Cone Penetration Testing (CPT) is a Dutch geotechnical instrument developed the better part of a century ago. Although it is conceptually simple – one pushes a steel cone into soft geology to determine its properties – it has come a long way in the ensuing decades and remains a critical tool of geotechnical engineers.

In recent years the offshore sector has ridden the growing wave of renewables development. Wherever there is open water, there is wind, and so sprawling farms of turbines have been sited just beyond the world’s coastlines. This has in turn led to new demands being placed upon the geotechnical professionals of the world, and old technologies have been adapted to new working environments.

The cone penetrometer is still that same critical tool, albeit more accurate than ever, but it is one that is being used in new ways entirely. To understand this, we need to take a step back and understand a technology that has been a critical part of the ground investigation toolkit for all of modern construction history.

The humble cone

Underground construction was once described as “an art for humble men” owing to the difficult nature of the ground. It is apt that the key ground sensor is so simple, at least at first glance. The CPT is carried out with an instrumented cone that spans about 40mm in diameter and is pushed at a set rate through softer geologies to a depth of metres, depending on site requirements. The basic cone measures resistance and friction, and provides insight in geotechnical parameters to determine, for example, the kind of foundations a structure might need.

The tip of the cone is where the work is done. Moving through the ground at 20mm per second, it measures the resistance of the soil, above that is the sleeve which measures the friction. Between the two some cones also accommodate a ‘pore pressure ring’, which gives the user information on hydrostatic water pressure. 

Gerry Sinjorgo, manager of Fugro’s transducer workshop in Nootdorp adds, “In the old days there was no inclinometer on the cones, and there is a story that on one project the operator pushed the cone down, only to have it emerge in a neighbour’s backyard. The owner screamed as the cone pushed against the window of her house.”

Today, in the centre of the cones, there is an inclinometer, which measures the inclination of the cone and so avoids unfortunate encounters with the local community. Sinjorgo has seen this and numerous other advances in his 37 years at Fugro.

Initially conscripted into the Dutch Royal Navy before working offshore, Sinjorgo spent most of his career in the workshop and is now head of the department. He leads on cone production, cone maintenance, cone calibration, and research and development into new types of cones. When he started nearly 40 years ago, they were fabricating maybe 100 cones per year, now it’s more like 400. Along with the servicing and calibration of the thousands of cones in existence. Which now include seismic sensors, electrical conductivity sensors, magnetometers and other bespoke sensing tools.

Sinjorgo adds, “To begin with it was a much simpler device, it started off back in the 60s with a mechanical cone, which was pretty much the only thing they had at that time. Next came the electrical cone which was also invented by a Dutch institute.

“Then Fugro was started with Kornelis Joustra. And he was really interested in the electric cone. But because it was a government institute, it took too long, so he was getting a little bit impatient. And that’s how it all sort of started. And then in around 2000/2002, we had the first digital amplifier built into a cone. And from then on, it’s taken pretty much a flight [sic.] as far as your data acquisition and your accuracy of the cone.”

A high-definition signal coming out of the cone improved the processing and data determination greatly. The more accurate the measurement, the more easily it can be correlated with other measurements – soil can be messy, difficult stuff to image. Industry standards are also always improving, and compliance with ISO 22476:2023 Geotechnical Investigation and Testing requires the highest measurement standards. The definition with a modern cone is 1kPa, or 100 times more accurate than early cones. This is particularly impressive considering the device must measure compressive strength ranging across five orders of magnitude, from a few kilopascals to hundreds of megapascals. All with the same accuracy.

Surf to turf

The investment in the offshore sector over the past two decades has seen developments in ground sensing technology. It is not hard to understand why; the seabed is an environment difficult and dangerous to access. The logistical effort and risks of manual testing are prohibitive, and so alternatives have emerged.

David Tindall is the global product owner for geotechnical equipment within the land team at Fugro. He is responsible for transferring technology from sea to shore. Tindall, who started as a CPT operator, adds, “I felt like a lot of the technology we were using is quite old, and has been in place since the nineties, if not earlier.”

The actual data and collection of it is not Tindall’s primary concern. It’s the handling and logistics of handling the cones that he is interested in. The first boon of this 20-years’ technological boost for land is a new CPT rig called Deep Drive.

“It’s a tracked machine, picture an excavator, but rather than having a cabin or a bucket, it has a continuous drive system and a coil. The continuous drive system pushes the coil into the ground. That’s where you’re collecting your CPT data.”

The 22-tonne rig can push harder with the CPT than usual – thankfully Sinjorgo and his colleagues are already developing more robust cones as a priority for the offshore sector – and the coil system means the operator isn’t having to manually add 1m rods on to the back of the cone system. A coil automates it all, controlled from a computer. This is much safer for the operator.

“CPT operators have had repetitive strain injuries. So, wrist injuries, elbow injuries, through years of screwing rods on and off. This completely eliminates that risk. The system allows for performance improvements too.

“We have a mud flush system on the deep drive, so we can pump lubricating fluid behind the cone. And that reduces the amount of friction on the rod on the coil, which allows us to push deeper, how much deeper depends on the ground conditions, obviously. In some comparisons we’ve done it’s been up to 30% deeper.”

Turning the tide

The land-based geotechnical sector is not purely dependent on its marine counterpart to provide technological developments. In the Netherlands, the originator of the CPT, the nation’s famous dikes and levees were threatened by a combination of droughts and flooding. Seemingly contradictory challenges brought about by climate change.

“Now we get very extreme weather events: prolonged periods of drought, and very intense rainfall events,” says Barbara Snacken, Senior Hydrology Consultant at Fugro. And about 10 years ago she was looking at the dry summers and wet winters that were causing problems in the Netherlands.

“Basically, there is enough water, but it falls at the wrong times. So, what you want to do, when that water falls, is to store it locally as much as possible,” says Snacken.

The local geology is helpful in storing this water. The Netherlands has a pancake geological structure of sand layers separated by impermeable clay layers. The water can be stored in aquifers in the sand. But these are complicated – not homogeneous – anisotropic filtration properties mean that water might infiltrate faster in one direction compared to another. The more permeable locations, layers and vectors are desirable.

“About 10-12 years ago I was looking for a new tool that would allow me to get more information on variations in permeability over depth.”

Research in Germany led to the hydraulic profiling tool (HPT), an injection flow logging tool. It measures the pressure required to inject a set flow of water into soil as the probe increases in depth. This was taken back to the Netherlands and in combination with a device to detect subsurface soil contaminants called a membrane interface probe (MIP), was used to develop a new tool called AquiSense.

“AquiSense is a ground penetration tool. And it’s based on the CPT technique, with added sensors. So it’s a standard CPT cone with, additionally, three piezo rings, or piezo sensors. On top of that, you have electric conductivity elements, which measure the electric conductivity of your ground and groundwater together. And on top of that, you have an HPT screen, which gives you an idea of the permeability of your subsurface.”

AquiSense is being used for one critical failure mechanism in dikes. A mechanism that, in previous years had, in an ignorance that might be darkly amusing to modern readers, been blamed on moles. This was due to the mole tunnel-shaped sand craters that were found after dike failures. 

We now know this to be the “Backward Erosion Piping Effect”. It occurs when there is a large differential in water height on the two sides of a dike. The difference in head can cause a little pipe to develop on the side with the lower load. This can work its way towards the opposite side of the dike, until a critical point is reached, and the dike fails. 

AquiSense allows for the characterisation of aquifers more accurately, keeps project costs and carbon impacts down, and quantifies risks.

The more efficiently and cost-effectively water management projects and dike improvements can be implemented in the Netherlands, the better. The environmental challenges and project cost restrictions escalate over time.

The ground itself never changes, but it always presents an unknown of varying degrees to construction projects. As technologies advance, risks can be contained, and uncertainties reduced.