Abstract This book deals with landslides, earthworks (cut and fill slopes), retaining structures and erosion protection on mountain roads and embraces planning, feasibility study, investigation, design, construction, improvement and maintenance. Non-arterial roads constructed in hilly and mountainous areas are usually characterized by low traffic volume, and are low-cost in the approach adopted in their design, construction and maintenance. This book focuses on these roads but many of the techniques described are equally relevant to more highly trafficked roads and high-investment infrastructure including railways and pipelines. The reason for this is that the techniques of geomorphology and engineering geology, which constitute much of the discussion and illustration contained herein, are among the most valuable tools applicable to any linear infrastructure project in complex and unstable terrain. This is especially true for remote locations where information is frequently lacking on ground conditions. This book also focuses on the humid tropics and subtropics where heavy seasonal rainfall is responsible for a high incidence of slope instability. Nevertheless, large parts of this book will also be of interest to practitioners working in higher latitudes. Environmental issues of mountain road construction and maintenance are not addressed per se although many of the engineering considerations relating to land use and vegetation cover, earthworks stability, spoil disposal, drainage and erosion are also highly relevant to environmental protection. TRL (1997) discusses environmental and social impact considerations of mountain roads and further review is given in, for example, Corbett & Gaviria (2003) ; Dhakal et al. (2010)

Abstract Road construction and improvement projects (usually comprising widening, pavement reconstruction or resurfacing and improvements to horizontal geometry) are conventionally subdivided into the following stages: feasibility study; preliminary design; detailed design; construction; and operation and maintenance. Low-cost road projects located in flat or gently rolling terrain typically incur costs in the following proportions (though percentages can vary significantly from project to project): 1% feasibility study; 2% design; and 97% construction. For new roads in hilly and mountainous terrain several alignment options may exist, each with its own implications for length, ease of construction, stability and cost. Decisions made over alignment selection and the choice of cross-section can have profound effects on the cost of construction and the performance of the works during operation and maintenance. Investments in desk studies and engineering geological field investigations during the feasibility study and design stages can assist this decision-making and help avoid otherwise unforeseen ground conditions and stability problems during later stages. It is recommended that the opportunity be taken during these early stages to carry out these studies, especially in difficult and complex terrain. A cost distribution between the three main project stages might then be of the order of 5%, 5% and 90% respectively. For example, in the case of the Arun III hydropower access road in Nepal, where comprehensive preparatory studies were undertaken, the combined cost of the feasibility study and design amounted to

Abstract Soils in hilly or mountainous areas are normally divided into two types: in situ weathered soils and transported soils. Table A3.1 provides a simplified classification and description of the common soils encountered in the humid tropics and subtropics and their engineering behaviour, based mainly on Fookes (1997) .

Abstract When the design, construction and maintenance of mountain roads are required to accommodate landslides and difficult ground conditions, decisions need to be made based on the assessment of risk. Risk management requires a balance to be struck between acceptable risk of blockage, damage or loss and affordable cost of risk reduction. However, before these decisions can be made, an assessment of landslide susceptibility and hazard is usually required.

Abstract ‘… if you do not know what you should be looking for in a site investigation, you are not likely to find much of value’ ( Glossop 1968 , p. 113). The term site investigation is conventionally used in civil engineering practice (e.g. Dumbleton & West 1974 ; Weltman & Head 1983 ; Hawkins 1986 ; Fookes 1997 ; bxsI 1999 ; Simons et al. 2002 ) to describe a range of studies and investigations undertaken to assess the topography, geology, geomorphology and geotechnical ground conditions of a site or an area for the purposes of engineering design. In hilly and mountainous areas, landslide and slope stability assessments usually form important elements of these studies, and are often undertaken as part of a terrain evaluation . This terrain evaluation includes office-based desk studies and field-based assessments, and comprises techniques designed to investigate, classify and interpret: landscape and landforms; geological structure, rock types and soil types; geomorphological processes, ground conditions and geohazards (including landslides) prior to embarking on any subxsurface ground investigation; groundwater conditions; and surface drainage patterns. As described by Lawrance et al. (1993) , a site investigation comprises terrain evaluation followed by subxsurface (or intrusive ) ground investigation principally by trial pitting, drilling and boring and laboratory testing. In all applications, it is important to review and interpret existing information and to carry out remote sensing and field mapping (Sections B2.2, B2.3, B3.3

Abstract Desk studies are most critical at the initial feasibility and planning phase of new road construction projects ( Dumbleton & West 1974 ). Decisions made at this early stage on the selection of alignment and the approach to design and construction are critical to scheme costs and to the future stability and operation of a road. While the advent of satellite imagery and geographical information systems (GIS) technology in particular (Section B2.7) has meant that desk studies have become potentially far more wide-ranging than they were even 10 years ago, the traditional desk study still remains valid and important, and often provides the bulk of information. The traditional desk study essentially combines data sources that are conventionally available in paper format, namely topographical maps, geological maps and aerial photographs (although digital maps and orthorectified photographs are now much more common place). Table B2.1 lists typical information that can be obtained from these three principal data sources, though their availability varies significantly ( Hearn 2004 ). Topographical and geological maps are normally available through government agencies and usually small-scale mapping can be downloaded from the internet prior to embarking on field investigations, either under license or for a fee. Unfortunately, published geological maps in many countries are small scale and show Formation-level (stratigraphic age) information only; the distribution of rock types, information that is most relevant to engineering, is often not shown. Even where larger scale geological mapping is available it usually pays ‘very little attention to surface formations.

Abstract Reconnaissance surveys are usually carried out to establish the main topographical, geological and engineering criteria that will influence the selection and design of a new alignment (e.g. Brunsden et al. 1975 ) or the key issues that need to be addressed in the case of a road improvement project (environmental and social impact reviews also form critical elements of these surveys). Reconnaissance surveys also allow validation of the desk study interpretations, and provide field information that can then be used to calibrate the desk study outputs.

Abstract The term ground investigation refers specifically to the investigation of subxsurface soil, rock and groundwater conditions, either through intrusive methods (principally boreholes and trial pits and the associated soil/rock sampling and testing) or surface and borehole geophysics. Ground investigation methods are described in numerous textbooks including more recently Clayton et al. (1995) , Simons et al. (2002) , Cornforth (2005) and Bond & Harris (2008) , for example. Ground investigations for low-cost roads can often be low on the priority list. They are sometimes seen to be costly and time consuming, providing little information of any value. However, if planned and implemented correctly, they can yield valuable information when compared to the cost of design. Ground investigations are undertaken for new roads to determine: typical soil and rock profiles to calibrate and augment the terrain models, terrain classifications and field mapping referred to earlier (Sections B2 & B3); specific soil and rock profiles in the case of deep cuts for site-specific design; foundations for structures, such as bridge piers and abutments and large retaining walls; depth and geotechnical composition of existing landslides and the design of remedial works; and material type and depth for borrow areas. Ground investigations are undertaken for existing roads when examining: landslides that were not stabilized during construction; and new landslides and road failures that have occurred since construction. In all cases, desk studies and