Fugro investigates subsidence near Emmen

At the end of 2019, the municipality of Emmen, the province of Drenthe and water board Vechtstromen commissioned Fugro to investigate the causes of subsidence and its possible relationship to damage to homes near Nieuw-Amsterdam and its surroundings. At the end of 2019, the municipality of Emmen received dozens of reports about subsidence and damage to houses, barns and pavement near Nieuw-Amsterdam and Erica. There was great concern among residents about the condition of their homes and also the safety of, for example, gas pipes. Following a tender, Geo-data specialist Fugro conducted ground surveys.

Fugro collected a lot of relevant information: historical canal levels, groundwater levels, building foundation methods, soil structure and data from the TNO DINO-Loket and the InSar satellite. In addition, the genesis of the area - a former peat colony - was studied and hand drilling and probing was carried out at 10 homes. Monitoring wells were also placed to monitor groundwater levels and possible effects of oil and gas extraction and vehicle vibrations were examined.

Lomulder: 'We have identified three soil types, each with a different susceptibility to settlement, showing different subsidence rates. Here the presence or absence of peat layers at ground level and deeper peat and clay layers plays a major role. This explains the variation in damage. The damage is caused by subsidence due to the soil structure and groundwater subsidence. This land subsidence occurs naturally as the upper peat layer oxidizes - say: evaporates - during dry periods. Local groundwater withdrawals can also play a role in the subsidence of deeper peat layers. This is being investigated further by the water board.'

The oxidation of the upper peat layer mainly affects damage to gardens, road pavements and outbuildings or barns. Houses are founded on deeper layers and damage to them occurs, depending on the foundation method, mainly due to subsidence of deeper peat and clay layers due to drop in groundwater level or head. Damage worsens if houses are founded and/or weighted in different ways (e.g., by a dormer). Oil and gas extraction, earthquakes and water level management could not explain the large differences in subsidence. Surface water level management played no role in subsidence. Vibration from vehicles is not in itself a cause of damage, but can be the final push in homes and foundations that are already under pressure from peat oxidation or subsidence of deeper peat layers.

Fugro advises residents to consult a structural engineer and to have soil investigations carried out for new construction or remodeling plans. The conclusions of this study are also of great importance for other peat colonies, because the soil structure found in New Amsterdam occurs in many more locations.

Robin Lomulder, senior consultant at Fugro: 'As the Geo-data specialist, we have all the essential disciplines at our disposal to get a complete picture and make a good analysis. This unique combination of specialties allows us to work efficiently and draw on the extensive experience and know-how within our organization.'