When restumping it is worth having an understanding of the choices and compromises. Restumping is an expensive process usually more expensive than a nice new car , and an understanding of what is happening between the ground and the stumps, can help you choose the best solution for you, and save a lot of money later.
You could argue that historically houses in Queensland were built on stumps because it was hot. Apart from a belief that airflow under the house would cool it, digging a flat platform on a stony site, a hundred years ago, by hand, was tough on a hot day. Digging holes was not much fun either, so they did not go down deeper than they had to, usually 2 or 3 feet.
Soil shrinks and swells over the years, and the seasons, and with the drying caused by putting a house over land, so that the rain only falls on the edge posts, and the internal area becomes very dry. The constant shrinking and swelling of the soil, together with gravity normally causes the perimeter stumps to sink, and the internal stumps are often a bit better behaved. Fungus works on the stumps and they become pencil ended, and the weight of the house drives them down. Stump life varies, on dry stable sites some stumps are still functional with a bit of packing after a hundred years. Many houses have been restumped, but practise was up to the 80’s to pull out the old stump, and put the new one in the same hole ( it was still hot ). Shallow stumps were a bit prone to movement on some sites, but people were not so worried, if you could not close the back door after rain, it was not so serious, because you were happy to leave it ajar for the cat anyway.
Several things have happened since, like fussiness, kitchens and dingos. People became fussy, they wanted to close their doors all year round. People changed from a couple of cabinets and a free standing kitchen table to master chef kitchens, with marble tops and millimetre clearance self closing doors. The people who dug holes were also getting tired of it being so hot, so they started using Dingo’s , small stand on loaders that can dig real deep real fast. House movement became less tolerable and the apparent solution was to go deeper. So when a restump was done, the new holes tended to be deeper than the old ones, a soil test was used to gauge how much.
Sites are graded to A – for rocky sites that hardly move S for stable sites that move a little like sand or gravel, M for clay sites that move around a bit, H for clay sites that move around a lot, and E for extreme sites that Ipswich and the Downs are famous for, where in dry weather cracks open up in the surface, and a snake can drop down and re-appear a long way away. Australian Standard AS 2870 is the main guide on designing foundations.
In 90 % of cases a stump which is 1 to 1.5 metres deep will bring movement down to an acceptable level, at a reasonable cost. Perhaps after 5 years a little packing will be required, and maybe every 10 or 20 years after that, but the cost of a re-level is a lot less than the cost of painting a house, and that occurs much more frequently. However on some H, E and problem sites, there may still be more movement than ideal.
You could fit adjustable heads, which make the adjustment easier. These are a bit ugly, and may lead to some vibrations in the house, but are an economical choice. A skilled householder or a contractor, could adjust them a few times over the first 5 years, and then at say 5 yearly intervals after that and not break the bank. Sometimes they require packing as well and can be more unsightly, although we have a new design which largely solves that problem too. However, if you are building in under the house they are not an option, as adjustment is no longer possible. The problem of leaning posts remains. It is quite rational and standard practice on an existing house to have the central stumps somewhat shallower than the perimeter stumps, as there is less movement there, and arguably at a lesser depth than the code requires. New builldings have stumps all at the same level, even though an argument could be made that the internal posts should be deeper, as they would be more subject to drying and shrinkage. I have never seen that done, most people would think it was an odd arrangement.
Going deeper has limitations too though. While in theory at the depth of influence ( 1.5 m in Brisbane, 2.3 m in Ipswich and a lot more on the downs ) the soil no longer moves as it is unaffected by seasonal changes, it is not that simple. The supplement to AS2870 for example recommends against very deep posts, there are good reasons for this. Firstly cost, having very deep posts is pointless if the return is marginal. Especially for restumps the house may be low, and deep posts may not be practical, and remember the soil in there is pretty stable anyway. Also in bad soils, the soils move sideways as well as up and down, a stump embedded deep in the ground is pinned at one end, and as the surface slides the post leans, which is all but un-correctable.
A better solution is to put the whole house on a raft of concrete which moves up and down and sideways with the soil, and takes the house with it, but as everything moves together there is no apparent movement in the house. This is a great solution if you are building in under the house. More expensive, but you are doubling the size of the house and almost eliminating the possibility of problems ( with time the house may still tilt a little, but rarely enough to notice, which in extreme cases can still be rectified by ground injection) .
What if your house is low, or you do not want a slab, it is possible to replicate the slab with ground beams, these are a good solution too, however quite expensive, and you do not have the benefit of having the new area under the house.
When restumping, we always recommend you consider the benefits of a lift. You may not need to install a structural slab at the time, we have a design that can be installed later, or you may opt for a futuer timber platform floor, which has a number of other advantages. Even if you do not build in, the value of the house goes up when you go to sell. But consider all the costs, plumbing, stairs and the fact that you will have to walk up further every day carrying groceries. Have a look to see if it improves your views, and consider what more use you could make of the area under the house, before deciding.
Like all engineering problems, restumping is a compromise, between cost, benefit and risks that cannot be completely ruled out.