Incomat^® Crib as erosion protection for spillway

The Gailtal valley is a region in Austria that has been affected by flooding several times in recent years - most recently in October 2018 with massive damage and devastation. The flooding was caused by a combination of heavy rainfall and dam failures. The floods caused dams to breach or overflow uncontrollably, causing water and material such as mud, timber and rocks to cause severe damage throughout the Gailtal valley. In order to prevent future flood damage, the existing dyke and the banks of the river were reinforced and secured by raising the dams. In order to control the overflowing of the Gail in the event of flooding, an overflow section was planned and constructed near Rattendorf, whereby it was necessary to protect the reinforced dyke against erosion and hydraulic effects during flood events.

To prevent damage that could jeopardise the stability of the dyke, it was necessary to protect the overflow channel with an erosion-resistant surface layer. The chosen system was Incomat^® Crib 10.200, which is characterised by its ease of installation and long service life. Incomat^® Crib consists of two layers of woven geotextiles (PA/PE) that are connected in a grid pattern. After filling with concrete, the system takes the form of a tubular grid with permeable intermediate areas. In order to prove the performance of the system, the Vienna University of Technology carried out physical model tests in 2010, which demonstrated the erosion resistance of overflow channels covered with Incomat Crib at flow velocities of more than 10 m/s and a specific discharge of up to 2.5 m³/(s*m).

The total length of the dyke to be covered was 150 metres with a width of approx. 24 metres, which corresponds to a total area of approx. 3,600 metres2 . Six prefabricated mats were assembled off site and then joined together on site using industrial zip fasteners to form a large contiguous area. The mats were then filled with high flow concrete with a flow diameter of ≥ 65 cm and a maximum grain size of < 8 mm. The filling of the mats began at the foot of the dyke and was continued in five stages towards the top of the dyke to reduce the forces on the Incomat^® fabric. Filling took six days and achieved a filling rate of approx. 700 m2 /day, with a total of 430 m3 of concrete being used.

Incomat^® Crib is a geotextile formwork that reduces the need for additional formwork, saving time and money. The cross-section was backfilled without joints, reducing the risk of erosion of the dyke core. The entire length of the dyke with an area of 3,600 m2 (4,300 m2 concrete mats) was backfilled within six days working eight hours per day, which reduced both the cost of hiring equipment and the working time of the personnel on the construction site. The highly permeable filter points allow soil moisture to penetrate into the surface layer allowing vegetation to grow. After filling the Incomat^® mats with concrete, they were covered with ≥ 60cm of soil and planted with vegetation to blend in with the surroundings. The natural look was requested by the client and blends aesthetically into the landscape.