Challenges of subsurface drainage of urban roads in low lying areas : A materials perspective
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The current pavement design criteria states uniform base course material for both low and high lying areas, which in practice experience different moisture during normal service. However, quite often, low lying areas are not properly drained, exposing the pavement structure to higher moisture content than the base material can withstand for a given applied load. The objective of the current study was to evaluate performance of the most commonly used materials for base construction namely CRR, lateritic gravel and their combination on drainability and capacity to withstand load under saturated conditions. Two roads in Kampala city were selected both containing low and high lying sections for establishment of relative performance where field DCP and coring was done on 11 points (5 at one and 6 at the other) at intervals of 200m considering inner wheel track, between the wheel tracks and outer wheel track. A total of 11 mix proportions 100- 0% to 0% - 100% (Laterite – Crushed stone) were prepared and classical tests carried out three (3) times repeatedly at varying energy levels of 62 blows and 30 blows and obtained results were compared with specifications where 5 mixes 40-60% to 0 -100% (Laterite – Crushed stone) were selected for soaked CBR testing to ascertain their strength in saturated conditions. Three (3) out of the five selected mixes 20 – 80%, 10 – 90% and 0 – 100% (Laterite – Crushed stone) with higher soaked CBR values of 76%, 75% and 70% were further selected and used in design of layer thicknesses to ascertain the best mix proportion for use. CRR was found not to be suitable for use in flood prone areas due to their significant reduction in strength based on CBR when saturated. The mixture with 20% laterite and 80% CRR was recommended as the best material formulation based on drainability and load carrying capacity. Observed from the study was the inadequacy of the layer thicknesses used by KCCA in sections of low lying areas. There is need to investigate performance of a wider variety of gravel and aggregates types from different regions in low lying areas to see how they can be improved upon.