HOW YOUR HOUSE ON STUMPS MAY BEHAVE DUE TO FLOODING AND FLOODWATER

Once the floodwaters had subsided following the October 2022 event, many homeowners who have houses supported by stumps have been surprised to find that their previously flat floors now resemble the rolling hills found in Victoria’s High Country.

TYPES OF HOUSE FOOTINGS

Before we dive any further, lets take a quick look at the different types of common house foundations found across Victoria and Many Parts of Australia.  For a full explanation of all things related to house footings and the vast array of footings that are available out there, take a look at THIS link.

For the purposes of this discussion, we will be focusing on two different types of common domestic footings:

  • Stump Footings
  • Slab footings

THE STUMP FOOTING

Stump foundations are a very common solution for houses which use structural timber for the floor support structure.

Stump foundations can either be timber, concrete, and sometimes less commonly; steel.

The stump itself generally sits upon a concrete pad which provides a flat bearing surface and assists to spread the load evenly on the founding soil.

A termite cap is then provided at the top of the stump which protects the structure above (usually comprised of timber) from termite attack. The image below illustrates a schematic arrangement of this type of footing system…

Cross-sectional view of a typical stump foundation wtih joists, bearers and sub-floor

This type of foundation is sometimes called a “sub floor foundation”. The free space created between the floor structure and natural ground surface is called a “sub-floor” space, hence the appropriate name for this type of foundation system.

THE SLAB FOOTING

Another very common footing type for houses, particularly in Australia, is the slab footing or more commonly referred as a stiffened raft slab (there is also a variation of this called the waffle slab foundation however we won’t confuse the situation further at this point).

A stiffened raft foundation is constructed from concrete and comprises a slab, generally 100mm thickness (4 in.) supported by ground beams.

Reinforcing is provided to both the slab and ground beams. Ground beams are provided around the full perimeter of the house and internally at regular intervals in both directions in a crisscross type arrangement.

PERFORMANCE OF A STUMP FOOTING SYSTEM UNDER FLOODWATER AND FLOODING EVENTS

Houses across many parts of Victoria (including suburban and rural areas) can be commonly founded upon reactive clay soils.

The “reactivity” of some types of clay soils refers to its tendency to shrink as its moisture content reduces and swell/expand when its moisture content increases.

This makes it tricky when you want to live in a house, with a flat floor, and walls that don’t crack.  The good news is that Australian design codes provide guidance on the proportion and sizing of footing systems (for both slab and stump type arrangements) with this reactive nature in mind.  When a house’s footing system is proportioned and designed in accordance with these standards, the effects caused by this reactivity and subsequent movement are controlled to within acceptable limits.

This is provided that relatively consistent moisture conditions are maintained under and adjacent to your house’s footings.  Obviously its impossible to keep the support soil’s moisture content 100% consistent throughout the entire year with rainfall intensity changing with the seasons throughout the year.  However, there are a few things a homeowner can do to keep their foundations operating at an optimal level which may include

  •  Keeping your down pipes in well maintained condition with no cracks or defects
  •  Having large mature trees clear from your house (trees which have large and significant root systems)
  • Not allowing water to pond or stand adjacent to your property for long periods of time
  • Ensure rainfall and other water from hoses and taps can freely drain away from the perimeter of your house

A flooding event can bring significant quantities of water in, around and under your house depending on the severity and height of the floodwater.  This can have significant influence on your footings, particularly if your house is supported by stumps.  The best way to understand these effects is to walk through a timeline of an example house from before it is constructed, right up until the flood event…

With reference to the numbered images below, the following sequence is quite common for many houses that were affected by flooding in country Victoria which are supported by stumps.

  1. Before the house is built, the site is prepared and cleared ready for construction. At this time, the moisture level of the soil across the property is uniform and at “equilibrium” as all surfaces receive generally the same exposure to seasonal rainfall events throughout the year.
  2. The house is constructed on the property
  3. Over several years the soil surface directly beneath the dwelling dries due to evaporation and lack of exposure to rainfall events throughout the year.  During this time, the soil beneath the interior zones of the dwelling may shrink due to reduction in moisture (remember the reactive nature of clay soils).  Older houses may have even been re-stumped during this time if the stumps have exceeded their design life.  Re-stumping and therefore re-levelling of the floors after this initial shrinkage can actually set the scene for a bad outcome once a flood event arrives…
  4. A flood event occurs which can result in standing water several metres above natural ground level for several days.
  5. Due to the standing water of the flood event, the moisture level within the soil across the entire property increases.  The main factor which influences the subsequent movement in dwellings founded on stumps is the difference in moisture change within the soil exterior to the dwelling compared to underneath the dwelling.
    The areas exterior to the dwelling have retained a relative degree of high moisture content due to rainfall events throughout the year.  The interior zone directly beneath the dwelling however can be bone dry and ready to soak up water like a sponge.
  6. The difference in moisture change results in a differential expansion within the soil under the dwelling compared to the soil surrounding the dwelling.  This results in “heave” to occur under the more lightly loaded stumps.  Interior stumps which directly support load-bearing walls may experience slightly less heave as they are pinned down by an opposing vertical force due to roof and wall loading.  A significant differential movement is generally experienced around the perimeter of the dwelling due to:
  • The perimeter stumps not being founded upon significantly dry soil compared to the interior stumps thus resulting in less heave effect.
  • The perimeter stumps are usually the most heavily loaded stumps in a dwelling as they support the ends of the roof trusses and the perimeter walls therefore the “pin-down” load is the highest.  By comparison, the first row of interior stumps are generally the most lightly loaded stumps as they support only the self-weight of the floor at ground level.

Timeline of a house on stumps with joists, bearers and sub-floor and effects of soil moisture condition. Sequence of events prior to flood event.

Timeline of house on stumps with joists, bearers and sub-floor and effects of soil moisture condition and subsequent soil movement. Sequence of evens during and after flooding.

By comparison, the support soil directly beneath a slab foundation is protected during a flood event. The pressence of the slab footing itself prevents significant moisture changes from occuring within the support soil structure. This is the key reason why houses and structures which are supported by slab foundations were found to perform far superior to those with stump footings following the October 2022 Victoria flooding.

The expansion/heave of the clay soil can be readily seen when inspecting the ground surface of the soil. The following image is an example taken from a house in Rochester after the flooding event. The the image is taken within the sub-floor zone of a house supported by a stump footing arrangement and the photo was taken relatively central to the dwellings footprint area…

Photo taken next to a stump at a property in Rochester Victoria relatively central to its plan footprint. Evidence of significant soil expansion due to reactive clays is evident with surface cracking appearing at ground surface level.

As evident above, the effects of heave and expansion in reactive clayey soils after a flooding event can have significant impacts on a house’s plumbness . This movement can result in floors which appear significantly unneven under foot and has been a cause for major concern for many residents throughout rural Victoria.

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Quentin Suckling is our founding director.  Prior to starting Sheer Force Engineering, he spent almost 2 decades working as a practicing Structural Engineer at Tier 1 engineering consulting firms delivering multiple billions of dollars worth of projects and managing large multi-disciplinary engineering teams. View More Posts

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