UNDERPINNING

Caterpillar under house
Excavation happening before Underpinning starts

 

UNDERPINNING Meaning Of

Underpinning means to “Pin Under”. Pins of concrete or steel are placed under walls or footing to provide these elements additional support. Usually an “Underpinner” is called in after the existing foundations have failed or are about to fail, Some times the “Underpinner” is called to beef up existing footing load bearing capacity as new renovations above are going to add additional loads.

This type of work has significant risk involved hence the need for experienced professionals.

 


Common Reasons Underpinning is Needed

  • Improve fragile underlying soils
  • Underpin with deep foundation elements to bypass weak soil
  • Raise settled  slabs or foundations
  • Stabilize sinking soil site
  • Treat expanding or collapsible soils

Foundations can experience settlement from compressible underlying soils or lateral movements from adjacent excavations, requiring various foundation rehabilitation approaches to stop or correct the movement. As foundation specialists, we have experience with the full range of rehabilitation techniques to provide effective foundation rehabilitation.

A foundation might be underlain by compressible soils or debris that consolidate or decompose over time. Good soils may be weakened as they slide into compromised underground utilities underlying a foundation. In mining subsidence regions, sinkholes can cause a building to settle as the soils moves into a void . Stabilizing the underlying soil can often provide the required foundation support necessary.

Foundation underpinning is an option to treating the soil. Foundation underpinning techniques bypass the problem soils by installing structural elements to transfer the building’s load to underlying good soils or bedrock. Underpinning and jacking off concrete or metal screw piles can be used to raise settled structures. Underpinning foundations can often be designed so that the completed job is below-ground ensuring aesthetics of the structure is not compromised.

Traditional UNDERPINNING Methodology

Usually starts with excavation to find the bottom of existing footings and then determine how deep we need to go to accommodate the needs we have. The soil profile will determine the way we go about the underpinning works as the methodology is quite different for sand, clay or below water table. We may need to shore up the soil, carry footings to rock or dig down to new levels for a basement.

 

Chemical UNDERPINNING Methodology

Try and locate the bottom of footings or slab in the affected areas and start pumping in urethane foam through small holes in concrete slab or external wall footings, hoping any voids will be filled and cause an uplift pressure on footings or slab.

Urethane foam Cell Structure

Microscopic view
Bubble gas pressure holds structure

 

Traditional Concrete piers going to underpin ground floor Slab

Concrete ground piers
Ground Piers

 

Load bearing        screw pile

 

      Typical sectional concrete underpin of walls to extend walls down.

The Advantages and disadvantages of Chemical underpinning.

 

The advantages of Polyurethane foam underpinning:

You don’t have to get your hands dirty. Very little physical effort is involved.

It’s not invasive. Only small holes made to inject product through

It’s a quick process and a quick dollar

It does a great job of filling voids

Can quickly raise a slab to the  desired level.

For a full list of advantages read the numerous engineering articles, blog/forum discussions and lab tests financed by the purveyors of these chemicals.

 

The disadvantages of polyurethane foam underpinning

The installer is charging by the quantity of polyurethane used, so arbitrary estimated volumes one is quoted can often lead to a nasty surprise.

The product takes the route of least resistance so you will occasionally find it runs next door and fills the neighbour’s sewer pipe void. You can’t see where the product is going and you are paying for it

The product takes a while to react and gas to expand so over lifting damage can occur

The product shrinks over time. It may shrink 25% over a month. Some even more where the operator doesn’t get the correct mix at the right temperature

Polyurethane is toxic and highly flammable. It is promoted as environmentally friendly.

 

The advantages and disadvantages of Traditional underpinning.

 

Advantages of Traditional Underpinning Methods:

You get a fixed price quotation upfront.

The physical underpin products of concrete and steel are relatively inert

The methodology of underpinning is to transfer the house footings load to a much deeper level where the soil has good bearing capacity and is uninfluenced by soil moisture variations. The methodology of the foam injection is to pressurize the poor surface soils for a quick fix forgetting that the poor soil will continue to compress/subside over time and still be affected by moisture fluctuations

You can do a controlled jacking of foundations if required.

 

Disadvantages of Traditional Underpinning Methods:

Is invasive, makes a mess and requires a lot of physical effort to get the job done.

 

The only constant appears to be that the chemical companies are continuously reformulating and adding new names – without any clear declarations of the reformulated results. So while the chemical companies may talk about improved performance, the 2% shrinkage number is still their printed claim. In reality take your chances.

Read the following references for health ramification of urethane, mandatory product data sheets, and experiences of customers using urethane

Polyurethane Information:

https://www.livestrong.com › article › 297783-urethane-foam-health-effects

https://nj.gov/health/eoh/rtkweb/documents/fs/1986.pdf

https://www.epoxysupplies.com.au/image/catalog/AAAMISCJ/SDS%20500T%20Tetrathane.pdf

https://www.farnell.com/datasheets/1519680.pdf

https://forums.whirlpool.net.au/archive/2087403

To check out what your needs are:

Call  0411212211

Email   info@bestunderpinning.com.au