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Retaining Wall Terminology

This is the material (placed behind the wall) that applies the load the wall must resist. Some backfills exert low lateral forces and hence the wall itself may be built more economically. The designs for Koppers Logs have assumed that a better class of backfill (than is usually available from the site itself) is used to reduce the overall cost.

Additionally it is desirable that the backfill does not settle with time (consolidate). While the backfill could be compacted by a roller, the cost of temporarily supporting the wall against the lateral forces generated is not normally warranted in domestic construction. So the desirable properties of a backfill should be for it to exert low lateral forces, to easily consolidate and to be free draining. 

For this reason, the soil placed behind the wall (for a distance equal to at least the wall height) should be selected for its superior properties.

Batter is a slope measurement.  1:4 means 1 horizontal to 4 vertical (in metric or imperial measure).
See also surcharge angle.

Changing the ground level can affect the stability of the earth, adjacent buildings and drainage. Many local councils insist on plans being submitted for approval before commencing work. Often these require a Consulting Engineer’s certification that the design is appropriate for the particular circumstances.

The posts of a cantilever wall stand almost vertical. They are embedded sufficiently into the ground so that they can resist the horizontal forces generted by the weight of the retained soil (posts cantilever out of the ground). Therefore, it is important that the embedment is sufficiently deep considering the ground’s strength and the load imposed by the retained soil. It is also important that the concrete around the post is sufficiently compacted so that the post will not move in the hole.

A cohesive soil is one that sticks together and includes the clays, clay-shales, silty-clay, sandy-clay, loams, etc. Primarily it resists load in direct proportion to its cohesive strength.

The function of the concrete backfill is twofold.  Since it forms a monolithic ring around the pole, the stresses exerted on the foundation material are less as the force is spread over a larger area. Concrete is stressed by the pole but, being relatively strong, it can resist the load with no deformation.  Other hold backfill materials are difficult to compact and yield when load is applied, resulting in a rotated wall.

Where the watertable is below the bottom of the post hole, a gravel layer should be placed to allow any accumulation of moisture to drain away. After placing the post, concrete backfill must be thoroughly compacted.

There are two methods of concreting, each suiting a particular procedure. Where posts are temporarily supported (by propping) the concrete should be a normal 20 MPa strength concrete with a slump of up to 60mm (this is the type most frequently supplied by a readymixed concrete company and would also be readily obtained using bagged pre-mixed concrete). Props can be removed the next day. 

Alternatively, a zero slump mix can be used. This has the same proportions of ingredients except for water. Water (equal in weight to 25% of the cement content) is added and mixed.  he post is held in line while the stiff concrete is rammed with a heavy rammer around the pole. The concrete should be placed in layers no thicker than 150mm and rammed thoroughly before adding the next layers.

With both systems, the top of the concrete should be finished to drain away from the posts. Usually the last 100mm of the backfill should be an impervious soil (eg. clay) to prevent water entry around the post and to facilitate bedding the lowest wales.

The critical zone refers to either an increased loading exerted on the wall by the extra mass of a building or the like, or movement of the ground which can cause movement of a building or the like. This occurs because the wall and ground share the work in resisting horizontal forces and both must move to result in an economical wall.

If buildings are within this zone then the wall must be designed for forces greater than those in the tabulations. The loadiing is called the earth pressure at rest and is beyond the scope of the tables.  Accordingly, a Consulting Engineer should be engaged to design the appropriate wall.

Cantilever walls can fail in a variety of ways but the most common are posts of insufficient strength resulting in fracture or extreme deflection or foundations of insufficient depth (embedment) resulting in rotational failure.

Embedment depth for cantilever walls is critical. A small reduction in depth results in a significant increase in stress within the foundation material. Additionally a reduction in post hole diameter causes extra soil stress and increases the likelihood of a foundation failure.

Fences often need to be constructed at the top of retaining walls as they are on boundaries or to prevent people falling over the wall. Apart from using extended posts to form fence uprights, fence posts can be bolted to either the inside or outside of the wall. Placing fence posts in the backfill without attachment to the wall can sometimes result in a sagging fence line as the backfill consolidates.Usually retaining walls are constructed solely on the property of the person who caused the instability (eg. levelling a platform for building purposes).  Fences, on the other hand, are usually built on the boundary. This presents a problem where a person on the high side erects a wall to retain filling right to the boundary. Then a fence, built on top of any sloping wall, will not be on the true boundary.

This is the earth which must resist the laterial forces. For a cantilever wall, it is the ground into which the post is embedded and for gravity type walls it is the ground on which the footing rests. It is important that it is sufficiently strong and that it will not consolidate. Any filling is normally not acceptable as foundation material.

A geotextile is a synthetic fabric designed to be used in the ground (geo). In retaining wall construction, geofabrics are used as separation membranes, stopping one material from flowing into another due to gravity or groundwater. If a fine grained soil flows into the voids between large grains eg. silt flowing into a gravel, it can leave a large hole behind (called piping). In the Koppers design guides, geofabrics are used to prevent fine material flowing between the walling, effectively reducing piping. Bidim A12 (from Geofabrics Australiasia Pty Ltd), or equivalent, is a suitable and economic geofabric for log wall construction. Geotextiles should be used with any large post and wale walls and with all pallisade walls.

Used under the post to encourage any water to drain away into the ground that may otherwise pond around the post and increase the risk of decay.

It has become common for hardware stores to offer steel hardware zinc plated rather than hot dip galvanized.  Zinc plating results in a thin uniform layer of protection which is cheaper and more attractive than hot dipping in molten zinc bath. It also means that parts of fine tolerances (eg. bolts and nuts)  fit together with a minimum of fuss. However, in landscaping, the abundance of moisture, together with the nature of the treatment salts, require that all steel parts be protected as fully as possible. This means that all steel shall be hot dipped galvanized to provide cathodic protection. Additionally, parts passing into the treated timber (such as bolts and deck spikes) should be well greased (eg. crude petroleum jelly) or even tar epoxy coated to provide a barrier to moisture, oxygen and electrical conductivity.

A non-cohesive soil is one in which soil grains do not stick together eg. sand, gravel, building rubble. It resists load by its internal friction, so called because its resistance increases with increasing weight from above.  Generally, non-cohesive soils tend to have more uniform strength than cohesive soils.

Where rock is encountered at the foundation level, often the appropriate solution is to use a free standing wall such as crib or tieback. If the rock can be drilled economically and the rock is competent (say equivalent to weak concrete) then cantilever posts may be embedded. The hole should be drilled at the recommended diameter to an embedment depth equal to twice this diameter. The shape of the excavation should be parallel sided to prevent the post rotating out.

Slab posts are only useful for low cantilever wall heights. They are used with the flat face against the horizontal walers, enabling the wale joins to be concealed. Because wood has been sawn off the pole in the most stressed regions to produce the slab, slabs are much weaker than poles. As a general rule for light duty walls, a slab 100mm between flat faces can be substituted for a 100mm dia pole (the slab having come from a 125mm round).

Any extra vertical load on top of the retained soil results in increased lateral force on the wall. This extra load can be in the form of a sloping backfill surface (often referred to as a surcharge angle) temporary or moving loads eg. building material (temporarily stored) or a vehicle, a building or swimming pool or other retaining walls stepped one above the other.

Tie-back walls are a type of closed face crib without a complete rear face. Lack of the rear face enclosure prevents them acting as efficiently as a crib and consequently are appropriate for low wall heights. They suit terracing and integration with the garden. These have to be custom designed by engineers.

Twin posts, instead of one, are usually not an efficient solution as more volume of timber is needed to provide the same strength.  However, for some cantilever walls it may be appropriate to use twin posts especially if the wall changes direction. The posts can be put together to conceal horizontal waler ends or can be placed apart to make for more economical waling. Post diameters need to be 80% of that nominated for single posts (since the strength of the post is a function of the cube of its diameter).

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