An ROV offers the solution against underflow


For the umpteenth time, the construction pit in the Juliana Canal between Berg and Obbicht filled with water on February 23, 2023. The cause of this is that water passed under the sheet pile wall. Work was underway here to widen the canal in South Limburg. However, this project has been on hold for almost a year now. Thanks to preliminary inspections with an ROV, this recurring problem can be prevented.

For two months, work was done on widening the canal in the dry cofferdam in the Juliana Canal, until the cofferdam filled up with water, within seven minutes. All employees working at the time managed to save themselves and all their materials just in time. After research, Deltares concludes that the water from the canal side has flowed under the sheet pile wall. So there was a case of underflow.

Systematic research                                                                                                                                                                                      Deltares has conducted research with the aim of learning from this situation and preventing it in the future. They looked at measurements, video images and eyewitness accounts. Careful examination of, among other things, the monitoring well data and the drawn sheet pile walls confirms that water has passed under the sheet pile wall. After this, the wall collapsed due to the water pressure. What would be a good addition is to carry out preliminary inspections. This can be done with an ROV, which can see everything clearly underwater.

Underflow                                                                                                                                                                                                  Underflow occurs when there is a large water level difference. This can be caused when the water on one side is higher on the other side. The flow then occurs when the soil is well-drained. This happens very slowly in the beginning, but the longer the underflow continues, the faster the leaching occurs. The flow can be prevented by ensuring that there is no water level difference. At that moment the cause would be gone. If it is not possible to prevent the water pressure difference, poorly permeable material must be placed around the sheet pile wall. This restricts the flow. Is the ground around the dam made of permeable material and is there a water level difference? Then there will always be a current under the wall.

Excavators at work in a construction pit

Not the first time                                                                                                                                                                                                           This isn’t the first time something like this has happened. Something similar happened in September 2020, between Stein and Urmond. The presence of old, underground pipes was then taken into account, which meant that the sheet pile walls could not be installed at their full depth. The water then flowed under the sheet pile walls, causing the construction pit to fill up.

ROV during inspection                                                                                                                                                                                               To prevent these types of incidents, using an ROV during an inspection is a good solution. This underwater drone can investigate everything underwater before the project starts. For example, you could see in advance whether there is a passage somewhere in the sheet pile wall through which water can pass, or whether the sheet pile wall is already provided with material that does not allow water to pass through properly. An ROV can provide good insight into what is happening underwater using the 4K camera, sonar, bright lights and thickness gauge. This gives you a clear picture of whether a specific project is safe to carry out.

Watch the video below to see how we inspect a sheet pile wall:

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