Last year we had an extended holiday. We wanted to observe the solar eclipse, but we took the opportunity to sail and drive across the United States. Sure, the eclipse was certainly one of the most impressive events. But the above picture was haunting my mind throughout my vacation. Is it familiar to anyone? Is there anyone an avid follower of ‘Who is the Mole’? Yes, these are the Painted Hills. In season 17 part 8, this is where Sanne Wallis de Vries mixed up the alphabet and Diederik Jekel had to leave the show. As our plans were already to visit the state of Oregon, we decided to see the location. Most striking are the coloured bands of sediment. Each colour represents a different geological age. After erosion, the layers became exposed. Usually, these sediment layers are covered and submerged, invisible to the dredge contractor or operator.
However, these layers can be made visible by performing a soil investigation. The bottom is probed with a Cone Penetration Test or CPT. It measures two parameters: the undrained shear strength and the cohesion. The shear strength can be measured by pushing a cone through the soil. The cohesion by pulling a sleeve through the hole the cone just created. This way the parameters are uncoupled. Imagine being able to identify the resistance of a hull separated in form friction and skin friction!
Sometimes also other measurements can be performed on the same sensor: water pressure, acidity, conductivity, light reflection; all attributing to a better understanding of what is beneath the surface.
The values for shear strength, cohesion and optional other parameters can be plotted in the same diagram against the penetration depth. Correlating the parameters can reveal what type of layers are there. Very easily clay and sand can be identified. Also gravel layers are recognised. Stones and debris are sometimes hidden as soundings that were aborted. Keep an eye out for signs like these, as they have a major impact on your project. The more soundings available, the better the project estimation. There is a PIANC guideline for the number of soundings for the area1. Anything less is meaningless.
A soil investigation report comprises a number of these diagrams and possibly also core samples for establishing a particle size diagram. If the client attaches such a thorough soil investigation report, we can work on it to select the best dredging equipment for the project and advise on an optimal working method.
The data of the Cone Penetration Tests can be used also as input for survey programs like Navguard. e.g. If a certain layer has to be removed, the depth of that layer can be presented to the operator as the design depth. So, NavGuard can be an excellent tool to dredge exactly what is needed, without extra work and costs. Certainly an option that will pay back quickly.