The ribbon test is performed only on the material passing the No. 40 sieve. The sample prepared for use in this test should have a moisture content that is slightly below the sticky limit. Using this material, form a roll of soil about 1/2 to 3/4 inch in diameter and about 3 to 5 inches long. Place this material in the palm of your hand and, starting at one end, flatten the roll, forming a ribbon 1/8 to 1/4 inch thick. This is done by squeezing it between your thumb and forefinger (fig. 16-7). Handle the sample carefully to form the maximum length of ribbon that can be supported by the cohesive properties of the material. If the soil sample holds together for a length of 6 to 10 inches without breaking, the material is then considered to be both highly plastic and highly compressive (CH). If the soil cannot be ribboned, it is nonplastic (ML or MH). If it can be ribboned, it is nonplastic (ML or MH). If it can be ribboned only with difficulty into short lengths, the soil is considered to have low plasticity (CL). The roll test and the ribbon test complement each other in giving a clearer picture of the degree of plasticity of soil.

The wet-shaking test is performed only on the material passing the No. 40 sieve. In the preparation of a portion of the sample for use in this test, enough material to form a ball of material about 3/4 inch in

diameter is moistened with water. This sample should be just wet enough that the soil does not stick to your fingers upon remolding or just below the sticky limit. For testing, the sample is then placed in the palm of the hand and shaken vigorously. This is usually done by jarring the hand on the table or some other firm object or by jarring it against the other hand. The soil is said to have given a reaction to this test if, when it is shaken, water comes to the surface of the sample producing a smooth, shiny appearance. This appearance is frequently described as livery (fig. 16-8).

The sample is then squeezed between the thumb and forefinger of the other hand. As this is done, the surface water quickly disappears and the surface becomes dull. The sample becomes firm, resisting deformation, and cracks occur as pressure is continued. Finally the sample crumbles like a brittle material.

The vibration caused by shaking the soil sample tends to reorient the soil grains, decrease the voids, and force water, which had been within these voids, to the surface. Pressing the sample between the fingers tends to disarrange the soil grains and increase the void spaces. The water is then drawn into the soil. If the water content is still adequate, shaking the broken pieces

causes them to liquefy again and flow together, and the complete cycle may be repeated. This process can occur only when the solid grains are bulky in shape and noncohesive in character. Very fine sands and silts fall into this category and are readily identified by the wet-shaking test. Since it is rare that fine sands and silts occur without some amount of clay mixed with them, there are varying degrees of reaction to this test. Even a small amount of clay tends to retard this reaction greatly. Some of the descriptive terms applied to the different rates of reaction to this test are as follows:

. SUDDEN OR RAPID. A rapid reaction to the shaking test is typical of nonplastic, fine sands and silts. A material known as rock flour that falls into the silt-size ranges also gives this type of reaction.

. SLUGGISH OR SLOW. A sluggish reaction indicates slight plasticity such as might be found from a test of some organic or inorganic silts or silts containing a small amount of clay. Even a slight content of colloidal clay imparts some plasticity and materially slows up the reaction to the shaking test. Extremely slow or no reaction is typical of all inorganic clays and of the highly plastic organic clays.

. NO REACTION. Obtaining no reaction at all to this test does not indicate a complete absence of silt or fine sand.