Soil Testing: Simple test determines moisture content

Soil testing, as we've demonstrated recently, is an integral part of road projects. Knowing what kind of soil exists in a project area will affect how a road is built and what materials will be used.

David Coronado, a transportation engineering specialist with ADOT's Soils and Aggregates Lab, says soil tests "help engineers to understand the soil's properties, stability, and bearing capacity. The best soil is free-draining but with enough clay for cohesion and stabilization, but not so high a clay content that the water is prevented from draining."

There are a number of different soil tests ADOT's Soils and Aggregates Lab performs, but let's look at one in particular: the T90 soil test. Coronado says the test is almost like playing with putty.

The technician measures 20 grams of fine, dry soil into a ceramic dish, adds water, and mixes it until it forms a putty-like consistency. From the mixture, a 10-gram ball is selected and then divided into smaller balls weighing no more than 2 grams each. Each ball is rolled back and forth on a clean glass plate until it breaks into small pieces. The fragments are collected, weighed and baked in an oven.

Once dried, the soil pieces are weighed, and the moisture content is recorded. Knowing the moisture content of this test (T90) along with its counterpart (T89), helps determine whether the soil is sandy, silty, or clay-rich, guiding engineers in selecting appropriate materials and supports for a project.

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