compaction of soil and laboratory determination of optimum moisture content

Compaction of soil

PRINCIPLES OF COMPACTION:

THIS IS THE PROCESS OF PACKING THE SOIL PARTICLES BY REDUCING THE AIR VOIDS IN THE SOIL MASS BY MECHANICAL MEANS. SHORT DURATION REPETITIVE LOADING IS THE REAL REQUIREMENT FOR CMPACTION.

          THIS IS ACHIEVED UPTO A DESIRED EXTENT , MOSTLY BY THE ROLLING OF THE SOIL GRAIN. WHEN WATER ADDED TO A DRY SOIL MASS , THE SOIL GRAINS GET SURROUNDED BY A FILM OF A ADSORBED WATER. IF MORE WATER IS ADDED , THIS FILM BECOMES THICKER, SOIL GRAINS CAN EASILY SLIDE OVER EACH OTHER.

          THUS, WATER ACTS AS A LUBRICANT IN THE COMPACTION PROCESS, AND A CLOSER PACKING OF SOIL GRAINS IS ACHIEVED ON ROLLING, BY THE EXPULSION OF AIR FROM THE VOIDS.

          IF MORE WATER IS CONTINUED TO BE  ADDED, THEN WATER STARTS HINDERING THE CLOSER PACKING OF GRAINS. IN THE LIMITING STAGE FOR A PARTICULAR AMOUNT OF WATER CAUSES MAXIMUM LUBRICATION, WITH OUT BECOMING EXCESS TO CAUSE HINDERANCE IN COMPACTION BY OCCUPYING SPACE FOR IT’S OWN MOLECULES. THIS LIMITING MOISTURE IS CALLED OPTIMUM MOISTURE CONTENT (OMC) AND CORRESPONDING DRY DENSITY IS CALLED MAXIMUM DRY DENSITY OF THE SOIL GRAINS.

LABORATORY DETERMINATION OF OPTIMUM MOISTURE CONTENT:

VOLUME OF THE STANDARD PROCTOR MOULD (1/30) CU. FEET. ( PROCTOR’S 1933)

STEPS ARE: 1. TAKE EMPTY MOULD WEIGHT ( SAY M1 gm.).

2. TAKE 3Kg. SOIL, ADDING 2 TO 5% OF WATER BY WEIGHT.

3. DIVIDE THIS WET SAMPLE INTO 3/5 LATER TO COMPACT EACH OF 3/5 LAYER AFTER GIVING 25 BLOWS WITH A STANDARD RAMMER OF 5.5Ib/12Ib , ALLOWED TO DROP UNDER THE GRAVITY FROM  A HEIGHT OF 12 /18 INCHES.

4. THE COMPACTED SOIL IS THEN REMOVED FROM THE BASE PLATE AND MEASURED THE MASS (SAY M2gm) , (M2-M1) GIVES THE MASS OF WET SOIL.

5. CALCULATION STEPS ARE:

MASS OF MOIST SOIL IN THE CYLINDER= (M2-M1), VOLUME OF MOULD = (1/30) cft.= 944 cm³  = V.

DENSITY OF MOIST SOIL= (M2-M1)/V= ρ gm/ cm3 (say). THE UNIT WEIGHT γ = (ρ/ρw) × γw = ρ× γw . THEN THE DRY UNIT WEIGHT OF SOIL=

γd = γ/(1+w) =(M2-M1) × γw / V×(1+w) .

HENCE, γd = (M2-M1) × 9.81 / 944×/(1+w)                        kN/m³

 6. SOME AMOUNT OF COMPACTED SOIL IS KEPT INTO OVEN TO GET THE MOISTURE CONTENT IN EACH TRIAL.

7. THIS STEP IS REPEATED BY 3 TO 4 TIMES TO GET THE CURVE DRY DENSITY V/S WATER CONTENT SAME PERCENTAGE OF WATER IS ADDED SYMUTANEOUSLY IN EACH REPEATING STEP.