Reinforcement and Heightening Drontermeerdijk
Important Information
Project Name: Reinforcement and Heightening Drontermeerdijk
Date: Project start December 2019
Client: Boskalis Nederland
Location: N306 from Roggebotlock/N307 to Reevelock, municipality of Dronten
Scope: Heightening the flood defence by approximately 2.7 meters over a 3 km section.
Project description
Between December 2019 and March 2021, Boskalis Nederland, on behalf of Waterschap Zuiderzeeland, reinforced and heightened the excisting flood defence along the Drontermeer in Flevoland.
As part of the Dutch Ruimte voor de rivier program, the Reevediep was realized as the final project of the IJsseldelta program, south of city of Kampen. This was one of the measures taken by Rijkswaterstaat and the provinces of Overijssel and Flevoland to to ensure a sufficiently high level of safety against flooding in the Kampen-Zwolle region for the medium-term. The Reevediep acts as a bypass for the river IJssel, which flows through Kampen, during high water levels. Through the Drontermeer, water from the IJssel can be safely and swiftly discharged into the Ketelmeer/IJsselmeer. To ensure the discharge could take place unhindered, it was necessary to move the Roggebotlock in the N307, as part of the primary water barrier, approximately 3 km southward, just beyond the point where the Reevediep flows into the Drontermeer. The new Reevelock was constructed at this location. Consequently, the flood defence from Roggebotlock/N307 to the new Reevelock had to be heightened and reinforced.
The reinforcement and heightening of the existing flood defence along the Drontermeer took place from Roggebotlock/N307 over a distance of approximately 3 km in the direction of Elburg. The heightening of the flood defence was around 2.7 meters.
"By understanding each other's challenges, interests, and risks, we were able to work together towards an optimal solution. In particular, Section 1, the connection to the N307, was a focus area. This involved a somewhat complex phasing with spatial limitations and a tight schedule. By collaborating on this (considering all variables such as preload height & area, construction time, rest time, measurement regime, and ongoing assessment and adjustments), we were able to work pleasantly together and achieve an optimal result," according Rogier Scholten, Boskalis.
SOCOTEC Geotechnics, formerly Inpijn Blokpoel engineers, provided the geotechnical site engineering for the project on behalf of Boskalis Nederland from December 2019 to October 2021.
Project Activities description
SOCOTEC Geotechnics, early in the realization phase, reviewed the available geotechnical data to optimize the geotechnical (baseline) parameters for the analyses to be performed during the execution. A monitoring plan was then drawn up for the geotechnical site engineering to manage the identified risks posed by the heightening works for the project. The monitoring plan had the following main objectives:
- Ensure the safe construction of the heightening works (stability during construction).
- Achieve the desired residual settlement to meet the related requirements set by the client.
During the contruction phase, the behavior of the subsoil was monitored by installing pore pressure meters and settlement plates. The pore pressure meters were automatically measured, and this data was made available online. The settlement plates were measured by Boskalis Nederland's Survey department with a Total Station according to the measurement schedule included in the monitoring plan.
With the measured pore pressures, each time a new heightening layer was to be realized, it was verified whether this was acceptable from the perspective of slope stability. When this was the case, this was documented in a release document for acceptance by the client, Waterschap Zuiderzeeland.
Using a residual settlement prediction, the timing was determined for when:
- The clay, used as a preload, on the crest of the flood defence could be removed, and the clay could be used as a protective cover layer on the outer and inner slopes of the flood defence.
- The final profile (finalizing the height of the sand core and applying the protective cover clay layer on the slopes) could be realized.
- The foundation and pavement of the N306 on the crest of the flood defence could be constructed.
Criteria related to the final height at completion and sufficiently minimal residual settlement of the crest of the flood defence (safety against flooding) and minimal residual settlement of the longitudinal and cross-surface flatness of the N306 pavement (traffic safety) played a role here. These matters were documented in a release document for acceptance by the client, Waterschap Zuiderzeeland.
Ultimately, the planning and phasing depended on the outcomes of these geotechnical analyses.
The N306 Roggebotlock - Reevediep project connected to the adjacent N307 Roggebot – Kampen project on its northern section. During the contruction phase of the N306 Roggebotlock - Reevediep project, the adjacent project was still in the design phase. This necessitated adjustments to the final design and the design for the construction phase for the northern section 1 of the project, the connection with the N307, during execution of the project. Settlement and stability analyses were conducted to ensure that the embankments for the design could be realized within the available time and that the residual settlement requirements were met.
Finally, in August 2021, a compaction test was conducted at the Stobbeweg connection for the sand that had been applied. Due to circumstances, it was not possible to perform this test in the usual way using sample rings, where the results of the sample rings would be compared to the Proctor densities specified in the request specifications. Instead, a satisfactory assessment was carried out based on the available cone resistances from cone penetration tests, confirming that the sand compaction was sufficient end was therefore approved.
Due to the extensive and precise monitoring, it was possible to prepare release documents with reliable results. As a result, all release documents were accepted by the client, Waterschap Zuiderzeeland, in one go, and the project was completed within the set project requirements, safety standards, and tight schedule.
