What’s hiding under the Botanical Garden?

About 90 students have been probing TU Delft’s Botanical Garden last Tuesday and Thursday. Relying on a combination of techniques, they reconstruct the garden’s geological history.

Going down! Students push a hydraulic powered drill as part of their geophysics field days. (Photo: Sam Rentmeester)
Going down! Students push a hydraulic powered drill as part of their geophysics field days. (Photo: Sam Rentmeester)

"We have used CPT data in calculations, but this is the first time we see how the technique works", said Marije van Hell. She is a bachelor student of Applied Earth Sciences at the Faculty of Civil Engineering and Geo Sciences. Together with other students she has been watching graphs grow on a computer screen while a hydraulic press drove a cone with sensors deep into the ground.

CPT or Cone Penetration Test is one of the presented techniques during the geophysical field days. The method registers the resistance and the friction of a steel cone as it is forced into the ground. "At about 16 metres they will hit the sand layer from the last Ice Age", said technician Alber Hemstede who supports the students in deploying various techniques. Hemstede knows that the compact sand layer is as far as the CPT technique will go.

Hemstede shortly introduced the other techniques that are available to the ninety students from the Bachelor course Introduction to Geophysics and Remote Sensing: With a hand auger, students take samples up to 4-5 metres deep. A Ground Penetrating Radar (GPR) transmits microwaves into the soil and records the reflections. Electric Resistivity measurements with evenly spaced steel pins over more than 80 metres produce geological information up to ten metres deep. Seismic measurements of vibrations caused by hammering a beam produce yet another type of depth information.

Students will match these various sources to an 18 metres long drilled core sample. "Then they may notice that a high value in the CPT data coincides with a thin layer of sand", said associated professor Dr. Guy Drijkoningen (CEGS).

Drijkoningen, who has led the field days before, knows what his students will encounter: sand, shells, peat and clay. These layers tell the geological history of Delfland since the last Ice Age when sea levels were 110 metres below their current value. At the end of the Ice Age, rivers began to flow, sediments began to form, plants grew and were drowned by the rising sea. Rivers and tides moved to and fro over the emerging delta, washing away the soil in one place and depositing banks and gullies elsewhere. In broad terms, this geological history is what's hiding under TU's Botanical Garden.