Longitudinal slopes

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Assuming that you have chosen the best route to minimise the amount of earthworks required, you may need to cope with small scale undulations or steep slopes.

Undulations

For small scale undulations, the easiest technique is to transfer material from the high points into the low points, forming a series of rises and falls within the required gradient.

coping with path undulations

Steep slopes

If space allows, short slopes can be reduced in gradient by cutting and filling material to achieve the required gradient, otherwise you will need to create ‘zigzags’ to increase the length of the path up the slope at a shallower angle. The use of zigzags means that you will need to adopt the techniques for dealing with cross slopes, in order to form a bench for the path. The turns will need to be wide enough to allow all users to negotiate the bend and additional earthworks will be required to form a level ‘platform’ for the turn. These turns are good places to include resting places.

Calculations
The length of the path that will be needed as zigzags is based on the height gained and the average gradient of the path. First measure the total height of the slope to be climbed using a level or clinometer (height = Sin(angle°) x distance). The length of path is then calculated using:
Gradient of path as a percentage: length = height rise ÷ gradient (%)
Gradient of path as a ratio: length = height rise x ratio (1: G)
For a 20 metre height gain with a path surface slope of 5% or 1:20 you will need 400m of path
(400m = 20 ÷ 0.05 (5%) or 20 x 20)
You then need to divide the length of path by the width of the path corridor, to determine how many ‘legs’ you need (e.g. on a 100m wide corridor, you will need 4 legs, each of 100m plus 3 platform turns, in order to gain 20m height with an average gradient of 5%).

 

The following table gives some typical figures that may help with estimating your needs:

Average path gradient Length of path needed to gain 20m 100m path corridor 50m path corridor
5% 400m 4 legs, 3 turns 8 legs, 7 turns
6.67% 300m 3 legs, 2 turns 6 legs, 5 turns
8.3% 240m 3 legs, 2 turns 5 legs, 4 turns
10% 200m 2 legs, 1 turn 4 legs, 3 turns

 

The path should be pegged out accurately on site to ensure correct gradients and remember to add on the ‘platform’ for turns in your calculations of materials required and length of path. Avoid using ‘symmetrical’ zigzags of the same length and consider blocking potential short-cuts (with vegetation, boulders or even barriers) as these will lead to rapid erosion of the slope. Make sure that drainage from higher levels does not interfere with lower sections, by extending the outfall / soakaway points beyond any lower section of path.

Long Slope Dainage

Tips
Earthworks to adapt the gradient should be considered as a last resort – try finding an alternative line that does not require cutting or filling
Earthworks may disturb sub-surface water flows and so drainage issues not found during the site assessment survey may become apparent. In particular, cuttings will attract water and suitable drainage provision will be required
Allow a minimum of a 500mm verge between a path edge and a side slope. If space does not allow such a verge, consider whether a hand rail will be required
Ensure that newly formed side slopes have a maximum gradient of 50% or 1:2 (66% or 1:1.5 for stronger soils). All slopes should be dressed with topsoil and preferably turfs or appropriate grass seed mix. Using turfs will rapidly establish and regenerate vegetation and avoid leaving large scars of bare earth. A vegetated slope is also less susceptible to slipping. If turfs are scarce, lay those that are available along the foot (toe) of a slope since growth here is particularly important for slope stability. If side slopes must be made steeper due to width restrictions, use some form of slope stabilisation or retaining wall
Edges of new slopes should be curved into existing slopes to avoid harsh, straight cut lines
If you need to dig borrow pits to generate fill material ensure they are left tidy and present no hazard to users who stray off the path. Ideally, fill borrow pits with unsuitable soil taken from the path route and cover with excess turfs
To reduce the landscape impact of earthworks, consider using additional planting to hide unavoidably large cuttings

 

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