Regional Seminar Proceedings 1990

6.1 GRADING DEMONSTRATION

RRM Mbeya are using a double axle towed grade manufactured by Arthur Garden in Zimbabwe. The grader is towed by a 75hp tractor. It is used for removing corrugations on earth roads or returning gravel from the shoulders to the carriageway, on gravelled roads.

When in operation the grader uses 5 passes on each side of the carriageway with a maximum cut of 10cm. It can cover a distance of 2km of road per day. The team working with the grader consists of the tractor driver, one or two grader operators and two labourers. The job of the labourers is to remove stones and grass from the centre of the road once the grader has passed. They are assisted by the maintenance lengthmen on the road who also ensure that all drains are kept open during grading. RRM Mbeya would hope to grade each road, under maintenance, 2 or 3 times per day.

During the small demonstration the depth cut by the grader was much less than the maximum of 10cm, therefore this figure depends very much on ground conditions.

6.1.1 THE FOLLOWING DISCUSSION RESULTED FROM THE GRADING DEMONSTRATION

Questions were raised which could be tested in the future.

• Would it be possible to have a group of labourers working behind the grader raking the soil into shape and therefore reducing the number of passes of the grader.
• Could the tractor be used for grading, then in the latter part of the day tow a roller to compact the loose soild of gravel which has been brought back onto the carriageway by the grading operation?

There are fears that the road surface can be damaged by traffic when rain falls on an uncompacted newly graded road.

The possibility of using the towed graders for heavy grading was generally not recommended by the participants. Arthur Garden also produces a single axle towed grader which would be easier to manoeuvre but needs to be carefully teamed up with the correct tractor.

6.1.2 EXPERIENCES FROM DIFFERENT CONTRIES

In Mozambique they have been using a light tractor towed blade for very light grading. The advantage of this blade is the ease of transportation from site to site. RRM Tanga are also doing some work with a blade attached to the tractor. In Botswana the normal maintenance set up is one tractor, one trailer, one towed blade plus 10 men. In Zimbabwe one overseer, 20 men with one tractor, one trailer and one blade are responsible for the maintenance of 300 km of road.

6.1.3 GENERAL

Training of grade operators is important for the correct results on the road and for the care of the grader itself. In Mbeya the operators spent 2 days in the classroom followed by 3 months training in the field. They are issued with tools to carry out routine checks on the graders including the frequent tightening of bolts as they have a tendency to work loose. Blades must be regularly replaced to ensure a good cutting edge.

6.1.4 CONCLUSION

It is vital to team the correct power and weight of tractor with the correct weight and size of grader. A light tractor will not be able to pull a heavy grader trying to make a deep cut. Any purchase of grader should be carefully considered in terms of the job it is required to do and then the most suitable tractor for towing. If a project is confined to the use of a particular tractor then this must have a bearing on the choice of towed grader. More information on grader/tractor compatibility is available from EMP/INFRA, ILO Geneva.

6.2 REHABILITATION AND GRAVELLING ROAD SITE

The road visited which was under rehabilitation, was a loop road and therefore work was started at both ends. The first group started in May 1989 and the second in September 1989. Thirteen kilometres have been rehabilitated, gravelled and compacted. The productivity is 1800 mandays per km (md/km) for road construction and 1300 md/km for gravelling. There is no shortage of labour and rehabilitation can progress at the rate of 1 to 1.5 km per month however gravelling was progressing at a rate of 0.8 km per month. At the time of the site visit rehabilitation had stopped and the labour force were concerntrating on excavation of gravel.

Graders had not been used on this site as there was a plentiful supply of labourers and a good quality finish to the road surface.

The visit to this site afforded the opportunity to look at the rollers and trailers in use at RRM Mbeya.

6.2.1 COMPACTING ROLLERS

Two types of rollers were on site. The first was the pneumatic tyred towing roller with the top tray which could be filled with earth for extra weight and the second was a towing vibrating roller. The pneumatic tyred roller is performing well in most soil conditions however on the road site visited there were areas where the vibrating roller was more effective.

An alternative discussed for use on labour-based sites was the pedestrian roller which would be suitable for the speed of progress on labour-based sites.

RRM Mbeya have also done compaction trials using an ox-drawn deadweight roller. Using the sand replacement test the ox-drawn roller gave 85% compaction inmoist conditions dropping to 65% in dryer conditions. Compaction took place after every step of rehabilitation thus the maximum thickness to be compacted was 10cm. In Kenya they found that a manually pulled roller gave as good results as the animal drawn and was an easier option than engaging animals. Botswana do not use compaction equipment but rely on traffic compaction, however they have a much longer dry season that in Southern Tanzania. In Rukwa traffic compaction only had been used but this may only be possible on lightly trafficked roads. The Kenyan Minor Roads Programme are not using compaction equipment but rely on vehicles despite the much higher traffic flows. Various reports from TRRL have shown good results can be obtained from traffic-only compaction, however this has been based on low traffic RARP roads in Kenya. Lesotho has found compaction, using 1.25 tonne vibrating rollers, to be essential under all circumstances because of poor soil types.

It was noted that projects should be very careful in deciding their compaction requirements at an early stage, based on both traffic and soil conditions in their particular circumstances. Traffic only compaction is not always satisfactory, but mechanical compaction represents a major initial recurrent investment.

6.2.2 TRAILERS

Two types of trailer were looked at and discussed. The first was the Kenyan trailer and the second a recently purchased trailer from Tinto Industries Harare. Both trailers were non-tippping. The Kenyan trailer has had a lot of design problems and has had to have extensive repairs carried out in the workshop. This trailer is very heavy when loaded (4.5 cu.m.) and the axle location places a large proportion of the weight on the tractor. However the most popular feature of this trailer is the low loading height. Until a major redesign has been carried out this trailer would not be recommended because of the damage to the tractors and the need for constant repairs.

The Tinto trailer has a good basic design combined with good quality workmanship. There were some weaknesses in the bolted connecion for the drawbar so RRM added welds in their own workshop. They are also not entirely satisfied with the opening mechanism for the side doors (which are to facilitate unloading) but have approached the manufacturers to reassess the design of these features. The one disadvantage is that the loading height is greater that that of the Kenyan trailer. The Tinto trailer had only been is use for two months at the time of the site visit and therefore a futher report should be made aftr a longer period of service.

6.3 ACTION TO BE TAKEN

A request was made that all project staff should carry out tests if they are using compaction equipment. This would give figures for the future discussions on compaction methods and remove the present subjectivity when discussing animal drawn versus tractor towed etc.

NOTE:Tractor and Trailer combinations are also discussed in Section 5 of this report under the heading of "Haulage of Materials".