Regional Seminar Proceedings 1990

2. DISCUSSION ON LOW COST STRUCTURES

2.1 DRIFTS

There is only a slight difference in cost between culverts and drifts. Drifts are slightly cheaper and they have the advantage that in most cases the road can be kept open during construction.

The approaches to drifts are important as they can cause damage to vehicles and give the road users a low opinion of drifts if they are too steep. A recommended slope for approaches would be 5 %.

The importance of properly constructed aprons, especially the down stream protection, should always be emphasised. Some guidance is needed on the size and depth of the upstream and downstream protection.

2.2 AVAILABILITY OF SKILLED PERSONNEL

Availability of skilled personnel to construct dry stone masonry structures and cement bound stone arches. How can you train masons who are inexperienced in building dry stone masonry structures?

In nepal untrained workers worked with experienced masons until they had acquired sufficient skills to work independently. It would be possible for ILO to arrange short term consultancies for a skilled mason to give demonstrations and train local workers in building with stone. This training could be either in dry stone work or in more complicated cement bound masonry such as stone arch bridges.

Another source of information on low cost structures and stone arch bridges would be Indian Government publications and/or Indian University texts.

2.3 BURNT BRICKS

Burnt bricks may be used in place of stone where stone is not locally available. However care must be taken with the quality of the bricks and the implications for deforestation for the burning of the bricks.

2.4 TOOLS FOR MASONRY WORKS

In most cases road construction programmes have used the locally available tools. There is a listing of masonry dressing tools in the "ILO Tools Manual" if tools were to be procured by a project.

2.5 STONE PITCHING ON RARRIAGEWAYS

Stone pitching has been used successfully in different countires, to assist on steep sections of road. If cobbles or setts are to be used they must be placed properly to be effective and durable.

2.6 TIMBER ALTERNATIVES

The use of timber for bridges is a wide topic and would have to be tackled as a separate issue.

In Makete, Tanzania the roads project has successfully used timber for constructing culverts, at the cost of US$25 per culvert, with a life expectancy of five years.

Using timber for a large number of structures may pose a problem for future repair or replacement of decks as where previously there may have been a plentiful supply of timber deforestation may drastically reduced supplies.

2.7 UNREINFORCED CONCRETE PIPES

None of the participants reported problems with performance of the unreinforced concrete pipes, however, there were a few reports of breakages during transportation and breakages due to poor backfilling of the culverts on site.

Culverts can either be produced in casting yard or adjacent to the road site. Having a casting yard close to the engineers office may allow closer supervision and greater quality control. A casting area at site may reduce the need for transport if located close to the sources of sand, gravelor stone for aggregate. In addition the delivery distance for the finished culverts would be reduced. Quality could perhaps suffer from less frequent supervision and less skilled staff. There are arguments for both options but the expense of transport should be carefully considered when deciding on the location for casting unreinforced culverts.

Dissatisfaction was expressed over the quality of the concrete pipe coulds. Some participants were of the opinion that those moulds being produced at present should be used, despite some problems, whereas other participants felt there should be a continued search for better quality moulds.

The Zambia project has ordered moulds from Malawi but they have not been tried out yet. Kenya and Tanzania projects are using locally produced moulds.

There are two criterion for producing better quality moulds:- a good design, and good quality control during manufacture. An incentive to ensure quality control during manufacture would be the placing of large orders rather than small orders for six or eight moulds only.

2.8 ALTERNATIVE STRUCTURE

As a general principle all road structures should be designed and costed with alternatives. The decision on which type of structure is to be built will therefore be based on an economic comparison of each proposal.

2.9 AGREED PLANS FOR ACTION, LOW COST STRUCTURES