Underpinning, or repair and strengthening of faulty foundations.

There are quite a few different methods of underpinning damaged, faulty or subsiding foundations.  I have split this page into the sections below.

Intro and causes of foundation faults.

A subject like underpinning a home, can range from fairly simple to highly complex.  It is not in the normal scope of a DIY handyman.
For this type of job it is always best to get independent technical advice.
By this I mean if the problem is large get a structural engineer's report.   Don't just accept the word of a contractor, who seems to have an understanding of the problems involved.
Never the less that being said I will now give you some of my thoughts on the subject, in the hope that you may pick up some knowledge that will help with some of your particular problems.  Just treat this page (and indeed this website as a whole) as general advice and ideas on the subject.   This is not a how to manual!

Before starting any underpiining work it is essential to find out what caused the damage..  Finding the cause is the key to finding the solution.

• Simple bad workmanship, poor quality control on the job and wrong materials used for the foundations. Poor compaction, missing rebar, wrong dimensions etc.etc.  I have underpinned foundations that were built using beach sand and cement.  The sulphides in the sand took about 30 years to break down the quality of the concrete, but eventually this damage threatened the stability of the house.
• The foundations not being designed to suit the bearing capacity of the soil.  Certain areas are known to have certain soil characteristics and the professional designers/engineers in a particular location know these areas and design accordingly, but there are always exceptions. For instance, I built an addition to a house that was built on reclaimed land, virtually a land fill dump.  There was no mention of this fact on the original drawing for the house. The original builder had just gone ahead with the original plan in spite of the fact that the ground conditions were not really adequate..

• Not uncommon where I live is the action of tree roots, planting large trees too close to a house can actually damage the foundations. One answer is to build a root barrier, but by far the best is to remove the tree.
• Abnormal flooding eroding the sub grade.  After a permanent repair is made it is usual to create some sort of retaining wall to stop it happening again. 
• In certain areas mining activity can cause settling or subsidence in otherwise sound soil.

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A true story, check your foundation material before building.

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Traditional method, new concrete under the old.

Underpinning - Traditional style underpinning of a railway bridge foundations.

• The photo above is of a job that I did in 1974.  The foundations of a railway bridge on the North Australian Railway had been eroded during a bad wet season by the creek that the bridge spanned. The bridge had sagged a couple of inches or so and with this method we were able to jack the bridge up to the correct level again by lifting off the "pig stye" type supports.  It was then easy and safe for underpinning to be done.
• So in this case water erosion was the cause of the damage.
• The underpinning took the form of excavating under the existing mass concrete footings that the bridge's steel trestle supports sat on and forming and pouring deeper and wider new concrete pads.

1. We excavated down to a solid base between the old foundations and used it to re prop the bridge structure to keep it secure from further damage and to keep it operable.  We had iron ore trains using it every couple of days.
2. Only one pad at a time was worked on and finished, before moving on to the next.
3. We excavated sideways under one of the old foundation to a solid base again.
4. Formwork was done and a new concrete pad was poured.
5. The last part of the job was reinstating compacted fill to the level of the old creek bed.

So the propping came first, then the existing structure was worked on, one pad at a time for safety reasons.  It would have been easier and cheaper to do them all at once, but not safer.

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Traditional method, supporting a brickwork wall.

Underpinning - A section through a brick wall showing the method of supporting it with needles and props.

1. The needle itself is the part that goes through the wall and it is always a compromise.  The shorter it is, the more load it can carry for a given section (size).  The longer the needles are then the more room to work.
2. If steel needles are used, always use a timber packer between the steel and the brickwork.  This compresses and lets the bricks sit on a broad area.  Steel directly masonry could feel solid but it could be sat on a dag of mortar that could turn to dust if load is applied.
3. Sole plates, wide pieces of timber are essential to hold the props and to spread the load.
4. This type of propping is very similar to propping for concrete formwork, so I won't go into detail here, maybe you would like to see my concrete pages for details of bracing etc..
5. Above I have shown the inside props supported by the floor slab, but sometimes if this is also in a bad condition, then the slab has to be cut out and the load taken by the soil inside.  In this case it may be advisable to compact the soil by mechanical means before using it to support a large wall.
6. Lets say that this is a two storey gable end wall.  It obviously depends on the extent of the movement that has already happened, but I would also be thinking about a few raking shores to support the wall, room permitting.  Using steel props or scaffold pipes the cost could be very reasonable compared to the other option of loosing a large section of wall.

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Traditional method, excavation and pouring.

Underpinning - The excavation and pouring concrete sequence.

I can't think of any circumstance that would allow anyone to try underpinning the full length of a wall, and then pour the concrete in one hit.  The work has to be done in sections.  Once again it is a compromise between time and cost on the job, offset against safety.

A practice that I have applied to dozens of situations when doing contracting work is to run through the "best case - worst case" scenario.  For a job like this I think on these lines.

• "What is the best that can happen if I excavate under the wall in six metre sections ?"
  • I save "X" amount of cash, we finish the job two or three days earlier, the customer is happy.
• "What is the worst that can happen if I excavate under the wall in six metre sections ?"
  • It is literally unthinkable, a wall collapse, a man trapped in a trench under the rubble of the wall.  All other considerations pale into insignificance.

So here I can't say how far apart to do each section, whether to prop or not prop, etc.  Each job is different, but safety should always be the main concern.

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Partial Underpinning a foundation.

Underpinning - An offset foundation

In certain case this could work very well, it is easier to do and a lot safer than the previous method.  It all depends on the job.
We did similar pad foundation like this increase the strength of houses to resist uplift forces on the roofs, when the cyclonic building code came into being.

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Bored or drilled, cast in-situ concrete piles

Underpinning - Sketch of a drilled concrete pier

This method of underpinning is obviously less intrusive and safer that the previous examples.  It also has the possibility of being able to lift the building somewhat, thus maybe closing up the cracks, leveling doors and window heads etc.  The piers are typically two to three metres deep.

• A series of holes are drilled along the length of the existing foundation.
• Some hand working is done to create a bearing surface under the old foundation.
• Say every second or third one is partly filled with concrete.
• After the concrete in these holes is set a small but powerful hydraulic jack is used to lift the existing foundations.
• The machine that augers the holes, quite often has the jack as an accessory and it is drive by a hose connected to the machine hydraulic system.
• The gap is packed with steel packers and the jack withdrawn and used again.
• When the correct height is reached and the foundation securely packed they are filled with mass concrete, or with a re-bar cage and concrete.

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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 crack in concrete, and we always visual feedback that the epoxy was indeed traveling along the crack from point to point.

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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.

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Micro pile, 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.

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