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Skim Pit

Problem:

AMG, Inc. was commissioned to help a large soybean crush plant replace an in-ground hexane skim pit during a 14 day shutdown.


Background

  • A skim pit is used to separate hexane or a soybean oil/hexane mixture from water. Hexane is a highly volatile and combustible solvent used in the extraction process.
  • The existing cast-in-place concrete skim pit was severely deteriorated and would have to be demolished and removed because it was located at the same place that the new skim pit needed to occupy.
  • The new skim pit would need to be more resistant to attack by the fatty acids contained in the residual soybean oil than the original.
  • The client had a design that he wanted to use. It consisted of a cast-in-place concrete pit measuring 36’ x 18’ x 12’ deep with five internal chambers separated by under flow and over flow baffles.
  • The entire skim pit demolition, removal, and reinstallation had to be accomplished in nine days because the shutdown was only 14 days and the plant required three days for purging before construction could begin, and two days for recharging after construction was completed.

Solution:

AMG decided to use a segmental precast concrete skim pit built in 4 segments, none of which exceeded 9’ in width or more than 100,000 lbs in weight to facilitate shipping. The precast segments were match cast by a B4 certified precast fabricator and shipped to the plant ready for assembly during the shutdown. After the segments were placed in the excavation and aligned, they were drawn together and locked tightly in place by 24 post tensioning strands fed through ducts cast into the segments. Finally, the skim pit was made water tight by pressure injecting urethane foam into the three construction joints between the four segments.


Results:

In order to complete the concrete skim pit in nine days, the solution required atypical construction techniques. Because the client was unfamiliar with these techniques, he requested a number of preliminary feasibility, constructability, and construction cost checks to confirm the viability of the approach to his satisfaction before giving approval to proceed.

The first step was to identify specialty subcontractors for each facet of the work.

  1. A precast contractor was found to fabricate and ship the precast segments. This contractor was certified as level B4 by the Precast / Prestressed Concrete Institute (PCI). B4 Certification indicates the precast fabricator has experience in fabricating the most complex form of concrete bridges, such as segmental precast concrete bridges.
  2. A post-tensioning contractor, also experienced in building segmental precast bridges, was responsible for inserting the prestressing tendons, drawing the segments together, prestressing the skim pit, and injecting cement grout into the tendon conduits to permanently bond them in place.
  3. A construction chemicals application contractor was located that was familiar with and equipped to inject the urethane sealant into the construction joints and apply the reinforced urethane liner to the inside of the skim pit.
  4. An erection contractor was found to lift the segments off the trailers and place them in the excavation.

Once these contractors were identified, a project budget cost was assembled from their provisional quotations based on preliminary sketches of the project. When the feasibility and cost of the project approach were demonstrated to the client’s satisfaction, final construction drawings were prepared.

The project execution came together with only couple of challenges.

  1. One of the proposed general contractors offered a voluntary deduct alternate to use cast-in-place construction if the client would add five days to the schedule. The client compared the construction cost savings if the alternate was accepted to the lost revenue if the plant was not in production for another five days. The decision was to reject the alternate as a non-starter.
  2. The successful general contractor wanted to use a small local precast contractor to fabricate the segments to minimize shipping cost. The local contractor was disqualified because he did not have the required PCI certification nor did he have the experience or the in house hoisting equipment necessary to handle the pieces.
  3. The demolition and excavation contractor was a separate prime contract to the client, not under the general contractor. He was required to extend the excavation a couple of feet in plan after he had finished final grading the major portion of the excavation. The finished grade in the extended area was a couple of inches higher than the original area and consequently one precast segment, which straddled the discontinuity, did not sit level enough to mate with the other segments. After some jacking and shimming, the problem was resolved and the construction was completed on time. Guide rails were added under the segments of the skim pit to ensure proper alignment, regardless of the levelness of the excavation.