Tag Archives: raft slab

Soil Tests – What it Means

Soil tests are used to help structural engineers get some information about the soil on your block.

Structural engineers need to know if there are any problems with the soil on your block so that they can design your house footings correctly for those site conditions.

The main soil problems are:
Soft soil
Loose soil
Overly wet soil
Clay soil (reactive clays)

Of these, the soil problem that has been getting the most bad press lately is reactive clay soils resulting in Slab Heave. Reactive clays change in volume substantially when they absorb and release moisture. The change in volume results in the ground surface moving up and down and this can cause damage to a house.

Reactive clays are classed as follows:
Class “M”: a moderately reactive clay. We see this site classification about 35% of the time. Ground movement isn’t ‘too bad’ and house slabs can easily be designed for this soil. Ground surface can move vertically between 20mm and 40mm between wet and dry conditions (seasons).

Class “H”: a highly reactive clay. We see this site class around 20% of the time. Ground movement is a bit more serious and care needs to be taken designing the footings and maintaining good drainage around the house. The footing and slab code starts introducing special requirements for drainage and for protecting pipes from the movement that is likely to occur.

Class “E”: an extremely reactive clay. We see this site classification about 2% of the time. Very special care needs to be taken with the footing design and extra precautions are needed by the builder and homeowner (for the life of the building) to protect the house from slab heave. This is a life-long soil condition and future homeowners need to be aware of the limits and disclaimers in the footing design.

Firm sandy sites are classified Class “S”. These are my favourite sites. Nice and easy to design, there is no reactive clay movement to worry about. We see Class “S” sites about 15% of the time.

Soft soils, loose soils,wet soils and other problems site are classed as:
Class “P”: a problem site. We see this site class about 28% of the time. Usually the soil test has further clues that describe the problem with the soil. You’ll need an engineer to design your footings to solve these problems. Some of the common “Problems” are:

  • Uncontrolled fill. Extra soil that has been placed on your block that either hasn’t been compacted properly or that doesn’t have the paperwork (compaction tests) to show that it has been placed and compacted properly.
  • Soft soil. Soil could be soft because it is loose or unusually moist. Soft soil may not be strong enough to support the weight of your new building without extra precautions being taken.
  • Abnormal moisture conditions. If the soil tester has identified the potential for abnormal soil moisture changes on your block, they’ll explain the reason in the report.

Soil tests for house sites are carried out in Australia to comply with Australian Standard AS2870. You don’t need to be familiar with this standard unless you are a builder, an engineer or a certifier. However the disclaimers and information that is on the soil test and on the engineer’s plans is often derived from the rules in AS2870. You need to read these rules, understand them, and pass the information on to the next home owner. The soil under your house is there for the life of your building.

How to arrange a soil test

This is what you should do when you need to arrange a soil test for a new house or a building extension:

  • Get a copy of the site plan showing any existing buildings and any new buildings.
  • Email the site plan to your soil testing company. Ask for a quotation for a site classification (or soil test to AS2870).
  • Approve the quotation.
  • Be available to give the soil tester access to your site. Be aware of any underground services so that the drill rig does not damage underground infrastructure.
  • You should receive the soil report as a written report in pdf format. Forward a copy of the soil report on to your designer and/or engineer.

Find a soil tester in Mackay, Queensland.

Find a soil tester in Brisbane, Queensland.

Or use one of our recommended soil testers.

By Matt Cornell
For Cornell Engineers – Structural Engineers
A Structural Engineering Blog

Slab Heave Hits Melbourne – and now Sydney

Slab Heave in Melbourne

A recent article by Simon Yohanson in The Age, Melbourne has highlighted the ongoing problem faced by home owners in new Melbourne subdivisions – slab heave.

“Thousands of suburban home owners facing financial ruin

Read more: http://www.theage.com.au/victoria/thousands-of-suburban-home-owners-facing-financial-ruin-20140607-39q4z.html#ixzz345UdtsEg

Yet, if you’re a Melbournite affected by slab heave, it is of little comfort to know that you are not alone.

Slab Heave in Sydney

There’s been a spike in users from Sydney hitting our website looking for information about slab heave. Welcome – and don’t panic! There’s plenty of information on the web about slab heave and how you can minimise its effect. I hope we can help provide some of that information for you.

What is Slab Heave?

See our post: What is slab heave?

Who is Responsible?

Melbourne, Sydney and Brisbane homeowners – if your house is within the builder’s warranty period, the builder and the developer are responsible for fixing slab heave – but they will try to re-assign the blame. For your best chance of having your house fixed you MUST be prepared to eliminate conditions that could be considered to contribute to the problem.

If your home is outside the builder warranty period, it is your responsibility to fix slab heave. There’s plenty of information on this site on how to improve conditions around your house. If you need more help or a recommendation for an engineer please ask. I’ll try to find an experienced residential engineer that can help you.

The Claim Process

If you are within the builder’s warranty period, this is the claim and fixing process:

  1. Monitor the damage. See my post about how to add some science to your monitoring and writing a ‘crack diary’. This will improve your chances of getting a good outcome.
  2. Improve the drainage conditions around your house. This is your responsibility as a homeowner and you can not get out of this responsibility. Read this post and follow the recommendations in “Do this First”.
  3. Make yourself aware of the insurance claim process for your state. Knowledge is your best weapon against slab heave.
  4. Make a written complaint to the builder outlining the damage, when it occurred and that you want the builder to fix it. Do not mention legal action. At this stage you want to work with the builder – not against the builder. Your complaint MUST be in writing and you MUST keep a copy as evidence. Send it by email and request a read receipt; or send it by registered post; or send it by facsimile. Include a history of the damage (including photos you took and a copy of your crack diary).
  5. Allow the builder fair access your property to assess the damage. A clever builder will engage a residential structural engineer almost immediately. Have your crack diary available for inspection or for copying but do not give your crack diary away.
  6. The builder should, but doesn’t have to, give you a copy of the engineer’s report. If any aspect of the engineer’s report is unclear, ask the builder if you can speak to the engineer directly. If you disagree with any aspect of the engineer’s report, consider getting an independent interpretation of the report, or better still, an independent engineer’s inspection. Read here about what to ask any engineer that enters your property.
  7. Follow the recommendations of the engineer’s report to the letter.
  8. Allow the builder access to the property to make any repairs required in accordance with the engineer’s report.
  9. Continue to monitor the cracks and keep your crack diary up to date.

Warnings!

Warning 1: Fixing slab heave is a slow process. You will not see improvements in crack width or floor slope occur very quickly. It takes a long time for heavy clay soils covered by a building to stabilise.

Warning 2: Bumping this problem up to litigation will slow the resolution process down infinitely. Most builders take the hands-off approach when lawyers get involved.

Warning 3: If you haven’t built your house yet – and your builder is using a waffle slab, carefully read and implement the builder’s and engineer’s advice on foundation maintenance (management of soil moisture around and under your house).

The Forums Might Help

We contribute to the HomeOne forum from time to time. Check out this long discussion on slab heave.

Summary

Slab heave can be avoided and it can be fixed. You have to be patient and you have to improve the site conditions around your property.

If you’re from Sydney or Melbourne and your house is suffering slab heave, I know your pain. I wish you all the luck in the world.

By Matt Cornell
A Structural Engineering Blog

Slab Heave – Waffle Slab vs Conventional Raft Slab

Slab Heave

Slab heave occurs in a house slab when the ground moves more than your slab can stand. Waffle slabs and raft slabs are both intended to moderate this ground movement to acceptable limits. The slab doesn’t normally crack, but if the movement in the slab is more than the brickwork and/or plasterboard walls can tolerate: they crack.

Slab Heave – Waffle Slab vs Raft Slab

Which is better? A waffle slab  or a raft slab?

Waffle slab setup before concrete

The quick answer:

A Raft Slab

Why?

  • Waffle slabs are built on top of the ground and need great drainage and really hard ground for the life of the building.
  • Raft slab footings are dug into the ground, have more perimeter stiffness and more tolerance to poor ground conditions than waffle slabs.
  • When turf and gardens are placed around waffle slabs, water can easily flow under the slab – that’s bad!
  • Poor site drainage is easily disguised. Even if the ground surface looks like it grades away from a waffle slab, builders often use sand fill to bring up ground levels before turfing. Sand is porous and allows water to flow up to and then under a waffle slab. This is a major cause of slab heave in waffle slabs.

Are Waffle Slabs Legal

Waffle slabs are legal and are their design is covered by AS2870 so engineers are entitled to specify them. However their use comes with a lot of provisions that relate to site drainage that must be observed for the life of the building.

Your best protection is to read the engineers plans and observe the rules. If you haven’t built yet, find out what the rules are and whether they will restrict your plans for gardens, trees, lawns and swimming pools.

Waffle Slabs Need Better Drainage

Waffle slabs are a lot less tolerant to poor drainage:

  • Site drainage is ULTRA-important but builders often only improve site drainage after the house is built. Water lying near a building under construction is against the engineer’s rules and can cause slab heave.
  • The site drainage rules apply for the life of the building but this important information often isn’t passed on to future owners.
  • Waffle slabs let moisture flow under buildings. This is the opposite of what is required for good performance of a house slab.

Damage from Slab Heave

Signs of slab heave include:

  • Gaps under walls
  • Damaged cornices
  • Uneven floors
  • Doors jamb and don’t close properly
  • Diagonal cracks in brickwork
  • Diagonal cracks in internal walls

Raft Slabs

You may have realised that we prefer raft slabs at Cornell Engineers. Want some reasons to pick a raft slab over a waffle slab? Check out this post.

Want to know more?  Here’s the link to our video on slab heave.

 

 

Slab Cracking – Waffle Slab vs Conventional Raft Slab

Slab Cracking

The age old question has been raised again this week. Should you go with a waffle slab or stick to a conventional raft footing and slab when building a new house?

Terminology

For those unfamiliar with the terms, refer to our page of Structural Engineering Terminology

Today I’d like to address just the issue of slab cracking.

Waffle Slab vs Raft Slab

Which slab is more likely to crack? The waffle slab or the conventional raft footing?

Check out this post: Why does concrete crack?

The Good News

Concrete cracks are very common. How do you know if the cracks in your slab are bad cracks?

The good news is that a crack in a house slab doesn’t mean the concrete has failed.

Concrete does crack. It’s rare to see a new house slab that doesn’t have some cracks in it.

Hairline cracks generally won’t affect the strength of your house slab because they often don’t penetrate right through the concrete. They are often surface cracks and are controlled by proper placement of the slab reinforcement (around 30mm to 40mm below the surface).

Hairline cracks in older slabs tend to fray and might appear wider at the surface but when I have inspected core samples taken through older cracks, once again the crack stops at the reinforcement.

Shrinkage Cracking

Typical Shrinkage Crack in Concrete

Of the slabs that I have inspected, the majority exhibited shrinkage cracking. (What is shrinkage cracking?) Either no curing was used, it was a hot day and the bleed water evaporated or the slab was over-worked and the bleed water was pushed away during screeding.

The same cracks will occur regardless of whether is is waffle slab or conventional slab. So no clear winner yet.

Plastic Shrinkage Cracks

The other slab cracking that we see in residential slabs, but less frequently, is parallel lines following the mesh at around 20 centimetre centres in both directions.

This is caused by poor

Plastic shrinkage cracks caused by concrete draping over the steel reinforcement.

compaction of wet concrete and the concrete slumping over the mesh reinforcement. Sometimes, in hot weather, it is also caused by the concrete drying out quicker around the hot mesh.

Again this cracking can occur on both types of slab if the mesh isn’t cooled or the slab concrete isn’t vibrated. Still no winner in the raft slab vs waffle slab shoot out!

Pure Shrinkage

The only type of cracking that might be different between waffle

slabs compared to conventional slabs would be pure shrinkage caused by the concrete trying to shrink in volume as it cures.

These cracks don’t follow the mesh and sometimes start in internal corners. You will also see shrinkage in long, thin slabs where there are no control joints.

In waffle slabs the slab can shrink freely because there is less restraint by the the ground to the slab contracting. In conventional slabs, the edge beams in the ground stop the slab shrinking in overall length. Engineers use heavier mesh in larger house slabs to counter these shrinkage forces. So waffle slabs just took the lead!

Overloaded Slabs

Concrete slabs will crack when they are overloaded. The steel reinforcement in concrete slabs is there to control the width of cracks under normal conditions. When a slab is overloaded, the steel stretches and cracks become visible.

A stronger slab system can take more load before it cracks. In theory there’s no real winner here because waffle slabs and raft slabs are designed for similar loads and will behave similarly when overloaded.

However raft slabs are cast against the ground whereas waffle slabs are cast onto polystyrene void formers and strips of concrete. The raft slab edges back a point. An overloaded raft slab is less likely to crack because it is cast onto the ground.

Who’s the Winner?

So, are waffle slabs less likely to crack than conventional raft slabs? My opinion is a reserved yes. The problems that cause cracks in slabs affect both slab types, but there should be less shrinkage stresses and fewer cracks in a waffle slab but a raft slab is less likely to crack if it is overloaded.

When to Worry

The Australian Standard AS2870-2011 gives advice on when slabs cracks are bad enough to cause concern – and often this is when you’ll need an engineer to help solve the problem.

Distinct cracks: around  2mm wide and accompanied by 10mm to 15mm change in offset from a 3m straightedge centred over the defect.

Wide cracks: 2-4mm cracks and accompanied by 15mm to 25mm change in offset from a 3m straightedge centred over the defect.

Gaps in slab: 4mm-10mm wide cracks and more than 25mm change in offset from a 3m straightedge centred over the defect.

Need More Advice?

Need more advice about cracking in waffle slabs and raft slabs? Contact Us or Get a Quote.