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Geotechnics

Nature of the soil

If a building must be built in a seismic risk zone, the ground must be probed to an important depth. Certain soils such as the alluvia or clays can be liquefied when they are subjected to an earthquake shock waves. It is necessary to avoid building at this place then or, failing this, establishing deep foundations which are anchored in the subjacent rock. Clay soils can inflate or be packed according to whether they are in wet or dry phase. The vertical movement extent of these soils can reach a few decimetres and the pressure exerted on the foundations can cause cracks, even ruptures. Soils with strong organic content such as peat can, with time, be in tension under the load of a building so to represent no more but a fraction of their initial volume, in fact causing structural breakdown. Other soils of low cohesion tend to be concealed under the load.

Once construction is completed, the soils can behave differently whether they were embanked, recomposed, drained or sprinkled, in short disturbed in any manners. Sometimes, the soil located under a construction project varies so much from one place to another that it is likely to pack differently, so that the building will undergo the consequences. Thus it is necessary to analyze the soil and the subsoil in order to determine the construction feasibility from a technical and economic point of view by calling upon the geotechnical engineers.

So, in the case of a solid substrate of low depth, the foundations will be more concentrated. On the other hand, when rocks or soils become less and less resistant while moving away from the surface, the foundations will have to be wider, so as to distribute the pressure more evenly due to construction weight.

Groundwater level

The foundations installation becomes complicated when it must sit in the ground-water, because water can undermine the excavation walls and cause them to collapse. In order to reduce this risk, a sheet piled wall can be built and inserted until the refusal corresponding to the excavation perimeter. A lining can also be installed, then water pumped so as to cause a drop in the ground-water level in order to consolidate or to retain the excavation walls and thus to avoid any collapse.

Elements of a Building

The main elements of a building include :

• The foundations, which make it possible for a construction to rest on the ground while supporting it and ensuring its stability;
  
• The structure or framework, which ensures the air stability of the work, supports all the loads applied and transmits to the foundations the stress due to the building weight, the occupation loads and the constraints exerted by the wind, snow, and earthquakes, etc.;
  
• The load-bearing walls which can be integrated into the structure, as well as the posts, the beams and the floors which define the framework;
  
• The interior partitions or load-bearing partitions, which sometimes can be integrated into the structure;
  
• Environment control systems: heating, ventilation, air conditioning, lighting and soundproofing;
  
• Vertical circulation systems: elevators, escalators, staircases and stairwells;
  
• Service tracks, which can comprise subsystems such as the internal communications, soundproofing, closed-circuit television or telephone wiring networks;
  
• Energy and water-supply systems as well as drainage waste systems;
  
• The building envelope, made up of the frontage, the gables and the roof, which separates the interior from the exterior of the construction and protects it from various stress: rain, wind, heat, cold, noise, sunlight, etc. It plays a fundamental part in energy saving.
  

Loads Imposed to a Building

They are classified as "dead" and "live" loads. Dead loads include the weight of the building itself, as well as all the building major items. Dead loads act directly downwards and are additive starting from the top of the building down. Live loads include wind pressure or the weight of snow, seismic forces, vibrations caused by machinery, movable furniture, stored goods and equipment, occupants and forces caused by temperature changes. Live loads are temporary and can produce pulsing, vibratory or impact stresses. In general, the design of a building must accommodate all possible dead and live loads to prevent the building from settling or collapsing and to prevent any permanent distortion, excessive motion, discomfort to occupants, or rupture at any point.

Fondations

The structural design of a building depends greatly on the nature of the soil and underlying geological conditions and human modification of either of these factors.

Types of Fondation

The most common types of foundation systems are classified according to their degree of depth. The shallow foundation systems reach a depth not exceeding three meters under the bottom of building, such as the raft foundations and spread footings, whereas deep foundations extend to more than three meters of depth below the building, like piles and caissons. The foundation chosen depends on the strength of the rock or soil, magnitude of structural loads, and depth of groundwater level.
The most economical foundation and the oldest is the reinforced-concrete spread footing, which is used in areas where the subsurface conditions present no unusual difficulties. It consists of concrete slabs located under each structural column and a continuous slab under load-bearing walls.

Raft foundations are typically used when the building loads are so extensive and the soil so weak that individual footings would cover more than half the building area. A raft is a flat concrete slab, heavily reinforced with steel, which carries the downward loads of the individual columns or walls. The load per unit area that is transmitted to the underlying soil is small in magnitude and is distributed over the entire area. For large raft foundations supporting heavy buildings, the loads are distributed more evenly by using supplementary foundations and cross walls, which stiffen the raft.

Piles are used primarily in areas where near-surface soil conditions are poor. They are made of timber, concrete, or steel and are located in clusters. The piles are driven down to strong soil or rock at a predetermined depth, (this depth is determined thanks to geotechnics). Each cluster is then covered by a cap of reinforced concrete. A pile may support its load either at the lower end or by skin friction along its entire length. The number of piles in each cluster is determined by the structural load and the average load-carrying capacity of each pile in the cluster. A timber pile is simply the trunk of a tree stripped of its branches and formerly was used for great constructions such as bridges or churches (Cathedral of Strasbourg). It was substituted during the 19th century by the concrete pile, longer, mechanically and chemically stronger. For extremely heavy or tall buildings, steel piles, known as H-piles because of their shape, are used. H-piles are driven through to bedrock, often as far as 30 m (100 ft) below the surface. Although they are more expensive, the cost is usually justified for large buildings, which represent a substantial financial investment.

Caisson foundations are used when soil of adequate bearing strength is found below surface layers of weak materials such as fill or peat. A caisson foundation consists of concrete columns constructed in cylindrical shafts excavated under the proposed structural column locations.