More than 90% of the road network in developing countries is unpaved and comprises of either gravel or earth roads. Such roads are prone to erosion leading to the formation of potholes, rills and gullies. Many studies have been undertaken on soil erosion, but only a few are focussed on earth roads. A systematic analysis of research on erosion of soils in earth roads was undertaken to draw out lessons that can be learnt. 564 studies were assessed. Of these only 85 were relevant to earth roads. Most significant erosion driver was rain. In addition to rainfall duration and intensity, findings were that the key factors that affected soil erosion in earth roads were soil type, clay content, soil plasticity, particle size distribution and degree of the surface layer compaction as well as traffic loading and speed.

More than 90% of the road network in rural areas of the developing countries is unpaved and comprises of either gravel or earth roads. Such roads are prone to erosion leading to the formation of potholes, rills and gullies. Many studies have been undertaken on soil erosion, but only a few are focussed on earth roads. A systematic analysis of research on erosion of soils in earth roads was undertaken to draw out lessons that can be learnt. 564 studies were assessed. Of these only 85 were relevant to earth roads. Most significant erosion driver was total rain, rainfall duration and intensity. Further findings were that the key factors that affected soil erosion in earth roads were soil type, clay content, soil plasticity, and particle size distribution; degree of the surface layer compaction, and traffic loading and speed. To challenge and validate the findings of the literature, the main factors affecting erosion were controlled during laboratory erodibility tests of a sand soil mixed with its 0%, 5%, 10%, 15% and 20% china clay and of a granular subbase material. The soils were compacted at their maximum dry densities and subjected to a simulated rainfall intensity of 30mm/hr for 30 minutes, and at 0% and 6% slopes. Interestingly, laboratory results agreed with the literature. Erodibility increased with increase in rainfall duration and slope gradient. In sand -china clay mixes, erodibility reduced with increased clay content. An increment of 5% china clay corresponded to a reduction of eroded sediment of about 8% to 12%. The subbase soil material was very less erodible due to its robust particle size that resisted detachment due to rain drops. Moreover, it was observed that most soils were detached and eroded within the first 15 minutes of the rainfall after which detachment reduces.

Earth roads in rural areas of the developing world are key engines to the development of countries. They give access to education and health services, sustain agriculture and businesses, and promote social interactions between communities. However, earth roads suffer substantially from poor engineering and funding for construction and maintenance. Rainfall is probably their most dangerous enemy resulting in soil particle detachment leading to the loss of surface material. A laboratory rainfall simulator was used to identify the performance of an earth road surface compacted at the maximum dry density against rainfall energy and surface flow. Under the rain intensity of 30mm/hr, erosion increased with rain duration from 0 to 30 minutes. Fine sand (0.06 – 0.02mm) and medium sand (0.02 – 0.6mm) particles eroded faster than coarse sand (0.6 – 2mm) and gravel (> 2mm) particles of the sediments collected at 5 minutes intervals of time. Additionally, a 20cm x 20cm photograph at the same place was analysed using ImageJ software and showed reduction in number of particles from 18554 at 10 min to 5803 at 25 min as smaller particles had eroded in the meantime.

Rainfall simulators have been used for erosion research for more than 50 years now. These are widely used in agricultural soils to assess the infiltration capacity and porosity of soils, and hence learn lessons on the potential of plant roots to penetrate those soils. Recently, rainfall simulators have been very useful to investigate soils detachment by both the raindrops kinetic energy and the subsequent flow shear stress. This has led to notable advances in the understanding of the failure of infrastructures such as unpaved roads due to surface soil loss and formation of erosion features, buried pipes and facilities due to removal of fill materials, as well as bridge scour and embankments failures to mention a few. To help conduct a thorough and rigorous research, rainfall simulators must produce raindrops of the same size as those produced by the natural rainfall. Calibrating rainfall simulators satisfying this key demand of raindrops sizes in the range of 1 mm to 6 mm posed challenges for years, and therefore led to inconsistencies in results from different studies. In this paper, an economical rainfall simulator which can be used for assessing erodibility of soils in unpaved roads was developed. The flour method technique was used to determine the sizes of the raindrops. The mean raindrops sizes were found to be 3.0 mm, 3.2 mm, and 3.5 mm, respectively for the rainfall intensities of 30 mm/hr, 51 mm/hr and 68 mm/hr falling through 2.0 m. In the same order of rainfall intensities, raindrops hit the surface of the tested surfaces by 193.5 µJ, 244 µJ and 301 µJ kinetic energies, which were sufficient to initiate detachment in soils of D50 ranging from about 0.4 mm to 1.5 mm compacted to maximum dry density.

Erosion of soils seriously challenges the sustainability and safety in unpaved roads. It leads to faster deterioration of these roads by formation of rills and gullies in the running surface. Many factors related to the soil properties, rainfall parameters, and road geometry affect erodibility of soils at the surface of unpaved roads. However, little is known about the relationships between those factors of erodibility for a single rainfall event. This paper models the contributions of soil properties, intensity and duration of the rainfall, and road's length and gradient to the quantity of eroded soils from unpaved roads. For a 30-minute duration and two consecutive days; rainfall intensities of 30 mm/hr, 51 mm/hr and 68 mm/hr were used to test the erodibility of soils. The tested bed surfaces were set at slopes of 0% and 6%, in a small (large)-scale testing box of 0.6 m (1.2 m) x 0.3 m x 0.17 m (length x width x height). RapidMiner Studio software was used to predict quantities of eroded soils based on the measured eroded soils under the same influencing factors of erodibility. Six predictive models were developed based on the first-and second-day rainfall events. The predictive models can perform well with the Nash and Sutcliffe's coefficients of efficiency (ME) ranging from 0.62 to 0.74. Also, clay content and mean particle size of the surface soils, rainfall intensity and slope gradient were the most contributing factors to the quantity of eroded soils from unpaved roads.