Turtles all the way down

But everyone who hears these words of mine and does not put them into practice is like a foolish man who built his house on sand Mathew 7:26

And yet sand is a popular base material under foundations :P

Our pool has a number of retaining walls. These walls sit on concrete footings. These footings in turn need to sit on something solid. On some sites the existing subgrade is sufficient, but it is common to over excavate and replace the over excavated material with gravel, crushed rock, or sand. Reasons for this include:

  • Provide a layer that is relatively unaffected by changes in moisture due to the weather (particularly important for reactive soils).
  • Provide a level surface for the concrete and bridge over any surface weak spots.
  • Preventing any mixing of damp surface clay/dirt with the wet concrete.
  • Providing a solid surface for reinforcement chairs.

But the primary reason is to improve the bearing capacity.

When you stand on the ground, the greatest pressure is on the soil directly under your feet. As you go deeper, the load spreads out laterally and the pressure decreases. For a footing therefore, the soil immediately beneath it is the most vital. Replacing 200 or 300mm with well compacted crushed rock will provide greater bearing capacity than without it.

For our footings, we are using what is commonly referred to as road base. It comes in a variety of classes, but it is essentially crushed rock and includes sizes from 20mm all the way down to fine rock dust. It looks like this:

Road base

A few meters of class B road base
Roadbase is a well graded material, consisting of various sized particles containing coarse and fine aggregate, that when mixed and placed correctly enable compaction of the material into a layer that resists distortion due to weather cycles of wetting and drying as well as deformation due to vehicular mass and movement. Cement Concrete & Aggregates Australia

Our engineer requires a minimum of 100mm under each of the footings, but we have around 150-170mm for good measure.

Compact, Compact, Compact

Crushed rock like this is very compactable. The fine dust and small pieces fill the gaps between the larger pieces and provide a very hard surface for the concrete. To achieve good compaction, we use a plate compactor.

Plate compactor

Mid-sized plate compactor

This compactor weighs around 90 kg, vibrates several thousand times per minute and hits with a centrifugal force of around 1500 kg. It doesn't take long to turn the lose crushed rock in a surface that feels very much like concrete:

Compacted road base on lower retaining wall trench

Compacted road base on side stepped footing trench

Unfortunately the mechanical compactor could not be used in all areas. The area where the new storm water and sewage pipes were laid and the area where the pool plumbing pipes pass under the rear retaining wall footing, all required manual compacting. For come reason the local tool shops don't stock manual tampers, and so I was left to make my own:

Homemade tamper

Homemade tamper

It's a slow and laborious process, but if you're looking for a serious bicep workout, I'd highly recommend it.