Foundatiions

This picture reminds us that reliability begins with foundations. This building was built near a beach, and the ground has proved inadequate; perhaps a hurricane contributed.

Every bridge has to rest on the ground in at least one place. The supports have to be placed so as not to move unacceptably, which means that the stresses must be reduced to values that can be supported by the ground. On hard, strong rock, the supports can be narrow, but in weaker ground, a wider foundation may be necessary.

These three pictures show a piece of foam plastic which has been strained by pushing objects against it, to represent a pillar resting on the ground. The strains are revealed by the square graticule that was drawn with a fibre pen. From the distortions we can deduce the following facts –

The strains are concentrated near the point of application.

The strains are spread over a large area.

There is tension, as revealed by the curved upper edge, which is

longer than the original straight edge.

There is shear, as revealed by the angles which are no longer

ninety degrees.

And of course there is compression.

The final configurations are those which minimise the total strain energy.

This picture tries to give a rough idea of the way that pressure diffuses through the ground under a heavy weight. A more exact picture could be made by drawing contours of equal stress. The ground has to able to withstand the stresses at all points without giving way, either quickly or by creep. In any volume where this is not the case, the ground must be replaced by a structural material which is designed to take the load and spread it into the ground at a supportable magnitude. This type of diagram cannot be exact, because the behaviour of the ground is dependent on its type. Even something as simple as the contact pressure under a pier is not constant across the section, and the behaviours with clay and sand are actually opposite.

If you tread on wet sand on a beach, you can often see the sand around your foot apparently drying out, because the grains have been disturbed, allowing more water into the gaps. The size of this area reveals the spread of forces around an object on the ground. Sand is, of course, not a good material on which to build, though damp sand can be used by children of all ages to build surprisingly large and complicated structures.

One way to work out the size of an excavation is to dig out a weight of soil equal to the weight of the structure that is to be supported. This is an empirical rule.

The "leaning tower of Pisa" is a well-known example of the difficulty of soil engineering.

Problems with soil include –

Differing types of soil

Variation of properties within a site

Variation of properties with time –

Variation of water content

Effect of flash flooding

Melting and freezing of included water.

Click here for a web-site dealing with earth structures and related matters. It also includes numerous links to web-sites about engineering and science.

Some of the problems that be encountered in building foundations are –

Depth of poor ground above firm rock

Depth of water above ground, which itself may be poor

Speed of water flow

Tidal variation in depth of water.

Practical Examples

Queenhill Bridge crosses the river Severn in a region where the alluvium is very deep. Parts of the bridge are as far below ground as the superstructure is above it. The foundations had to go deep in order to find good ground.

Here is a picture of Telford’s bridge over the river Severn at Over, near Gloucester. After the centring was removed, the crown sank about ten inches, or 25 cm, but the bridge was used continuously and safely from 1829 to 1974. It was then replaced by a wider bridge which could take greater volumes of traffic. The ground in the area is alluvial, and for this reason, Gloucester, like Worcester, is built almost entirely on the east side of the river.

We can calculate how much ground movement will produce a given drop if we assume that the structure is rigid apart from a central hinge. The diagram below shows how this works.

The rate of change of the rise R as a function of the span S is given by the equation –

dR / dS = – S / 2R.

So for very flat arches, the sag can be large for a small change in span. Furthermore, the flatter the arch, the greater the outward thrust. This is discussed in the page about arches.

For example, if the span is 30 metres, and the rise is 3 metres, and the span increases by 5 cm, the sag will be 5 X 30 / 2 X 3 in cm, which is 25 cm, or 10 inches.

The opposite situation arises with a suspension bridge: the cables have to be anchored to prevent them pulling out of the ground. Here at Clifton, the wrought iron chains go into sloping tunnels in the ground, where they are securely anchored. Where the ground is unsuitable for anchoring cables, massive anchorages have to be built; these rely on sheer weight to hold the cables. The Humber bridge has an anchorage of this type.

The beautiful tower of Cirencester church has buttresses which go right down into the ground in the west wall. Because of difficulties with the ground, threatening the tower, this wall had to be taken down on the south side in order to add the buttresses. The NW and SW corners of the tower are also well buttressed. Should you climb the helical stairway to the top to see the splendid view of the town and country, you can be quite sure that the tower will stay upright.

There is actually something peculiar about these buttresses – they are straight. If you look at the page about the funicular you will see that the buttresses should logically curve towards the ground. In the case of Cirencester we can imagine that the builders wanted to anchor the buttresses as far from the tower as possible, in the hope of finding better ground.

Perhaps appearance entered into the design, since a window was required, and curving the buttress around the window would have looked rather strange. On the other hand, the builders might not have understood the flow of the forces at all, as everything seems to have been done empirically in the middle ages. In the first picture, we see that the buttress actually reaches the ground perilously near the corner of the building, but in fact there probably isn’t much thrust left in it by that point: it has probably gone into the lower part of the wall.