Underpinning

December 4, 2019

What is Underpinning?

Underpinning is the process of lifting or supporting the footing of a building or structure, so as to provide a more stable foundation.

Underpinning is commonly used to stabilise houses when they experience severe cracking.

There are several different types and methods of underpinning. Two of the most common are:

  • Traditional concrete underpinning, which involves piering or excavating under a footing to install a concrete support for the footing.
  • Chemical underpinning, using an expanding urethane foam or similar to create piers in the soil under a footing, to provide lift to it.

These methods are quite different and are best used in different situations, the choice dependent on the cause of the cracking.

Traditional concrete underpins

Traditional concrete underpins used to be dug by hand and were typically 1m cubes of concrete placed under a footing beam. The footing beam could then be jacked up, using the concrete underpin as a stable base.

However, the cracking in some houses is related to seasonal moisture variation in the soil which can extend down to over 3m in depth. This seasonal moisture variation is the drying/shrinkage of the clay soils over summer and the wetting/expanding of the clay soils over winter. Hence, due to the depth of soil moisture variation, the 1m cube concrete block also experiences some lift and settlement over the seasons. Hence, a 1m cube underpin may not be a stable foundation.

It is better practice to install a concrete underpin as a concrete-filled bored pier to a depth of at least 4m, founding the pier in soils which are stable over the seasons. Please note that this depth may vary due to other considerations, such as the permanent water table depth, soil conditions or shallow rock, proximity of trees, etc.

The footing under the building may then be jacked off the underpin if jacking is viable.

The location of underpins will affect the decision to use traditional concrete underpins or another method, as it is preferable to use a machine such as a small excavator to auger or dig the underpin. It may not always be possible or practical to position an excavator where required to dig the underpin – you don’t want an excavator in your hallway!

Jacking over underpins

Underpins are often installed as a stable base so that the footings of the building can be jacked up and re-levelled. This can only be done using traditional concrete underpins.

However, there are many limitations to this jacking, which include:

  • The concrete strength of the footing. If this is too soft, the footing concrete may crush rather than lift.
  • If the footing is bluestone rather than concrete, it may not be recommended to jack as it is not structurally continuous, or strong
  • The footing may crack if lifted too far, which in turn may cause additional cracking to the building over
  • Soil suction on the footing may prevent lift.

Chemical Underpinning

Chemical underpinning is undertaken by injecting an expanding urethane foam (or similar) into the soil at selected locations under the footing. This creates a pier of foam in the soil to lift and support the footing over. This type of underpinning incorporates jacking.

Chemical underpinning has some advantages and some disadvantages when compared to traditional concrete underpins. These are discussed below.

The advantages of Chemical Under-pinning are:

  • It can be done relatively quickly – only one day is required for a normal house, whereas a week may be required using traditional concrete underpins.
  • The method provides a large degree of control over the lift undertaken to the footing.
  • The injection can be undertaken at numerous points close together.
  • The injection can be undertaken under internal walls more easily than the installation of traditional concrete underpins.

The disadvantages of chemical underpinning are:

  • The chemical is injected using a wand or tube inserted into the soil under the footing. This limits the founding depth of the pier and the resultant pier is likely to be founded in soil which may show seasonal lift and settlement. This is undesirable (see above).
  • The system requires the use of a very high-pressure pump.

Which underpinning method to choose? Chemical or Traditional?

The decision whether to use traditional concrete underpins or chemical underpins should depend on the reason for the movement and cracking occurring, and location of the cracking.

If the movement is due to seasonal wetting and drying of soils around the exterior of a house, traditional concrete underpins are better as they are founded at a deeper depth.

If the movement is in the centre of a house and is not related to seasonal wetting and drying of the soils, chemical underpins may be a better option.

Underpinning Costs

The relative cost of chemical underpinning vs traditional concrete underpins is variable and depends on many factors, but the order of magnitude of both options is similar. The exact relative costs are best explored with individual contractors.

Magryn recommends both types of underpinning, with the choice depending on the details and requirements of the individual project. We recommend that you engage a structural engineer to review any project which may require underpinning, to ensure that:

  • You receive an unbiased and professional opinion on the need for underpinning.
  • You receive a report documenting the scope of underpinning you require. You can then take this scope to several underpinners to obtain easily comparable quotes. The report should detail the number and location of underpins required, as well as the details of these underpins (depth, size, reinforcement, etc).

Magryn would be pleased to assist you by providing an underpin report and design for your house. We can even recommend quality reliable contractors to undertake the work.

Give us a call on 8295 8677 and discuss your problem and requirements with us.




Is Your Home Cracking Up?

December 3, 2019
Filed under: House Cracking — Tags: , — Magryn @ 2:53 pm

Why do houses crack?

Houses are a rigid structure, built on top of a footing, which in turn rests on the soil under the house.

When the soil moves, the footing over can bend and this causes the house structure over to move and crack.

What causes soil to move?

Many things can cause the soil to move. For clay type soils this generally involves a change in moisture content. As clay soils dry, and shrink, or when they get wetter and swell. This change in moisture content can be due to many different reasons, including:

  • Seasonal change – soils get wetter from winter rains and dryer in summer.
  • The effects of trees drawing water from the soil.
  • Leaking pipes under the house. This can include sewer pipes, stormwater pipes, and water supply pipes.
  • Poor stormwater management around the house, allowing roof stormwater to pool around the house.
  • A change in the water table level in the ground.

Some clay soils are more “reactive” than others, meaning they shrink and swell more for the same moisture content change.

Sand Soils

Sand soils on the other hand, are not reactive, and don’t change volume with a change in moisture content. However, sandy soils have other problems. Sandy soils, particularly in Adelaide are along a coastal strip and are generally old sand dunes. These dunes were placed by wind action and the sand is poorly compacted. Hence when you build a house on top of them, they can compact and settle, which may cause the house to crack.

What impacts the amount of cracking?

The extent a house may crack will depend to some extent on three factors:

  1. The soils under the house. Very reactive (or heavy) clay soils will move more with a change in moisture content, starting the cracking process.
  2. The footings of the house are very important, as the stiffer the footings, the less movement and cracking of the house over. Generally, there are several different categories of footings, which work (or don’t work) to different degrees:
    1. Bluestone footings on houses built before around 1910. These are just large rocks in the ground and are very flexible with very little structural integrity. These footings are very flexible.
    2. Concrete strip footings, which are reinforced concrete beams, generally just under the walls of the house. These are in houses with timber floors. They are better than bluestone footings, but still very flexible in comparison to modern footings.
    3. Modern footing systems (generally on houses built after around 1980). These can be concrete raft, waffle pod or grillage raft. These all incorporate a concrete slab with concrete beams built integral with the slab. They are quite stiff and perform the best.
  3. The type of house construction over the footings. The more flexible the house structure, the better it will accommodate some slight movement without cracking. Typically, the following applies:
    1. the most flexible type of house structure is timber-framed and clad, as in a weatherboard type of house.
    2. a brick veneer house with internal timber framing and plasterboard lining, with a single external skin of brickwork is generally good, if the external brickwork has movement control joints (ie is articulated) to absorb some movement.
    3. a full brick house which has double-leaf external brick walls and single leaf internal brick walls, and has brickwork movement control joints (internally and externally) is more rigid. This type of house construction with the movement control joints is rare.
    4. the most rigid type of house construction is a full brick house with double-leaf external brick walls, and single leaf internal walls without any brickwork movement control joints included. This type of house construction was very common before the 1960’s.

The more rigid the house structure, the more it will tend to crack.

That’s not a crack!

It is better to have a house that has no cracks rather than a cracked house. However, most houses (particularly in Adelaide) show some cracking.

Cracking can be divided based on its severity:

  • slight cracking, which are cracks less than 1mm wide. These are generally not of a structural concern.
  • moderate cracking, which are 1 to 5mm wide.
  • severe cracking, showing cracks wider than 5mm. These may be a structural concern.

If a crack is causing you concern, then it should be looked at and reviewed by an experienced structural engineer.

Are cracks dangerous?

Generally, cracks in houses are not dangerous, but there are exceptions in severe cases.

  • If cracks are extensive, this can loosen brickwork or individual bricks, which may fall.
  • If the cracking causes walls to lean, the walls may become unstable and fall.
  • If the movement in the house is excessive, this may loosen the ceiling panel, causing it to fall.
  • If the cracks are very wide (greater than 20 or 30mm) this indicates severe movement and instability in the house.

Of course, the appearance of cracks in your house can reduce its potential sale value, should you wish to sell it.

If you are concerned, you should get your house reviewed by an experienced structural engineer.

What will happen if I do nothing?

Most cracks have some seasonal influence, meaning that they change with the seasons. Generally, they get wider over winter and tend to close up over summer.

Hence they open and close over the year, and tend to grow in width and length over the years, getting worse.

If you do nothing, they will tend to grow until they become a structural problem, and ultimately compromise the structural integrity of the house.

So the sooner you do something to control them the better.

The exception to the above is cracks which appear suddenly and grow very quickly (eg they appear and grow to 20mm wide over six months). These are often due to another external cause and require urgent attention.

What can I do to repair a house that’s cracking?

There are many things that you can do to repair a house that is showing movement and cracking – some are expensive and some are not.

It is important that any action you take is targeted towards what is causing the cracking, so that you reduce the cracking and don’t waste your money.

It is important to realise that it is unlikely that you will eliminate all future cracking and your aim should be to reduce it to a maintenance level, where regular patching and painting is all that is required.

Some of the work that can be done to control cracking in a house can include:

  • collect and dispose of stormwater properly, so that it does not soak into the soil around your house
  • ensure that there is perimeter waterproof paving all-around your house
  • remove large trees that are too close to the house. This may require approval from council for significant or regulated trees, or co-operation from your neighbours
  • check sewer, stormwater and water supply pipes for leaks and repair as necessary
  • underpinning. This is often used as a last resort, as it is expensive and invasive. Also, underpinning will not prevent all future cracking and may cause cracking in some areas not currently showing cracking. Please refer to our (future) blog on underpinning methods and recommendations.
  • install articulation joints in brickwork and timber framed walls/ceilings, to provide movement locations
  • installation of reinforcement into brick and stone walls across existing cracks
  • general crack repair in walls.

We recommend that you seek and receive professional advice in regards what is required in your particular case in regards remedial work to reduce and limit future cracking.

What can Magryn do to assist you?

We have years of experience in building movement assessment and recommending appropriate remedial works.

We will get one of our experienced engineers to visit and inspect your house internally and externally. We may also:

  • take a level survey of the floor of the house to determine the extent of vertical variation
  • take a borelog of the soils on site to determine how reactive the clays are (how much they shrink and swell with changing moisture content) and how dry they are at depth
  • review previous work undertaken on the house and the history of the cracking.

Cracking Report

We then write you an engineering report which notes:

  • What the current extent of damage and cracking was at the time of the inspection.
  • Any site factors which may be influencing the house and cracking.
  • A discussion of what is causing the movement and cracking of your house.
  • A list of recommended remedial work which should be undertaken. This may or may not include underpinning – if it does, we will include an underpin plan and design, which you can take to different underpinners to obtain quotes on the work.

Our fees for the above are time-based, and dependent on how much time we spend on your job.

If you would like to book in for an engineer to attend and inspect your house, or just talk to us regarding your house, please give us a call on 8295 8677 or contact us here.