What dry unit weight of soil corresponds to 95% relative compaction for this material

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Identify the importance of compaction in Geotechnical Engineering


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Laboratory Objectives:

1. Determine the maximum dry unit weight and corresponding moisture content of a soil sample by the Standard Proctor Compaction Test.


Educational Objectives: 

Upon completion of the lab students should be able to:

1. Define compaction.

2. Relate the degree of compaction to the energy of compaction.

3. Identify the importance of compaction in Geotechnical Engineering and related disciplines of Civil Engineering.

4. Determine the maximum dry unit weight and corresponding moisture content of a soil sample by the Standard Proctor Compaction Test.



Compaction is a mechanical densification of the soil. This densification is obtained by reducing the air voids in the soil. Removing the air requires mechanical energy. Examples of mechanical energy used to compact in the field include sheep foot roller, smooth wheel roller, and vibratory compactors. Compaction improves soil strength and reduces soil compressibility, which is important in the construction of highways, airports, and other structures.

The degree of compaction of a soil sample is measured in terms of its dry unit weight. When water is added to the soil during compaction, it acts as a lubricating agent between the soil particles. The soil particles slip over each other and move into a densely packed position. Therefore, the dry unit weight after compaction first increases as the moisture content increases. However, as moisture content continues to increase, water (which is less dense than the soil particles) will displace soil particles, and the dry unit weight will begin to decrease.

In 1933, Proctor developed a laboratory compaction test to determine the maximum dry unit weight of compaction for a soil, which can be used for specification of field compaction. Therefore, lab results can be used to obtain the required compaction in the field.

Relative Compaction, RC, is defined as the ratio of the dry unit weight in the field to the maximum dry unit weight as determined in the lab. RC = d The goal of the Proctor lab is to determine dm In practice, RC for a specific project will be explicitly defined in the contract and specs. Typically RC varies from 90-95%.



1. Oven-dried sample

2. Standardized compaction mold

3. Standard Proctor compaction hammer

4. Large flat pan

5. Balance(s)

6. Straight edge

7. Moisture tins

9. Drying oven



1. Break up any lumps larger than about one quarter inch in the soil sample. Be careful not to break individual soil grains.

2. Determine the weight of the proctor mold and base plate without the extension.

3. Attach the extension to the mold.

4. Add water to the oven-dried soil and mix thoroughly such that the moisture content is approximately 4%.

5. Fill the mold slightly more than 1/3 full (not including the extension) with the soil sample and compact evenly with 25 blows from the standard proctor hammer.

6. Repeat the previous step two more times so that the compacted soil specimen extends just above the top of the mold (not including the extension).

7. Remove the extension without breaking the soil specimen. If the soil extends more than ¼ inch above the top of the remold, you must discard the specimen and repeat the step.

8. Using a straightedge, trim the protruding soil off of the top of the mold.

9. Determine and record the weight of the base plate, mold, and soil sample.

10. Remove the mold from the base plate

11. Remove the soil sample from the mold.

12. Take a representative soil sample from the center of the specimen and determine the moisture content of the specimen used during this test.

13. Break up the rest of the specimen and replace it in its original container. Reassemble the mold and extension.

14. Add moisture to the soil and mix thoroughly.

15. Repeat Steps 4-13 for each of the remaining points: 8%, 12%, 16%, and if needed, 20%.


Calculations and Analysis:

1. Calculate the moist unit weight for each trial performed in the procedure:


where:   ca       Moist unit weight of the compacted soil specimen

WS = Weight of the compacted soil in the mold (lb.)

V = Volume of the mold (1/30 ft³ )


2. Calculate the moisture content, w, of the compacted soil for each trial performed

3. Calculate the dry unit weight of the compacted soil for each trial performed:


where:dy= Dry unit weight of compacted soil

w=moisture content (decimal form)


4. For each moisture content determined, estimate the maximum theoretical dry unit weight. This would occur if all air voids in the soil sample were removed during compaction, and is a function of moisture content:



where:cc=Theoretical unit weight of soil sample with zero air voids

cd=Unit weight of water

SG = Specific Gravity of Soil Solids (assume 2.70)


5. Plot a graph of dry unit weight versus moisture content. Determine the maximum dry unit weight and corresponding optimum moisture content. On the same graph, plot the theoretical lines corresponding to S = 100%, and S = 80%.



1. What dry unit weight of soil corresponds to 95% relative compaction for this material?

2. What range of water contents must the field water content be within if it is to be plus or minus 3 percent of the optimum moisture content?



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