In the construction of bridges and flyovers, retaining walls often come into the picture because of the need to raise a mass of soil to a higher level to permit movement of vehicles underneath. This mass of soil must be supported by a retaining wall.
The design of retaining walls considers the backfill soil as retained earth in dry state during design as this gives more economical design. However, in the event of rain or rise of water table, the backfill soil becomes flooded and this causes the unit weight of the submerged soil to rise thereby creating more pressure on the retaining wall.
To avoid failure of the wall due to increased pressure, there should be provision for drainage. This drainage can be achieved with the use of the following:
- Weep holes: These holes are usually 0.1m diameter and spaced at about 1.5m to 3m in the horizontal direction. To avoid backfill material washing into it and clogging it, it is provided with filter material.
- Perforated pipes: These pipes are laid near the base of the retaining wall. The water is collected from the backfill and discharges at a suitable place at the bends. Filter materials such as cloth or geotextile fabric is provided around the pipes to avoid clogging. Clean-outs are also provided for the pipes to enable cleaning when clogged.
Note
- Fine-grained soils cause large earth pressures against retaining walls and are rarely used as backfill material. Good draining granular materials should be used especially along the sliding wedge portion of the wall.
- In a situation where the fine-grained soil must be used, a filter of coarse permeable material should be placed behind the retaining walls to prevent the development of excess pore water pressures.
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