Ground insights reduce uncertainty on former sugar beet site

Fast-track subsoil insights for future-proof urban planning

A factory in Groningen used to process sugar beet into crystalised sugar. From 1913 until the factory’s closure in 2008, freight lorries would offload their cargo of sugar beet on a daily basis. The sugar beets would then be washed to remove the attached ‘tarra’ soil, which then flowed into ‘flow’ fields – basins where the sediment could settle.

The history of this industrial process has given rise to a range of challenges and risks, which the municipality wants to map out before the construction of a large residential development, ‘De Suikerzijde’, begins.

The municipality required accurate information about:

• the subsoil (composition, settlement behaviour and strength)

• a water-management solution

• the minimisation of soil transportation, along with the associated CO2 emissions.

Industrial history

De Suikerzijde is a new district in the city of Groningen. It will include a large residential development located on the former sugar beet factory’s drainage fields.

Over the years there has been considerable disturbance to the soil on site: entire drainage fields have been filled, re-excavated and then replenished. Some are still full of water.

Given the site’s long industrial history, the municipality of Groningen had concerns about:

• how the subsoil would react to the new load

• the risk of settlement in the longer term

• the behaviour of rainwater collected in water drainage and infiltration solutions (wadis).

Project requirements

To reduce uncertainty, in 2020 the municipality contracted our multi-disciplinary team to conduct a thorough investigation of the subsoil.

The first homes are due to be built in the north-east corner of the site – the drainage fields in this area were filled in around 15 years ago. The municipality wanted the ‘tarra’ soil to remain in place and to minimise soil transportation to the site.

Site investigations

Our comprehensive investigations included traditional soil drilling and cone penetration tests on site, supplemented by geophysical surveys and laboratory testing.

We constructed two artificial test embankments, fitted with settlement measurement sensors. The test embankments enabled us to monitor continuously (and in real time) how the subsoil behaves when pressure is applied. One of the embankments had vertical drainage, which enabled excess groundwater to be drained more quickly; the other did not.

Our infiltration tests at the wadis confirmed the low permeability of the ‘tarra’ soil, owing to its high clay content.

We visualised the results from the test embankments and the infiltration tests in real time using VirGeo®, our online portal. This gave us more accurate insights into the expected settlement behaviour, in terms of its size and progression over time; and into the effect of heavy rainfall on the wadis.

Accurate insights with 3D ground model

We input all our research data, along with a height map of the terrain, into our advanced, user-friendly 3D ground model. This gave us accurate insights into local variations and a very detailed view of the subsoil.

Our 3D ground model can easily be converted into a building information modelling or geographic information system model for future use by the contractor.

Turning soil insights into sustainable impact

Our findings confirmed that the 'tarra’ soil had low permeability to water, so the wadis risked overflowing after heavy rainfall. To mitigate this risk, we recommended the municipality to construct drainage channels from the wadis to an adjacent canal and to continue monitoring the groundwater in real time.

By combining our research findings, including those from the test embankments, we were also able to provide an accurate settlement forecast. We used this to calculate the exact quantity of sand that would need to be transported to site, to prepare the area for construction.

Our calculations resulted in a significant reduction in soil transportation requirements and related CO2 emissions; 931 tonnes which is equivalent to a CO2 emission saving of 15 transatlantic flights.