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Foundations - every bridge needs them

A foundation has to create a smooth transition allowing the internal forces to flow between the bridge and the ground. Foundations are the 'bridges' or links between the bridge and the ground.

The picture on the left shows the underside of the approach spans of the Second Seven Crossing near Bristol, England. The brdige bearings are immediately under the bridge deck and sit on piers which take the load down to the permanent caisson (which has a safety railing around its top).

If a foundation doesn't work for some reason - if the ground gives way, settles, consolidates or crumbles - then the whole bridge may fail. So foundations are very important even though they are largely unseen.

There are really only two types of bridge foundations - although each has many variations - 'spread' foundations and 'piled' foundations.

Both types must transfer forces into the soil beneath the bridge. So before we look at these two types let's briefly consider the soil beneath.

When bridge engineers talk about soil they don't mean the topsoil in which you grow your garden plants. That soil will not support any weight coming from bridges or any other structures - so it has to be removed. To a bridge engineer soil is the material that you find after the topsoil has been removed.

It is largely sand, clay or a mixture of sediments and deposits that come from the disintegration of rock. It lies mainly above the layers of rock that are studied by geologists. Clearly if the bed rock is close to the surface foundations are much easier.

As you might imagine the variety of engineering soil is enormous and for many centuries has defied attempts at scientific understanding. Now we have the engineering science of soil or 'geotechnics' which bridge designers of foundations can use. Engineering soils have solid grains, water and sometimes air and are usually classified by size as gravel, sand, silt or clay.

The solids are particles. They range in size from clay where they are the size of dust, through the type of sand you find on a beach, to large boulders.

Clay is 'sticky' or cohesive because the particles are so fine there is an attraction force between them. The particles cannot easily be separated out by filtering or allowing them to settle out - they are colloids where the molecules are larger than those in a solution but smaller then those in a suspension. Milk and paint are common examples of other colloids.

Sand particles are bigger and so sand is not sticky. The particles slide over each other - but they are rough and so the sliding is resisted by friction between particles.

In summary the shear resistance of dry sand is almost entirely a friction and interlocking between the grains. Finer grained soils have some 'stickiness' or cohesion so the shear resistance depends on the friction and on cohesion.

Karl von Terzaghi is widely known as the father of the soil mechanics and the science of building foundations which we now call geotechnics.

He took the ideas of early pioneers, like Frenchman Charles-Augustin de Coulomb who published a theory of earth pressure in 1773.

When Terzaghi started to think about this problem he realised that one key idea was missing - the pressure between the soil particles in contact with each other. He called this the effective stress. Terzaghi's breakthrough was to realise that this determines the ability of soil to resist load. But he also realised that this pressure depends on the pressure in the water in the soil. He called this the pore water pressure because it exists in the pores of the soil.

He defined the total stress as the stress applied to the soil and defined the effective stress as the difference between total stress and the pore water pressure.

In quicksand, for example, all contact between the soil particles has been lost - the effective stress is zero, the water dominates and the quicksand flows just like water - but a very dense water.

Terzaghi laid the scientific and engineering foundations for the building of bridge foundations

Spread Foundations


The loads, the forces, from the bridge are spread over an area large enough for the ground to bear them.

Spread foundations are rather like the snowshoes you might wear to prevent you sinking into deep snow - the stereotype is like a tennis racquet - in fact the French term is raquette neige.

The principle of a spread foundation applies if you were ever unfortunate enough to find yourself caught in quicksand. The best advice is not to struggle but to lie flat and still on the surface to try to spread your weight. If no one is around to help pull you out then try to swim - quicksand is liquefied sand and so is much denser than water. Swimming in it will be very hard work but it could save your life.

Spread foundations are often called strip footings if they are the bases for each pier or abutment. Caissons (see below) are used build larger spread foundations under water.

Piles


Piles are long slender columns in the ground that transmit the loads to lower depths where they can be carried.

They are several types
  • Driven Piles: manufactured concrete or steel piles are knocked into the ground - like massive nails driven into timber by hammering.


  • Preformed Driven Cast in-situ Piles: a steel tube with a closed end is hammered in and filled with concrete.


  • Driven Cast in-situ Piles: a steel tube is hammered in and as it is filled with concrete the tube is withdrawn.


  • Bored and Cast in-situ: a hole is drilled into the ground and then filled with concrete - often with a cage of steel reinforcement


  • The choice of pile depends on the ground conditions and soil strata.

    If there is bed rock some way down then the piles can be driven or cast so that the loads are taken directly to the rock - they are known as end-bearing piles. Bored piles are usually end bearing with sometimes quite large diameters.

    However if there is no bedrock near enough to the surface then the piles may be driven or cast into stiffer ground. The piles are then designed to transmit the load to the ground through the friction between the surfaces of the pile and the ground. Driven piles may be end bearing or friction piles or a combination of both.

    A tension pile is used to transmit a pull out force rather like an anchor. Groups of them are often used to hold down the cables at the end of a suspension bridge. They act a bit like a plug that you might use to fix a shelf bracket into a brick wall.

    There are two main criteria. First the soil must not fail. Second any settlements must be controlled.

    There are a number of types of piles commercially available.

    Caissons and Cofferdams


    It is not easy to lay foundations under water. Caissons are large watertight chambers sunk into the river bed to enable men to work in the dry.

    The picture on the left shows one of the caissons for the Forth Railway Bridge. At the very bottom - around the perimeter and at each side in the diagram - there is a cutting edge that sinks into the ground. You can see the men digging in a working chamber. The floor above their heads is fitted with air locks so that the chamber is under compressed air. This helps to keep the water out. When the excavation reaches the required depth the whole lower chamber is filled with concrete - the ceiling is removed and the bridge piers are built on top.


    Cofferdams are water tight shields built around an underground underwater structure such as a dock wall. The water is pumped out so that work can be carried out in the dry. If the head of water is low then steel sheet piles suffice even though they will leak - the water can be continously pumped out.

    Bearings


    Bridges do move because of changes in temperature, traffic movement and braking, wind loads, shrinkage and creep of materials, movement of fooundations and earthquakes.

    The job of a bearing is to accommodate these whilst supporting the bridge and transmitting the loads from the bridge to the foundation.

    There are three basic types.

  • Elastomeric: An elastomer is a natural or synthetic rubber material which can deform under load then regain its original shape when the load is removed. A pot bearing has an elastomeric disk completely enclosed in a steel pot. A fixed bearing can accomodate rotation only.
    A guided bearing allows rotation and movement in one direction. A free bearing allows rotation, longitudinal movement, and transverse movements.

  • Plane sliding: Plates (often of PTFE - a low friction polymer) slide over each other so that only vertical loads are resisted with no accomodation of rotation.


  • Rollers: Only movements in the direction of the rollers are accomodated.
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