND GEOTECHNICAL OVERVIEW
Livian Teddy1 and Petrus Chanel2
ABSTRACT: Polytechnic of Sriwijaya (Polsri) will build a 2-story parking building, ground floor as car
parking and upper floor as motorcycle parking. With the building load and soil conditions, it is planned to use
bored pile as the foundation. But underneath the additional parking, the existing pool is to be preserved because
the water will be used as a source of firefighting water. With this situation where 4 columns will be sitting on the
pool slab, ‘raft foundations’ or ‘pile raft’ are considered. It should be calculated in advance, whether the
allowable bearing capacity and settlement will be exceeded and whether the thickness pool floor strong enough
to withstand the load of columns. This paper discuss the result of the study.
Based on the CPT data, the estimated allowable bearing capacity ‘raft foundation’ at depth of 1,5 m is 0,5
kg/cm2, theoretically sufficient to carry out the load, but stress concentration should be analysed. Simple
structural calculation and finite element analysis is carried out and the results of analysis show that the
combination of pile foundation and the ‘raft foundation' can be implemented.
Keywords: raft foundation, pile raft, foot plate, allowable bearing capacity, punching stress
INTRODUCTION
Polytechnic of Sriwijaya (Polsri) will build a 2story parking building, ground floor as car parking
and upper floor as motorcycle parking. Considering
the building load conditions and the result of soil
investigation, it is planned to use bore pileas
foundation. But the location of the existing pool is to
be preserved because the water will be used as a
source of firefighting water. With this condition, it
should be studied to have 4 columns of the building
sitting on the pools slab, such as ‘raft foundations’ or
whether bored pile can be added below the pool slab
and the foundation will be pile raft system. It should
be analysed in advance, whether the foundation
allowable bearing capacity and settlement are
exceeded and whether the thickness pool floor strong
enough to withstand the column loads.
This paper presents the result of the study,
although perhaps just a simple experience that might
be able to inspire solutions to other similar cases.
RAFT FOUNDATIONS
Raft foundation or mat foundation is the bottom
of the raft-shaped structure that extends to all parts of
the base of the building. This section serves to
1
2
transfer building loads to the ground (Christiadi,
2002). The design of raft foundation consists of 2
methods (Mahdi):


The conventional rigid method
The conventional rigid method is simple
computation but is limited to rafts with relatively
regular arrangement of columns.
The finite element method utilizing computer
programs; The finite element method can be
used for the analysis of raft or pile raft
regardless of the column arrangements, loading
condition and existence of cores and shear
walls. The study used computer program
PLAXIS as an approach.
In the conventional method, spreading the
pressure on the base of the raft foundation, calculated
equation :
q = P/A ± Mx/Ix*y ± My/Iy *x ≤ qall
(1)
Where :
P = total number of foundation load (KN)
A = total area of raft foundation (m2)
x,y = consecutively coordinates at any point on a raft
made x, y axis direction through the central
area of heavy foundation (m)
Lecturer, University of Sriwijaya Palembang, livianteddy@gmail.com, INDONESIA
Geotechnical Engineer, PT Geotechnical Engineering Consultant, Bandung, INDONESIA
C10-1
600
G
I
720
720
720
K
650
2090
PLAT DUIKER
J1
PLAT DUIKER
cor beton
K-175
RABAT
-0.18
R.RUMAH
TANGGA
+0.20
1
-0.18
gorong - gorong
-0.15
-0.15
K
720
820
720
manhole-4
manhole-3
Kolam
r.mesin
tower air
650
980
J-1
175 175 225
manhole-5
I
cor beton
K-175
POS SATPAM
148 200
300
buis beton Ø 60 cm
400
110
buis beton Ø 60 cm
200
buis beton Ø 60 cm
R.SOPIR
+0.20
R.TEKNISI
LISTRIK
+0.20
290
L
PLAT DUIKER
Jalan Lingkungan Kampus
600
155
0c
2a
600
656
0b
600
3a
4c
600
1b
450
4b
300
6a
BM (50.000)
NK
215
DENAH LANTAI DASAR
a
Skala 1:300
300
600
600
0b
600
600
155
656
720
I
720
buis beton Ø 60 cm
buis beton Ø 60 cm
PLAT DAK BETON
cor beton
K-175
buis beton Ø 60 cm
400
G
WC
400
320
400
G
400
400
H
120 200
358
150
1370
K
2090
PLAT DUIKER
PLAT DUIKER
PLAT DAK BETON+2.70
PARKIR LANTAI 2
+ 4.00
L
720
PLAT DAK BETON
720
-0.18
L
820
K
tower air
980
1370
r.mesin
J
390
I
0c
1b
2a
450
RABAT
695
3a
4b
4c
6a
3845
PLAT DAK BETON
PLAT DUIKER
NK
215
PLAT DUIKER
300
Jalan Lingkungan Kampus
600
0c
1b
600
155
556
0b
600
3a
600
2a
6a
450
4b
300
4c
695
3845
DENAH LANTAI ATAS
b
Skala 1:300
Figure 2. Parking Building Plan,
a). Ground floor plan, b). Upper floor plan
A
300
450
BS 30X60
BS 30X60
600
600
BS 30X60
BS 30X60
BS 30X60
390
BS 30X60(A)
BS 30X60(B)
BS 30X60(A)
BS 30X60(B)
BS 30X60(A)
BS 30X60(B)
BS 30X60(B)
BS 30X60(B)
BS 30X60
BS 30X60(A)
330
390
BS 30X60
600
KOLAM EKSISTING
BS 30X60(A)
BS 30X60(A)
BS 30X60(A)
BS 30X60(A)
325
980
BS 30X60(A)
BS 40X80(A)
BS 30X60(A)
325
BS 30X60(A)
BS 30X60(B)
BS 30X60(A)
BS 30X60(B)
BS 30X60(A)
BS 30X60(B)
BS 30X60
BS 30X60(B)
BS 30X60(B)
BS 40X80
= 2 for corner column
= 3 for exterior column
= 4 for interior column
600
BS 30X60
BS 30X60
BS 30X60
B
BS 30X60
BS 30X60
BS 30X60
BS 30X60
450
BS 30X60
BS 30X60
BS 30X60
300
BS 30X60
BS 30X60
BS 30X60
Figure 1. Critical punching shear perimeter
BS 30X60
BS 30X60
BS 30X60
BS 30X60
b
BS 30X60
BS 30X60
BS 30X60
BS 30X60
600
600
600
600
720
BS 30X60
720
C2
656
3845
The concrete strength for punching is the least of
following three value :
1. qup = 0,316*  fcu/c
(5)
2. qup = 0,316*(0,5+c1/c2)*  fcu/c
(6)
3. qup = 0,8*(0,2+ *d/U)*  fcu/c
(7)
a
155
L
300
180
C1
300
manhole-2
manhole-1
GDG.INVENT
390
J
720
325
695
The applied punching shear stress qup equals :
qup = Qup / U x d
(4)

300
PLAT DUIKER
180
The critical perimeter is at d/2 (d=foundation
thickness) from the face of the column. The
critical shear ‘U’ is calculated as shown figure
below and applied punching load Qu is obtained
after subtracting the load of the punching area
(axb) by ultimate pressure at this point qsu.
Thus:
Qup = Pu – qsu (axb)
(3)

600
RABAT
WC
400
400
G
400
400
H
150 187
320
358
320
600
PLAT DAK BETON

600
PLAT DUIKER
As for the design of shear punch:
The punching load for each column is calculated
by multiplying the applied working load with the
load factor.
Pu = 1,5 x Pi
(2)
Pu = column ultimate load (KN)
Pi = column working load (KN)

0c
600
0b
2a
450
2
300
1b
3a
4c
4b
6a
695
PLAT DUIKER
Ix, Iy = moment of inertia of the x-axis and y-axis
(m4)
ex, ey = eccentricity in x and y direction
q = pressure on the base of the foundation due to the
load on it (KN/m2)
qall = allowable bearing capacity (KN/m2)
REVISI DENAH SLOOF & PONDASI
a
SKALA 1 : 120
BUILDING DATA
300
B 50X100(A)
300
B 30X60
300
600
B 30X60(B)
B 30X60(B)
B 30X60(B)
B 30X60(B)
B 30X60(B)
980
720
B 50X100(A)
B 50X100(A)
B 50X100
B 30X60
300
600
b
B 50X100
B 50X100(A)
B 30X60(B)
B 30X60(B)
300
300
B 50X100(A)
B 50X100
B 50X100
B 30X60
300
600
300
B 50X100
B 50X100(A)
B 50X100(A)
B 30X60(B)
B 30X60(B)
B 30X60(B)
B 30X60(B)
300
300
B 50X100
B 50X100
B 30X60
225
300
B 50X100
B 50X100(A)
B 40X80
B 50X100
B 30X60
450
300
B 50X100(A)
B 50X100
B 50X100
B 50X100
225
300
B 50X100
B 50X100(A)
B 30X60
300
300
B 50X100
B 30X60(A)
B 30X60(A)
390
225
B 50X100
1370
225
B 30X60
300
300
600
DENAH BALOK LT.01
SKALA 1 : 150
Figure 3. a). Foundation & tie beam plan,
b). Upper floor beam plan
C10-2
720
300
Ground floor plan functioned as car parking can
be seen that there is a pool position (hatching mark)
(Figure 2a). While the upper floor functioned as a
motorcycle parking (Figure 2b). In tie beam and
foundation plan (Figure 3a) can be seen placement
foundation foot plate (P1 to P4) on the pool floor
plate. In pool section (Figure 4) in addition to the
pool there is a floor plate, to support the weight of
the pool water is also fitted beam 0.4 x 0.8 m.
30 58
20 60 20
5
87
40
sloof 30/60
PLAT LANTAI KOLAM T=20CM
325
330
73 15
dilubangi & diperbaiki
dg. produk ex. SIKA
107
162
147
sloof 30/60
20
895
15
58 30
dilubangi & diperbaiki
dg. produk ex. SIKA
153
895
15 73
1070
BALOK 40X80
POTONGAN A-A
SKALA 1 : 100
263
600
600
2395
600
332
30 58
dilubangi & diperbaiki
dg. produk ex. SIKA
15
58 30
dilubangi & diperbaiki
dg. produk ex. SIKA
20 60 20
162
147
sloof 30/60
448
5
sloof 30/60
PLAT LANTAI KOLAM T=20CM
500
500
73
500
448
2395
BALOK 40X80
BALOK 40X80
73
2570
BALOK 40X80
BALOK 40X80
POTONGAN B-B
SKALA 1 : 100
Figure 4. Pool sections
For soil investigation results of the data that is
generated two 2 point sondir (CPT) (Figure 4) and 1
point SPT (Figure 5).
Figure 6. SPT graph and drilling log
Figure 5. CPT graph
Figure 7. Position of columns on the pool
GEOTECHNICAL INVESTIGATION
Summary from the results of CPT and SPT soil
layers at the site plan as follows:
Table 1. Summary result
Depth
N
qc (kg/cm2)
Soil Layers
0,00 – 3.20
3
5
Soft clay
3.20 – 4,80
12
6 - 13
Stiff clay
4,80 – 10.0
47
25 – 65
10.00-end of
drilling
54
80
Very stiff to
hard clay
Clay shale
ANALISIS AND DISCUSSION
Based on the CPT data, by a simple calculation
allowable bearing capacity ‘raft foundation’ :
qall = qc / 20 and at 1.5 m depth qall = 0,25 Kg/cm2.
Combination of DL + LL load on each column
are:
P1 = 860 KN
P2 = 940 KN
P3 = 940 KN
P4 = 940 KN
TOT = 3680 KN
STEP 1
q = P/A + Mx/Ix*y + My/Iy *x
A = 24,55*9,55 = 234,45 m2
Ix = BL3/12 = 9,55*24,553/12 = 11.775,4 m4
Iy = LB3/12 = 24,55*9,553/12 = 1781,9 m4
Total vertical = P1+P2+P3+P4 = 3680 KN
x = 1/3680*(4,42*3680) = 4,42 m
ex = x – B/2 = 4,42 – 9,55/2 = -0,355 m
y = 1/3680*[(3,62*940)+(9,62*940)+(15,62*940) +
(21,62*860)] = 12,42 m
C10-3
ey = y – L/2 = 12,42 – 24,55/2 = 0,145 m
Mx = Ptotal* ey = 3680*0,145 = 533,6 KNm
My = Ptotal* ex = 3680*(-0,355) = -1306,4 KNm
q = P/A + Mx/Ix*y + My/Iy *x
= (3680/234,45)+(533,6/11.775,4)*y+
(-1306,4/1781,9)*x
Weight concrete pool + pool water = 0,25 kg/cm2
Allowable bearing capacity qall netto ‘raft foundation’
= 0,5 kg/cm2 – 0,25 kg/cm2 = 0,25 kg/cm2 = 25
KN/m2.
Table 2. Pressure on soils under the columns
Point
P1
P2
P3
P4
x (m)
-0.355
-0.355
-0.355
-0.355
y (m)
9.20
3.20
-0.28
-8.80
q (KN/m2)
16.37
16.10
15.94
15.56
mark
ok
ok
ok
ok
The results of calculation shows, P1 to P4
columns no load exceeds allowable bearing capacity
<25 kN/m2.
STEP 2
To obtain a stable column placement on the pool
floor, foot column must be widened as foot plate to
reduce the punching stress on the pool floor plate.
Pu = 1,5*Pi = 1,5*P4 = 1,5*940= 1410 KN
Foot plate thickness (d) = 50 cm
Foot plate quality of concrete = K-300
a = C1 + d = 500+500 = 1000 mm
b = C2 + d = 500+500 = 1000 mm
u = 2*(a+b) = 2*(1000+1000) = 4000 mm
C1=500
a=1000mm
C2=500
P4 = 15,56 KN/m2 (see table 2)
quP4 = 1,5*15,56 = 23,34 KN/m2
Punching stress :
Qup = Pu - quP4*(a*b)
= 1410 – 23,34*(1*1) = 1386,66 KN
qup = Qup/(u*d) = 1386,66/(4000*500) = 0,69 N/mm2
Concrete strength :
1). qup = 0,316* fcu/c
= 0,316*(30/1,5) = 1,41 N/mm2
2). qup = 0,316*(0,5+c1/c2)* fcu/c
= 0,316*(0,5+0,5/0,5)*(30/1,5)
= 2,12 N/mm2
3). qup = 0,8*(0,2+4*d/u)*  fcu/c
= 0,8*(0,2+4*0,5/4)*(30/1,5) = 2,5 N/mm2
Table 3. Summary of punching shear at foot plate for
each columns
Point
P1
P2
P3
P4
Pi (KN)
860.0
940.0
940.0
940.0
Pu (KN) q (KN/m2) qu (KN/m2) qup (N/mm2)
1290.0
16.37
24.56
0.63
1410.0
16.10
24.15
0.69
1410.0
15.94
23.92
0.69
1410.0
15.56
23.34
0.69
mark
ok
ok
ok
ok
Punching stress foot plate foundation (0,63 to 0,69
N/mm2) < foot plate concrete strength (1,41 N/mm2)
 OK. Means thickness foot plate strong enough to
withstand the load P1 to P4 column.
STEP 3
To check, whether thick slab strong enough to
withstand the load of the foundation foot plate.
Pu = 1,5*Pi = 1,5*P4 = 1,5*940= 1410 KN
Pool floor plate thickness (d) = 20 cm
Pool floor quality of concrete = K-225
a = C1 + d = 1500+200 = 1700 mm
b = C2 + d = 1500+200 = 1700 mm
u = 2*(a+b) = 2*(1700+1700) = 6800 mm
C1=1500
b=1000mm
Figure 8. Critical punching shear perimeter foot plate
a=1700mm
C2=1500
b=1700mm
Figure 10. Critical punching shear perimeter pool
floor plate
Figure 9. Foot plate foundation section
C10-4
P4 = 15,56 KN/m2
quP4 = 1,5*15,56 = 23,34 KN/m2
Punching stress :
Qup = Pu - quP4*(a*b)
= 1410 – 23,34*(1,7*1,7) = 1342,55 KN
qup = Qup/(u*d) = 1342,55/(6800*200) = 0,99 N/mm2
Concrete strength :
1). qup = 0,316* fcu/c = 0,316*(22,5/1,5)
= 1,22 N/mm2
2). qup = 0,316*(0,5+c1/c2* fcu/c
= 0,316*(0,5+1,5/1,5)*(22,5/1,5)
= 1,62 N/mm2
3). qup = 0,8*(0,2+4*d/u) * fcu/c
= 0,8*(0,2+4*0,2/6,8)*(22,5/1,5)
= 0,98 N/mm2
FINITE ELEMENT ANALYSIS
The proceeding discussion is mainly on the
structural overview of the foundation system with
raft only. Hence, it is required that geotechnical
analysis is to be presented with raft and with pile raft
where the pile is selected a strauzz pile with 300 mm
diameter at 6m depth, with one column sitting in one
pile. To carry out the geotechnical analysis, 2D finite
element analysis has been conducted using Plaxis
computer program. The following is the results of
analysis.
Table 4. Punching stress pool floor plate
recapitulation for each foot plate foundation
Point
P1
P2
P3
P4
Pi (KN)
860.0
940.0
940.0
940.0
Pu (KN) q (KN/m2) qu (KN/m2) qup (N/mm2)
1290.0
16.37
24.56
0.90
1410.0
16.10
24.15
0.99
1410.0
15.94
23.92
0.99
1410.0
15.56
23.34
0.99
mark
ok
ok
ok
ok
Thus :
Punching stress plate floor pool (0,90 to 0,9
N/mm2)
<Pool
concrete
strength
(1,22
N/mm2)OK. Means thickness foot plate strong
enough to withstand the load foot plate foundation
from P1 to P4 column.
The following table is soil parameters used for analysis.
No.
Material Set
Depth
N-SPT
γunsat
γsat
E
Soil and Interface
(m)
(blow)
(kN/m3)
(kN/m3)
(kN/m2)
1
Soft Clay
0-2.8
3
14
18
2100
2
Stiff Clay
2.8-4.8
12
16
20
3
Hard Clay
4.8-7.8
47
18
4
Very Hard Clay
7.8-16.5
54
5
Concrete
-
-
C
φ
(kN/m2)
(o )
0.45
37.5
0
8400
0.35
300
0
22
32900
0.2
400
0
18
22
37800
0.15
500
0
24
24
2.1E7
0.15
-
-
The sequence of loading analysis is following the
steps
1. Excavation and construction of raft and piles
2. Reset displacement to zero
3. Application of column load
4. Application of water load on the pool
ν
maximum bending moment is 90.8 kN-m/m which is
still below the structural capacity of the concrete slab.
Analysis with pile raft reduce the settlement to
12.82 mm which is acceptable. The shear force is
higher -85.57 kN/m but the shear is resisted by the
piles, hence it will not damage the concrete. And the
maximum moment is 35.52 kNm/m, much less
compared to the condition of raft only.
CONCLUSIONS
It can be seen from the finite element analysis,
that by raft only, the risk of settlement is 27 mm
which is a little bit above the requirement [25 mm].
The maximum Shear force is -57.68kN/m and
From the above calculation with the additional
burden of four columnns above the pool floor plate,
apparently carrying capacity of the soil below the
base floor pool plate is not exceeded. So is thick
foundation foot plate and pool floor strong enough to
withstand the load of columns from structural point
of view.
The results of finite element analysis also proved
that all of the structural load can be carried out by the
existing slab both by raft or pile raft. The decision of
C10-5
the engineer is to use raft only with slightly higher
settlement above the requirement.
This study, although very simple, gives a clear
understanding on the use of pile raft and raft on this
case. Structural analysis combined with geotechnical
analysis are important tools from design point of
view rather than only from each perspective.
REFERENCES
Christiadi H, Hary (2002), “Teknik Pondasi 1”,
Penerbit Beta Offset,Yogyakarta
Mahdi, Hisham Arafat (….), Design of Raft
Foundation,
modul
kuliah
Faculty
of
Engineering-Sixth of October University.
C10-6