286 



SOME MUNICIPAL ENGINEEEING PBOBLEMS IN THE TEOPICS 



Diagonal 

 crossings 



Kain-water 

 drainage 



" Fig. 85 shows a block in the third class or native quarter with the 12 feet wide 

 conservancy lanes traversing it. An enlarged plan of the Double Street Crossing is also 

 shown." 



From the detail of the double diagonal crossing referred to it will be seen that the 

 shape of some of the plots makes them very inconvenient, especially in the third class 

 residential or business quarter of the town, where the buildings are usually erected on the 

 frontage. In many cases the owners of such plots have kept the buildings back 

 somewhat and planted the corners as gardens, the effect being excellent. 



Owing to the very arid climatic conditions during the greater part of the year, 

 combined with the high winds, it is extremely difficult to maintain the surface of the 

 macadamised streets. Experiments have been made with various mixtures of tar and 

 pitch as binders, but it was found that the limited funds available precluded their use 

 meantime. In spite of the destroying action of sun and wind, fair results have been 

 obtained with a mixture of Nile mud and sand, especially in places where the subsoil 

 is damp, due to the presence of salt. Further experiments with crude petroleum and 

 other materials are in progress, however. 



The Embankment and the streets running north and south, which are exposed to the 

 prevailing winds, are troublesome to maintain as are also those constructed without a 

 proper bottoming. The foundation is Nile mud, a clayey soil which is exceedingly hard 

 when dry but becomes soft when damp so that in the absence of efficient bottoming the 

 upper crust breaks up under heavy traffic. This is what happens in the case of those 

 streets bottomed only with small stones which sink into the foundation. 



The metalling used has been principally weathered fragments of iron-stone broken to 

 2'' cubes, but recently fine grained grey granite has been laid in some of the main avenues. 



The distressing effect of the glare from roads in the Tropics metalled with light- 

 coloured materials is well known. The glare from streets metalled with the iron-stone or 

 grey granite is not uncomfortable when blinded in the ordinary way with sand and mud, 

 and it is even lessened by the application of some of the special "binders" referred to. 

 The selection of colours and materials to modify the effects of the light and heat rays 

 of the sun is dealt with in reference to "Buildings" (Section III). 



The drainage of the rain water is often a problem in the Tropics, as the rains occur 

 generally during only a part of the year and are then exceptionally heavy. It is usually 

 necessary, therefore, to provide a special rain water drainage system. At Khartoum the 

 rainy season is from July to September, the remainder of the year being practically dry. 

 The amount of rainfall recorded varies from a maxinuim of 188 mm. (7'4') in one season to 

 a minimum of 15 nun. (-59") in another season, the maximum recorded for any one day 

 being 45 mm. (1'8"). 



The system adopted meantime is that of open trenches, as reconnnended by 

 Mr. C. E. Dupuis,' M. Inst. C. E. At the street crossings steel pipes are used and the 

 main trenches are carried along the less important streets, and across open spaces where 

 posssible. The inconvenience of the trenches is thus reduced to a minimum. 



As very few of the streets are metalled to the full width between kerb stones the 

 trenches are excavated entirely through soft material which is replaced after the rains. 

 Fig. 86 shows the details of the system. To reduce the depths of the main trenches the 

 outfalls to the river are comparatively frequent, and, as they are all somewhat below high 

 flood level, the trenches can only be emptied by pumping if heavy rains occur during 



extreme flooch 



' Late Inspector General, Sudan Irrigation Service, now Adviser to the Egyptian Ministry of Public Works. 



