SOIL COMPACTION
3. It is generally best to allow the contractor to select and use the appropriate equipment to achieve the desired compaction
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102 CHAPTER 5 SOIL COMPACTION
5 . 9 S U M M A RY
Compaction—the densifi cation of a soil by expulsion of air and forcing the soil particles closer together—is a popular method for improving soils. The laboratory test to investigate the maximum dry unit weight and the optimum water content is the Proctor test. This standard test is used in most applications. For heavy loads, the modifi ed Proctor test is used. Various types of equipment are available to achieve speci- fi ed compaction. You need to select the appropriate equipment based on the soil type and the availabil- ity of the desired equipment.
Self- A ssessment
Access Chapter 5 at http://www.wiley.com/college/budhu to take the end-of-chapter quiz to test your understanding of this chapter.
Practical Example
E X A M P L E 5.5 Interpreting Standard Proctor Test Results and Specifying Field Compaction Equipment
The standard Proctor test for a gravelly sand (24% gravel, 76% sand) to be used as a base course (a soil layer above the existing soil) of a highway embankment is shown in Figure E5.5a.
FIGUREE5.5a 21.0
20.5 20.0 19.5 19.0 18.5 18.0 17.5
17.00 2 4
Water content (%)
6 8
Dry unit weight (kN/m3)
(a) Specify the compaction criteria for the fi eld.
(b) Recommend fi eld compaction equipment that would achieve the desired compaction.
(c) Specify an appropriate quality control test.
Strategy Because the soil is a gravelly sand, it is best to specify compaction dry of optimum.
Solution 5.5
Step 1: Determine maximum dry unit weight and optimum water content.
The maximum dry unit weight and optimum water content are 19.6 kN/m3 and 5.8%, respectively.
Step 2: Specify dry unit weight and water content.
Specify 95% standard Proctor test to be compacted dry of optimimun (Figure E5.5b).
gd518.6 kN/m3; w54.4%
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Assume Gs5 2.7, where necessary, for solving the following problems.
Theory
5.1 Discuss the effects of not dropping the hammer in a standard Proctor test from a height of 305 mm. Plot a graph of energy versus hammer height from 250 mm to 400 mm in steps of 50 mm to discuss the effects. Assume the mold, hammer mass, number of layers (3), and num- ber of blows (25) are all constants.
5.2 (a) Plot the theoretical dry unit weight versus water content using the following data.
Water content (%) 4 5 8 10 12 Degree of saturation, S (%) 20 30 60 70 75 (b) Determine the maximum dry unit weight and opti-
mum water content.
Problem Solving
5.3 A soil at a mining site is classifi ed according to USCS as GW-GM.
(a) Would this soil be suitable for the base course of a road?
(b) What type of fi eld compaction equipment would you recommend?
(c) How would you check that the desired compaction is achieved in the fi eld?
(d) Would you specify compaction dry or wet of opti- mum? Why?
5.4 The water content in one of five standard Proctor test samples is 15.2%. The corresponding degree of saturation is 85%. Calculate the dry unit weight if Gs 5 2.67.
5.5 The data from a standard Proctor test are shown in the table below.
(a) Determine the maximum dry unit weight and the optimum water content.
(b) Plot the zero air voids line.
(c) Determine the degree of saturation at the maxi- mum dry unit weight.
Diameter of mold 101.4 mm Height of mold 116.7 mm Mass of mold 4196.50 grams Specifi c gravity, Gs 2.69
E X E R C I S E S
EXERCISES 103
FIGUREE5.5b 21.0
20.5 20.0 19.5 19.0 18.5
95% Maximum dry unit weight Maximum dry unit weight
18.0 17.5
17.0 4.4 5.8 6.7
0 2 4
Water content (%)
6 8
Dry unit weight (kN/m3)
Step 3: Determine fi eld method of compaction.
The soil contains a larger proportion of sand than gravel. From Table 5.1, a vibrating roller is excellent for sand and good for gravel. Specify a vibrating roller.
Step 4: Specify quality control equipment.
Either the sand cone or the nuclear density meter is suitable.
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104 CHAPTER 5 SOIL COMPACTION
Unit weight
determination Water content determination Mass of
can and Mass of can Mass Mass of wet soil wet soil and dry soil of can and mold (grams) (grams) (grams) (grams) 5906 108.12 105.10 42.10
6013 98.57 94.90 40.90
6135 121.90 114.70 42.70 6156 118.39 110.50 42.50 6103 138.02 126.80 41.80
Practical
5.6 A fi ne-grained soil has 60% clay with LL 5 220%, PL 5 45%, and a natural water content of 6%. A standard Proctor test was carried out in the laboratory and the following data were recorded.
Diameter of mold 101.40 mm Height of mold 116.70 mm Mass of mold 4196.50 grams Specifi c gravity, Gs 2.69
Unit weight Water content determination determination
Mass of Mass of
can and can and Mass Mass of wet soil wet soil dry soil of can and mold (grams) (grams) (grams) (grams) 6257 105.05 103.10 42.10 6356 100.69 97.90 40.90 6400 114.71 110.70 42.70 6421 134.26 128.50 42.50 6400 109.34 104.80 41.80 (a) Determine the maximum dry unit weight and opti-
mum water content.
(b) If the desired compaction in the fi eld is 95% of the standard Proctor test results, what values of dry unit weight and water content would you specify?
Explain why you select these values.
(c) What fi eld equipment would you specify to com- pact the soil in the fi eld, and why?
(d) How would you check that the specifi ed dry unit weight and water content are achieved in the fi eld?
5.7 Standard Proctor compaction test results on a sandy clay (35% sand, 55% clay, and 10% silt), taken from a borrow pit, are given in the following table.
Water content (%) 4.2 5.1 7.8 9.2 12 Dry unit weight (kN/m3) 16.9 18.1 19.6 19.5 18.5
The sandy clay in the borrow pit has a porosity of 65%
and a water content of 5.2%. A highway embankment is to be constructed using this soil.
(a) Specify the compaction (dry unit weight and water content) to be achieved in the fi eld. Justify your specifi cation.
(b) How many cubic meters of borrow pit soil are needed for 1 cubic meter of highway fi ll?
(c) How much water per unit volume is required to meet the specifi cation?
(d) How many truckloads of soil will be required for a 100,000-m3 highway embankment? Each truck has a load capacity of 22.5 m3 and regulations require a maximum load capacity of 90%.
(e) Determine the cost for 100,000 m3 of compacted soil based on the following:
Purchase and load borrow pit material at site, haul 2 km round-trip, and spread with 200 HP dozer 5
$15/m3; extra mileage charge for each km 5 $0.5/m3; round-trip distance 5 10 km; compaction 5 $1.02/m3. 5.8 A sand cone test was conducted for quality control dur- ing the compaction of sandy clay. The data are as follows.
Calibration to fi nd dry unit weight of the standard sand
Mass of Proctor mold 4178 grams Mass of Proctor mold and sand 5609 grams Volume of mold 0.00095 m3 Calibration of sand cone
Mass of sand cone apparatus and jar
fi lled with sand 5466 grams Mass of sand cone apparatus with
remaining sand in jar 3755 grams Sand cone test results
Mass of sand cone apparatus and jar
fi lled with sand 7387 grams Mass of excavated soil 2206 grams Mass of sand cone apparatus with
remaining sand in jar 3919 grams Water content of excavated soil 9.2%
(a) Determine the dry unit weight.
(b) The standard Proctor maximum dry unit weight of the sandy clay is 17.8 kN/m3 at an optimum wa- ter content of 10%. The specifi cation requires 95%
Proctor dry unit weight at acceptable water contents ranging from 9% to 10.8%. Is the specifi cation met?
Justify your answer.
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105 C H A P T E R