Introduction to this book on assessing slab heave


I have decided to write this E-Book in aid of structural engineers that have been asked to assess and diagnose slab heave in residential houses.

The intention is not to tell you how to suck eggs. That has been done. However it is intended that this guide will provide a uniform assessment and diagnosis methodology of slab heave across the state, and perhaps the country.

This document is a “work in progress”. I have opened up this manuscript to comment rather prematurely but with the mindset that this information is open to criticism, review and particularly revision.

In this context I invite honest opinion and review, comment, feedback and criticism. For in all the time that I have assessed and diagnosed slab heave as a professional structural engineer, an open source of this type of information has not been easily accessible.  I would like to change that.

It is my intention to create an E-book that I will keep available on my website and freely distribute to all those that need it. I promise to attribute all comments that I receive in the Appendix – so please honour me with your real name and keep your comments clean. However I reserve the rights to this work so please understand if your comments do not make it to the final wording.

My Background

My experience with diagnosing and assessing slab heave has been established in Queensland since 1995. For the most part, the reporting and assessing criteria were specified by the Queensland Building Services Authority (QBSA), now named the Queensland Building and Construction Commission (QBCC).

The QBCC has a rigid proforma that they use to guide engineers in preparation of a uniform investigation and report structure.  I would like to recognise QBCC for their thought-leadership in the area of slab heave. The various experts at QBCC offices across Queensland and their deep understanding of the issue of slab heave is a wonderful, sobering reminder that we live in a great state, with non-unique problems, that we as a collective have a great deal of knowledge about. Current and previous QBCC investigators are therefore expressly invited to comment on this ebook also.

My First Slab Heave Investigation

My first ever slab heave investigation was on a cattle property north of Springsure, Queensland, Australia. My client was the QBSA. The house was a single level, concrete masonry house with timber framed internal walls built on a raft slab.

The QBSA does an excellent job of preliminary assessment and diagnosis of slab heave. At the time, building inspectors carried water levels to every inspection, and if there was any indication of damage due to slab surface movement, slab surface levels would be recorded.

The home-owner had lodged a complaint with the QBSA about cracking in her house. The builder had attended the property on at least one occasion to inspect and repair damage in the cornices and plasterboard walls, however new damage was still occurring and the existing cracking was getting worse. The house was moving and the owner had no way of fixing it.

We arranged an independent soil test and assessed the original design drawings. The design was fine and the original soil test was accurate. We asked the home-owner to stop watering her lawn. No real change was noticeable 6 months later. She told me on one re-re-inspection that the gap under her walls was so big she could ‘fit her toe under the wall up to the corn.”

After 12 months of monitoring, we decided to recommend a cut-off trench around the house. The work was done, and we did a monitoring inspection six months later. The gaps were closing!

Twelve months after the work had been completed we assessed that the ongoing movement was small enough to recommend rectification and repainting.

I have never been back to that property.  The house has been stabilised and is performing well.

The Background Problem to Slab Heave

The background problem that structural engineers will agree on, that isn’t formally mentioned as often as it should be,  is that houses are being constructed from stiff and fragile building materials. These materials are being chosen for their aesthetics over their durability.

Brickwork panels – particularly single skin panels of brickwork – are incredibly fragile. In fact single skin panels ARE more fragile than brick veneer walls for one simple reason – if the wall moves and cracks you can see through it! Disaster!

A similar amount of movement in a brick veneer wall will produce the same crack, but seeing daylight through a crack is often a home-owner’s spur to ask for help.

The other problem is plasterboard sheet walls fixed to skinny sticks of timber that are nailed together to form wall frames. The ability of a length of wall to absorb differential movement along a wall line is severely limited because the wall frame and plasterboard has insufficient stiffness to resist longitudinal bending forces. The predominant longitudinal bending strength of a wall is attained by the very wall cladding that is susceptible to movement – plasterboard.

The use of stiffer, or better still, more ductile, building materials for internal and external walls should be considered for areas where slab heave is a common problem.

The obvious comment here is that slab heave is a differential moisture problem and the prime movement resisting portion of a modern house structure is the footing system not the wall framing. However this guide is not limited to assessment and diagnosis of differential movement in raft slab and waffle slab footings – it also encompasses houses built on low stumps and piers.

Be Thorough

Slab heave in buildings is an emotional time for homeowners. How other emotions will run even higher if they case progresses through to the courts of law or tribunals. Be away from your first conversation until your final investigation that the case you are investigating may end up in a court and act accordingly. Professionalism and thoroughness at every turn our your best defence if you are ever called to the stand.

It is much better to take too many photographs during an inspection then to few. It is also much better to take report more information that is required during inspection than too little.

Notes during inspections and photographs and records of your assessments will be invaluable to you if you are ever called to give expert testimony in a court case.

There is a whole chapter dedicated to giving expert as an expert witness. There are also numerous references and online guidelines presented. Remember that as an expert witness your duty is to the court and to providing a fair and I’m biased account of the information that you have gleaned. As an expert witness it is also your responsibility to assess and diagnose the information that you have assembled.


Rule out Subsidence over Council Trenches

  1. Determine from survey data or local authority information the location of local authority services. Any service that runs under the building or within 2m of the building should be investigated further to determine if the footings are in the zone of influence for the excavation.

Assess the Independent Soil Test

Before you use the site classification report, check it for consistency and errors (don’t you do this already?).

  • Check that the address is correct and the correct allotment was tested.
  • Check that the photo is of the correct house.
  • Check the date of the report and the date of testing are as expected.
  • Review the bore logs.
    • Check the DCP results.
    • Review the depth of fill (if any).
    • Check the recorded soil types.
    • Check the moisture profile and compare it to the DCP results.
    • Check that the site has been classified to AS2870 correctly.
  • Confirm that Hs has been reported in the report.
  • Confirm that an accurate ys  has been recorded in the report (not a 10mm or 20mm range!).
  • Confirm that a valid site classification has been given that matches the Iss, ys, and site conditions.
  • Confirm that Atterburg limits have been provided if you requested them.

Rule out Consolidation of Loose Soil

Consolidation of soft soil does not cause slab heave. The rectification process for a house that has been constructed on unconsolidated fill or soft, loose sand is totally different to slab heave rectification.

Beware low DCP results in moisture softened clays. Many independent soil tests return a “P” site classification simply because the soils are moist and the DCP rod penetrates the soil easily.

  • A Class “P” result is NOT the end of a slab heave investigation.
  • A Class “P” result does not necessarily indicate footing bearing failure.

Correlate the cause of the Class “P” site classification with the bore logs and soil moisture profiles.

  • Do the DCP numbers improve to achieve more than 100kPa allowable bearing capacity in dryer soils?
  • Do the borelogs indicate soft or loose soils?

If loose soils are found on the site, check whether movement in the building has been caused by consolidation of the loose soils. Refer to the chapter on Loose Soils.

Assessing the Soil Moisture Profile

The soil moisture profile should be presented in tabular value with numbers down the column representing soil moisture percentages at 500mm increments.

Unless you are very familiar with the soils in the allotment, the soil moisture contents are a relative indication only. Wet clay soils can vary in consistency and strength immensely. Therefore use the soil moisture percentages as a basis for comparison between bore logs and previous site classifications.

A relative increase in soil moisture content between the original soil report and the independent soil report with slab dishing is a preliminary indication of slab heave under the external footings.

A relative decrease in soil moisture content between the original soil report and the independent soil report with slab doming is a preliminary indication of slab heave under the middle of the slab.

 Look for Soil Moisture Correlations

Next look for correlations between soil moisture conditions and slab surface levels:

  • Look for high slab surface levels and external sources of high soil moisture such as poor drainage, sprinkler heads, over-watering of lawns.
  • Look for low slab levels and external drying conditions such as sheltered ground, large trees within their mature height of the building, extensive paved areas with good drainage.

Assess the Original Structural Drawings

Assess the original structural drawings in relation to the construction standards applicable at the time of design and construction. Check the following:

  1. Slab thickness and slab reinforcement.
  2. External beam depth and reinforcement.
  3. Internal beam depth and reinforcement.
  4. Internal beam spacing.
  5. Handling of internal beams and reinforcement continuity at re-entrant corners.
  6. Slab detailing at step downs.
  7. Starter bar size and spacing.
  8. Concrete grade and slump.
  9. Pier depth and diameter.

Can any of the design errors or deviations from the standard details be correlated to the location of movement and damage observed in the building?

Correlations in Slab Movement and Damage

Lastly consider correlations in slab surface movement and slab damage.

  1. Do gaps between cornices and walls correlate with low floor areas?
  2. Do gaps between walls and slabs correlate with low slab surface areas?
  3. Do high slab levels correlate with locations where wall cladding has bowed in our out?

Assessment of Slab Levels

AS2870-2011 has the following table of movement limits. Assess the slab surface levels for compliance with these limits as

Leave a Reply

Your email address will not be published. Required fields are marked *

How to Identify and Fix Slab Heave