Acceptable Manual Tests

Manual analysis of soil is conducted to determine quantitative as well as qualitative properties of soil and to provide more information in order to classify soil properly. It includes the following steps:

1. Plasticity: Mold a moist or wet sample of soil into a ball and attempt to roll it into threads as thin as one-eighth inch in diameter. Cohesive material can be successfully rolled into threads without crumbling. For example, if at least a two-inch (50 mm) length of one-eighth inch thread can be held on one end without tearing, the soil is cohesive.
2. Dry strength:
   • If the soil is dry and crumbles on its own or with moderate pressure into individual grains or fine powder, it is granular (any combination of gravel, sand, or silt).
   • If the soil is dry and falls into clumps that break up into smaller clumps, but the smaller clumps can only be broken up with difficulty, it may be clay in any combination with gravel, sand, or silt.
   • If the dry soil breaks into clumps that do not break up into small clumps and that can only be broken with difficulty, and there is no visual indication that the soil is fissured, the soil may be considered unfissured.
3. Thumb penetration: The thumb penetration test can be used to estimate the unconfined compressive strength of cohesive soils.
   • Type A soils with an unconfined compressive strength of 1.5 tsf can be readily indented by the thumb; however, they can be penetrated by the thumb only with very great effort.
   • Type C soils with an unconfined compressive strength of 0.5 tsf can be easily penetrated several inches by the thumb and can be molded by light finger pressure. Conduct this test on an undisturbed soil sample, such as a large clump of spoil, as soon as practicable after excavation to keep to a minimum the effects of exposure to drying influences. If the excavation is later exposed to wetting influences (rain, flooding), change the classification of the soil accordingly.
4. Other strength tests: A pocket penetrometer or a hand-operated shearvane can also obtain estimates of unconfined compressive strength of soils.
5. Drying test: The basic purpose of the drying test is to differentiate between cohesive material with fissures, unfissured cohesive material, and granular material. The procedure for the drying test involves drying a sample of soil that is approximately one inch thick (2.54 cm) and six inches (15.24 cm) in diameter until it is thoroughly dry:
   • A sample that develops cracks as it dries is an indication of significant fissures.
   • Samples that dry without cracking are to be broken by hand. If considerable force is necessary to break a sample, the soil has significant cohesive material content. The soil can be classified as an unfissured cohesive material, and the unconfined compressive strength should be determined.
   • If a sample breaks easily by hand, it is either a fissured cohesive material or a granular material. To distinguish between the two, pulverize the dried clumps of the sample by hand or by stepping on them. If the clumps do not pulverize easily, the material is cohesive with fissures. If they pulverize easily into very small fragments, the material is granular.

This test is based on the thumb penetration test described in the American Society for Testing and Materials (ASTM) Standard designation D2488, "Standard Recommended Practice for Description of Soils (Visual-Manual Procedure)."

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