Title: Moisture-Unit Weight Relationships (Compaction Test)
Aim: To familiarize the student with the laboratory compaction test and to obtain the moistureunit weight relationship for a given compactive effort on a particular soil.
Objective: In preparing the geotechnical aspect of structures it is important to improve the soil strength via compaction. This experiment seeks to find the maximum dry unit weight/density (γdmax/ρdmax) and corresponding optimum water content (wopt) for a clay soil sample consisting of particles that have passed the US No. 4 sieve.
Introduction: Compaction of a soil generally increases the shear strength, decreases its compressibility, decrease its permeability and minimize long-term settlement. During this lab we used the Standard Proctor Compaction Test, which uses the application of mechanical energy to compact the soil sample at different water contents. The experiment simply requires accumulatively adding water and weighing after compaction. By deduction, the heaviest sample will have the greatest dry unit weight/density. The purpose of the experiment as stated above is to find this maximum dry density value, ρdmax that is, the level of compaction required for the soil to have maximum shear strength and minimum compressibility. The relative compaction (RC) is defined as: RC =
x 100(percent) This is the term used to compare the in situ compacted soil to the laboratory compaction
curve. This value may be more or less than 100 percent, therefore it is possible that for a given project it may be specified as greater or lower based on the required strength properties. There is no field compaction counterpart to the laboratory impact method of soil compaction. In spite of that, the standard compaction test is widely used to determine the compaction criteria.
The soil mass involved in the compaction process starts as a three-phase system of soil, water and air. The aim of the process is to obtain a two-phase system of only soil and water as the high pore pressures reduce the shear strength of the soil. It is practically impossible to have a zero air void condition therefore the compaction curve always falls below the curve of zero-air-voids dry unit density. To find the zero-air-void density, the following equation is used:
ρw is the density of water (1g/cm3) Gs is the specific gravity of soils(2.7)
Procedure: 1) Air-dry soil was obtained, on which the compaction test was to be conducted. All the soil lumps were broken down. 2) The soil was passed through a U.S. No. 4 sieve and all of the minus 4 material was collected in a large pan. This should be about 15 lb (7 kg) or more. 3) Enough water was added to the minus 4 material and mixed in thoroughly to bring the moisture content up to 2%. 4) The weight of the Proctor mould + base plate (not the extension), was determined. 5) The extension was then attached to the top of the mould. 6) The moist soil was scooped into the mould in three equal layers. Each layer was compacted uniformly by the standard Proctor hammer (5.5 lbs falling at a height of 12”) 50 times before another layer was added. Each layer was scratched with the spatula forming a grid to ensure uniformity in distribution of compaction energy before adding an additional layer to form a continuous sample.
7) It was ensured that the layers of loose soil that are being poured into the mould extend slightly above the top of the rim of the compaction mould at the end of the three-layer compaction. 8) The top attachment of the mould was removed and a straightedge was used to trim the excess soil above the mould. 9) The weight of the mould + base plate + compacted moist soil in mould, was determined. 10) The base plate was removed from the mould and a hammer was used to knock the compacted soil cylinder out of the mould. 11) The mass of the moisture can to be used was determined. 12) A sample from the soil cylinder obtained in Step 10 was collected in the moisture can, and the mass of the can + moist soil was determined. 13) The moisture can with the moist soil was placed in an oven to dry for at least 24 hours or until a constant weight was obtained. 14) The rest of the compacted soil broken down by hand and mixed with the leftover moist soil in the pan. More water was added and mixed to raise the moisture content to the desired percentage (increments of either 2% or 2.5 % were used). 15) Steps 6 to 12 were then repeated until there was a decrease in the weight of the mould + base plate + moist soil. 16) The mass of the moisture cans + soil samples, (from step 13) were then determined after a period of at least 24 hours.
Results: Results from a Standard Proctor Test. Wt. of Cylinder 2002 g Cylinder No. K/K1 Cylinder Volume 1311.93 cm3 Table 1: Cylinder Information Compaction Test Results 1 2 Run No. 3731 3826 Wt. of Sample & Container [g] 1729 1824 Wet. Wt of Sample [g] 1.31791 1.39032 Moist Density [g/cm3] 3 5 Moisture Content % Dry Weight 4.27459 5.92126 Actual Moisture Content% 1.26388 1.3126 Dry Density. [g/cm3] 4.27459 5.92126 Moisture Content At Saturation wsat% 2.42063 2.32784 Dry Density Table 2: Compaction Test Results
Can No. Wet Weight Gross [g] Dry Weight Gross [g] Wt. of Water Ww [g] Tare Weight [g] Dry Weight of Soil [g] Moisture Content%
Water Content Data 8 H4 81.03 96.9 78.67 93.14 2.36 3.76 23.46 29.64 55.21 63.5 3 5
H3 110.25 103.81 6.44 31.45 72.36 8
Table 3:
OPTIMUM WATER CONTENT %
11.8
MAX. UNIT DRY WEIGHT g/cm3
1.41
Table 4 : Summary Data
3 3942 1940 1.47874 8 8.89994 1.35789 8.89994 2.1769
4 4074 2072 1.57935 11 11.7952 1.41272 11.7952 2.04783
1 H 116.55 85.67 106.76 77.1 9.79 8.57 23.76 14.28 83 62.82 11 13.5
5 4009 2007 1.52981 13.5 13.6422 1.34616 13.6422 1.9732
Dry Density(g/cm3)
Dry Density Vs Moisture Content 1.44 1.42 1.4 1.38 1.36 1.34 1.32 1.3 1.28 1.26 1.24
Dry Density Vs Moisture Content
0
5
10
15
Moisture Content (%) Chart 1: Dry Density Vs Moisture Content
Zero Air Voids Curve 3
Dry Density(g/cm3)
2.5 2 1.5 Zero Voids Curve
1 0.5 0 0
5
10
Moisture Content (%) Chart 2: Zero Air Voids Curve
15
3
Dry Density(g/cm3)
2.5 2 Dry Density Vs Moisture Content
1.5
Zero Air Voids Curve
1 0.5 0 0
5
10
15
Moisture Content (%) Chart 3: Showing The Zero air voids curve and the Dry Density vs Moisture content.
CALCULATIONS The sample calculations listed below are all taken from RUN No. 1.
Volume of mould
= πr2h = 1311.93cm3
Wet wt. of sample
= wt. of sample & cylinder – wt. of cylinder = 3731 – 2002 = 1729 g
= wet wt. gross – dry wt. gross = 81.03 – 78.67 = 2.38 g
Wt. of water
= dry wt. gross – tare wt = = 55.21 g
Dry wt. of soil
=
Moisture content % dry weight
= = 4.27 %
=
Unit wet wt.
= =1.32 g/cm3
Unit dry wt.
= = 1.26 g/cm3
=
= 4.27%
Discussion: During construction, compaction is essential to increase the shear strength, decrease the compressibility, decrease its permeability and minimize long-term settlement in the soil. It is difficult to check those objectives directly therefore they are checked indirectly by finding the optimum moisture content and dry unit density. By finding the optimum moisture content and maximum dry density it allows civil engineers to gauge the soils strength. When soils close to 0% moisture content it can only be compacted by so much but as water is added, the dry unit weight increases because the water lubricates the particles making compaction easier. As more water is added and the water content becomes larger than the optimum values, the void spaces become filled with water so further compaction is not possible. Beyond this point, the dry unit weight decreases as shown in Chart 1. The points at which this begins to happen is described as the optimum moisture content and the maximum dry unit weight. In this test the Optimum Moisture Content and dry unit weight were found to be 11.8% saturation and 1.42 g/cm 3. For any given water content and soil solids, the zero-air-voids dry unit weight represents the best possible compaction. As shown in the Chart 3 the actual compaction curve was below the zero voids curve which is an expected result. For a given soil and water content the best possible compaction is represented by the zero-air-voids curve on Chart 3. The actual compaction curve will always be below it. Compaction of soil increases the density, shear strength, bearing capacity, thus reducing the voids, settlement and permeability. Hence the optimum moisture content and the maximum dry density are useful in the stability of field problems like earthen dams, embankments, roads and airfields. Compaction in the field is controlled by the value of the optimun moisture content determined by laboratory compaction test. In other words, the laboratory compaction tests results are used to write the compaction specifications for field compaction of the soil. Sources of error Water may not have been thoroughly absorbed into the dry soil. As a precaution an adequate period should be allowed to mature the soil after it is mixed with water
Each layer of soil may not have been the same depth into the collar of the mould as a precaution proper care should be taken to make sure that each layer is nearly equal in weight. To avoid stratification each compacted layer should be scratched with spatula before next layer is placed. Human error in operating the hand rammer, it is impossible to apply the same compaction energy to each layer. A possible precaution that could be taken is to ensure that the same person applies the blows to each layer. Another will be to ensure the rammer blows are uniformly distributed over the surface. Improvements More accurate results could be obtained if the manual compacting hammer was replaced with a mechanical arm along with using a fixed height and fixed force to uniformly compact the sample.
CONCLUSION Within the limits of experimental error, it was found that the maximum dry unit weight was 1.42 g/cm and the corresponding optimum moisture content was 11.8%. 3
REFERENCES
Standard Proctor Compaction Test Specifications (ASTM D-698)
Craig, R. F. (2004). Soil Mechanics (7th edition). Taylor and Francis, April 2004
Smith, Ian (2006). Smith's Elements of Soil Mechanics (8th Edition). WileyBlackwell.
Mitchell, James K.. (1993). Fundamentals of Soil Behavior. Second edition. John Wiley and Sons, New York.
http://www.engr.uconn.edu/~lanbo/CE240LectW011fundamentals.pdf
