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Recently, URETEK ICR was asked to review a project involving lifting and stabilization of 10 precast transformer pads at a wind farm located in South Dakota.

The pads had settled to varying degrees with 8 of the 10 having settled 1 (±0.5) inch and requiring only stabilization and the two remaining, with settlement in excess of 3.5 inches, requiring lift and stabilization.

QUICK FACTS:

Location: South Dakota Wind Energy Farm

Problem: Settled Transformer Pads at Base of Turbines

Project Considerations: Transformers + Pads, Weight = 29,000 lbs. ea.

All the transformers were mounted on individual 15 ft. x 8.5 ft. x 8 in. thick precast pads which were partially supported on precast cabling vaults and lean concrete structural support on the unloaded side of the pad and compacted earth on the loaded side. The settlement to the loaded side of the transformers caused the pad to tip on the inside edge of the concrete vault.

Settlement had occurred within one and a half years of installation and was determined to be due to combination of factors including insufficient compaction surrounding the transformer pad location and freeze thaw cycles.

 

Voids were present in varying degrees under all of the slabs.

Concerns about how to return the two worst transformers to level, coupled with questions about material containment in the target area and mechanical stresses on the precast transformer pad were addressed and described under Proposed Solution.

PROPOSED SOLUTION: 

URETEK ICR proposed two solutions. The first addressed stabilization of soils below each of the transformer pads. The second ensured a sure, steady, and complete lift of the two worst pads to level, coupled with void fill and soil stabilization.

The eight pads which had settled <1.5 inches were slated to have URETEK DEEP INJECTION® work performed with the goal of stabilization of sub soils to prevent future settlement. Although lifting back as close to original grade was a goal, it was not specified as the ability of the soils to keep the material contained could not be determined prior to application.

• A single depth injection was performed at 4 locations parallel to and approximately one foot from the settled edge of the pad.

• A secondary injection was performed by angling the URETEK DEEP INJECTION® probes at an approximate 45˚ angle, placing the injection point under the transformer pad.

It was determined prior to submission of the proposed solution that the precast transformer pads were capable of bearing the weight of the transformer while under lifting stresses from the outside unsupported edge of the pad. Therefore mechanical lifting options were incorporated, in the two worst cases, to provide a an assisted lift assuring that material containment issues were alleviated.

• Four 20 ton mechanical lifting devices were dug in and spaced along the settled side of the pad with load distribution taken into consideration.

• Lifting surfaces were covered with wood to prevent direct contact with the pads.

• The mechanical lifting system easily lifting the slab to within 1 inch of desired grade.

• Pads were chocked and the lifting devices un-weighted to ensure the slab would remain at desired grade during void filling operations. Upon verification of stability, closed cell expanding polymer material was injected into the void space below the pad and allowed to set.

• Finally, all chocks were removed and URETEK DEEP INJECTION® process was used as described above.

Three days were allotted for project completion.


DOING THE PROJECT: 

URETEK ICR personnel arrived on site after completing required site specific safety training and began the project. At the start, URETEK ICR worked with the customer to locate high-voltage power lines to ensure no accidental drilling into and/or through underground cabling. URETEK ICR also completed the insertion of required injection tubes in the specified pattern and depth – to ensure adequate soil densification and material containment. During injection of the first pad, material containment was superb. Although small amounts of material did make it to the surface, it was within scope and didn’t present any environmental or personnel hazard. Each pad was brought back to level. By the end of the first day, two pads had been leveled and all injection probes driven around four additional pads in preparation for the second day’s work.
The URETEK ICR crew staged the first of the two mechanical lifts. Hydraulic equipment was installed and hooked up in less than 30 minutes. The mechanical lifting was accomplished in under one minute and within 30 minutes the pad had been void filled and stabilized to approximately one inch below desired grade. URETEK Deep Injection® completed the soil stabilization using the same probe pattern as was performed on day one. The URETEK Deep Injection® process also lifted the surrounding soils which, in turn, returned the pad to desired grade in under two hours. The second of the two mechanical lifts was performed in the same manner and with equal success. Three additional pads were stabilized, void filled, and returned to desired grade for a total of five on Day Two. At the end of day two, a total of seven pads had been lifted and stabilized.
The three remaining pads were lifted to desired grade on Day Three achieving the anticipated project completion window and exceeding the project expectation of soil stabilization only by successfully returning all pads back to desired grade and remaining within budget estimates set forth for each transformer pad.

PROJECT SUMMARY:

URETEK ICR counts this as a successful project. This is due to these factors:

1. The project was completed within the tight timeline and budget – with no surprises! (This project required 3 days)

2. The use of the URETEK Deep Injection® process was able to successfully lift and stabilize all 10 transformer pads, exceeding the original customer requirement

3. The customer now knows of a fast and cost effective solution to settled transformer pads.

BENEFITS:

URETEK ICR’s repair strategy cost substantially less and took substantially less time than other considered repair methods. Our crews were able to the project in low (10 degrees) temperatures, when the customer needed it done, thereby maximizing the productivity of the wind farm and reduce any future transformer maintenance or repair costs. And, because our materials are environmentally friendly (NSF 61 Certified) the repair will not contribute to any future ground or water contamination issues.

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