Scarcity of good building land will often necessitate building on areas of fill. A variety of materials can be found in filled sites, ranging from quarry and mining waste to household and industrial refuse. Sites filled with refuse can give rise to problems of internal combustion, methane gas and other toxic chemicals; therefore building on these should be avoided whenever possible.
If the fill is fairly shallow then the most sensible option is to use piled foundations. The augured pile described earlier is often not suitable in fill if large stones and rubble are likely to be encountered and an alternative method is to use a driven pile.
One option is to use a driven pile made up of individual hollow pre-cast concrete sections, typically 300 400mm diameter. Using a special crane the pile is driven down into the ground adding extra sections as necessary. It has reached its correct depth when repeated hammer blows only produce minimal downward movement of the pile; this is known as a ‘set’ and is specified by an engineer.
As the fill naturally consolidates over the years there may be a downward force on the piles due to the friction of the ground against the pile sides. This ‘down-drag’ is rarely even and the resulting differential movement can cause cracking of the building.
In practice it is difficult to sleeve the whole length of a pile and several manufacturers prefer to coat the pile sections with a bituminous compound during manufacture. Under slow rates of strain the bituminous compound acts as a viscous fluid and reduces the down-drag (or up-lift) on the pile.
Deeper fill is best dealt with by the use of rafts and, as explained earlier, the raft spreads the load from the walls over the whole ground floor area. Some movement is to be expected and it is therefore essential to make sure that the services which enter or leave the property have flexible connectors immediately adjacent to the external wall.
Rafts, when designed for poor-quality ground, or ground where subsidence is expected, can be very expensive and have to be designed by structural engineers. However, they are a fast form of construction with minimal excavation and are sometimes also used on soft clays as an alternative to the reinforced wide strip foundation.
The pictures show a simple raft foundation formed from 150mm reinforced concrete slab and a more complicated raft foundation with downstand beams.
Occasionally it is possible to provide some form of ground treatment and use traditional strip foundations. On very large housing sites this can be cheaper than the use of rafts or piling. There are a variety of methods which attempt to increase the stability and bearing capacity of the ground. One method, called vibro compaction, involves the use of a crane-mounted poker which is driven into the ground.
A spinning eccentric weight inside the poker causes it to vibrate and this helps to compact the surrounding ground. The poker is then slowly extracted from the ground at the same time as sand is pumped through the poker to fill the void.
The operation is then repeated at 2 or 3m intervals to form a regular grid across the site. Vibro-replacement is another ground treatment; a treatment more suitable for cohesive soils. A special poker or piling rig forms a grid of stone columns in the ground, at the same time compressing the surrounding soil and increasing its density.
The stone piles act as weak columns transferring the building loads to a firmer strata. An alternative form of ground treatment is called dynamic compaction. This sounds a very grand title but in fact just involves dropping a weight of several tonnes on to the ground from a crane. It is not suitable if there are existing buildings in the immediate vicinity.
