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Recent publications

Workreport 2017-38



Assessment and Optimization of Foundation Layer Design at Large-Scale


Schatz, T., Hassan, M., Marjavaara, P.



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A total of 13 large-scale, foundation layer tests were performed over three distinct test phases. Foundation layer design was assessed and optimized with respect to a number of different parameters. These included construction material, installation density, foundation layer thickness, backfill block stack assembly and stability relative to as-built and under infiltration inflow conditions, installation and loading on a representative, excavated tunnel floor and interaction between foundation layer and bentonite backfill blocks with inflow wetting.

The Phase 1 foundation layer tests, undertaken in a surface facility, were made to evaluate and study three different candidate foundation layer materials (Milos granules, Cebogel QSE extruded pellets and roller compacted pillow-shaped MX-80 pellets). No problems regarding block stacking under dry conditions or stack stability under dry or wet conditions were encountered with any of the foundation layers regardless of material type.

The Phase 2 tests were carried out in a component test area in ONKALO to assess the performance of Cebogel QSE pellet foundation layers on drill-blasted, rock tunnel surfaces and to examine foundation layer installation and block stacking during infiltration inflow. No difficulties were encountered with respect to either foundation layer installation or block/ballast stacking on the rough tunnel surface under dry or wet conditions.

In Phase 3 of the project, tests were performed in a surface facility on Cebogel QSE pellet foundations layers with backfill block loads including one to two layers of actual bentonite blocks in positions of closest proximity to the foundation layers. The Phase 3 tests also examined the feasibility of using thinner foundation layers as well as system performance against higher infiltration inflow rates. In these tests erosion of bentonite blocks was observed, but the block stacks remained satisfactorily stable over the inflow durations. Notably, this stability was observed with a thin foundation layer (100 mm) against a high inflow rate (0.6 L/min).

One interesting finding from the foundation layer tests is that wetting inflow from underneath the layers will migrate upwards and this upwards migration will continue as long as additional bentonite material is encountered above.


Backfill,, foundation layer, granular bentonite, bentonite pellets, performance, early inflow, wetting, erosion


WR 2017-38_web (pdf) (24.6 MB)


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