Terrain assessment, drainage and erosion - Geotechnical Investigation and Design Tables - page 190

Table 15.5 Development procedures based on slope gradients only.

Vert. : Horiz. Deg. ◦ Grade % Slope risk Comments on site development

1V : 2H 27 50 Very high Not recommended for development > > >

1V : 2H to 1V : 4H 27 to 14 50 to 25 High Slope stability assessment report

1V : 4H to 1V : 8H 14 to 7 25 to 12.5 Moderate Standard procedures apply

1V : 8H 7 12.5 Low Commercially attractive

< < <

15.6 Terrain categories
•

Categorisation of the terrain is the first stage in its assessment.

Table 15.6 Terrain categories.

Terrain category Slope Common elements

% Deg. ◦ Vert. : Horizontal

Steep hill slopes 30% 16.7 1V : 3.3H > >

High undulating rises 20-30 11.3-16.7 1V : 5.0H to Ridges, crests and upper

1V : 3.3H slopes

Moderate undulating rises 10-20 5.7-11.3 1V : 10H to Mid slopes

1V : 5H

Gently undulating to level plains

<

10% 5.7 1V : 10H Lower and foot slopes

15.7 Landslide classification
•

The different slopes have a different potential for landslides.

•

This does not cover rock falls, which was covered in previous chapters.

Table 15.7 Typical landslide dimensions in soils (Skempton and Hutchinson, 1969).

Landslide type Depth/Length ratio (%) Slope inclination lower limit (Deg. ◦ )

Debris slides, avalanches 5-10 22-38

Slumps 15-30 8-16

Flows 0.5-3.0 3-20

15.8 Landslide velocity scales
•

Rapid landslides cause greater damage and loss of life than slow landslides. See
Table 15.8.

15.9 Slope erodibility
•

The slope erodibility is controlled by the grades and type of soil. The latter is
provided in later tables.

•

The minimum gradients are usually required for drainage purposes, eg 1% gra-
dient for drainage - a cleansing velocity, but higher velocities are required to
minimise flood conditions on higher ground.