Sustentação (arte de renovação de fundações)

Support (art of foundation renovation)

June 5, 2024 Roberto Magalhães

Underpinning is a process of strengthening the foundation of an existing structure, which can be a building or another type of structure.

This is a good technique used during and after construction to improve the foundation. Generally, the existing foundation and soil conditions are improved to support more load. This is a method that can be used to improve the foundation.

Support is a technique that must be carried out with great care and careful monitoring of the work. Incorrect approaches, procedures, methods, etc. can lead to structure failure.

This is usually done to the existing structure. If not done correctly, it can result in structural failure, serious damage to the structure, etc.

Purpose of support – Why we do support

There are many reasons why we can discuss this topic. Some of the most important reasons are listed below.

The original base is not strong or stable

If the original foundation is not strong enough to support the load of the superstructure, it must be improved to prevent failure. Additionally, if differential settlement occurs due to inadequate foundations, improvements can be made below the foundation to increase the bearing capacity of the soil.

When the use of a framework changes

It is possible to change the use of buildings. If loads increase due to these changes, shoring is carried out as an improvement to the foundation.

Changing the properties of the soil under the foundation

For compressible soils, decomposing soils, etc., their properties change with time. As a result, the foundations may settle. Under these conditions, improvements in soil conditions are made.

Taking into account incorrect soil properties

Taking into account incorrect soil properties or incorrect classification of soils leads to incorrect assessment of soil design parameters such as bearing capacity, surface friction (positive friction and negative friction), etc.

This can lead to excessive settlement of the building during the construction phase.

Excavation work for adjacent structures

The excavations had a significant impact on the existing structure. Adequate support could avoid/minimize these effects. However, the most appropriate support method must be chosen.

Increase the load capacity of the foundation

If it is necessary to increase the loads acting on the foundation, appropriate support methods are used.

For example, if we need to add a new floor to the existing structure rather than building it separately, the foundation improvement will be accomplished using the methods described in this article.

An improvement is more cost-effective than building a new structure

As explained previously, building a new structure will always cost more money and other resources.

For example, when we construct a new building, we need to utilize the available space. However, this is not a problem if we add a floor to the existing building.

Let us now discuss the basic methods available to address the above challenges.

Essentially, there are two methods.

Temporary support methods
Permanent support methods

Temporary methods include ground freezing, which is carried out in tunnel construction when excavations are carried out below the water table, and groundwater control, which is carried out to control the movement of groundwater when construction presents greater difficulties.

Now let's move on to the most important part of this article.

The following figure shows the available and commonly used methods.

As indicated in the table above, there are three main types of support methods.

Geometric base
Injection support
Support with stakes

Let's discuss some of the important methods in detail.

Support by grouting

Injection methods are often used to improve foundations and are particularly popular in gradual soil improvement.

If the floor has voids, the mortar can penetrate more easily.

Cement grouting

Cement grouting is effective in filling voids in granular soils where there are voids into which cement mortar can be transported. This is a useful technique in foundation improvements.

However, there are now many other methods that can make this process much more effective. Therefore, cement grouting is not used as often compared to other methods.

The following advantages and improvements can be observed in the cement injection process:

Reduces soil permeability as voids are filled with cement mortar
It supports the foundations
Stabilizes the soil under the foundation and improves its resistance.
This method can be used as excavation support

Chemical pressing

Chemical grouting converts granular soil into a very hard form with very high strength.

This type of injection is often used to improve granular soils. When injected into the soil, they unite particles, improving the soil's support capacity.

Chemicals such as acrylamides, polyurethanes, acrylates, epoxies and sodium silicates are commonly used for chemical grouting.

Furthermore, these methods are identical to cement grouting and similar benefits can be seen.

Blasting process

Jet grouting is also called in-situ soil-cement column method.

Let us discuss the steps to be followed in jet grouting as shown in the figure above.

A hole is drilled to a depth that requires improvement (stabilization and reinforcement) of the soil.
Drilling can be done with a rotary device or with a water jet
Then cut the soil with a high pressure jet (20-70 MPa) using a rotating nozzle with a diameter of 2-3 mm.
During this process, the mortar liquid is mixed with the soil.
Then, a soil-cement column is gradually created, which solidifies the soil as it hardens.

There are basically three different grouting techniques.

Single rod
Double rod
Triple bar

The following figure shows the blasting process carried out for a project.

As we can see in the image above, grouting can be used very effectively in construction; Furthermore, it serves as a support method.

Below are some of the benefits of jet blasting.

Reduces structure settlement
This is possible up to a depth of 80 m.
This method can be used as excavation support
Reasonable cost savings
Can also be used in confined spaces
High security

Compaction injections

This is a soil improvement method that increases the density and stiffness of the soil. This is possible with loose, dense and granular soils.

Generally, a low-slump mortar is injected into the soil layer at high pressure. The injection pressure is in the range of 1-4 MPa .

Pressure injection displaces the surrounding soil and fills it with the injected mortar. The depth of the improvement can be greater than 10m depending on the equipment available.

The injection rate can be around ^4-6m3 per hour. However, this may vary depending on the nature of the soil.

The following possible uses and advantages can be highlighted:

Suitable for loose soil, bendable soil, liquefied soil, etc.
Compaction grouting reduces foundation settlement
The soil's resistance increases and, as a result, its bearing capacity increases.
Increase soil density

Geometric support

These methods were used when it was necessary to widen the foundations, increase the bearing capacity of the soil beneath the foundation, or increase the stiffness/density of the soil beneath the foundation.

There are five main methods discussed in this article as indicated in the table shown at the beginning.

Continuous support of strip foundations
Needle Beam Support
Support and support
Pynford Fundamentals
Rise

Let's discuss each of these methods in detail.

Continuous support of strip foundations – pit method

This process is also known as pillar support, pit support or pit pillar.

It is also sometimes called the mass concrete bearing method.

These are very old methods of improving foundations when they fail or begin to settle. This method is often used to improve the foundation under brick walls.

In this case, the ruin is not a collapse, but rather the formation of cracks in the building or structure in question due to settlement.

This method proceeds as follows:

First, pits are dug under the foundation at regular intervals.
The distance between the pits can be 1-2 m, depending on the nature of the brick wall. If the quality of the bricks is very poor, this distance cannot exceed 1.0 m.
The spacing of the wells is also determined by the curvature of the brick wall above the well.
The pit walls can be supported with wood if necessary.
Digging depth can be adjusted depending on soil conditions. Excavation work is typically completed with a layer of soil that has the required bearing capacity.
The pit is then filled with solid concrete.
If other materials such as brick masonry, masonry or prefabricated concrete blocks, etc. are used, they should be used with cement mortar.
The final space between the backfill and the foundation (50-100 mm) can be filled with a strong mixture such as cement:sand in a ratio of 1:3.

Needle Beam Support

This method is used to improve foundations, especially when loads are unevenly distributed.

There are two main methods in which we can use the needling method.

Simply supported beam
cantilever beam

In these other methods, we cut a hole in the wall to support it through the beam running through the wall.

The type of beam can be reinforced concrete, wood or steel. Steel beams are preferred because they are easier to construct/fix compared to reinforced concrete beams.

Cantilever Needle Beam

Needle beams supported on piles

Shallow depth, needle beam supported

The needle beam supports can be prefabricated piles, driven piles or other supports supported on the ground as indicated in the figures above. The choice of type of support depends on the size of the loads that each support must support.

Support and support

To support the foundation, the tension or load on the foundation must be released. Therefore, walls are supported in different ways to reduce the axial force on the foundation.

The following illustration shows one such method that can be used in foundation improvements.

Pynfored Fundamentals

This is a very old method of improving foundations.

Mainly used to support brick walls.

Supports placed at a short distance serve to support the load exerted by the walls. The base of the column is reinforced with strong mortar to prevent settlement.

The following image shows a wall supported in a similar way.

Advancement methods

This method is often used in reinforced concrete structures. The lifting operation is carried out not only to improve the foundation, but also to correct the pillars.

The pillars are supported by jacks placed on the concrete layer or concrete slab placed for this purpose. An upward force equal to its axial force is exerted on the support.

This could be done by raising the beams around the column. For high axial loads, raising the ground floor is not sufficient as the beam may fail due to shear. In these situations, the number of floors to be supported must be verified through calculation.

Support with stakes

As indicated in the table above, there are many methods that can be used to support the work.

Let's list all these methods.

driven piles
Press batteries
Leaky batteries
Continuous Perforated Pile
Small diameter cuttings
Root cuttings and net-shaped root cuttings
Micropiles

Let's briefly discuss each method.

driven piles

Driven piles are more useful in supporting the work as they are easy to install and installation takes less time.

However, there are also significant disadvantages to consider.

Installation can only be done on the outside of the building or structure.
Excessive vibration during installation and problems such as settlements, cracks, etc. in existing structures.
Old construction can be damaged by vibrations.
Sufficient space is required for installation.

Press batteries

Pressed piles are a good alternative to driven piles as they solve some of the problems.

Mainly this prevents floor vibrations that occur during installation.

Pressed piles are segmented piles made of concrete or steel.

As column loads increase, it becomes very difficult to make improvements to the foundation.

Leaky batteries

Construction-related vibrations and noise are comparatively low with bored pile foundations. Therefore, this method can also be used in sensitive areas.

However, installation also requires free space and a higher ceiling height.

Therefore, bored piles are also erected outside the structure when the foundations are supported. However, there are methods by which installation can be carried out at low heights. Tripod equipment can be used to install the sign posts.

For more information about foundations, you can refer to the following list of articles.