Civil Engineering Reference
In-Depth Information
Figure 13.37: Raft foundation. Source: Bugge 1918
systems the level of the groundwater has been lowered, and because of this, fungus
attack on the foundations will occur, causing a settling of the buildings.
The simplest form of raft foundation is a layer of logs laid directly onto the ground tied
to logs laid across them. Masonry columns or perimeter walls are built on this foundation,
and around the edges layers of clay are packed in. Raft foundations were probably in com-
mon use around the seventeenth century and quite normal up to about 1910, when they
were slowly replaced by wide, reinforced concrete slabs.
In pile foundations the raft is replaced by vertical logs, which are rammed down into
the ground. It is usual to lay three or four horizontal logs onto the piles to distribute the
weight evenly, before building the walls. The weight of the building and the bearing
capacity of the earth decide how close the piles need to be to each other. Foundations
for smaller buildings usually have thinner piles, from the thickness of an arm down to
the thickness of a finger. To distribute the load, a filled bed of round stones may be
used.
In sandy earth lacking soil the piles above ground level can be taken to a bottom
plate. This can provide a simplified solution in certain cases, but even with good impreg-
nation and high-quality timber it is doubtful that the foundation will hold longer than 75
years.
Structural walls
Timber buildings are usually associated with load-bearing timber walls. It is nec-
essary to differentiate between light and heavy structures. The most important
aspect of lightweight building is the framework, which is economic in the use of
materials and takes advantage of the tensile and compressive strengths of timber.
