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As outlined in Section A2, there are several factors that determine the selection of route corridors and the design of the alignment within them. These include road network planning and traffic forecasts, construction and maintenance costs and engineering, socio-economic, environmental and political considerations. The following discussion focuses on route corridor and alignment design in relation to topography and geometric standards, and describes how these considerations interface with those of slope stability and ground conditions.


The choice of cross-section on mountain roads is of critical importance. Even small increases in road width on steeply sloping ground can have a major impact on earthworks volumes and the need for retaining structures. An increase of road pavement width from 5 to 6 m, for example, can increase construction costs by as much as 50%; formation widths commonly adopted for roads in lowland terrain may be difficult to justify economically in hilly and mountainous areas. Furthermore, the greater the road width the greater the disturbance to the natural hillside and usually the larger the volumes of spoil required to be disposed of. These outcomes may exacerbate slope stability problems and necessitate a higher maintenance commitment in later years.

Many road projects involve the upgrading of existing roads through a combination of widening and improvements to the horizontal and vertical geometry. Widening into the cut slope will require slopes to be completely reformed and will invariably reactivate or trigger slope instability and erosion. Widening into the hillside can also be quite disruptive to traffic if the road is to be kept open at the same time. By contrast, widening on the outside edge of the road may encounter difficulties with the stability of fill slopes and the suitability of foundations for retaining structures (see discussion in Section A4.3).

For a new road in hilly or mountainous terrain there are essentially three main choices of cross-section: full-cut, part-cut/part-fill and full-fill, as shown in Text box C2.1. The cost

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