Retaining walls - Geotechnical Investigation and Design Tables - page 260

Table 20.13 Wall type and facings required for cut slopes.

Consideration Wall type and facing required

Slightly to
distinctly

Distinctly to
extremely

Extremely to
residual

Rock weathering Fresh to slightly

Typical cut slope
Maximum slope angle

1V: 0.01H 1V: 0.27H 1V: 0.58H 1V: 1.00H 1V:1.73H

~90 75 60 45 30

Design if adverse jointing
or space limitations

Vertical
wall

Battered
wall

Reinforced slope

Type of facing Active facing Passive facing No facing

•

Berms for maintenance may be required with a steeper slope.

•

Actual slope is governed by the rock strength, joint orientation and rock type.

•

Rock trap fences/netting may be required at any slope.

20.14 Drilled and grouted soil nail designs
•

Soil nails are either driven or drilled and grouted type. The latter has a larger area
and tensile strength, and with a larger spacing.

•

An excavated face of 1.0 to 1.5m is progressively made with soil nails installed
with a shotcrete face before excavating further. About 5 kPa cohesion in a clayey
sand has show to be sufficient to allow 1m of excavation to proceed.

•

For soils without sufficient cohesion the order can be reversed ie, shotcrete before
nailing.

Table 20.14 Drilled and grouted nails - typical designs (adapted from Phear et al., 2005 and Clouterre,
1991).

Material type Typical Facing Length Area per nail Nails per m 2

(m 2 )

slope angle type

Weak rocks 70 to 90 ◦ Hard 0.6 to 1.0 H 1.5 to 2.5 0.4 to 0.7

Soils 70 to 90 ◦ Hard 0.8 to 1.2 H 0.7 to 2 0.5 to 1.4

Natural soils 45 to 70 ◦ Flexible 0.6 to 1.0 H 1 to 3 0.3 to 1.0

Natural soils and fills 30 to 45 ◦ None 0.8 to 1.2 H 2 to 6 0.1 to 0.5

•

Typical strength of a drilled and grouted nail is 100 to 600 kN.

•

Table assumes a level ground at the top.

•

In high plasticity clays the length may need to be increased to account for creep.
An active bar ( ie bar with a plate) instead of a passive facing (ie bent bar) may be
required.