Slab heave is a problem that has plagued the building industry increasingly for many decades. Many people outside the building industry don’t understand what slab heave actually is and unfortunately, many inside the industry don’t either. This article will explain what slab heave is, the conditions required for it to occur, who is responsible for it, and measures builders can put in place to prevent and attempt to fix the damage caused by slab heave.
Slab heave is caused when dry ground under or near the edge of a slab becomes wet and the clay swells so much that it lifts the edge of the slab. The centre of the slab stays dry and shows little movement. This can be deceptive as often the damage is seen in the centre of the house. This is explained by the fact that as the edge of the slab lifts, the frame at the edge is also pushed up, and with the modern long span roof trusses the damage is transferred to the middle of the house along with damage near the edge. Hallways and kitchens can sustain substantial damage with large plaster cracks and cupboard and vertical wall movement. Brickwork cracking can also occur but to a lesser extent as the articulation joints in the brickwork absorb some of the brickwork movement. Windows and doors often begin to jam and cornices detach from the plasterboard.
There are three factors required for slab heave to occur to a substantial degree:
1. Highly reactive to extremely reactive soil
2. Dry initial moisture profile (before the build starts)
3. A source of moisture (poor drainage, leaking pipe, heavy rainfall, etc.)
Extreme climate conditions over the last 20 years have greatly added to the number and intensity of slab heave cases. In particular, extended droughts have set the conditions for the second factor and the extensive highly reactive basalt plains in the west and north of Melbourne provide the first factor. Poor building practises, poor landscaping and very wet years between droughts account for the third factor.
The 2020-21 year to date has been a very wet year around the outer suburbs of Melbourne and new slab heave cases are being recorded in the highly reactive west and Northern suburbs.Some of these heave cases are less than 2 years old and potentially could have a lot of movement to go depending on the climate.
Let’s run through a typical slab heave scenario that has occurred numerous times over the last 20 years. First, an area of highly reactive soil is subjected to below-average rainfall and so dry soil moisture conditions are created. The house block is cut flat usually for a waffle slab which instantly creates poor drainage. Then, the builder builds a small artificial batter up against the slab which can be made anything such as gravels, tan bark or loose soil, and is poorly compacted. The batter allows moisture in around the footing but restricts evaporation therefore increasing the moisture at the edge of the slab. Once the roof is constructed, if temporary downpipes are not connected then any rainfall or moisture is discharged in a concentrated area onto the fill batter up against the footing which further increases the moisture and the soil begins to swell. If the soil is not graded away as required in the Australian standards, then further moisture can collect around the edge of the slab. I have recorded a soil moisture increase of over 70% on a typical building site compared to a neighbouring vacant block in the west of Melbourne.
In the worst scenarios the sewer pipes break or leak and cause extreme heave which is much greater than heave caused by stormwater or drainage issues.
There a few factors that are not well understood that explain the extra heave resulting from sewer breaks which we have researched and talk about in our seminars to builders and engineers.
This a very important reason for builders to use flexible plumbing to allow for ground movement and prevent extreme heave created by sewer leaks.
Failure on several fronts allows excessive moisture to occur on site and therefore for abnormal moisture conditions to develop:
1. Builders not grading the soil away from the slab and builders not installing and maintaining temporary downpipes after roof construction.
2. Building inspectors not calling out builders for not grading the soil away from the slab.
3. Geotechnicians and engineers underestimating the reactivty of the soil and not performing lab testing.
4. Some geotechnicians not identifying extremely dry soil as “abnormal moisture conditions” during below average rainfall in drought periods.
5. The VBA, which has been described as a toothless tiger, not overseeing and enforcing the regulations. In 30 years of full time fieldwork on and around new housing estates I have seen the VBA less than a dozen times.
6. Builders not checking the integrity of the plumbing before handing over the house to the owners.
7. Builders making eaves as an optional extra rather than as standard design.
8. Builders not installing roof truss and top plates correctly restricting flexibilty. Articulation joints not clear of obstructions.
9. Builders introducing square set plastering which has very little tolerance for slab movement.
10. Owners adding to the poor moisture conditions by not properly constructing a protective path around the perimeter. Poor landscaping choices being made that keep adding to the wet soil moisture around the foundation. Not properly informing clients of the correct landscaping and the potential for slab heave could be construed as a failure of the builder.
Implementing building practises that include grading the soil away from the slab and installing temporary down pipes during construction. Plumbing inspections and testing at several stages. Educating owners about the importance of maintaining a landscape that tries to minimise moisture change around their new home. A properly constructed protective path should be incorporated as an essential part of construction on a highly reactive site. The VBA enforcing the rules and regulations regarding the Australian standards and appropriate construction methods and supervision. Australian standards need to change to incorporate real time climate information which would require ground movement calculations to adjust to extreme enviromental conditions. This would add cost to the residential industry but only when required to help prevent the long term extra costs and hardship resulting from slab heave. The geotechnical industry needs to be more aware of significant soil moisture variations occurring during drought and wet periods. Increased reactivity and moisture testing should be conducted to track long term soil moisture changes and to alert geotechnicians and engineers when to adjust the ground movement model.
The thing to do is to remove the source of excessive water which may be from several origins. Landscaping and drainage should be correctly designed and implemented. The plumbing system should be checked and any leaks fixed. Agi drains or cut off drains may be needed to prevent subsurface moisture from entering the area surrounding the footings. Soil moisture conditions may return to a normal range once the sources of moisture have been removed. This may take months, years or even decades to happen – such is the nature of highly reactive low permeability clays. The slab is unlikely to then return to the original level as this would require the soil to dry out to drought conditions and the slab reinforcement will be permanently bent out of shape. The goal is to stabilise the upward slab movement and the best case scenario would be some partial downward movement with eventual stabilisation.
Drillology (previously JM Fieldwork) has trialed several experimental methods to stabilise heaving slabs with success, verified with a series of floor levels over several years. Two different methods were used, one with electro osmosis and the other with dry chemical stabilisation.
Drillology has done several presentations for major builders explaining the causes, prevention and remediation of slab heave which have all been well received. If you are a building company and would like a presentation to site supervisors, management, contractors or any staff, please contact us and we can arrange a presentation at your office. We also belong to HEDRA (Housing Engineering Design Research Association) and do regular presentations on slab heave and various topics involving residential foundation design and construction.