UNDERPINNING IN FOUNDATIONS
Underpinning is the act of strengthening
an existing foundation system in an existing building.
It is usually done in one of the following situations:
In the third case, the danger to the building can come simply from deep excavation in an adjacent plot. As each foundation has a cone of soil below it that supports it, if a very close new excavation cuts into it, the foundation can lose some or all of its bearing capacity.
- the building is showing signs of sinking into the ground or cracking, indicating that the foundation system is not able to bear the weight of the building
- the building is either being enlarged or converted for use in a new type of activity, resulting in heavier loading than it was designed for
- a large new building with deep foundations or basements is being constructed close to an existing building, causing problems for the existing building
In the third case, the danger to the building can come simply from deep excavation in an adjacent plot. As each foundation has a cone of soil below it that supports it, if a very close new excavation cuts into it, the foundation can lose some or all of its bearing capacity.
types of underpinning
Underpinning can be done in one of two ways: one, improve the foundation system by enlarging the existing foundations or adding new foundations, and two, to improve the properties of the soil below the foundations without touching the building at all.
A key aspect to consider is how much disturbance to the building the underpinning method will cause. As you will see below, some methods will cause extreme disruption to the ground floor and external areas of the building.
All such interventions must be done only by qualified structural engineers after much study and calculation, as the operation can cause serious structural damage or total collapseif not done correctly.
A key aspect to consider is how much disturbance to the building the underpinning method will cause. As you will see below, some methods will cause extreme disruption to the ground floor and external areas of the building.
All such interventions must be done only by qualified structural engineers after much study and calculation, as the operation can cause serious structural damage or total collapseif not done correctly.
part a - improving the foundation system
Foundations may be improved in a great number of ways, and a creative engineer can sometimes find a new way to transfer the weight of a building to the soil.
underpinning method 1: pit method
This is done in the following way:
The most important thing to understand is that this is done insmall sections at a time, not for the entire building. Each section could be from 8' to 12' long (2 - 4m approx) depending on the size of the building. The new sections are interspersed among the old, thereby making the whole process much safer.
Lets talk about each step in greater detail:
Step 1: a temporary foundation is constructed to bear the weight of the building, so that the existing foundation is relieved of its weight and can be worked on. This is usually done so as to cause minimal disruption inside the building. However, this usually means significant disruptions to the ground floor of the building, as the floor will have to be dug up to install the temporary foundations.
The building is then supported onneedle beams. A needle beam is a new beam that is constructed at right angles to the existing foundation, and is pushed into the existing foundation wall (a hole is cut for it first) like a needle through cloth. A number of such needle beams are constructed at regular intervals so as to bear the weight of the building above. For temporary needle beams, it is convenient to use steel I-beams that can be re-used at every new site. Once the beam has been pushed in, the gap between it and the foundation wall is sealed with strong cement mortar so that the wall can transfer weight to it.
Step 2: once the temporary foundation is in place and has been checked, a pit can then be dug by hand below and around the existing building. The existing foundation will then hang in mid-air. This is an operation that must be done very carefully, as it can be quite dangerous. It is important to note that the pit method can only be done if the soil at the site is a cohesive soil. A cohesive soil is one that holds its shape, and does not collapse when excavated or cut, which could kill the construction workers inside it.
Step 3: the foundation is then underpinned by filling the pit with concrete. It is very important that there be no gap between the new foundation and the old, as if there is, the building will settle into this gap, causing cracks and damage above. Therefore filling in the last few inches below the existing foundation is critical.
Step 4: Once the concrete has set, the temporary foundation can be removed, the earth filled in, and the internal floors repaired and re-tiled. The concrete is the new, enlarged foundation, that rests on soil at a greater depth, giving it a higher safe bearing capacity.
This whole operation is done in small sections at a time. For example, if the entire peripheral wall of the building is say 100 feet long, then this may be done in 10-foot sections at one time.
- A temporary foundation system is first constructed around the existing one.
- Then a pit is dug around and below the existing foundations, thereby relieving them of load-carrying duties.
- Then, a new foundation is built below the existing one by filling the pit with concrete.
- Once the concrete has set, the temporary foundations are dismantled, allowing the load to settle onto the improved foundation system.
The most important thing to understand is that this is done insmall sections at a time, not for the entire building. Each section could be from 8' to 12' long (2 - 4m approx) depending on the size of the building. The new sections are interspersed among the old, thereby making the whole process much safer.
Lets talk about each step in greater detail:
Step 1: a temporary foundation is constructed to bear the weight of the building, so that the existing foundation is relieved of its weight and can be worked on. This is usually done so as to cause minimal disruption inside the building. However, this usually means significant disruptions to the ground floor of the building, as the floor will have to be dug up to install the temporary foundations.
The building is then supported onneedle beams. A needle beam is a new beam that is constructed at right angles to the existing foundation, and is pushed into the existing foundation wall (a hole is cut for it first) like a needle through cloth. A number of such needle beams are constructed at regular intervals so as to bear the weight of the building above. For temporary needle beams, it is convenient to use steel I-beams that can be re-used at every new site. Once the beam has been pushed in, the gap between it and the foundation wall is sealed with strong cement mortar so that the wall can transfer weight to it.
Step 2: once the temporary foundation is in place and has been checked, a pit can then be dug by hand below and around the existing building. The existing foundation will then hang in mid-air. This is an operation that must be done very carefully, as it can be quite dangerous. It is important to note that the pit method can only be done if the soil at the site is a cohesive soil. A cohesive soil is one that holds its shape, and does not collapse when excavated or cut, which could kill the construction workers inside it.
Step 3: the foundation is then underpinned by filling the pit with concrete. It is very important that there be no gap between the new foundation and the old, as if there is, the building will settle into this gap, causing cracks and damage above. Therefore filling in the last few inches below the existing foundation is critical.
Step 4: Once the concrete has set, the temporary foundation can be removed, the earth filled in, and the internal floors repaired and re-tiled. The concrete is the new, enlarged foundation, that rests on soil at a greater depth, giving it a higher safe bearing capacity.
This whole operation is done in small sections at a time. For example, if the entire peripheral wall of the building is say 100 feet long, then this may be done in 10-foot sections at one time.
underpinning method 2: needle beam method
This is done in the following steps:
This method has the great advantage of not disturbing the inside of the building.
Lets now discuss each step in full detail:
Step 1: Use a small piling rig to make the micropiles close to the building. First check that the building is not so weak so the vibrations from the micropiling rig cause damage to it. When loaded, one pile will act in compression, and the other in tension.
Step 2: Excavate the area where the needle beam is to be constructed. Then cut a hole just large enough to take the cantilever needle beam in the foundation wall.
Step 3: Cast the RCC cantilever needle beam in situ, making sure to have a good connection between the micropiles and beam.
Step 4: Backfill earth into the open areas, and you are done.
- First construct two micropiles at a specified distance from the building. This is done with a micropiling rig.
- Then cut a hole just large enough to take the needle beam.
- Construct the RCC needle beam over the piles and inside the foundation wall.
- Once the beam has set, fill in the earth over the beam. The new foundation system can now take additional load.
This method has the great advantage of not disturbing the inside of the building.
Lets now discuss each step in full detail:
Step 1: Use a small piling rig to make the micropiles close to the building. First check that the building is not so weak so the vibrations from the micropiling rig cause damage to it. When loaded, one pile will act in compression, and the other in tension.
Step 2: Excavate the area where the needle beam is to be constructed. Then cut a hole just large enough to take the cantilever needle beam in the foundation wall.
Step 3: Cast the RCC cantilever needle beam in situ, making sure to have a good connection between the micropiles and beam.
Step 4: Backfill earth into the open areas, and you are done.
underpinning method 3: micropiling method
This is done in the following steps:
We can now talk about each step in detail:
Step 1: Use a micro-piling rig to construct micro-piles at an angle, just below the existing foundation. Before doing this, one should excavate earth adjacent to the existing strip foundation to find out exactly how deep it is.
Step 2: Excavate the portion of earth till the top of the micropile, ensuring there is enough space for a worker to use her tools.
Step 3: Now remove the earth between the micropile and the bottom of the existing foundation.
Step 4: Fill in this gap with concrete. Then repeat the whole process at regular intervals till the entire periphery is done.
- Construct micropile just below existing foundation at an angle, as shown in the sketch.
- Excavate earth till the top of the micropile.
- Then manually remove all earth between the pile and the existing foundation, creating a cone shaped void that rests on the micropile.
- Fill in this void with concrete. Once it sets, the existing foundation can transfer forces to the micropile, thereby dramatically increasing its capacity.
- Repeat this at regular intervals.
We can now talk about each step in detail:
Step 1: Use a micro-piling rig to construct micro-piles at an angle, just below the existing foundation. Before doing this, one should excavate earth adjacent to the existing strip foundation to find out exactly how deep it is.
Step 2: Excavate the portion of earth till the top of the micropile, ensuring there is enough space for a worker to use her tools.
Step 3: Now remove the earth between the micropile and the bottom of the existing foundation.
Step 4: Fill in this gap with concrete. Then repeat the whole process at regular intervals till the entire periphery is done.
part b - improving the soil below the foundation
In this section, we will talk about how to improve the properties of the soil below and around the existing foundations. This is another way in which we can improve the performance of the foundation system.
This method has the great advantage that the building is not disturbed. In some cases, it can be done while the building is occupied.
This method has the great advantage that the building is not disturbed. In some cases, it can be done while the building is occupied.
I really appreciate how well you broke down the underpinning process. This post is a great resource for anyone looking to understand foundation strengthening!
ReplyDeleteUrathane Solutions is one of the Underpinning in Melbourne specialists. Click for all info.