Soil borings are not boring

For stormwater infiltration best management practices (BMPs), understanding your soil prior to designing and constructing the practice is critical to the performance of the BMP. Many BMPs fail or do not achieve their desired level of performance because soils were not properly investigated. The following recommendations will help reduce the risk of failure due to poor design or construction.

1.       Take enough samples. The Minnesota Stormwater Manual recommends one boring for BMPs less than 1000 ft² in area and a boring for each additional 5000 ft² in area. This is the minimum number of samples. Before taking borings, review local soil maps and depth-to-bedrock maps to determine likely variability in soils and bedrock surface within the area (link here for more information). Take additional borings if any of the following conditions apply.

·         Multiple soil mapping units are likely to be present

·         Bedrock is likely to be within 10 feet of the surface

·         The seasonal high water table is likely to be within 5 feet of the surface

·         The site location exhibits moderate or high topographic relief

2.       Go deep enough. Borings should extend to a minimum depth of 5 feet below the bottom of the infiltration practice. If restrictive soil layers are likely to be encountered, extend borings to 10 feet below the bottom of the infiltration practice.

3.       Understand what you are looking for. Identification of low permeability or restrictive layers in soil is critical to proper design and construction of infiltration practices. The presence of any of the following may indicate the presence of a restrictive soil layer.

·         High clay content

·         Redoximorphic features

·         Change in particle size distribution

·         High blow counts

Since even a very thin restrictive layer in the soil profile will impede infiltration, the following are recommended practices in collection borings.

·         If recovery in a split-spoon sample is less than six inches, drill two feet deeper and perform another split-spoon sample.

·         If a confining layer is found, or there is low recovery in a split-spoon, reduce sampling intervals to every two feet. Record the presence of the most restrictive layer, even if that material is not the most abundant in the split-spoon sample.

·         If low recovery is encountered, explain in the field notes why the driller thinks this is happening.

·         Record blow counts during a Standard Penetration Test.

·         Include soil type, recovery, blow counts, and presence of water in field logs.

Additional resources

1.       Guidance on conducting soil borings – Minnesota Stormwater Manual

2.       New Jersey Stormwater BMP Manual – Appendix E

3.       New Hampshire Stormwater Manual: Volume 2 – see Section 2.4