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1 5. Conclusions and Recommendations
5.1 Conclusions
At present most of the local design engineers do not consider the impact of temporary construction cost on total construction cost of the bridge. This study was carriedout to find the impact of coffer-damming and de-watering cost on the total construction cost of bridge foundation and hence the total construction cost of the bridge. Also this study establishes valuable relationships among various bridge construction activities.
Analysis of the data collected from the Road Construction and Development Co. (Pvt.) Ltd., pertaining to recent bridge projects have been carried out. In the first category, construction cost for 11 completed bridges have been analysed to establish valuable relationship among various bridge construction activities. In the second category, construction (direct and total) costs of foundations of 16 bridges have been analysed to establish relationship between direct construction costs and total construction costs of foundations.
The first category of sample contains all three types of foundations, including three bridges of spread foundations, five bridges of pile foundations, a bridge of caisson foundations, a bridge having combination of spread and pile foundations and the other bridge having combination of all three types of foundations. It is therefore inappropriate to analyse the average relationship among different bridges having same foundation types, as the sample that represents it would become small. Also results should be read cautiously, because of the following reasons;
a) The small samples of bridges were analysed. b) All bridges are standard types, simply supported girder bridges, spans ranges from
16.2 m to 7.0 m. c) Bridges have been constructed at different point of times. d) Depth of foundations have not been considered.
5.1.1 Conclusions based on the study of cost break down of completed bridges
(i) The average direct foundation cost is around 39% of the total construction cost of
a bridge.
(ii) On average, the total foundation cost (Direct cost and coffer-damming and de-
watering cost) is around 50.5% of the total construction cost of a bridge.
(iii)Average total temporary construction cost is about 13.5%, and most of the cost components (around 11.5%) are influenced by coffer-damming and de-watering costs.
(iv) Average total foundation and sub-structure cost is around 64.5% and the average super structure cost is around 18.5% of total construction cost of a bridge. Vazirani and Ratwani (1992), Khanna (1981) have mathematically proved that in economical bridge design, super structure cost is equal or nearly equal to the total foundation and sub-structure cost. Comparing results deduced, with the above theory, this study shows that most of our bridge designs are not economical or optimal.
5.1.2 Conclusions based on the study of cost break down of foundations
(i) The total construction cost of foundation with spread footing is approximately over 200% of the direct foundation cost. Which indicates that the cost of coffer damming and de-watering etc are more than the direct cost of the foundation for spread footing.
(ii) The total foundation cost for piled foundation is 119% of the direct foundation cost. Which means that the coffer-damming and de-watering etc, account for only 19% of the direct cost of foundation. The total foundation cost for caisson is 120% of the direct foundation cost and it means only 20% needs for coffer-damming and de-watering etc. (Please refer the note given in section 4.3 for further clarifications.)
(iii) The cheapest form of foundation appears to be pile foundations. However, for foundations with depths less than 5 m it is not practicable to use piles, using pile driving method. Therefore for foundations up to about 4 m, spread footing appears to be the most suitable type. Above that depth pile is the best in-terms
44
of total foundation costs, which will result in the lowest total cost of bridge as well. Here the span of the bridge is assumed to be independent of the foundation type.
Caissons are likely to be the most expensive type of foundation for any depth.
However, there may be situations in which the most suitable type of foundation
can be the caisson, where the selection will not be based on the cost of
foundation. For example, if a bridge is to be constructed in build up area or place
where the depth of water is considerably high, the caisson type of foundation
should be chosen without considering the cost.
45
5.2 Recommendations
Based on the results of this study the following recommendations are made.
1. In designing foundations for Road Bridges, the option of Pile Foundation should
always be considered as it is found to be the most economical type of
foundation.
2. Action should be taken to review the design techniques of superstructure and sub structure.
3. A study should be carried out to find out the possibility of using new technology
to construct shallow pile foundations particularly when the depth of foundation
is less than 5.0 m.
4. A study can be carried out to construct single span bridge with longer span on
simple beam type abutment, found on pile (driven or bored) foundation or
spread footing with no huge abutment sections.
5. A study should be carried out to find out the optimum type of super structure, for example the span, as the Sri Lankan bridges seem to have a lower cost value for the super structure compared to the total construction cost.
46
6. References
Jha ,J. and Sinha , S.K. (1993). Construction Foundation Engineering (5th Edition),
Khanna Publishers , Delhi.
Khanna, P.N. (1981). Indian Practical Civil Engineer's Hand Book (8th Edition),
Engineers Publishers, Delhi.
RDA (1996).Road Development Authority, "Roads Materials Design and construction
Standard's Study, Bridges", Ministry of Highways, Sri Lanka.
RDA (1989).Road Development Authority, "Standard Specifications for Construction
and Maintenance of Road and Bridges", Ministry of Highways, Sri Lanka.
Vazirani.V.N. and Chandola, S.P. (1986). Railway Bridges and Tunnels (4th Edition),
Khanna Publishers, Delhi.
Vazirani, V.N. and Ratwani, MM. (1992). Concrete Structures (15th Edition),
Khanna Publishers, Delhi.
47
Appendix A
Design of Foundation and Abutment Sections
Concept of Design
a) Bed rock or hard strata are found in less than 3 m from ordinary water level (OWL)- upto that depth no alternative except spread foundation.
b) Bedrock or hard strata are found in more than 3-m depth from OWL- caissons can be chosen.
c) Bedrock or hard strata are found more than 5 m from OWL and over burden is more than 4 m • piles can be chosen.
Standards used
a) British standard BS 5400
b) British standard CP 110
c) Reinforced concrete design code • IESL
Type of Loading
H a type loading
I
Design of bridge abutments and foundations
Data used Clear & pan of the bridge
Type of bean s Width of the abutment Design load from deck per meter length of abutment Tractive force on abutment in meter length of abutment Live load surcharge Designed bearing pressure Density of concrete Density of Earth Angle of internal friction Ranking coefficient Permissible tensile stress in concrete Type of pile Type of abutment
Type of concrete Foundation slab, pile cap and caisson cap 37.5mm) For caisson Reinforcement used
: 15.2 : 16.15m standard psc : 9.6 m
: 210 KN
: 17.15 KN : 10 KN/sq.m : 700 KN/sq.m : 24 KN/Cu.m : 18 KN/Cu.m : 30° : 1/3
: 0.25 N/mm* : 355 x 355 mm A 2 precast RCC piles : Mass concrete 1:2:4(37.5 mm) with
20% (150x225) plums
: G. 25, (30% 19mm and70%
: G. 25(19 mm) : G.425 cold worked deformed high
yield steel.
Design procedure - Abutment
At each section the positions of resultants due to all vertical loads and horizontal forces were found and the compression and tensile stresses were calculated. The width of the section was designed, so that the maximum tensile stress should be less than the allowable tensile stress.
II
SELECTED SECTIONS OF SPREAD FOUNDATION AND ABUTMENT SECTIONS FOR AVERAGE DEPTHS
Depth of foundation 3 m from OWL
1,200
Depth of foundation 4 m from OWL
1,200
1,200
1,200
600
600
400
400
400
1,150
2,150 2,150
350
J II 6 I
2,750
1,200
1,200
1,200
Depth of foundation 5 m from OWL
1,200
1,200
2,150
All dimensions are in mm.
350 Reinforcement : Main bars 16 mm dia. T.S. at225C.C. Distribution bars : 12 mm dia. T.S. at 250 C.C.
Ordinars Water Level
3,550
III
Depth of foundation 6 m from OWL
1,200
1,200
1,200
1,200
1,200
550
600
40oJ
400
400
400
600
1,200
2,150
350
-ii ft 4,150
All dimensions are in mm.
Reinforcement : Main bars 16 mm dia. T.S. at 225 C.C. Distribution bars 12 mm dia. T.S. at 250 C.C.
OWL Ordinars Water Level
IV
SELECTED SECTIONS OF PILE FOUNDATION AND ABUTMENT SECTION
1,200
,400, 1,100 , 1,100 , i j i i
400 1,760 » 1,760
PILE LAYOUT
Reinforcement : 16 mm dia. T.S. at 225 C.C. in both for top and bottom nets of Pile Cap
directions
SELECTED S E C T I O N S O F C A I S S O N F O U N D A T I O N S A N D A B U T M E N T S E C T I O N
1,200 2,250 . 200 . 2.250
•4 M M »i
3,550
9600
CAISSON LAYOUT
Reinforcement : 16 mm dia. T.S. at 225 C.C. in both directions for top and bottom of caisson cap
All dimensions are in mm.
V
Appendix B
Nine bridge design engineers attached to RDA and RCDC were interviewed based on a structured questionnaire as shown here. According to the details provided by the design engineers all bridges are standard type, concrete deck supported on spread or pile foundations. The results abstracted from the questionnaire are analysed in the page VIII. Design engineers comment on specific question No.q and r are given in page IX.
The comment on question q and r imply that nonof the design engineer has done detail analysis of cost on temporary construction.
VI
OUESTIONAIRE FOR BRIDGE DESIGN ENGINEERS
(1) name of the bridge designed :-Year
(2) Detail of the bridge : - Concrete Steel
Span No. of Spans
(3) Type of Super Structure : - Solid Slab Simply supported
Continuous Girders ... Cantilevered
Arch
(4) Type of Foundation : - Spread Pile
Caisson Cylinder
Raft
(5) Did you consider the following factors in designing the bridge
Yes / N o Comment
a. Volume and nature of the traffic
b. Live load on the Bridge
c. Length and Width of the Bridge
d. Aesthetic appearance
e. Geographical and physical features of the area
f. Environmental factors particular to the site
g. Social problems ion location
h. Nature of the river
I. Straight, and horizontal approaches
j . Firm and well defined banks
k. Straight streamline and smooth flow
I. Minimum depth of water and width
m. Right angle crossing
n. Soil profile at the site
o. Suitable type of foundation
p. Depth required for proper founding
q. Cost of coffer damming and de-watering
r. Cost of other temporarily construction
s. Availability of resources
t. Method of construction
u. Economical and technical feasibility
v. Facilities available for construction and maintenance
w. Time for construction
x. Economy between feasible alternatives i
* If space is not available attach a separate sheet.
VII
summary of the results of the questionnaire No. Factors considered in design process Answer - { Yes - Y No • N > Total ol
Y N a) Volume and nature of the traffic Y N
i 1 i N N N N N N 2 7
b) Live load on the Bridge Y Y Y | N
i . Y Y N N Y 6 3
c) Length and Width of the Bridge Y Y Y I v i Y Y Y Y Y 9 0
d) Aesthetic appearance N N N N N N N N N 0 9
e) Geographical and physical features of the area Y N N Y Y N Y N 5 4
0 Environmental factors particular to the site Y N N j V N N N N N 2 7
g) Social problems ion location N N N y N N N N N 1 8
h) Nature of the river Y Y Y Y Y N Y Y N 7 2
i) Straight and horizontal approaches Y Y Y N Y N N Y Y 6 3
j ) Firm and well defined banks N N N N Y N N N N 1 8
k) Straight streamline and smooth flow N Y Y N Y N N Y N 4 5
1) Minimum depth of water and width Y Y Y 1 Y N Y Y 7 1 m) Right angle crossing Y Y Y N Y Y N Y N 6 3
n) Soil profile at the site Y Y Y Y Y Y Y N Y 8 1
o) Suitable type of foundation Y Y Y V Y Y Y Y Y 9 0
P) Depth required for proper founding Y Y Y V Y Y Y Y Y 9 0
q) Cost of coffer damming and de-watering N Y N Y Y N N Y Y 5 4
r) Cost of other temporarily construction N Y Y N Y N Y N Y 5 4
s) Availability of resources Y Y N Y Y Y Y Y N 7 2
t ) Method of construction Y Y N N N N N Y N 3 6
u) Economical and technical feasibility N Y Y Y N N Y Y N 5 4
v) Facilities available for construction and maintenance Y N N N Y Y Y N N 4 5 w) Time for construction N N N N Y Y Y Y N 4 5
x) Economy between feasible alternatives N Y N N N N Y N Y 3 6 Type of bridge C C C C C C C C C
Type of foundation BP SP SP SP SP DP SP SP SP
Span of the bridge in m 1 6 2 3 1 6 2 3 15.23 16.23 1 6 2 3 7 0 0 16 23 1 2 2 0 16 23
Number of spans 1 3 2 2 1 1 2 1 1
C - Concrete
SP - Spread
BP • Bored Pile
DP • Driven Pile
VIII
Comment on specific questions
Did you consider the Comments Answer
i
Cos
t of
cof
fer
dam
min
g an
d
de-w
ater
ing
Alternations are not considered N
i
Cos
t of
cof
fer
dam
min
g an
d
de-w
ater
ing
Considered but not analysed Y
i
Cos
t of
cof
fer
dam
min
g an
d
de-w
ater
ing Not considered (rock exposed) N
i
Cos
t of
cof
fer
dam
min
g an
d
de-w
ater
ing
Considered (shallow depth) Y i
Cos
t of
cof
fer
dam
min
g an
d
de-w
ater
ing
Considered but not analysed Y
i
Cos
t of
cof
fer
dam
min
g an
d
de-w
ater
ing
Not considered (no alternative foundation type) N
i
Cos
t of
cof
fer
dam
min
g an
d
de-w
ater
ing
Not considered N
i
Cos
t of
cof
fer
dam
min
g an
d
de-w
ater
ing
Not seriously considered (rock exposed) Y
i
Cos
t of
cof
fer
dam
min
g an
d
de-w
ater
ing
To decide the excavation cost but not analysed Y
Cos
t of
othe
r
tem
pora
ry
cons
truc
tion
s
No comments N
Cos
t of
othe
r
tem
pora
ry
cons
truc
tion
s
Considered but did not analysed Y
Cos
t of
othe
r
tem
pora
ry
cons
truc
tion
s Considered to provide by road but did not analysed
Y
Cos
t of
othe
r
tem
pora
ry
cons
truc
tion
s
Standard and simple bridge N
Cos
t of
othe
r
tem
pora
ry
cons
truc
tion
s
Allow existing traffic but did not analysed Y
Cos
t of
othe
r
tem
pora
ry
cons
truc
tion
s
Not required, new location N
Cos
t of
othe
r
tem
pora
ry
cons
truc
tion
s
To deviate traffic Y Cos
t of
othe
r
tem
pora
ry
cons
truc
tion
s
Not congested N
Cos
t of
othe
r
tem
pora
ry
cons
truc
tion
s
Not analysed Y
I X
