Table 1.

Summary and comments on the technical principles of the green roads approach to road construction

Technical principles (GIZ 2013)Comment‘Traffic light’ description
Construction during agricultural slack seasonOrdinarily construction should be during the dry season; this may or may not coincide with the agricultural slack seasonAmber
Phased construction: first year, 2 m wide track; second year, widened to 3.5 m; third year, widened to 4.5 mThis seems counter-productive as slopes will be cut three times, delaying the start point from which slopes can begin to self-stabilize or benefit from planting schemesAmber
Sectoral construction: road construction is divided into sections, 5 – 10 km in lengthThis would be consistent with the deployment of a number of small-scale contractors under the overall supervision of a main contractor or supervising engineerGreen
Staged construction: low-volume traffic to begin with, road widened for increased traffic laterThis is normal practice, although if higher traffic volumes are anticipated at the commencement then the road should be designed and constructed accordinglyAmber
Mass balancing: achieved by applying a cut and fill approach in spite of conventional ‘box cutting’Not all cut is suitable as fill. Use of hand-driven compaction equipment can be time-consuming and may not compact fill to the required density, although the phased construction approach allows for some natural compaction, thus allowing refilling in settled areas. There are many circumstances where a balanced cut and fill in each cross-section is not favoured, either on stability grounds or where side slopes are too steep (>30°) for unretained fill (Fookes et al. 1985; TRL 1997; Hearn 2011)Amber
Alignment planning: the alignment should avoid areas near active landslides, erosive rivers and gulliesThese guidelines regarding roadside development are consistent with good practiceGreen
Where appropriate (hill) alignments should be preferred against valley bottom ones and narrow serpentine-type switchbacks should be avoidedSwitchback sections of alignment corridor are sometimes unavoidable and often preferable to avoid unstable parts of the landscapeAmber
The road gradient should not exceed 12°12° is exceptionally steep where the road surface is not bitumenized and where the subgrade comprises residual soil or colluviumRed
Special attention should be paid to areas where (roadside) settlement development might lead to encroachment of hazard zonesAgreed regarding the control of roadside developmentGreen
Use of indigenous knowledge: (local people) can usually contribute extensive knowledge on areas where soils are soft and unstable or where marshy areas indicate high groundwater tables, which might induce landslidesLocal people can often provide valuable information but specialist advice should also always be soughtAmber
Dispersed water management: the approach aims at dispersed water management. In hilly terrain the cross-section of the road should therefore slope slightly outwards so that water is led away from the mountainsideRunoff should be controlled. No road surface can be constructed or maintained in such a way that water is evenly distributed along its length. An inward sloping road with a side drain is recommended with frequent culverts into existing stream channels with low-cost scour protection belowRed
Bio-engineering: after road completion adjacent slopes should be rehabilitated by planting vegetationBio-engineering should be an important element of slope protection and community extensionGreen
Supportive structures: at some locations roads must be protected by retaining structuresThis is almost inevitable in steep terrain and where excavated soils are vulnerable to failureGreen