| 57 CONTRACTING HEB Construction Waikato Rachel Graham Light footprint to stormwater work A riprap system was used at the Ulster Street Outfall in Edgecumbe Park, Hamilton, to protect slopes from scour and erosion. Lance Magee 021 998 503 Mark Magee 021 277 5479 Office 07 826 3044 office@proearth.co.nz Proud to be part of the HEB solution Johan Hakkenbroek’s greatest hope for his Ulster Street Outfall project is that in years to come people won’t even notice the work that has gone into it. The project’s aim is to slow the water flow through a stormwater culvert which was progressively undermining nearby trees in Edgecumbe Park, Hamilton. Now Johan Hakkenbroek, HEB Construction Waikato’s contract manager, hopes what people will see is a natural looking creek which winds the water through the park even when it is flowing high. Johan says after a collaborative discussion between HEB, the engineers at BBO, the client – Hamilton City Council, and the contractor, an innovative solution was agreed on. The solution proposed by Eugene Vodjansky, of BBO was a riprap. Something frequently used in the waterways of the United States and Canada to counter high speed flows, however seldom used in New Zealand on this scale. A riprap uses rocky material to protect shorelines, or steep slopes from scour and erosion. In this case around 80 four to six tonne rocks, in conjunction with a soil riprap mix which met rigorous specification was used to create a sealing basin. This, along with downstream weirs, dampen the water flows and stop the erosion. The four to six tonne rocks needed to meet the exact specifications necessary to withstand the velocity of the water. The large rocks were individually sourced from a number of quarries by HEB and required an incredible level of attention to detail. They all had to be a minimum 1200 diameter in any direction/cross-section and of a specific hardness/wear. Johan says the placement of the rocks and compacting using the soil mix was almost a black art, and required constant communication and frequent site visits by the designer Eugene Vodjansky. “It’s not like a normal bricks and mortar type of job, it is about how you place the rocks and how that changes the flow of the water, which in turn limits the effect of the flow and the erosion on the banks. “ “There is quite an art to it to getting the mix just right,” says Johan. “That is why we had the engineer out to the site on an almost daily basis. “The mix changed for different locations, and the ratio from big rocks to small rocks might change, and all of that is to do with managing the flow and speed of the water.” “It’s not like a normal bricks and mortar type of job, it is about how you place the rocks and how that changes the flow of the water, which in turn limits the effect of the flow and the erosion on the banks. There is quite an art to it to getting the mix just right.” The location presented another challenge says Johan, with the elevation from the laydown area to the creek covering a vertical drop of approximately 50 metres. “We had to use a little truck, a Morooka dump truck, to bring in each of these massive rocks. Each one had to go down individually down a track through the redwoods. “ He says they also endeavoured to create an environment which still allows eels to thrive and for the fish to still migrate upstream. The project was recognised in the Waikato Civil Contractors New Zealand Awards winning the award for projects under $1 million dollars. Johan says the project was satisfying to work on because rather than looking at the cheapest or least risky solution, they went for the solution which provided the best answer and best use of rate-payers money. He says the riprap technique has real benefits which can be used elsewhere. Johan says he is particularly pleased that the project doesn’t take away any of the natural beauty of the park, while solving the problem. “It does the job 100 percent, it looks natural and secures the future of the trees. For the stakeholder, it is money well spent. “In a couple of years, once it is all established you probably won’t even know the work has gone in there.”
RkJQdWJsaXNoZXIy NDc2Mzg=