Improvement of cobblestone road conditions versus riding quality in the City of Kigali

Abstract

The use of stones for paving roads is not new idea; it is dating from ancient times when people used horses as a mean of transport. When vehicles started to replace horses as a mean of transport, cobblestone roads started to be declined due to their uncomfortable riding quality. The poor riding quality is mainly caused by the unevenness of the surface finishing of these roads made with irregular and bumpy stones. In the City of Kigali, the surfacing using cobblestone have been adopted as alternative cheaper solution compared to surfacing using asphalt concrete or surface seal, because row materials are sufficiently available in quality and quantity. However, road users are not only suffering from poor riding quality of existing cobblestone roads but also from early degradation of these roads. The visual inspection conducted on existing cobblestone roads showed that these degradations are including displacement of cobbles, erosion of the surface joints, edge break, undulations and depression of the carriageway surface. The objectives of this study was to carryout the assessment of existing conditions of cobblestone roads towards riding quality and road durability, to analyse the cause of earlier degradation of the existing cobblestone roads and to propose the improvement for the construction technique of cobblestone roads that prevents the displacement of cobblestones and that reduce the stressful conditions of roughness. The assessment of the riding quality of existing roads has been conducted by using the bump integrator classified as a Response Type Road Roughness Measuring System (RTRRS) calibrated by MERLIN device; as a rapid, cheaper and accurate method. The riding quality was assessed in terms of roughness converted into International Roughness Index (IRI) and measurements were done at a speed of 32km/h. The study found that the level of roughness of existing cobblestones roads is greater than 11.76 IRI, which is beyond acceptable limits of 4IRI for paved roads and 8IRI for unpaved roads as set by Rwanda Transport Development Agency (RTDA) for roads constructed in Rwanda. It was also found that the displacement of cobblestones was caused by irregularities in shape and dimensions of manually shaped stones used as surface finishing of cobblestone roads. These irregularities reduce the confinement between cobblestones and the movement of vehicle tyres displace them easily.

vii The improvement of the surface finishing of cobblestone roads requires the use of stones with regular shape and dimensions. It was found that manual production cannot satisfy this requirement and the research recommended the use of mechanically cut stones on the surface finishing of the road. A regular Large paving block of 14cm of width, 20cm of length and 14cm height was recommended for use due to its advantages in production, since more quantities of Large paving blocks is produced in shorter period compared to blocs of smaller dimensions. Furthermore, Large paving blocks have another advantage in stone laying because they can be accommodated to any laying pattern. A kerbstone was recommended to have 20cm of width, 80cm of length and 20cm of height by merging local practice with the dimensions proposed in literature surveys. The research recommended using an arch layout as a stable laying pattern against stone displacement. Paving stones are laid on a bed course of sand of thickness 4cm after its compaction. The joints between the two adjacent laid stones must be less or equal to 2cm and must be filled with sand. During the construction of the pavement, it is required to control the alignment of laid stones and inequalities beyond ±5mm must be eliminated. The compaction of paved stones is required to increase the confinement between paved stones and this is done by using a vibrating plate of 200 to 600 kg of weight and of centrifugal force equivalent to 30KN. The use of the technique provided for in this research will benefit road users in reduction stressful conditions of driving and in reduction of vehicle operating cost