Earth Pressures - Retaining Walls

1. Active Earth Pressures - these are those pressures which tend to move the wall at all times and consist of the wedge of earth retained plus any hydrostatic pressure. The latter can be reduced by including a subsoil drainage system behind and/or through the wall.

2. Passive Earth Pressures - these are a reaction of an equal and opposite force to any imposed pressure thus giving stability by resisting movement.

Retaining Walls up to 6-000 High

These can be classified as medium height retaining walls and have the primary function of retaining soils at an angle in excess of the soil's natural angle of repose. Walls within this height range are designed to provide the necessary resistance by either their own mass or by the principles of leverage.

Design the actual design calculations are usually carried out by a structural engineer who endeavours to ensure that:

1. Overturning of the wall does not occur.
2. Forward sliding of the wall does not occur.
3. Materials used are suitable and not overstressed.
4. The subsoil is not overloaded.
5. In clay subsoils slip circle failure does not occur.

The factors which the designer will have to take into account:

1. Nature and characteristics of the subsoil(s).
2. Height of water table † the presence of water can create hydrostatic pressure on the rear face of the wall, it can also affect the bearing capacity of the subsoil together with its shear strength, reduce the frictional resistance between the underside of the foundation and the subsoil and reduce the passive pressure in front of the toe of the wall.
3. Type of wall.
4. Material(s) to be used in the construction of the wall.

Small Height Retaining Walls

Retaining Walls up to 1m High-2: Retaining walls must be stable and the usual rule of thumb for small height brick retaining walls is for the height to lie between 2 and 4 times the wall thickness. Stability can be checked by applying the middle third rule.

Retaining Walls up to 1m High

The major function of any retaining wall is to act as on earth retaining structure for the whole or part of its height on one face, the other being exposed to the elements. Most small height retaining walls are built entirely of brickwork or a combination of brick facing and blockwork or mass concrete backing. To reduce hydrostatic pressure on the wall from ground water an adequate drainage system in the form of weep holes should be used, alternatively subsoil drainage behind the wall could be employed.

Pile Testing

It is advisable to test load at least one pile per scheme. The test pile should be overloaded by at least 50% of its working load and this load should be held for 24 hours. The test pile should not form part of the actual foundations. Suitable testing methods are:

1 . Jacking against kentledge placed over test pile.
2. Jacking against a beam fixed to anchor piles driven in on two sides of the test pile.

Pile Caps

Piles can be used singly to support the load but often it is more economical to use piles in groups or clusters linked together with a reinforced concrete cap. The pile caps can also be linked together with reinforced concrete ground beams. The usual minimum spacing for piles is:

1 . Friction Piles † 1„100 or not less than 3 x pile diameter, whichever is the greater.

2. Bearing Piles † 750 mm or not less than 2 x pile diameter, whichever is the greater.

Cast In-situ Piles

An alternative to the driven in-situ piles.

Driven In-situ Piles

Used on medium to large contracts as an alternative to preformed piles particularly where final length of pile is a variable to be determined on site.

Steel Tube Piles

Used on small to medium size contracts for marine structures and foundations in soft subsoils over a suitable bearing strata. Tube piles are usually bottom driven with an internal drop hammer. The loading can be carried by the tube alone but it is usual to fill the tube with mass concrete to form a composite pile. Reinforcement, except for pile cap bonding bars, is not normally required.

Steel Screw Piles

Rotary driven and used for dock and jetty works where support at shallow depths in soft silts and sands is required.