Foundation building in Construction and Bearing Capacity

Anytime a Structure is built, all over the earth, will be necessary to determine how to transfer its loads to the underneath ground safely. This process is called foundation building. In engineering it is also defined as the application of soil mechanics and rock mechanics (geotechnical engineering) in the design of foundation elements of structures.

The main functions of foundation are:

• Avoid overloading the soil (causing unequal settlement) by distributing equally the structure’s load on the ground,
• Level surface for construction,
• Prevent structure failures related to natural forces, like earthquakes, floods, etc.
• Increase stability, prevent overloading and lateral movements.

The foundation design is realized by evaluating that soil below the foundation structure does not undergo shear failure, the settlement caused during the first service load or have to be within the limit and the pressure the soil can withstand without failure.

Types of Foundation

Foundation can be classified in two main categories: shallow foundation and deep foundation.

Shallow foundations, often called Footings, transfer the load into soil for about a meter or so. There are 2 main types:

• Spread footing that consists of strips or pads of concrete (or other materials) which extend below the frost line and transfer the weight from walls and columns to the soil or bedrock,
• Slab-on-grade where the load is transferred to the ground through a concrete slab placed on the surface.

Deep foundations are used to transfer a weight of a structure to the stronger layer of subsoil below. Heavy weights, like Skyscrapers, weak soils or site constraints like property lines require deep foundation. There are different types of deep footings, we’ll analyze all of them in the next paragraph.

Deep foundation focus

Driven foundations

Whit this method pile driver is used to insert piles into the soil. Materials used for prefabricated piles are wood, concrete (reinforced with rebars) or steel. Concrete piles have different shapes, like square, round and octagonal, while steel is piles or sometime beam section (h-pile). Driving piles has a high load-bearing capacity because the soil displaced by the piles compresses the surrounding soil, generating friction on pile sides.

Drilled piles

A borehole is drilled into the ground, then concrete is placed into the borehole to form the pile. This technique allows larger diameter piles that are necessary for dense or hard strata of soil. When the sides of borehole can slough off before concrete is poured a casing is applied. End-bearing piles require that drilling will go on until a sufficient depth and a strong layer has been reached.

Piled walls

Retaining walls provide special advantages where available working space dictates that basement excavation faces be vertical. In permanent works, these walls can be designed to sustain vertical loads in addition to horizontal forces. Contiguous walls are constructed with small gaps between adjacent piles, that is determined by soils strength.

Deep mixing/mass stabilization techniques

The increase of ground bearing capacity is obtained through binders, like cement, lime, flyash, or other. This solution is mainly used for clays or organic soils.

Diaphragm walls

Preventing a deep trench from collapsing can be reached by filling it with bentonite and then reinforced concrete panels, the joints can be watertight. This is used where basement and above ground works are carried out at the same time (top-down construction).

Caissons

This are suitable for structures below water level. Caissons are watertight retaining structures entered into the ground by removing material from the bottom.

Ground anchors

The very high loads are mechanically transferred from a tendon to the ground. Ground anchors can be tensioned by the applied load or pre-tensioned.

Bearing capacity

The capacity of soil to support the loads applied to the ground. Soil bearing capacity is the maximum average contact pressure between the foundation and the soil; over this level we can observe a shear failure in the soil. The theoretical maximum pressure which can be supported without failure is the ultimate bearing capacity; allowable bearing capacity is the ultimate bearing capacity divided by a factor of safety.

The bearing capacity is one of the major design criterion for civil engineering structures. Civils.ai provides a useful calculator to analyze soil bearing pressure of a building foundation: