Finding Good Subgrade

MARCH, 1937 ARIZONA HIGHWAYS

In Hubbard-Field press. The pin is removed and the air pressure is run up to 100 lb. per square inch for a period of one minute, then decreased to 75 lbs. per square inch, at which pressure the test is run. The load is then applied at the rate of 0.8 inches (40 turns) per minute; this rate is maintained until the gauge indicates a maximum has been passed. The apparatus is then removed and cleaned, a sample of the material from middle of the cylinder being retained for moisture determination. The test is now repeated, using the remaining batches. With practice the number of batches required can be greatly reduced.

CENTRIFUGE MOISTURE EQUIVALENT. A centrifuge moisture equivalent is then run on the material in the same manner as that used in the Bureau of Public Roads soil constants, but with the following variations: 25 grams of passing 3-mesh is used in order to have the C. M. E. value on the same material as used in the shear test, thus affording a basis for direct comparison with the moisture content in the shear tests. However, in the case of materials which will almost entirely pass a 40-mesh sieve, the 5 gram, 40-mesh C. M. E. is used.

INTERPRETATION OF RESULTS. The shearing loads are plotted against the corresponding moisture contents, and the intersection of the resulting curve with the C. M. E. percentage is noted. The curves produced by this method can be classified into five general types as shown in figures 1 to 5.

TYPE B-1 MATERIAL. (Fig. 1) This type of material shows very little or no decrease in shear strength with increase in moisture content. All materials of this type have shown shear values of above 1000 lbs. with all moisture contents. Of seven materials tested, six were of A-2 types, and one was A-5.

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TYPE B-2 MATERIAL. (Fig. 2) This type material shows a slight decrease in shear strength, followed by increased shear strength with increases in moisture content. All materials of this type have shown shear values above 1000 lbs. with all moisture contents. Of material tested, two were A-2 types and two were A-3 types.

TYPE B-3 MATERIAL. (Fig. 3). This type material shows a slight decrease in shear strength with increase in moisture content up to a certain critical point at which the shear strength begins to decrease very rapidly. The C. M. E. (pass 3-mesh) of the material lies in the first portion of the curve. The C. M. E. point is of a high order, usually above 1000 lbs. Of twelve materials tested, ten were of A-2 type, and two had soil constants of A-7 type but gradings of A-2 type. (Continued on Page 19)