This being the first blog entry, I thought I would share with you some information on how the ballfield here at Target Field is constructed. I started working for the Twins around the first of March in 2009. While many field materials were selected by then, some were still open for discussion, such as the infield dirt and the warning track. In an upcoming post, I will discuss our infield dirt and the process of developing that mix to big league specifications. Today, we will look at the foundation for the turfgrass and how it is built.

   The architect for playing field portion of the project was Brian Smith, from Populous. Mortenson Construction was the general contractor and builder of the entire facility here at Target Field. The sub for the field work was Fields, Inc. Target Field was essentially an asphalt parking lot prior to being a ballpark. When field construction began, the field area of a little less than 3 acres was excavated to 32" below what would be finish grade. The subgrade was compacted and verified by engineers. From there, approximately 12,500 tons of a 'select granular fill sand' was imported to the site. This sand ends up being 18" in depth and provides a stable base for the field to be built on, while still allowing for natural drainage. Once this phase was complete, the next 14" of the field base and rootzone was ready to be built. The field section illustration below is a helpful reference for the rest of the discussion.



illustration courtesy of: Populous.

   To install our drainage lines, Fields, Inc. and their project manager, Steve Peeler, were able to trench directly into the fill sand. This was a smooth process and meant we were not exporting any more waste material prior to the pea gravel layer. All of the lateral drain lines are 6" slotted ADS pipe and they are 10' apart. The lateral lines feed into an 18" collector pipe that runs from the 1st base foul territory area out to left center field. Lateral drain lines are all backfilled with washed pea gravel.

   The next phase was the importing of a 4" layer of washed pea gravel. This gravel, as well as the fill sand and the finish rootzone mix, all came from Elk River, Minnesota. Our suppliers there are Plaisted Companies and Peat, Inc. In all, 170 dump truck loads of pea gravel were brought in and spread across the entire field. Then the pea gravel was laser graded to closely match the finish grade of the field. The pea gravel passed stringent 'bridging' tests, to ensure that the rootzone mix would not migrate into it and potentially clog the drain tile. By building this way, we create a perched water table, which helps retain moisture and nutrients within the rootzone.

   Our contractor for the field heating was Tweet Garot, Inc. from Wisconsin. The heating materials used are the same as used in Green Bay by the Packers. There is approximately 40 miles of 1/2" diameter PEX tubing below the rootzone. As you can see in the illustration above, the tubing rests on top of the pea gravel layer. The field is divided into five heating zones: three in the outfield, the infield, and all of foul territory. The warning track is not heated, but all of the infield dirt, baselines and home plate are supplied with heat. Because of the amount of silt and clay in the 'dirt' areas of the field, our heating system is crucial in helping us have the field playable each year by April 1st. The heat pipes are supplied with a mix of 70% water and 30% glycol in a closed system, which means that the water/glycol mix recirculates and is always on site here.
By the last week of March, we have our soil temperature range set to 58-60* F, which is enough to get the turfgrass up and running. The heat is a great resource for us, but we still need sunlight for the grass to really get going in the spring each year.

   Next, just over 5,000 tons of an engineered rootzone mix was trucked down from Elk River to our ballpark. This was done after the irrigation system was installed by a local contractor, Mickman Brothers. One note on the photo above; it does not show the irrigation lines, which basically run through the pea gravel layer. One key to having a rootzone that meets USGA specs is having it tested, developed, processed and tested again with a large degree of quality control. We settled on a mix that is 93% sand and 7% peat. Both the sand and the peat are from Minnesota. The peat is a processed reed sedge peat moss, which helps us with nutrient and moisture retention within the turfgrass plant. As the rootzone mix is brought in and carefully spread across the field (so it does not disturb the heat pipes), an effort was made to keep it properly moist. Dry sand is difficult to grade and compact, so it is a challenge to keep it moist during a summer project. Fortunately, as Fields was close to doing final grading of the sand, we had a significant rain event that properly settled and saturated the rootzone. The field was laser graded with a tolerance of 1/4" over 25'. This essentially means that the grade survey checks the field every 25' and any difference in grade must be 1'4" or less. At that point, the field is ready for finishing the infield dirt work and sod install.

   That is a quick overview of how our ballfield is built here in downtown Minneapolis. Many people were a part of this great project. The Site Superintendent for Mortenson, Dave Mansell, was the key person in getting the entire facility at Target Field built. We also had great support on the playing field project from Brendan Moore of Mortenson, who was the field project engineer and Dr. Trey Rogers of Michigan State, who acted as a turf consultant for Mortenson. More to come in later blogs.
                                                                                                                     Larry