The breaking test is done only on the material passing the No. 40 sieve. This test as well as the roll test and the ribbon test, is used to measure the cohesive and plastic characteristics of the soil. The test normally is made on a small pat of soil about 1/2 inch thick and about 1 1/2 inches in diameter. The pat is prepared by molding a portion of the soil in the wet plastic state into the size and shape desired and then allowing the pat to dry completely. Samples may be tested for dry strength in their natural condition as they are found in the field However, you should not depend too much on such tests because of the variations that exist in the drying environment under field conditions. You may approximate the dry strength by such a test however, and verify it later by a carefully prepared sample. After the prepared sample is thoroughly dry, attempt to break it using the thumbs and forefingers of both hands (fig. 16-5). If you are able to break it, then try to powder it by rubbing it with the thumb and fingers of one hand.

The typical reactions that are obtained in this test for various types of soils are described below.

1. Very highly plastic soils (CH). The pat cannot be broken or powdered by finger pressure.

2. Highly plastic soils (CH). The pat can be broken with great effort, but cannot be powdered.

3. Medium plastic soils (CL). The pat can be broken and powdered with some effort.

4. Slightly plastic soils (ML, MH, or CL). The pat can be broken quite easily and powdered readily.

5. Nonplastic soils (ML, MH, OL, or OH). The pat has little or no dry strength and crumbles or powders when picked up.

This test is performed only on the material passing a No. 40 sieve. First, you mix a representative portion of the sample with water until it can be molded or shaped without sticking to your fingers. This moisture content is referred to as being just below the sticky limit. Next, prepare a nonabsorbent rolling surface by placing a sheet of glass or heavy wax paper on a flat or level support. Place the sample on this surface and shape it into an elongated cylindrical shape. Then attempt to roll the cylindrical sample rapidly into a thread approximately 1/8 inch in diameter (fig. 16-6). If the moist soil rolls into a thread, it has some plasticity. The number of times it can be rolled into a thread without crumbling is a measure of the degree of plasticity of the soil. Materials that cannot be rolled in this manner are nonplastic or have extremely low plasticity.

1. High plasticity (CH). The soil can be molded into a ball or cylinder and deformed under firm finger pressure without crumbling or cracking.

2. Medium plasticity (CL). The soil can be molded, but it cracks or crumbles under finger pressure.

3. Low plasticity (CL, ML, or MH). The soil cannot be lumped into a ball or cylinder without breaking up.

4. Organic material (OL or OH). The soil forms a soft, spongy ball or thread when molded.

5. Nonplastic soil (ML or MH). The soil cannot be rolled into a thread at any moisture content. From the thread test, the cohesiveness of the material near the plastic limit may also be described as weak, firm, or tough. The higher the soil is on the plasticity chart, the stiffer the threads are as they dry out and the tougher the lumps are if the soil is remolded after rolling.