Complete Excavation

This method can be used in firm subsoils where the centre of the proposed basement can be excavated first to enable the basement slab to be cast thus giving protection to the subsoil at formation level. The sides of excavation to the perimeter of the basement can be supported from the formation level using raking struts or by using raking struts pitched from the edge of the basement slab.

Basement Excavations - Perimeter Trench Excavations

In this method a trench wide enough for the basement walls to be constructed is excavated and supported with timbering as required. It may be necessary for runners or steel sheet piling to be driven ahead of the excavation work. This method can be used where weak subsoils are encountered so that the basement walls act as permanent timbering whilst the mound or dumpling is excavated and the base slab cast. Perimeter trench excavations can also be employed in firm subsoils when the mechanical plant required for excavating the dumpling is not available at the right time.

Basement Excavations Open

One of the main problems which can be encountered with basement excavations is the need to provide temporary support or timbering to the sides of the excavation. This can be intrusive when the actual construction of the basement floor and walls is being carried out. One method is to use battered excavation sides cut back to a safe angle of repose thus eliminating the need for temporary support works to the sides of the excavation.

In economic terms the costs of plant and manpower to cover the extra excavation, backfilling and consolidating must be offset by the savings made by omitting the temporary support works to the sides of the excavation. The main disadvantage of this method is the large amount of free site space required.

Gabions and Mattresses

Gabion: a type of retaining wall produced from individua rectangular boxes made from panels of wire mesh, divided internally and filled with stones. These units are stacked and overlapped (like stretcher bonded masonry) and applied in severa layers or courses to retained earth situations. Typical sizes, 1.0 m long x 0.5 m wide x 0.5 m high, up to 4.0 m long x 1.0 m wide x 1.0 m high.

Mattress: unit fabrication is similar to a gabion but of less thickness, smaller mesh and stone size to provide some flexibility and shaping potential. Application is at a much lower incline Generally used next to waterways for protection against land erosion where tidal movement and/or water level differentials could scour embankments. Typical sizes, 3.0 m long x 2.0 m wide x 0.15 m thick, up to 6.0 m long x 2.0 m wide x 0.3 m thick.

Types of Soil Nails or Tendons

A cost effective geotechnic process used for retaining large soil slopes, notably highway and railway
embankments.

Function: After excavating and removing the natural slope msupport, the remaining wedge of exposed unstable soil is pinned or mnailed back with tendons into stable soil behind the potential slip plane.

Types of Soil Nails or Tendons:

• Solid deformed steel rods up to 50 mm in diameter, located in bore holes up to 100 mm in diameter. Cement grout is pressurised into the void around the rods.
• Hollow steel, typically 100 mm diameter tubes with an expendable auger attached. Cement grout is injected into the tube during boring to be ejected through purpose-made holes in the auger.
• Solid glass reinforced plastic (GRP) with resin grouts. Embankment Treatment ~ the exposed surface is faced with a plastic coated wire mesh to fit over the ends of the tendons. A steel head plate is fitted over and centrally bolted to each projecting tendon, followed by spray concreting to the whole face.

Crib Retaining Walls

A system of pre-cast concrete or treated timber components comprising headers and stretchers which interlock to form a three-dimensional framework. During assembly the framework is filled with graded stone to create sufficient mass to withstand ground pressures.

Climbing Formwork or Lift Casting

This method can be employed on long walls, high walls or where the amount of concrete which can be placed in a shift is limited.

Cantilever Retaining Walls

These are constructed of reinforced concrete with an economic height range of 1200 to 6000. They work on the principles of leverage where the stem is designed as a cantilever fixed at the base and base is designed as a cantilever fixed at the stem. Several formats are possible and in most cases a beam is placed below the base to increase the total passive resistance to sliding.

Mass Retaining Walls

These walls rely mainly on their own mass to overcome the tendency to slide forwards. Mass retaining walls are not generally considered to be economic over a height of 1800 when constructed of brick or concrete and 1000 high in the case of natural stonework. Any mass retaining wall can be faced with another material but generally any applied facing will not increase the strength of the wall and is therefore only used for aesthetic reasons.

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.