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A Foundation For Success: Tips for Building a USGA Green Subgrade feet at a time. The soil should be compacted at each stage to eliminate any air pockets and reduce the risk of settling. If the subgrade material is wet and resists proper compaction, one option is to remove the base soil and spread it out to dry. Once the soil dries, it can be replaced and used to build a firm subgrade. If the soils are inherently unstable, it may be necessary to lay geotextile fabric over the finished subgrade to prevent the gravel layer from sinking into the soil. However, do not cover drainage lines with geotextile fabric; install the fabric either beneath the pipes or between the drainage lines.
A quality subgrade is essential to successful green construction. (Photo Credit: Brett Hochstein)
Like the foundation of any structure, a properly built subgrade is critical to the successful construction of a putting green. In many cases, building the subgrade is the most difficult part of the process but it also is the most important. When building a green to USGA construction guidelines, extra care must be taken to create a quality foundation. This will allow subsequent construction steps to be accomplished effectively. The USGA Recommendations For a Method of Putting Green Construction state that the subgrade should conform to the general slope of the finished grade. Placing grade stakes on 20-foot intervals typically provides enough information to achieve the general contours of the finished green. However, in cases where extreme precision is required a more intensive staking scheme may be necessary. During shaping, every effort should be made to ensure that the subgrade is stable and unlikely to settle or shift. If the subgrade is built from fill material it is imperative to build in stages, or “lifts”. When building in lifts, the final subgrade elevation is reached 1.0-1.5
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After the subgrade has been shaped, smaller equipment generally is used to smooth and further compact the subgrade. During this stage, any large clods (greater than one inch in diameter) should be eliminated and any low areas where water might accumulate should be filled. Cavity walls should be steep enough to prevent large differences in the depth of mix near the edge of the green. Vertical walls are easier to create when coring out an existing green, while sloped walls are more common when building a new green. A quality subgrade is essential to successful green construction; without it all future steps in the construction process will be compromised. For more information on putting green construction, refer to the USGA Green Section’s digital collections “Building the USGA Green: Tips for Success” and “A Guide to Constructing the USGA Putting Green”. Additional resources on putting green construction and other topics can be found on the Course Care section of USGA.org.