Retaining walls - Geotechnical Investigation and Design Tables - page 255

Table 20.6 ( Continued )

Wall Drainage measure Typical design detail for rainfall environment

height

1000 mm 1000 mm < >

5m Weep holes and Geotextile 75 mm Weep holes at 75 mm Weep holes at

> • • •

wrapped 150 mm perforated pipe 2.0 m horizontal and 1.5 m horizontal and

at base of wall with outlet. vertical spacing vertical spacing

Internal drainage system (staggered), and (staggered)

necessary 300 mm drainage gravel 300 mm drainage gravel

• •

Horizontal drains wrapped in behind wall behind wall

•

filter to be considered Typically 5 m long * 75 mm 5 m long * 100 mm with • •

with spacing of 5 m spacing of 3 m vertically

vertically and 5 m and 5 m horizontally

horizontally

•

Even walls above the groundwater table must be designed with some water
pressure. For a dry site a water pressure of ¼ wall height should be used.

•

Drainage layers at rear of gabions and crib walls (free draining type walls) are
not theoretically required. The 200mm minimum thickness of the drainage layer
behind these and the low height/low rainfall walls shown above is governed by
the compaction requirement more than the drainage requirement.

•

Compaction against the back of walls must be avoided, hence the use of a self
compacting “drainage layer'' is used behind all walls, without the need to compact
against the wall.

•

A geotextile filter at the back of the wall drainage gravel (if used) is required to
prevent migration of fines.

•

For intensity rainfall 2500mm and/or large catchments (sloping area behind

wall) more drainage systems than shown may be required.

>

Figure 20.2 Drainage of walls.