Gabions are utilised as a major component of retaining walls in all kinds of applications.
Their relatively low cost, flexibility and free draining properties have long been known; over the years as engineers have become more conscious of environmental needs, the natural appearance of gabion walls has come to be appreciated as an additional asset.
Construction of Gabion walls
For construction purposes, gabion retaining walls are considered as mass gravity flexible structures. The following principles need to be noted.
1. Horizontal forces
- Active soil pressures are calculated using a formula of the Coulomb wedge theory type or by graphical analysis (trial wedge).
- Hydrostatic pressure. Unless the gabions are made to be impermeable, full hydrostatic pressure is not allowed for, since the gabion wall acts as a highly efficient drain.
- Passive soil pressures. In most instances gabion walls are founded at or just (50cm) below ground level, and therefore no significant passive pressure is mobilised
Figure 1: Forces acting on a retaining wall
Pa – active pressure
Pv – active pressure (vertical component)
Ph – active pressure (horizontal component)
W – mass of wall
R – resultant
Rv – total vertical component
f – bearing pressure
2. Vertical forces
Self-weight of wall. It is usual to allow for ± 30% voids in the gabion stone fill when calculating the self-weight of the wall.
3. Stability calculations
- Stability against overturning. A factor of safety of 1.5 is normally considered sufficient.
- Foundation pressures. The foundation pressure diagram is assumed to be of a straight-line trapezoidal form: therefore the resultant of the calculated forces should pass through the middle third of the base. Since the flexible nature of a gabion wall is likely to produce a more uniform distribution of pressure, the maximum pressure calculated errs on the safe side.
- Forward movement. Resistance to forward movement should be calculated using the frictional resistance of a foundation soil-to-soil interface. A downstand, as indicated in Figure 1, may be located towards the back of the wall to mobilise the maximum pressure available.
- Additional checks. Walls higher than 4m should be checked for stability at intermediate levels at the discretion of the designer. To calculate frictional resistance, ∅ for gabion rock fill is taken as 45 deg.
Figure 2:
Example of gabion retaining wall construction. The cross-section shows the concrete screed and drainage channel, as well as the slight batter of the structure.
5. Layout Ideas for Gabion Walls
- In plan. Walls curved in plan are aesthetically pleasing, have a more natural appearance, and mask the almost inevitable surface irregularities.
- In elevation. Wherever possible, gabion walls should have a pattern of bonding similar to brickwork. This not only adds strength to the finished structure but improves its appearance by interrupting the noticeable thick vertical joints between units.
- In section. A slight batter (approx. 5%) improves the stability of the wall by swinging the centre of gravity towards the fill. It also helps to hide the slight irregularities of the face.
For further advice on any upcoming projects please contact us.
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