Forest Road Engineering Guidebook - Ministry of Forests

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Forest Road Engineering Guidebook 30 zones of ground water seepage may experience larger and greater frequency of slope failures or sloughing problems. The significance of observed seepage zones might dictate the application of special drainage measures to reduce the likelihood of slope failure during construction and the intensity of maintenance activities over the operating life of the road. As a general rule, without special drainage measures, the side slopes of poorly drained fills (e.g., fills composed of silty soils) should be constructed at angles that are flatter than the angle of repose to minimize the likelihood of slope failures. Cut slopes The design of cut slopes should consider and address factors such as the desired performance of the cut slopes, types of cut slope materials, issues about overall terrain stability, engineering properties of soils, seepage conditions, construction methods, and maintenance. In general, cut slopes will remain stable at slightly steeper angles than fill slopes constructed from like soil materials. The reason for this is the undisturbed soil materials in a cut (1) are often in a denser state than similar type materials placed in a fill, and (2) may contain sources of cohesive strength that further increases the shearing resistance of the soil. Cut slopes constructed at too flat an angle can be uneconomical in steep ground because of the large volumes of excavation. Steeper cut slopes may be more economical to construct in terms of reduced volumes of excavation. However, they can also be more costly from an operational standpoint because they require more road maintenance. For most forest roads, cut slope angles are generally designed to favour steeper angles to reduce the length of cut slopes, to minimize visible site disturbance, and to reduce excavation costs, provided that a somewhat higher level of road maintenance and likelihood of slope destabilization is acceptable for the site. In the latter case, prepare and implement a maintenance schedule that addresses the erosional processes acting on the exposed cut slope face (such as splash, sheet, rill, and gully erosion) and reduces the threat to drainage systems (as a result of cut bank slope failure redirecting ditchwater flows onto potentially unstable fill or natural slopes), user safety, and the environment. It may be necessary to construct flatter cut and fill slopes, or to build retaining wall structures to support cut slopes or fill slopes, in cases where slope stability problems are expected to be difficult to manage with maintenance measures alone.
Forest Road Engineering Guidebook Table 5. General guidelines for cut and fill slope angles for use in forest road design. Cut Slopes Fill Slopes Suggested Cut Slope Angles b for Cut Suggested Examples of Material Bank Height Examples of Material Fill Slope Types a < 6 m c Types a Angles b Coarse- Road cuts in loose to compact 1 1 Ú2 H : 1 V Road fills composed 1 1 Ú2 H : 1 V grained SANDS or SANDS and predominantly of Soils d GRAVELS (not cemented and SANDS, or SANDS and non-cohesive) GRAVELS, or drained mixtures of coarsegrained and finegrained soils Fine- Road cuts in loose SILTS, or soft 1 1 Ú2H : 1V for lower grained cohesive soils such as SILTY cuts Soils CLAYS or CLAYS (not to Road fills composed 2H : 1V consolidated and not cemented) 2H : 1V for higher predominantly of SILTS cuts or CLAYS e Road cuts in hard cohesive soils such as SILTY CLAYS or CLAYS (consolidated) 1H : 1V or flatter Dense Road cuts in dense to very 3Ú4H : 1V or flatter See above for coarse- See above for Glacial Till / dense GLACIAL TILL (i.e., basal grained soils coarse-grained Cemented till), or cemented SANDS or soils Sands and Gravels SANDS and GRAVELS Rockf Road cuts in strong, good quality 1Ú4H : 1V Road fill composed 1H : 1V ROCK masses with no significant to predominantly of weaknesses vertical individually placed and locked together (not dumped) good quality angular blasted or ripped ROCK Road cuts in other ROCK types 1Ú4H : 1V Road fill composed 1 1 Ú4H : 1V should be flatter to include any to predominantly of to weaknesses from the effects of 11Ú4H : 1V dumped angular ROCK 11Ú2H : 1V structural discontinuities in the or placed rounded rock mass, and other factors such as the strength of the rock material, and the spacing, aperture, roughness, filling, weathering, and orientation of discontinuities ROCK 31
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Endhauling road fill pullback Endha
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Office review tion prescription and
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• vehicle usage (4-wheel drive, A
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Care should be taken in verifying i
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Field observations (prescription in
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Forest Road Engineering Guidebook B
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Appendix 1. Field identification of
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Soil consistency Soil density The c
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Appendix 2. Vertical (parabolic) cu
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Design speed (km/h) Design speed (k
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Table A2.4. Vertical curve elements
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Forest Road Engineering Guidebook A
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• PWL and HWL, where applicable
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Appendix 5. Tables to establish cle
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1. To compensate for a cut at the c
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Appendix 8. Example field data form
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Appendix 10. Landslide risk analysi
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• the type of materials involved
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Table A10.2. Example of a qualitati
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Table A10.7. Example of consequence
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Forest Road Engineering Guidebook - Ministry of Forests

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