The Advances in Understanding Erodibility of Soils in Unpaved Roads

Abstract

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.

Calibration of the Simple Rainfall Simulator for Investigating Soil Erodibility in Unpaved Roads

“…Erosion rate is the measure of erodibility given by the ratio of dry sediment over eroded area per unit time. Generally, the results agreed with previous studies [7][8][9], [19] and highlighted the key elements governing understanding of erodibility of soils in unpaved roads, as can be observed in Tables 4 and 5. These elements are: Erosion rate is more dependent on the rainfall intensity and the area of the tested surface rather than the slope length.…”

“…Using the flour method to determine the size of the raindrops [3], [7], [15][16][17], the drop sizes of 3 mm, 3.2 mm, and 3.5 mm were found, leading to the kinetic energy from the raindrops of 193.5 µJ, 244 µJ and 301.1 µJ, respectively for 30 mm/hr, 51 mm/hr and 68 mm/hr rainfall intensities. The sizes of the raindrops were satisfactory to initiate erosion of soils of 0.4 mm < D50 < 1.5 mm [18], which were found appropriate at the surface of unpaved roads [9]. Importantly, the obtained raindrop sizes satisfied the range of the natural rainfall drops sizes (1.0 mm to 6.0 mm), as reported in previous studies [15] [19-27].…”

“…Three rainfall intensities of 30 mm/hr, 51 mm/hr and 68 mm/hr were used in this research. These were chosen deliberately based on IDF curves from countries of the Central and Eastern Africa, a region in the Sub-Saharan Africa where about 80% of the road networks are unpaved [9], [12][13][14] and thus in much need to control erosion in these roads. Using the flour method to determine the size of the raindrops [3], [7], [15][16][17], the drop sizes of 3 mm, 3.2 mm, and 3.5 mm were found, leading to the kinetic energy from the raindrops of 193.5 µJ, 244 µJ and 301.1 µJ, respectively for 30 mm/hr, 51 mm/hr and 68 mm/hr rainfall intensities.…”

“…High spatial uniformity of raindrops (> 90%) and easy control on drop size are attainable [2][3]. The raindrops of natural rains are reproducible (1 mm -6 mm) [1], [9]. The raindrops fall by gravity force [1].…”

“…An additional diagram for the detachment of the soils by surface flow velocity with respect to the particle size distribution of the soils was given according to Carey and Simon [31], as shown in Figure 10. The tested soils were synthesized by mixing gravelly sandy (GS) and very gravely sandy (VGS) soils with percentages of English china clay (ECC) to simulate the variability of soils at the surface of unpaved roads in terms of particle size distribution as well as the required plasticity index at the surface of unpaved roads of about 2% to 12% [7,9,32], as shown in Table 3.…”