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Civil engineering

Wind Turbine foundation stability

Wind turbine foundation stability – sensitivity study and proof of concept of designing wind turbine foundations based on numerical methods.

Wind turbine foundations in Finland are generally designed with analytical methods that only consider rock mass weight as the force resisting uplift. These methods partly or completely ignore the capacity of the rock mass to resist shear and the natural tendency of rock joints to dilate during shear and rock blocks to interlock when packed tightly and forced to slightly rotate due to uplift forces (rock anchors). Peikko Oy is introducing a more modern and efficient way of wind turbine foundation construction, by streamlining the process and using numerical methods to optimize the design. Numerical simulations were performed in this project to assess the sensitivity of the total force resisting overturning to rock mass parameters, joint orientations and anchoring lengths. RMCF performed result and performance comparisons between 3DEC, FLAC3D and PFC3D.

The simulation geometry included the jointed rock mass, the concrete slab of the wind turbine foundation, the flanges on top and at the bottom of the concrete slab and a part of the wind turbine tower. The top and bottom flanges were connected with post-tensioning bolts. Post-tensioned rock anchors were connected to steel faceplates on top of the concrete slab and extended to varying depths into the rock mass depending on the simulated variation. Also, the grouted length of the rock anchors was varied case by case.

Numerous simulation cases were run with varying joint orientations and joint and rock mass properties. As expected, some jointing combinations were much less resistant to pullout forces than others and joint surface properties play a significant role on the natural tendency of rock blocks to interlock. Site specific and sufficient geological surveys are critical to realistically simulate the pullout capacity of a rock mass and optimize design accordingly. In a jointed rock mass the interaction between distinct rock blocks and the resulting resistance to uplift is a very complex process which cannot in practice be realistically replicated with analytical approaches without needing to rely on unnecessarily large safety factors.

In general, the simulation results from 3DEC, FLAC3D, PFC and Peikko’s software, were found to be equivalent and matched displacement data monitored from an operational wind turbine.