What is Underpining ?

There are quite a few different methods of underpinning damaged, faulty or subsiding foundations. I have split this page into the sections below. Sometimes called in the US "Mud Jacking". This method of underpinning I must confess that I have never seen it done, I don't live in an area where it has become effective. As a result I am only surmising here. If any of my readers have any experience of this method I'd be glad of the feedback.

Modern methods, high pressure grout injection.


Underpinning - Injecting a grout mixture into the sub soil under the weak foundation.

Sometimes called in the US "Mud Jacking".  This method of underpinning I must confess that I have never seen it done, I don't live in an area where it has become effective.  As a result I am only surmising here.  If any of my readers have any experience of this method I'd be glad of the feedback.
- I can see it being used very efficiently if one of the other methods, like the previous one, has lifted the structure, then pressure injection of grout into the voids formed by the lifting process will greatly improve the repair strength.

- On it's own, I guess that only a very specific range of circumstances and or soil types make this method practical or cost effective compared to other methods.
- The web sites that I have seen claim that the pressure built up by the grout can lift the building back to it's original position.
- There must be a certain amount of guesswork involved in estimating the amount of time and the quantity of grout.
- At the same time I can imagine that many times the job may be finished and if the building has not shown any signs of lifting no one can really say if it has been effective.
- I have done pressure injection of epoxy into structural cracks in concrete, and we always got visual feedback that the epoxy was indeed traveling along the crack from point to point.

Modern methods,screw piles, micro piles etc.
There are various non intrusive methods of placing piles under weakened foundations to stiffen up and provide greater bearing capacity for the sub grade soil By non obtrusive I mean piles that are not dynamically driven into the ground by impact with any of the versions of the traditional pile driver.  Which in many cases could do more harm than good.

- Pressed piers are typically short sections of shaped steel that are pressed into the ground at an angle to go under the footing.  They are pressed by a hydraulic ram, with sections added until the capacity of the ram is reached.  They typically go into the ground up to about three metres.

- Helical or screw piles  are piles that are literally screwed into the ground by a machine.  Unlike a traditional augur that lifts the soil out of the hole, these piles screw into the ground and are left there.  There are many different varieties of these, and many are primarily designed for new construction.

Micro Piles & Mini Piles

Underpinning - A large jet grouting machine preparing to stabilize the foundations under an existing property, prior to excavation work alongside.  Photo thanks to Wiki Commons

Used mainly on commercial and civil engineering jobs, these small piles, 125mm to 300mm rely on the surface friction between gout and the surrounding soil.   Micro piles were first used in Italy and they have gained immediate acceptance in the field of stabilizing historic building because of their non-intrusive nature.
There are various sizes of machines, some small enough to work indoors.
The machine is capable of drilling through most substrates though it is seldom necessary to reach bedrock.
The hole is drilled, a steel casing with a central rebar is inserted.
The casing is withdrawn while at the same time grout is pumped in the hole.
The friction between many reinforced grout micro piles and the ground is what does the support.  Not that they have gone into a different harder layer of ground.
This type of underpinning work is also used extensively under existing buildings to stabilise them, prior to deep excavation work being done on the adjacent property.

Bored or drilled, cast in-situ concrete piles

THANKS TO BUILDER BILL
SOURCE: http://www.builderbill-diy-help.com/underpinning.html