Oh the horror! An old salt like me had to go camping during our summer holiday. Our daughter had her birthday during our road trip in the USA and she wanted to celebrate it by camping in the woods. Complying to her wishes we pitched a tent and roasted marshmallows. Meanwhile my mind was frantically searching for familiar clues to connect to my maritime heritage. Hammering down the tent pegs, it dawned to me: putting up a tent is basically a simple Standard Penetration Test.
Standard Penetration Test is one of the easiest soil investigations you could do1. All you need is a pipe and a hammer. You count the number of blows to hammer the pipe down and you have an indication of the effort it takes to cut the soil. This method completely ignores sophisticated parameters as e.g. undrained shear strength, porosity or internal friction angle. It is very crude in its results. On the other hand, the basic principle of driving the pipe into the ground is very similar to the cutting action of the pick points on a cutter. As such, it is a very good indicator for the performance of a cutter head. This is also the reason, why for initial discussions about the performance of a CSD, the SPT is a good starting point to ask the client. He might have a report like this already available, or he can easily perform the tests. Also ‘Sandy’ accepts SPT values for an indication of the soil quality2.
Be aware, that SPT’s are often not very deep. Of course, a full soil investigation report with a Cone Penetration Test is much more valuable. We can always translate the results from a CPT report into a SPT value. But the SPT information is not covering all the parameters to translate this to a CPT. Sometimes even an SPT report fails. And then it might be useful to discuss with the client on a qualitative level about the soil condition. Usually people have actually touched the soil, or at least can paint a mental picture of the soil conditions and these criteria might help to use the same descriptive language.
Hammering a rod into the soil is a relative cheap and quick method to collect the soil consistency. It can be performed everywhere, anytime, under most conditions. That is why it was also selected for the soil investigation on one of the most remote locations imaginable. Although it is still on my wish list of dream destinations, the prohibitive price tag of the ticket will prevent it for me to pitch up my tent over there. I just have to revel in the camping adventures of Neil and Buzz.
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.
Happy New Year everybody! I wish you all: health and happiness.
Last year’s new year’s resolution, was to start a blog and share with you my thoughts and experiences in dredging. As you are reading this, I succeeded so far. I really hope you enjoy these little stories on my professional interests and by now have already bookmarked my site to your favourites.
Last year we had the CEDA Dredging Days1 as a starter and continuing story arc for a couple of blog posts. Now I will probably only update you on the progress of the Dredging Management Commission, I am participating in2. Any other current events that will happen will also be covered as they come along. One big project worth covering will be the ¡VAMOS! project3. It will have its final tests and the total conclusion of the project. So, there will be some reports on that. We have several students working on interesting projects that will be highlighted on a case by case basis.
A real cornucopia of dredging related topics is our own Damen Dredging Experience4. Here in Nijkerk, we have a small museum with exhibits that explain the processes of dredging in a very tangible way. They are very suitable to fill in the gaps between the other stories.
I think, that with all the ideas I already have on the shelf, the ‘Chapters page’ of this website will nicely fill out and start to become the reference page I intended it to be.
The first stories that are on the shelf, are about my last year’s holiday. In our Christmas break, besides reading other author’s books (Did you also?), we were compiling our own picture book about our summer holiday in the United States. Sorry, I am a passionate dredger and even on my holiday I had some observations and experiences about dredging that I want to share with you also.