Types of Soil Investigation

If you ask a gardener, an architect or engineer, and an environmental consultant what a "soil investigation" is all about, you'll get three very different answers. Different types of soil investigations can be used to answer a number of very different, very important questions, such as: "Is this soil suited to the type of crops I want to grow? Is this soil suitable for supporting the foundation of a structure of the type and size I want to build? Is the soil or groundwater here contaminated with hazardous wastes or petroleum products?"

Geophysical soil investigations concentrate on the physical characteristics of the soil column, such as the depth to groundwater, each stratum's soil classification (according to the Unified Soil Classification System), moisture content and other physical properties such as particle size, moisture content and soil density.

A geophysical soil investigation's report of findings and recommendations should be based on the testing of soil samples obtained from preplanned, on-site soil boring locations. It should include, at minimum, data on the bearing capacity of the soil, foundation design recommendations, paving design recommendations, compaction of soil, lateral strength (active, passive, and coefficient of friction), permeability (as measured by infiltration testing) and frost depth.

Agricultural soil investigations concentrate on the viability of surficial soils (the topmost 2 to 3 feet of the soil column) to support food crops. These investigations may be carried out on existing or proposed cropland. Field work consists of collecting composite soil samples from a minimum of 10 locations per 40 acres of similar land. Areas that differ in soil type, appearance, crop growth or in past treatment should be sampled separately. Sampling locations should be limited to areas at least 50 feet from barns, roads, lanes, or fence rows.

Soil analyses useful in an agricultural soil investigation include analyses for organic matter, available phosphorous, exchangeable potassium, magnesium, calcium, soil pH, buffer pH, cation exchange capacity, percent base saturation of cation elements, soluble salts and sodium, sulfur, zinc, manganese, iron, copper and boron. If groundwater beneath the site is to be used for irrigation, a client may wish to analyze groundwater samples for many of these same parameters.

Environmental soil investigations are typically undertaken as a follow-up to any one of a number of previous investigations of the property. Among these are: (1) a Phase I environmental site assessment that has revealed a "recognized environmental concern" as defined by the American Society for Testing and Materials Standard E1527-05; (2) a previously documented spill or other known or suspected release of a hazardous substance or petroleum product for which assessment and cleanup is required by law. (Reference 4)

In these soil investigations, soil samples are collected at locations specified in the previous assessment, or at locations likely to have been the source areas for the suspected releases. Proper decontamination of soil sampling equipment is essential before and after each sample is collected in order to prevent cross-contamination of soil samples.

The types of lab analyses performed on each sample depends on the chemicals of concern. Samples cannot be tested for every compound known to man due to the prohibitive costs involved. Rather, each sample is tested for the presence of chemicals known or suspected to have been discharged in the area where that sample was collected.