Underpinning FAQ — Dubbo

It comes down to scope. Resin injection under a few sections is a smaller job than underpinning several footings, and both are well short of a full-perimeter underpin. Depth, access and the number of piers set the difference, and it is rare for every footing to need work. Ground conditions matter too. Council's natural resource snapshot lists Euchrozems, Red Earths, Red Brown Earths, Red Podzolic soils and alluvial soils across Dubbo, and groundwater depth and salinity can change the pier design and the concrete specified. Get a structural engineer's report first, so contractors quote against the actual cause, not just the symptom.

Hairline cracks under 1 mm that don't grow are usually cosmetic. Cracks wider than 5 mm, diagonal cracks running from the corners of doors and windows, doors or windows that have started binding, and cracks that come back after patching are the ones worth an engineer's look. Photograph each crack with a ruler in shot and the date, then check it again after a long dry spell and after heavy rain. Movement that opens and closes with wet and dry weather points to reactive soil.

It could be either, or both. Dubbo Regional Council's 2017 'Salinity in Your Backyard' brochure shows homes and offices with salt damage in South and Central Dubbo, including a groundwater tide mark on a South Dubbo wall. It also reports salinity in Firgrove Estate, Delroy Park, the CBD and villages such as Ballimore and Wongarbon. Brick, mortar or render that flakes, crumbles or grows a white crust low on the wall points to salt. Diagonal or stepped cracks higher up, sticking doors and sloping floors point to movement. Tell the engineer what you are seeing, because the repair for each is different.

For the urban area, yes. Dubbo Regional Council's Urban Salinity Monitoring Network, set up in 2004, has 129 groundwater bores across the urban area, including the Firgrove and Richmond estates. Groundwater level and conductivity, a measure of how salty the water is, are read every two months. The data is available at cost from Council's Sustainability Coordinator, and details are on Council's land salinity page. Local readings are a better starting point for pier depth and concrete choice than a guess. On the Daisy Hill lots listed in clause 7.19 of the Dubbo Regional LEP 2022, salinity must also be considered before development consent is granted.

Resin injection pumps expanding polyurethane resin under the footing to fill voids and lift it back towards level. It is faster, less invasive and cheaper, and suits moderate movement. Concrete underpinning means excavating below the existing footing in stages and pouring new, deeper piers or a beam underneath, the heavier fix for severe movement or where the load needs to reach deeper ground. Either way, the work is usually done from outside or under the house, so moving out is rarely needed. Where new concrete goes into salty ground, ask what strength and reinforcement cover it is designed for, as AS 2870 sets both for aggressive soils.

Underpinning fixes the footing, not the water that moved it, so the cause needs attention as well. Bureau of Meteorology averages for Dubbo Airport show about 590 mm of rain a year, with 1 mm or more on about 58 days, so the water you add yourself counts. Council's 'Salinity in Your Backyard' brochure notes that over-watering raises groundwater and brings salt to the surface. It advises fixing leaking taps, gutters and downpipes, keeping gardens away from buildings so moisture doesn't bypass the damp-proof course, and connecting stormwater to the street gutter rather than rubble drains. Ask the contractor about drainage as part of the same job.