The Waikato River is the longest river in New Zealand. Winding its way north from Taupo to Port Waikato, its powerful flow forges a path through canyons and farmlands. At Tahorakuri, just south of Golden Springs, where the Waimahana Marae rests upon the Waikato River’s southern shores, the Mihi Bridge straddles the river where it intersects State Highway 5. Built in 1956, the bridge abutments were most recently protected by a gabion structure, but high river flows had washed some of the gabions out, so more strengthening on the upper side of the river was required.
Due to the strength of the river flow and uneven surface of the bank beneath the surface, Waka Kotahi required a solution that was not only easy and safe for contractors to place but would conform to the contours of the sub-surface slope without the need for excavation work.
DESIGN
Submerged 3m below the surface and rising 3m above, 140 2t Cirtex Rock Bags were evenly distributed along the banks of the river to dissipate water flow and further protect the abutments. Flow velocities would be absorbed within the rock- filled bags, while the UV resistant polyester nets keep the rocks in place. Once settled on the riverbed, each bag locks in place as it rests upon the bag below, forming a natural structure.
Above the surface, the Cirtex Rock Bags were placed over where the water flows past the bank, giving further protection against scour and erosion of the soil slope surrounding the abutments. Vegetation can also grow within the rocks, blending and binding the structure into the natural surrounds.
INSTALLATION
Cirtex Rock Bags are easily filled using a steel frame, which can be supplied by Cirtex. Contractors were able to fill all 140 bags and store them onsite before carefully placing them one by one into the river. By looping the strap through the Cirtex Rock Bag connection and securing both ends of the strap to the crane, crew were able to safely direct the positioning of the Rock Bags from the bank, with the crane driver lowering the strap once each Rock Bag was placed so it could easily be disconnected, ready to be secured to the next bag.
In less than two weeks, all 140 2t bags were filled and placed within the Waikato River and along the bank. Now, when larger velocities flow, water will be dispersed around the abutments and embankments, further extending the life of Mahi Bridge and securing access through the region.
The Cirtex Rock Bags provided the following benefits to the project:
- Easy to fill and place upon any surface shape
- Dissipates water flow, preventing scour and erosion
- Can harbour subsurface ecological or above ground vegetated environments
