CHAPTER 6 SOIL COMPACTION Omitted Sections 6.6, 6.7, 6.8 SOIL COMPACTION In Geotechnical engineering practice, the soils at a given site are often less than desirable for the intended purpose. They may be: Weak (strength) Highly compressible Have a high permeability Solution Relocate the project Articulate design for structure members Stabilize or improve the properties of the soil The third alternative may be in most cases the most economical alternative. There are different techniques for improvement of soils (This subject is covered in details in CE 486 “Improvement of Geotechnical Materials”). We will consider in this course only compaction. SOIL COMPACTION Compaction is also very important when soil is used as an engineering material, that is the structure itself is made of soil. Ex. .Earth dams .Highways .Airfields .etc. Definition Compaction is the densification of soils by removal of air through the application of mechanical energy. The degree of compaction is measured in terms of its dry unit weight. SOIL COMPACTION Increases unit weight Increases shear strength Increases bearing capacity Increases stability of slopes of embankments Decreases settlement of structures SOIL COMPACTION You remember well-graded reduced Air Air Compaction Water Water Solid Solid SOIL COMPACTION General Principle The degree of compaction of soil is measured by its dry unit weight. When water is added during compaction it acts as a softening agent on the soil particles. gd(max) When the moisture content is Soil Solid gradually increased, the weight of the soil solids in a unit water volume gradually increases. Soil Solid Optimum moisture content (OMC) is the water content at g which the maximum dry unit weight d (max) is attained. SOIL COMPACTION Soil Solid water Soil Solid SOIL COMPACTION Types of Compaction Methods in the Laboratory •Impact or dynamic (The most common type) •Kneading •Static The laboratory test generally used to obtain the maximum dry unit weight of compaction and the optimum moisture content is called the Proctor compaction test. It is named after R. R. Proctor (1933) (engineer in LA). He established that compaction is a function of: 1.Moisture Content 2.Compactive Effort 3.Soil Type There are two methods or tests: Standard Proctor test (ASTM D-698 & AASHTO T-99) Modified Proctor test (ASTM D-1557 & AASHTO T-180) Standard Proctor Test • Mold 1/30 ft3 in volume • 3 layers • 25 blows • 5.5 lb hammer • 12 inch drop Mold Hammer The procedure for the standard Proctor test is elaborated in ASTM Test Designation D-698 (ASTM, 2007) and AASHTO Test Designation T-99 (AASHTO, 1982). Standard Proctor Test Standard Proctor Test Process of Compaction Several samples are mixed at different water contents Compact according to the compaction test (standard or modified). W g moist Vmold W = Weight of compacted soil in the mold 3 Vmold = Volume of the mold = (1/30 ft ) For each test find the moisture content of the compacted soil. The dry unit weight is given by g g moist d 1 w Plot g d vs. w From the plot, find OMC and g d (max) Standard Proctor Test In order to avoid a large number of compaction tests, it is desirable to begin the first test at a moisture content that is about 4 to 5% below the approximate optimum moisture content. Standard Proctor Test REMARKS 1. Each data point on the curve represent a single compaction test. 2. Four or five tests are required 3. The curve is unique for: - A given soil type - Method of compaction - (constant) compactive effort 4. gd(max) is only a maximum for a specific compactive effort and method of compaction. This does not necessarily reflect the maximum dry unit weight that can be obtained in the field. 5. Typical OMC are between 10% and 20%. Outside maximum range 5% to 40%. Standard Proctor Test 6. Increasing the compactive effort tends to increase the maximum dry density, as expected, but also decrease the OMC. (This is why the curve never be to the right of zero curve). 7. In practice less amount of water is used but higher compactive effort or vise versa. 8. For clay soils gd(max) tends to decrease as plasticity increases. 9. The approximation to field is not exact because the lab. test is a dynamic impact type, whereas field compaction is essentially a kneading-type compaction. 10. In the field, compactive effort is the number of passes or “coverage” of the roller of a certain type and weight on a given volume of soil. Standard Proctor Test Theoretical g d(max) The maximum is obtained when no air in the voids (i.e. s =100%) G g s g d 1 e w but wGs se for S 100% e wGs G g g s g w z av w 1 1 wGs w Gs Where gzav = zero air void unit weight. The relationship between gzav and w can be obtained as shown in the figure across. Compaction curve is always to the left of the zero-air-void curve. No matter how much water is added, the soil never g becomes completely saturated by compactiond (max). Standard Proctor Test To obtain the variation of gzav with moisture content, use the following procedure: Under no circumstances should any part of the compaction curve lie to the right of the zero-air-void curve. FACTORS AFFECTING COMPACTION Besides moisture content, other important factors that affect compaction are: 1) Soil type; 2) Compaction effort. 1. Effect of Soil Type Grain Size Distribution Shape of the soil grains Gs Amount of clay minerals Type of clay minerals Fine grain soil needs more water to reach optimum. FACTORS AFFECTING COMPACTION Effect of Soil type and gradation Fine grain soil needs more water to reach optimum. FACTORS AFFECTING COMPACTION Effect of Soil type and gradation Typical Values (kN/m3) OMC (%) Well graded sand SW 22 7 Sandy clay SC 19 12 Poorly graded sand SP 18 15 G g g Lows plasticityg clay w CL 18 15 z av w 1 1 wGs w Non plastic silt G s ML 17 17 High plasticity clay CH 15 25 Gs is constant, therefore increasing maximum dry unit weight is associated with decreasing optimum moisture contents. Do not use typical values for design as soil gisdhighly(max) variable. FACTORS AFFECTING COMPACTION Compaction Curves Encountered in Soils •The bell-shaped compaction curve is typical for most clayey soils. Typical •Some curves have more than one peak others have no peak. FACTORS AFFECTING COMPACTION 2. Effect of Compaction Effort Standard Proctor For the standard Proctor test • The standard Proctor mold and hammer were used to obtain these compaction curves. • For all cases the number of layers was equal to 3. g Compaction effort dmax wopt. EXAMPLE 6.1 TEXT IN SI UNITS EXAMPLE 6.1 TEXT IN SI UNITS EXAMPLE 6.1 TEXT IN SI UNITS EXAMPLE EXAMPLE Example (2nd Midterm Exam Fall 40-41) If you are checking the field compaction of a layer of soil and the compaction curve for the soil is shown in Figure 1. From the specifications, the dry density of the compacted soil should be at least 95% of the maximum value and within ± 1% of the optimum water content. When you did the sand cone test, the volume of the soil excavated was 1153 cm3. It weighed 2209 grams wet and 1875 grams dry. a. What is the compacted dry density? b. What is the field water content? c. What is the relative compaction? d. Does the test meet the specifications (explain)? e. If it does not meet, what should be done to improve the compaction so that it will meet the specifications? f. What is the degree of saturation of the field sample? g. If the sample were saturated at constant density, what would be the water content? Modified Proctor Test Modified Proctor Test (ASTM D-1557, AASHTO T-180) With the development of heavy rollers (also requirements of heavy aircrafts and trucks) and their use in field compaction, the standard Proctor test was modified for better representation of the field conditions. This is sometimes referred to as modified Proctor test. •Mold 1/30 ft3 in volume (same as for standard test) •5 layers •25 blows (same as for standard test) •10 lb hammer •18 inch drop Developed in WWII by U.S. Army Corps of Engineers to better represent the compaction required for airfield to support heavy aircraft. Modified Proctor Test Modified Proctor Standard Proctor Test Test Layer 5 Layer 4 Layer 3 Drop = 457.2 mm Layer 2 (18 in) Layer 1 Drop = 304.8 mm (12 in) hammer hammer = 2.5 kg (5.5 lb) = 4.54 kg (10 lb) Modified Proctor Test Standard Modified Proctor Proctor Test Test Volume of mold 944 cm3 944 cm3 # of layers 3 5 Mass of hammer 2.5 kg 4.54 kg Drop of hammer 30.5 cm 45.7 cm # of hammer blows 25 25 Compaction Energy for Unit Volume of Soil Standard Proctor Test (25)(3)5.5(1) E 12375 ft -lb/ft 3 592.5 kN m/ m3 600 kN m/ m3 (1/ 30) Modified Proctor Test (25)(5)10(1.5) E 56250 ft -lb/ft 3 2693.3 kN m/ m3 2700 kN m/ m3 (1/ 30) . Because it increases compactive effort, the modified Proctor test results in an increase of the maximum dry unit weight of the soil, and this is accompanied by decrease in the optimum moisture content. Note: In the field, compactive effort is the number of passes of the roller of a certain type and weight on a given volume of soil.