Unfavourable soil conditions cause of stopbank failure, report says

Unfavourable soil conditions are probably what caused the Rangitaiki River stopbank to fail during July’s floods, an independent report concludes.

However geotechnical engineer Dr Marianne O’Halloran, who wrote the report, notes a range of other factors that may have speeded up the process. “It is difficult to imagine more unfavourable conditions than those that occurred,” she says.

Dr O’Halloran considers the stopbank breached when water seeping under it broke through to the surface in the adjacent paddock. When that happened, the water flow increased and began to wash soil out with it. As a result, “pipes” formed under the stopbank, making it vulnerable to collapse.

Dr O’Halloran says the type and layering of the soil in the paddock were key to the stopbank failure. The land at the breach has a subsurface layer of pumice sand through which water is able to flow quite easily. On top of this is a less permeable layer of silty soil, which was found to be quite thin (as little as 400mm deep) in places.

The breach actually started in the paddock well out from the base of the stopbank, with water probably breaking through at a thin spot. “It appears the surface soil layers were heaved up by water pressure, allowing water and soil to escape,” she says. The pressure of water flowing from underneath meant the hole worked rapidly back towards the stopbank, causing the collapse.

Dr O’Halloran says, when the stopbank was topped up after the Edgecumbe earthquake in 1987, the designers “appear to have been unaware of the existence and significance” of the thin areas in the upper soil layers. However, even if investigations had been done, “the weakness in the stopbank system may not have been identified” because the areas could fairly easily have been missed by spot testing.

Dr O’Halloran says that, given the magnitude of the flood, the soil conditions would have been enough to cause stopbank failure on their own. However, other conditions may have accelerated the failure.

They include: The saturation of the ground by the high rainfall The fall of the ground away from the landward toe of the stopbank towards Hydro Rd. The taking of soil from the river berm when originally building the stopbank. The landowner recently tilling and then re-grassing the soil in the paddock. Good root structure may have given slightly more strength to the surface soils. The row of fence posts along the stopbank toe possibly penetrating to the high permeability soils

Environment Bay of Plenty chief executive Jeff Jones welcomed the report, saying the council felt it was necessary to have an independent investigation into such an important matter. He says council staff will now be working with Dr O’Halloran to identify the best steps to take next.

Mr Jones says the stopbank area that breached in July had shown no evidence of problems during the major flooding of 1998. Since that event, the council has carried out geotechnical tests on, and upgraded, several sections of stopbanks identified as vulnerable. These held up well during the July event, confirming the appropriateness of that work, he says.

A geotechnical investigation of the stopbanks upstream of the Transpower/Horizons substation, including the area where the breach occurred, was on the schedule for this year as part of a routine check. However, Mr Jones points out, this testing may still have missed finding the thin patches of soil. While Environment Bay of Plenty is very disappointed the stopbank failed, Mr Jones says it is good to know the reason was not related to poor construction of the stopbank itself. The report shows there was no evidence of erosion, cracking, seepage paths or poorly compacted layers within the stopbank.

The Flood A breach occurred in the Rangitaiki River stopbank just upstream of Edgecumbe on the morning of Sunday 18 July. The flood was the biggest experienced since 1944 and certainly since the stopbank was built. The large volume of water that flowed through the breach scoured out a hole nearly five metres deep. The hole extended inland about 150m from the stopbank. It caused serious flooding of farmland, the railway, roads and buildings downstream.

The Soils Stopbanks work best when they are built on (and from) low permeability soils that don’t let large volumes of water seep through them. Unfortunately most of the Rangitaiki Plains soils have a moderate to high permeability. However, many areas have a surface layer of silt, which restricts water flow. Dr O’Halloran says this is often the only reason there are not excessive flows under the stopbanks. “The rapid variation in soil types and layering makes it very difficult to design a stopbank system which can take into account all possible foundation conditions,” she explains.

The Edgecumbe Earthquake Dr O’Halloran does not think the Edgecumbe earthquake had a direct influence on the breach. However, because the stopbank was topped up afterwards to make up for land subsidence, it may have had an indirect influence, she says. When the stopbank breached, the water level was half a metre above what would have been the top of the older stopbank.

Matahina Dam The stopbank would have failed whether or not modifications were made to the river rate at the Matahina Dam, the report states.

The Report Writer This independent report on the Rangitaiki River stopbank breach was made by Dr Marianne O’Halloran of Ice Geo and Civil (Tauranga), a division of Ice Construction Ltd. It was reviewed by D. Dennison, principal geotechnical engineer for Opus International Consultants Ltd.

Dr O’Halloran is a widely respected geo-technical engineer with 21 years history in the industry. She has a Bachelor of Engineering, a PhD in Geotechnical Engineering and a post-graduate Diploma of Business Administration. She is a member of the New Zealand Institute of Professional Engineers and a Chartered Professional Engineer.

The Report: The report and other information is available on the Environment Bay of Plenty website at www.envbop.govt.nz under “flood update”. People can also view copies at the council’s office in Whakatane.

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