People look at the Hawaiian Islands and think to themselves, “This is just one big solid rock!” In reality, there is a lot of complex geology going on with the islands, and that can make building and maintaining foundations difficult.
The reality is lava rock is among the weakest in the world and decays the fastest of all the rock formations. Nature has ordered that the youngest material out of the earth falls apart first.
Volcanic rock decays and erodes quickly. The decay is the result of a subtropical setting with relatively high temperatures and humidity all year round. Volcanic rocks are high in iron content. In the hot, humid climate the iron rusts and the volcanic material falls apart. That is why the soil is red – because of the rusted iron. Then the high mountains catch the moisture riding in on the trade winds and much rainfall occurs at the higher elevations. This rainfall takes off down the steep hillsides and erodes the decaying rocks.
The eroding soil ends up on the flatter ground toward the beach and – especially on Oahu – this eroded soil is deposited in backwater called lagoonal deposits. Lagoonal deposits are quite soft and piling is required to support structures of any size.
Meanwhile in the intermediate elevations between the steep mountains and the lagoons the red dirt has formed and waiting for the developers who like to build houses on it. Some of the red dirt is what we call moisture active.
That is, it has the property of changing volume when the moisture content changes. When the volume changes, any light structures sitting on it (like your house) can go up or down. This is the source of the expansion and contraction of footings and floor slabs that are constructed on red dirt. A whole specialty revolves around curing, founding and repairing of light buildings on expansive soils.
The higher hillside areas give rise to development that seeks another kind of experience – the view. Hillside volcanic rocks can appear solid, but they are not necessarily so. Careful foundation design and construction is required on any hillside property.