Compaction refers to the hardening of soil that occurs when air is pushed out of the pore spaces between soil particles, usually due to foot or vehicle traffic. This can make it harder for plant roots to grow and for water to soak into the ground. If the bottom of a stormwater basin becomes compacted, it might not work as it should. Instead of allowing water to infiltrate and percolate downwards, compacted soil could lead to drainage problems, flooding, or poor water quality, with harmful effects like sediment running off into nearby areas.
One way to check if the soil is compacted is by doing a simple test called the Probing Wire Test (see diagram). For this test, you use a firm steel wire (15 1/2 gauge steel wire – e.g. survey marker flag, straight wire stock, etc.) about 18 to 21 inches long. Hold it by the flag end and push it 6 inches vertically into the ground. Observe if the wire moves easily without resistance or bending.
Prior to the application of topsoil, it’s important that the soil underneath (called the subgrade) isn’t compacted. Subgrade soils should be free of excessive compaction to a depth of 6 inches. If the soil needs loosening, it can be done through methods such as scarification or tilling to break up the soil, as long as there is no danger to underground utilities (cables, irrigation systems, etc.). (Another method, as specified by a New Jersey Licensed Professional Engineer, may also be acceptable.) Topsoil can then be applied to the de-compacted soil. This helps plants establish long root systems and healthy, long-lasting growth.
The Soil Erosion & Sediment Control Standard for Land Grading (19-1) addresses the potential for excessive soil compaction and provides remediation and mitigation guidelines based on the land’s intended use. It covers when to test for compaction and how to fix it when necessary, especially when planning to grow permanent vegetation. Explore this Rutgers Cooperative Extension “Understanding Soil Compaction” Fact Sheet for more information about soil compaction.