Brian Anderson The Western Front www.sunlive.co.nz

Recent reports of liquefaction problems in the Bay have worried some of the readers.

Many mistakenly believe that liquefaction is a consequence of an earthquake but liquefaction will occur when any seemingly solid, sandy soil reaches a certain level of water saturation. The soil turns to slurry which can pour or slump down a hillside or collapse the concrete foundations of a house. An earthquake, a lightning strike, thunder or even a vehicle backfiring can prematurely trigger the swift change from solid to liquid – but the basic cause is poor drainage and storm water control. It is not the water in the drains that is the problem – it is the water in the soil. Every impermeable surface, such as a roof, a driveway or a road, collects water – and unless it is dispersed correctly in approved manner from down pipes, drains and storm water culverts the risk of liquefaction increases dramatically.

Some local liquefaction examples have been caused by improperly fitted downpipes and soak holes. But others are in cheap residential developments on reclaimed estuaries. Houses are being written off because their concrete bases are tilting, and in some cases, splitting in two. Early developers believed that our estuarine soils were only a few metres deep but recent investigations have identified the estuarine soils can be up to 200 metre deep. Flooding does not cause the problem. More than 60 per cent of water from the Kaimai Ranges is travelling through soils.

We were told recently that to protect our versatile soils, flood-prone land was a legitimate site for housing. I have already challenged successfully the development of 40 houses on the McKinney Stream estuary just North of Katikati. The developer was planning to re-site 40 old houses from Waihi on to the estuary as an affordable housing project with absolutely no planning for storm water or flood control. At that time, I was able to show that the council had no rules or guidelines for building houses in flood-prone areas. It is great to see that in a recent committee report the council has at last acknowledged the absence of any policy on the building of houses on flood plains. When a policy is written they might be able to start writing guidelines and regulations that make sense.

Property owners with piled houses should not have to worry over liquefaction. Their houses can be re-piled. Houses built on concrete slabs are the most at risk. The best advice, for all home owners in the Bay, is to check all drains in your area, ensure gully traps are kept clear and all of the downpipes from your roof have been correctly connected to the storm water system. If in doubt call an expert. One person I know was worried and called in an expert. With the correct advice he was able to fix his problem. His neighbour saw this as a warning and called in the expert only to find the house was beyond repair and had to be demolished. Don't wait for the next earthquake. You can't do much to protect your property then but you can identify and fix any liquefaction risks now.