20 Windmills reinforced with Micropiles

Context

Currently, about 15,000 windmills are operating in Germany producing regenerative energy as an alternative to energy produced from nuclear power. Every third wind wheel in the world rotates in Germany. The windmill industry employs 45,000 people, has an annual double-digit growth and has an annual turnover of 4 billion Euros. The German steel industry supplies more steel to windmill contractors than to shipbuilders.

Also in the USA windmills for generating electric power are being built at an ever increasing rate. In addition to mechanical engineering problems, the structural engineering problem of anchoring the towers in the soil must be solved. One system uses double concentric rings of vertical bars (about 240 pcs. with lengths varying from 5.5 to 8.0 m) which serve as ground anchors and anchor bolts for the steel towers.

Solution

The bars vary from Ø 25 mm thru Ø 35 mm reinforcing steel grade 75 and Ø 26 mm thru Ø 36 mm prestressing steel grade 150 (St 1030 N/mm2). The bars are cast in a vertical torus formed by concentric corrugated steel pipes 3.05 m to 3.65 m in diameter. This patented system is inexpensive and fast. However, some of these foundations failed and were subsequently repaired using GEWI® Piles. The foundation failed because the wobbling of the windmill towers induced passive compression resistance to the top level soils which were incapable of the loads applied.

Therefore, 12 micropiles using 3 GEWI® Bars Ø 63.5 gr 80 (St 700 N/mm2) with a length of 13.7 m each were installed around each windmill. The tops of the bars terminated in a cluster plate which was embedded in a 1.2 x 1.2 m concrete circular perimeter beam. This beam was dowelled into the corrugated steel pipe. Some of the towers required plumbing (moving to vertical) prior to the micropile repairs. For this purpose, two pile-anchored abutment blocks were concreted near the tower foundation. An HDPE sheathed strand bundle was wrapped around the foundation corrugated steel pipe and each end was inserted into an abutment block. The strand bundles were tensioned by means of two jacks and the tower vertically plumbed.

DYWIDAG provided the bars for the repairs of about 20 windmills and more will be repaired in the course of 2006.