Importance of Correct Resistivity Figures

The structural engineers design required the piles to penetrate the sub-grade clay to a depth of 3.6m. This is to avoid the seasonally effected zone. Quite a depth, compared to the coast of NSW. The material is very reactive (high shrink/swell) so the design also included a void-former at the surface connection with the concrete.

We asked for the builder to provide resistivity figures for the clay as the west areas of NSW can be quite corrosive-aggressive with steel. The results came back mild, so we chose a pile with an appropriate wall thickness.

The builder provided the site scrape, and once on site we trialled the first pile, screwing normally. This proved ineffective as we encountered very hard material at shallow depths.

We then pre-bored a 200 mm hole to allow deeper penetration, which was fine until we hit rock (at 1m depth). Our machine (5 tonne) wasn’t strong enough to bore through the rock. This layer of rock had a high percentage of angular stones (100 – 150 mm diameter) in the clay matrix. Persevering and investigating, we inserted a 50 mm test hole (using our sampling continuous flight auger) through the layer and found another layer of rock fragments a further 1m down. Both these rock layers were about 800 mm thick each.

A bigger machine, then a much bigger (25 tonne) machine were brought on site to penetrate the layers of rock.

We found the application of limited moisture was enough to reduce the shear capacity of the clay matrix and allow our auger/helix to slide the rocks to the side and continue penetration. At the end of the days we were satisfied the house was going to be soundly supported.
What started as a two-day job occupied our time for seven days, but the outcome was secure and everybody is happy.