Nigerian Society of Engineers Annual Conference Proceedings ...

Nigerian Society of Engineers Annual Conference Proceedings “CANAAN 2011”

1

BRIDGE CONSTRUCTION: PATRONIZING INDIGENOUS EXPERTISE FOR EFFECTIVE TRANSPORTATION SYSTEM IN NIGERIA. BY O.A. APAMPA Moshood Abiola Polytechnic, Abeokuta. Email: [email protected] Tel: 08061664780

ABSTRACT The paper aims at promoting the patronage of indigenous expertise in the construction of bridges in Nigeria by highlighting the processes and challenges that led to the successful completion of a recently commissioned 4x20m span bridge in Ago Iwoye, Ogun State, Nigeria. It emphasizes the fact that though the bridge was built to international standards, the know-how was wholly Nigerian, and all materials including specialist items like bearings and expansions joints were fabricated or sourced locally. The paper concludes that the development and encouragement of indigenous expertise in bridge construction is vital to effective transportation system in Nigeria, and calls on government at all levels to, as a matter of deliberate policy, save foreign exchange and create job opportunities locally, by patronizing Nigerian engineering expertise in bridge design and construction. 1.0 INTRODUCTION The 37km stretch of road linking Ilishan and Ago Iwoye – both of them university towns in Ogun State, is crossed by 3 rivers, namely Ona, Erigba and Omi respectively. Crossing of these rivers was by means of 3 single lane bridges built about 1962 by the government of the old Western Region. The civilian government of the second republic initiated the construction of standard size bridges as replacement at about 1982, but this attempt did not go far until the third republic when the state government decided in the year 2005, to complete these

bridges using its direct labour agency – the Ogun State Road Management Agency. These bridges have since been commissioned and opened to traffic, the last one being Omi bridge, which was opened to traffic in November 2010. This paper discusses the process through which the bridges were constructed using Omi bridge as case study. It highlights the major challenges involved in a bridge of this nature and how these challenges were confronted by Nigerian Engineers and Technicians using very basic tools. The paper emphasizes the fact that the technology for constructing bridges of this category of bridges is fully indigenized and that the interest of the nation will be better served if government patronage for this category is exclusively reserved for truly Nigerian Engineering companies. 1.1 The Bridge Structure The Omi bridge is a 4-span bridge, each span 20m long, giving a total length of 80m. In the dry season, the Omi river narrows to a stream like flow, no more than 5m wide, but can widen to nearly the full 80m length at high flood, with a depth of more than 5m.

2

Figure 1: The Bridge Structure As shown in Figure 1, the bridge structure comprises of: 1No abutment on piled foundation 1No abutment founded directly on bedrock 3No piers founded directly on bedrock 32No precast concrete main carrier beams 4No concrete decks, each deck complete with diaphragm beams, 70mm thick precast concrete soffit shuttering slabs, 200mm in-situ reinforced concrete deck slab, walkways and parapet walls on each side. Each span being

separated from the other by means of an expansion joint. 1.2 Initial State of the Bridge Site At the time of taking over the site in October 2006, only two piers had been built before being abandoned by the previous contractor. For a 4-span bridge, it means 2No abutments, 1No pier and 4 spans of complete deck structure were required to be built, to bring the bridge to completion. Figure 2 shows the initial state of the bridge site.

Figure 2: Initial State of the Bridge Site (Omi).


3

2.0 METHODOLOGY 2.1 Site Investigation and Consultancy Services The firm of reputable Nigerian engineering consultants – Intecon Partnership was engaged to carry out an investigation of the site and sub soil and to produce the design and working drawings to complete the bridge. Principally, the soil investigation revealed a layer of bedrock at depth varying from 6m to 2m below the respective ground levels at abutment/pier locations. This formed the basis for the economic design of foundation types. While Abutment 1 was recommended for piling to get to bedrock some 6m below, footings for pier1 and abutment 2 were founded directly on

the bedrock by excavating 4m and 2m respectively. 32Nos 600mm diameter bored piles, about 6m long on the average, were done for abutment1 by an indigenous firm of foundation specialists based in Ibadan. 2.2 Site Organization and Equipment One key feature of this project is the leanness of the site in quantity of manpower and the simple nature of the tools employed. Figure 3 shows the site organizational chart, Table 1 shows the labour returns, while Table 2 gives the list of equipment.

Figure 3: Site Organizational Chart

S/n Category 1 Specialist Engineer (Bridges) 2 Assistant Bridge Engineer 3 Supervisors 4 Carpenters 5 Iron Benders 6 Helpers (Regular) 7 Helpers (Casual) 8 Driver Table 1: Labour Returns

Quantity 1 1 3 6 6 2 Varied according to need 1


4

S/n 1

Equipment Rotating Tilting Drum Mixer, 0.15lires capacity 2 Poker Vibrator 3 Water Pump 4 5KVA Diesel Generator 5 Welding Machine 6 Level Instrument with Staff 7 Dyna Pick-up 8 Wheel barrows, head-pans, shovels, water tanks, etc. Table 2: Major Equipment on Site

2.3 Action Plan for Project Completion Typical of bridges in this category, successful execution is hinged on two critical legs – training the water to your purpose and successful beam launching operation. The success of other activities is either derived or judged from these. 2.4 Working Round the Water This as can be expected, was a major challenge, as the water with a depth of 5m and very high currents appears impossible to work in at high flood, which is between July and October of every year - the height of the rainy season when the dam waters at the upstream Oyan dam are released. But an understanding of the seasonal nature of the water brought about the simple solution of scheduling the work such that all work that required staying on the ground, close to the water way was scheduled for the dry season while at the same time making all necessary preparation for rainy season work. The idea was to use one period of dry season to do all necessary foundation, abutment, pier and precast beams for 2 spans, and to use the succeeding rainy season for the corresponding deck work. Then to repeat this process for the remaining 2 spans. The sequence of work for completing two spans with this arrangement is as listed below: 2.5 Dry Season Work 1. Filling and leveling the first two spans to create a level working surface 2. Piling for abutment 1 3. Construction of Abutment 1 4. Construction of Pier 1 5. Ground preparation for precast beams 6. Casting of 16Nos precast beams 7. Procurement of bearings

Quantity 2No 2No 2No 1No 1No 1 set 1No

8. Launching beams of spans 1 and 2 9. Re-open the water channel 2.6 Rainy Season Work 1. Construction of diaphragm beams 2. Placement of precast concrete panels 3. Shuttering, reinforcement work and expansion joint etc. for 2 decks 4. Casting of decks of spans 1 and 2. The foregoing was the broad strategy with which the problem of working on the water was overcome. 2.7 Footings, Abutments and Pier The soil investigation report recommended a pile foundation for abutment1 to take the loads down to the competent bedrock stratum some 6m below. The depth of bedrock at the location of the other foundations ranged from 2 – 4m, therefore upon excavating the overlying soft sandy clay material, these other footings were constructed directly on bedrock. For abutment 1, piling consisted of 32Nos 600mm diameter bored piles accomplished with a motorized drilling rig, and employed the bentonite suspension technique to maintain the stability of the drilled holes. Placing of concrete was by means of tremie pipes to avoid segregation and ensure proper displacement of the drilling mud. The pile boring was sub-contracted to an Ibadan based specialist firm. The abutments were designed with counterforts, and one of them (abutment1) is actually 11m high. This height was a major challenge considering the basic nature of tools at our disposal (neither crane nor hiab was available). The solution adopted was to attain the 11m concrete height in lifts of 2.4m, that being the longer dimension of the plywood panel. Upon

5

striking the vertical form, the wall would be backfilled and prepared for the next lift. The pier was founded some 4m below the ground level. The usual problems of underground construction below the water level had to be grappled with and overcome using standard procedures – adequate side slopes for excavations, sump pits, pumps etc. 2.8 Beams and Beam Launching The main carrier beams are 20m long with a cross-sectional dimension of 300x1100mm. To hide any appearance of sag in the beam when in position, they were constructed with a bottom camber of 75mm at the middle. As will be expected, reinforcement for the beams was heavy and tightly packed, 3.25tons per beam,

Figure 4: Bridge Beams being prepared for launching

Figure 6: Beam launching in progress

giving a ratio of 492kg of reinforcement per cubic meter of concrete. The essential challenge here was to ensure that the concrete is sufficiently workable to go into all the narrow spaces around the reinforcement and to ensure proper compaction. For this type of bridge, launching of the precast beams is by means of mobile cranes. The choice of type and size of crane is one of the most important decisions in the task of beam launching. It is necessary to not only know the weight of beam, but also the envisaged operating radius the crane will be working in. With these parameters, a typical lifting chart such as shown in Figure 10 will reveal the crane capacity appropriate to the lifting operation.

Figure 5: Launching operation in progress

Figure 7: Launched beam in place [the ground

6

is reclaimed water way that had to be re-opened at the start of the rainy season] For Omi bridge, a 50ton truck mounted crane was hired from a dealer in Ibadan. The speed and ease of launching depends on the arrangement of the beams relative to their final

2.9

position and the experience and confidence of the crane operator. Between 2 – 8 beams were launched per day for each of the spans at different times as each span became ready. Figures 4 – 7 show some scenes from some of the launching operations.

The Bridge Deck

Figure 8: The Bridge Cross-section The bridge deck comprises of precast concrete panels 500x1200mm (acting as shuttering for the soffit) placed on the launched beams. This acts as the base and working platform for all activities on the deck (reinforcement work etc.) leading up to deck casting. The deck is designed with a 1.2m cantilevered walkway on each side. To avoid the problem of having to prop cantilevered deck at a height of more than 6m, the edge beams had been fitted with wooden edge brackets. The cantilever was

formed on this edge bracket. Upon completion of the necessary reinforcement and fixing of the expansion joint, the in-situ concrete deck 200mm thick was cast. 2.10 Special Items Elastometric Rubber Bearings: These are the usual bearings for bridges of this category. For Omi bridge, these were ‘free’ rubber bearing pads 450x600x50mm thick with 3layers of embedded steel plates. They were procured from a local manufacturer in Ibadan.


7

3.0 THE COMPLETED BRIDGE

Figure 9: The completed bridge 3.1 Quality Control 3.11 Concrete Concrete grade 30 was specified for the main carrier beams while grade 25 was specified for the rest of the bridge structure. Grade 30 was accomplished with mix ratio 1:1:2 batched by volume, while grade 25 was achieved with ratio 1:1½ :3, also batched by volume. Cube samples were taken as appropriate and tested at 7 and 28 days respectively at independent laboratories in Lagos. Average compressive strength recorded were 33N/mm2 and 27N/mm2 for the grades 30 and 25 respectively.

of these sort of bridges – reinforced concrete deck girder type. The successful completion of the bridge, to the standard to the standard to which it has been completed is a testimony to the ability of Nigerian engineers, technicians and craftsmen to rise to the occasion when appropriately challenged. Capital flight will be stemmed and the interest of the Nigerian nation will be better served if more such opportunities are given to Nigerians to demonstrate their skill and expertise in every sphere of human endeavour.

3.12 Steel The characteristic strength of the reinforcing steel was specified as 410N/mm2. Initially we tried the locally made bars (Ajaokuta quality brand) and found them generally in the range of 330 – 385N/mm2. Thereafter we resorted to imported brands only (Ukraine or Brazil quality). Samples were taken from every batch of reinforcement and subjected to axial strength tests. The ‘litmus test’ for the acceptance of any brand was a satisfactory test result.

BIBLIOGRAPHY

4.0 CONCLUSION AND RECOMMENDATION The paper presented a summary of the methods and procedures adopted in the construction of the 4x20m span Omi bridge, Ago Iwoye, Ogun State. It is a documentation of, and a testimony to the availability of every single human and material resource required for the construction

APAMPA, O. (2011). OMI BRIDGE CONSTRUCTION: TASKING THE INGENUITY OF INDIGENOUS EXPERTISE. TECHNICAL REPORT, ABEOKUTA: UNPUBLISHED. BROCKENBROUGH, R. A. (2003). HIGHWAY ENGINEERING HANDBOOK. NEW YORK: MCGRAW-HILL. GUPTA, B. A. (2009). ROADS, RAILWAYS, BRIDGES, TUNNELS AND HARBOUR DOCK ENGINEERING. DELHI: STANDARD PUBLISHERS DISTRIBUTORS. LAWAL, H. (2001). CROSSING CREEKS AND SHALLOW RIVERS: ECONOMICAL RC MULTIPLE SPAN BRIDGE SOLUTION. . JOURNAL OF CIVIL ENGINEERING, NIGERIAN INSTITUTION OF CIVIL ENGINEERS. , 16-30.

8

THE STEEL CONSTRUCTION INSTITUTE. (1994). STEEL DESIGNER'S MANUAL.

LONDON: BLACKWELL SCIENCE.

APPENDIX

Figure 10: Typical Crane Lifting Chart

9