CPT at Muttons and Herringfleet Mills

Published: 25th September, 2020
CPT truck at Mutton’s Mill, Halvergate

We’ve been out a couple of times recently at the end of this summer to have a look at what lies beneath the marshes. A few of the iconic Broads drainage mills that we are restoring, as part of our Land of the Windmills project, have begun to lean over the past few decades. It’s not really that surprising as these large structures are built on top of soft peat laid down long ago. Our plan is to right the mills by driving piles deep into the ground, past the peat to give them a better foundation on which to stand.

A smaller CPT tracked vehicle at Herringfleet

In August, we conducted our first CPT (Cone Penetration Testing) of the ground below Mutton’s mill and in September we probed the ground below Herringfleet mill. CPT allowed us to quickly determine the condition of the soils where these mills sit in the least disruptive way possible. These tests tell us a lot of what we already knew from decades of borehole samples, that this peat is deep! We probed down to 15-16 metres before we hit any of the ancient sands that were laid down thousands of years ago when the marshes were a marine environment.

Live digital read out showing the friction and pore pressure of the probe near 15m depth

The CPT contractor employed a unique vehicle that is part truck and part hydraulic ram. This special bit of kit is able to drive the probe, which sits at the end of a long metal shaft, down to significant depths. In fact, at Herringfleet we were able to probe down to 25 metres (more than 80 feet!)  It was really cool to watch the readouts as the probe was driven deeper and deeper below the marshes. We were able to see just how wet the peat soils really are (it is a “wetland” after all). At the end of the test, the shaft and probe are pulled back up and once the truck moves away, all that is left is a small hole with a diameter of only a few centimetres. Over the next few days, the soil expands and fills in the hole, hiding the fact that anything happened at all.

If you’re interested in learning more about the deep landscape of the Broads, check out our Section on Broads Geology written by Geologist Tim Holt-Wilson or have a look at the British Geological Survey’s Map showing the Broads area.

Remains of a probe reaching a depth of 15m at Mutton’s Mill.