Lab report on Standard Proctor Compaction test

The standard proctor compaction test is used in establishing a relationship between moisture content and dry density for the soil under controlled conditions. The standard proctor compaction test was first developed by R.R. Proctor (1933) for assessing compacted fill and from then it became a universal standard test. Compaction of soils is a very essential procedure in geotechnical engineering, it helps in improving the strength of the structures such as height way construction and airports. Therefore, the standard proctor compaction test is a procedure used to achieve the dry unit weight of a soil which passes sieve #4. It is also an indicator test for the specification of field compaction.

In the construction of engineering structures, loose soils must be compacted to increase their unit weights. Compaction of soil increases the strength of the soils, which led to an increase in the bearing capacity of foundations constructed over them. Compaction also increases the stability of the slopes of embankments and reduces the amount of undesirable settlement of structures.

In Geotechnical Engineering, the soil is a particulate medium contain pore spaces, which may or may not filled with water. When the soil is subjected to external forces, due to the high void ratio, the soil particles will be pushed to fill the voids spaces, which then led to soil deformation. Therefore, it is necessary to decrease the void spaces of partially saturated loose soil deposits through compaction of the soil to improve strength, reduce compressibility and conductivity.

Equipments used

Compaction mould

Standard proctor hammer

U.S. No.4 sieve

Jack

Steel straightedge

Drying oven

Balance sensitive to 0.01 Ib

Large flat pan

Moisture cans

Description of test procedure

1. Approximately 10 Ib of air-dry soil for which compaction test was carried out was obtained and all the soil lumps were broken.
2. The soil was sieved on U.S. No 4 sieve and all minus 4 large material were collected in a large pan.
3. Enough water was added to minus U.S. No 4 sieve materials and mixed thoroughly to obtain a moisture content up to 4 % to 5 % below the estimated optimum moisture content.
4. The weight of the proctor mold + base plate (W1) was the determined.
5. The extension was attached to the top of the mold and placed on the solid ground.
6. The moist soil was poured into the mold in three equal layers. Each layer was compacted uniformly 25 times using a standard proctor hammer before the next layer of loose soil was poured into the mould. The layers of loose soil were poured into the mold such that each layer after compaction have about one-third volume of the mold and the end of the three-layer compaction, the soil slightly extend above the top of the rim of the compaction mold.
7. The top of the mold was removed carefully making sure not to break off compacted soil inside the mold.
8. Straightedge was used to rim the excess soil above the mold until the top of the compacted soil is even with the top of the mold.
9. The weight of the proctor mold + base plate + compacted moist soil in the mold (W2) was the determined.
10. The base plate was removed from the mold. Compacted soil cylinder was then extruded from the mold using a Jack.
11. The mass of the moisture can (M3) was determined.
12. The moisture sample from the moist soil extruded was collected using a moisture can and the mass of moisture can+ moist soil (M4) was determined.
13. Moisture can with moist soil was placed in oven to dry into a constant weight.
14. The rest of the compacted soil was broken to U.S. No. 4 sieve by hand and mixed with the leftover moist soil in the pan. More water was then added and mixed to raise the moisture content by approximately 2%.
15. Procedure 6 to 12 was then repeated. The weight of the proctor mold + base plate + compacted moist soil in the mold (W2) was first increase with increase in the moisture content then decreases. The tests were then carried out until at least two successive down readings were obtained.
16. The mass of moisture cans + soil samples (M5) was obtained in the next day.

Results

Table 1: Data for compaction test

Item

Test No.

1

2

3

4

5

1. Weight of mold and base plate W1 (Ib)

9.25

9.25

9.25

9.25

9.25

2. Weight of mold and base plate + moist soil, W2 (Ib)

13.006

13.284

13.494

13.45

13.387

3. Weight of the moist soil, W2-W1 (Ib)

3.756

4.034

4.244

4.200

4.137

4. moist unit Weight ϒ= W2-W1/1/30 (Ibft3)

112.68

121.02

127.32

126

124.11

5. moisture can number

D5

D21

D31

D41

D51

6. mass of the moisture can, M3(g)

60.49

62.22

61.76

63.09

64.25

7. mass of can+ moist soil, M4 (g)

101.58

106.36

122.51

126.10

137.25

8.Mass of can + dry soil, M5 (g)

96.05

99.95

112.58

114.70

123.60

9.moisture content, W (%) = (M4-M5/M5-M3) *100

15.55

16.99

19.54

22.09

23

10. Dry unit Weight ϒd (Ibft3) = ϒ/ {1+W (%)/100}

97.56

103.44

106.51

103.20

100.90

Table 2: Data for Zero-air unit weight calculation

Specific gravity of soil Gs

Assumed moisture content W (%)

Unit weight of water ϒ w(Ib/ft3)

Zero-air unit weight ϒzav (Ib/ft3)

2.68

10

62.4

131.9

2.68

12

62.4

126.5

2.68

14

62.4

121.6

2.68

16

62.4

117.0

2.68

18

62.4

112.8

2.68

20

62.4

108.7

Sample calculations

Weight of the moist soil=W2-W1 (Ib)=13.006-9.25=3.756 Ib

moist unit Weight ϒ= W2-W1/1/30 (Ibft3) =3.756/1/30=111.68 Ibft3

moisture content, W (%) = (M4-M5/M5-M3) *100= [(101.58-96.05)/ (96.05-60.49)] *100=15.55%

Dry unit Weight ϒd (Ibft3) = ϒ/ {1+W (%)/100} = [112.68/1+(15.55/100)] =97.56 Ibft3

ϒd (theory-max) = ϒzav =ϒw/W (%)/100 +(1/Gs)

Where ϒzav= zero-air unit weight

ϒw=unit weight of water

W (%) =moisture content

Gs=specific gravity of soil

ϒzav =ϒw/W (%)/100 +(1/Gs) = {62.4/ (10/100+1/2.68)} =131.9 Ib/ft3

Discussion

Using equation 10.8, W opt= [1.95-0.38]log 600 (PL)=0.894 PL

Where PL=14.1

W opt=0.894*14.1 ≈10.61%

Using equation 10.10, ϒd max (Ib/ft3) =6.361[ϒd max KN/M3] ≈144.27e-0.0164PL

PL=14.1

ϒd max (Ib/ft3) = 144.27e-0.0164*14.1≈114.5 Ib/ft3

W opt from the lab=19.54% and ϒd max from lab =106.51 Ib/ft3 as shown from the graph.

The difference between observed optimum moisture content W opt and estimated W opt is equal to (19.54%-10.61%=8.93%). The difference between observed maximum dry unit weight ϒd max (Ib/ft3) and estimated maximum dry unit weight ϒd max (Ib/ft3) is equal to (114.5 Ib/ft3-106.51 Ib/ft3 =7.99 Ib/ft3). The discrepancy between the observed and estimated values are caused by errors while carrying the experiment.

Possible sources of errors include, wrong measures of samples, improper reading and recording of the data, wrong interpretation of graphs and incorrect calibration of instruments used. The discrepancy between the observed and estimated values can reduced in future by eliminating all possible sources of errors.

Conclusion

Compaction of soil is an important process which helps in achieving various physical properties required for proper soil behavior under loading. For example, proper compaction of highway embarkment or earthen dam decreases the probabilities of its settlement by increasing the shear strength of the soil, reduces soil permeability and increases soil density.

Standard proctor compact test was carried out successfully and all the objectives were satisfied. The curves that relate moisture content with dry unit weight and zero-air unit weight of the soil was also obtained.

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