Part C: Design and Construction
Published:January 01, 2011
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
Figures & Tables
Slope Engineering for Mountain Roads
This book provides a complete guide to the study, design, construction and management of landslide and slope engineering measures for mountain roads, with an emphasis on low-cost. The geographical focus of the book is on the tropics and sub-tropics, but is also highly relevant to other regions where heavy rain, steep slopes and weak soils and rocks combine to create slope instability. The causes and mechanisms of landslides are described, and the hazards they pose to mountain roads are illustrated. Methods of desk study, field mapping and ground investigation are reviewed and illustrated, with an emphasis on geomorphological and engineering geological techniques. The design and construction of alignments, earthworks, drainage, retaining structures, the stabilization of soil slopes and rock slopes, and the control of erosion on slopes and in rivers and streams are covered. Slope management as part of road maintenance and operation is reviewed, and procedures for risk assessment and works prioritization are described.