Foundation

Published by Basilio on

The foundation is the part of the structure that is in direct contact with the ground and serves to transmit its loads to the ground. There are three types of foundations, superficial, semi-deep and deep.

  • Shallow foundations are those that are made close to the surface. Footings and foundation slabs correspond to this type. Shallow foundation systems are the most common and economical, although they need a resistant layer near the surface of the ground.
  • Semi-deep foundations are those that are made shallow. To this type correspond the foundation pits, retaining walls below grade, and micropiles.
  • Deep foundations are made when the resistant stratum is well below the surface level. These can work in two ways; per tip, when they transmit the loads to the tips of the pillars. And by lateral friction, when the loads are transmitted by the sides of the pile.

Types of surface foundations:

Shoe:

The footing is the most common foundation system. It consists of a wide concrete prism located under the pillars of the structure. Its function is to transmit the stresses to which the rest of the structure is subjected to the ground and anchor it.

There are several types of shoe:

Isolated footing:

Serves as the basis for specific structural elements, this footing expands the support surface until the ground supports the load transmitted to it without problems.

The insulated footings are braced with reinforced concrete bracing beams with a section below the footing.

They can be made of mass concrete, that is, without reinforcement, if they have a considerable depth (they are the so-called rigid footings).

Arming the lower part: A mesh made up of crossed corrugations is made; the separation between corrugations must not exceed 30 cm.

Combined shoe:

diagrama cimentación zapata corrida

When two footings are close together, they fuse to form a combined footing, which supports several piers.

Also when it comes to a perimeter column with large bending moments, the ground pressure may be insufficient to prevent the foundation from overturning. The way to solve it is by joining or combining the footing of this column with the closest one.

Running or continuous shoes:

zapata de hormigón

They are made to support load-bearing walls, or pillars aligned relatively closely, in low, medium or high resistance terrain.

They are also used in dividing walls to better distribute the weight and not invade the neighbor's property.

Foundation slabs:

cimentación losa de hormigón

When the ground does not allow the foundation by footings, either because of its constitution, or because the calculation of the footings exceeds the 50% of the surface of the area to be cemented, the foundation is made by slab.

A foundation slab is a concrete plate supported on the ground which distributes the weight and loads of the building over the entire support surface.

Slabs are a type of shallow foundation that has very good behavior in not very homogeneous soils that with another type of foundation could suffer differential settlements. They are also used in soils with very little bearing capacity.

In essence, a foundation slab behaves like a plate that undergoes vertical forces on its underside and the settlement or vertical deflection of the slab. For a slab of uniform thickness the expression is:

w (x, y) Represents the vertical descent or settlement at each point on the plate.

qs Represents the surface load, usually concentrated on the pillars that support the plate.

kb It is the ballast modulus, which measures the stiffness of the ground.

The ballast modulus being a magnitude associated with the stiffness of the ground:

 D It is the flexural stiffness of the slab.

Once the function that gives the vertical settlements has been determined, the bending moments in each direction:

formula momentos flectores

From the bending moments the correct thickness of the slab is determined and the armor needed for the slab.

Types of semi-deep foundations:

Foundation pits:

cimentación semiprofunda

Foundation pits are semi-deep foundations that are made when the ground is too soft to make a shallow foundation.

The particularity of the foundation pit is that it is being built as it sinks into the ground. The cross section of the well is generally circular, but there are also square, rectangular or elliptical sections.

Micropiles:

cimentación micropilotes

The micropile is a foundation element, resistant to tensile and compressive forces; It is made up of a steel tube placed inside a hole drilled in the ground and received in it by means of an injected cement grout.

The resistant element of the pile is a tubular-shaped reinforcement that is placed in the hole with threading or welding of the different sections.

Micropiles can be used in the following cases:

-As reinforcement of foundations for the expansion of buildings.

-In underpinning of buildings.

-Reinforcement and support of existing foundations for excavation of basements by means of micropile screens.

-Deep foundations on properties with terrain not suitable for conventional piles.

Types of deep foundations:

Piles:

A pile is a constructive element used for foundation works, which allows the loads to be transferred to a resistant layer of the soil, when it is at a depth such that it makes a more conventional foundation using footings or slabs unfeasible, technically or economically.

The denomination is applied when the construction method consists of making a hole in the ground to which, once finished, a reinforcement will be placed inside and later it will be filled with concrete.

It has the shape of a column placed vertically inside the ground on which the element that transmits the loads (pillar, pile cap, slab ...) rests and transmits the load to the ground by friction of the shaft with the ground, supporting the tip in stronger layers or by both methods at the same time. The piling system has been used in the past to build in swampy areas, using wooden piles and driving them into the flooded land. As the wood was not exposed to water, it did not decompose. It is the main foundation system in the city of Venice.

Piles in situ

esquema pilotaje hormigón

The denomination is applied when the construction method consists of making a hole in the ground to which, once finished, a reinforcement will be placed inside and later it will be filled with concrete.

Due to the way the casting is executed, there are basically two types of piles: extraction and displacement piles. An extraction pile is carried out by extracting the ground, while the displacement pile is carried out by compacting it. In both cases, different techniques are used to maintain the stability of the excavation walls.

Silo displacement in situ pile

Usually as shallow piloting working to the tip, supported by rock or hard layers of ground, after traversing soft layers. Also as piling working by shaft and tip in medium or loose granular soils, or in soils with alternating coherent and granular layers of some consistency

Displacement in situ pile with gravel plug

Usually as piling working by shaft on medium compact granular soils or on soils with alternating coherent and granular layers of some consistency.

Extraction in situ pile with recoverable casing

This type of pile is executed by excavating the ground and using a sleeve (metal tube as formwork), which prevents the excavation from collapsing. Once the emptying is completed, and as the pile is being concreted, the sleeve is gradually removed, which can be reused again.

In-situ extraction pile with lost jacket

It is executed by the same system of the in situ type of extraction with recoverable casing, with the difference that the metallic jacket is not extracted, but is permanently attached to the pile.

In-situ drilled pile without casing with thixotropic muds

It is an extraction pile, in which the stability of the excavation is entrusted to the action of thixotropic sludge. Usually as piloting working to the tip, supported by rock or hard layers of ground. When soft layers are crossed that are maintained without detachment due to the effect of sludge.

In-situ drilled pile without casing

esquema pilotaje hormigón

Auger for the execution of piles.

Usually as a pilot working on the edge, supported by a layer of hard coherent terrain. Also as piling working by shaft in coherent terrain of firm consistency practically homogeneous or coherent of medium consistency in which no detachment of the walls occurs.

Auger for the execution of piles.

Usually as a pilot working on the edge, supported by a layer of hard coherent terrain. Also as piling working by shaft in coherent terrain of firm consistency practically homogeneous or coherent of medium consistency in which no detachment of the walls occurs.

In situ pile drilled and concreted by central auger tube

Usually as piloting working to the tip, supported by rock or hard layers of ground. Also as piloting working by shaft and tip in soils of compactness or medium consistency, or in soils with alternate layers, coherent and granular of some consistency.

These are piles by displacement of the earth by means of a continuous auger. Subsequently, the concreting is carried out by pumping through the central tube existing inside the auger.

This system is suitable for soft and unstable soils and with the presence of water. The reinforcement is introduced once the pile has been drilled and concreted, which generates the inconvenience that due to the density of the concrete, the reinforcement length does not exceed 7.00-9.00 m.

Driven piles

cimentación pilotes hincados

It consists of introducing precast concrete elements similar to light poles or metal sections by means of pilings in the ground.

Said elements are placed vertically on the surface of the ground and later "driven" into the floor with blows from the "hammer", this causes the element to descend, penetrating the ground, a task that lasts until the depth of the stratum is reached. resistant and the soil “rejection” occurs in the case of a pile that works by “point”, or reaches the design depth, in the case of a pile that works by “friction”.

Precast piles

Precast piles belong to the category of deep foundations, they are also known by the name of precast piles; They can be built with ordinary reinforced concrete or with prestressed concrete.

Conventional reinforced concrete piles are used to work under compression; Prestressed concrete works well in tension, and is suitable for sheet piles and when they must be submerged under water. These piles are driven into the ground by means of blows made by a hammer or with a metal shovel equipped to drive the pile.

Its section is usually square and its dimensions are normally 30 cm x 30 cm or 45 cm x 45 cm They are also built with hexagonal sections in special cases. They are made up of two reinforcements: a longitudinal one with four 25 mm diameter rods, and a transverse one made up of rod stirrups with a minimum section of 8 mm. The pile head is reinforced by fences 5 cm apart over a length of one meter. The tip is reinforced with a special metal piece to facilitate driving.

Operating principle

The piles transmit the loads they receive from the structure to the ground through a combination of lateral friction or shaft resistance and resistance to penetration or point resistance. Both depend on the characteristics of the pile and the terrain, and the ideal combination is the object of the project. For a circular pile, concreted in situ and supported, the lower tip of which is on a layer of appreciable resistance, the sinking load is given by:

H, Pile height and diameter.

what, Overload [kN / m2] on the foundation foundation of the piling.

γ, μ, specific weight of the ground and friction coefficient ground-pile.

 ρt, admissible pressure on the stratum on which the pile tip rests.

In many cases, the theories that allow estimating shaft resistance and tip resistance are empirical. In other words, they are the result of a statistical analysis of the behavior of certain piles under certain ground conditions. Therefore, it is extremely important to know the origin and the conditions under which certain calculation formulas are valid.

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