The 99 MW Greenwich Wind Farm, located in northern Ontario, is built on a massive granite rock layer known as the Canadian Shield, which features a hardness level of 28,000 pounds per square inch. Given the site’s location, project developers quickly concluded that the standard method used to secure wind turbine foundations – typically consisting of large spread-reinforced concrete footings, as shown in Figure 1 – would not work in such extreme conditions. Therefore, the situation dictated the need for an alternative.
After several tests were conducted, the project’s balance-of-plant contractors opted to use rock-anchored foundations for the site, which involve securely anchoring the turbine foundation while minimizing rock excavation and concrete used to reduce the environmental impact.
When site conditions call for the use of something other than mass concrete foundation systems, developers are beginning to turn to pre-stressed anchor foundations or micropiles. The rationale is simple: If the soil conditions are not properly planned for in advance, overturning failures can result, as depicted in Figure 2.
This method can minimize instances of fatigue and provide savings of up to 75% reduction in foundation area, 40% reduction in concrete consumption and 70% reduction in reinforcing steel, as evidenced in Figure 4.
In order to test and post-tension these foundations, a groutable void form (GVF) has to be used. The GVF has the function to temporarily separate the pile from the pile cap during stressing and testing to a load higher than the design load in compression and tension while also allowing for some range of movement. After post-tensioning, the GVF is filled with cement grout in order to also transfer compression loads to the pile, as shown in Figure 3.
Using a post-tensioning procedure with the GVF method allows for the better control of foundation settlement and prevents overloading the soil-bearing capacity with a prescribed sequence of post-tensioning. Further, post-tensioning in balanced anchor groups permits gradual load transfer into the piles without overstressing the soil. Lastly, the settlement of the foundation cap is minimized, as the piles take over the compression load from the cap. Fatigue failures can be eliminated since the post-tensioned tension and compression loads are higher than the design loads and, therefore, minimize or eliminate movements during load changes.
During construction, the micropiles or anchors can be installed before or after placing the concrete foundation cap. The advantage with the first method is that a larger drill hole diameter can be achieved using the open or cased hole drilling method and that the GVF can be whet set into the upper cement grout body of the pile or anchor. The difficult part is the exact pre-locating of the piles or anchors before the foundation cap is placed. In average, there are about 20 piles and anchors for each foundation.
The advantage of the second method is that the piles or anchors are drilled through exactly located sleeved holes in the foundation cap with a pre-installed GVF under the foundation cap, as shown in Figure 5.
The disadvantage here is that the drill hole size is determined by the smaller sleeve, inside the foundation slab, and that it is providing sufficient cement grout under the GVF. The smaller sleeve size is dictated by the bearing plate size of the pile or anchor.
However, this problem does not exist using the grout injection bore (IBO) method using hollow bars. In this case, the cement grout is injected simultaneously during the drilling operation. The drill bits have small-diameter side holes through which the grout will exit under high pressure, such as during jet grouting, and will flush a larger drill hole than the drill bit size, creating a sufficient cement grout body underneath the GVF. The IBO method also greatly improves the condition of the ground around the micropile and, therefore, creates a favorable higher skin friction capacity. Three types of post-tensioning steel tendons are 7 Wire, 270 KSI low-relaxation strand; solid, 150 KSI high-strength bars; and IBO hollow Titan bars.
Strand and solid bar anchors are used primarily for foundations on rock, or foundations on soil, when a larger drill hole is required and the anchored piles are pre-drilled. Strand anchors have the advantage over bar anchors when large capacity and long anchors are required, eliminating the extra cost of multiple bar couplers.
IBO hollow bars are used for foundations on soil with pre-installed pipe sleeves inside the foundation and pre-installed GVF underneath the foundation. The jet grouting effect during drilling will flush a larger drill hole under the GVF and greatly improve the shear strength of the soil around the micropile.
Stressing of the anchors, simultaneously in pairs, will pre-stress the piles and post-tension the foundation before the GVF is filled with cement grout.
The amount of anchors stressed in pairs is dictated by the compression load capacity of the soil. When this load is reached, the GVF is grouted. After the grout has reached sufficient strength, the stressing procedure can continue. w
Marketplace: Bolts, Anchors & Tensioning
Using Anchor Foundations In Difficult Soil Conditions
By Horst Aschenbroich
Most wind farm foundations typically employ large spread-reinforced concrete footings. However, some soil conditions might dictate using an alternative.
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