Table of Contents

What is Underpinning?

Underpinning, or foundation repair, is the act of strengthening a foundation. This occurs when the building is showing signs of sinking or cracking (indicating that the current foundation system is not able to bear the weight of the building), the building is being enlarged or converted (which results in heavier loading than the original design planned for), or if a large new building with deep foundations or basements is being constructed close to an existing building (causing problems for the existing building) [1].

Pit Method

This is done in small sections at a time. Depending on the size of the building, each section could be 8ft to 12 ft (approx. 2m – 4m). Steps on how to carry this out are listed below [1].

A temporary foundation system is first constructed around the existing one [1].
Then a pit is dug around and below the existing foundations (relieving them of load-carrying duties).
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.

Step 2 [1]	Step 3 [1]

Needle Beam Method

This method has an advantage as it does not disturb the inside of the building. Steps on how to carry this out are listed below [1].

First construct two micropiles at a specified distance from the building. This is done with a micropiling rig [1].
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.

*Original Look [1]*	Step 1,2, & 3 [1]	Step 4 [1]

Micropiling Method

Construct the micropile just below existing foundation at an angle [1].
Excavate earth till the top of the micropile ensuring there is enough space for a worker to use their tools.
Manually remove all earth between the pile and the existing foundation, creating a cone shaped void that rests on the micropile.
Fill in the gap with concrete. Once it sets, the existing foundation can transfer forces to the micropile which increases the capacity.
Repeat this process at regular intervals.

Step 3 [1]	Step 4 [1]	Finished Look [1]

Additional Reading

Deep Foundations

Foundation Building Code

House Foundations

Shallow Foundations

References

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