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Soil Compaction.

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Presentation on theme: "Soil Compaction."— Presentation transcript:

1 Soil Compaction

2 Compaction Compaction is the method of mechanically increasing the density of soil by removal of air. Dry density (gd) is the measure of the degree of compaction. Each measure a different thing

3 Why Compaction? Increase bearing capacity
Increase stability of slopes of embankments Reduce compressibility Reduce permeability Reduce volume changes Prevent frost damage

4 Laboratory Compaction Tests
Why Test?    Provide moisture density curve identifying optimum moisture. Compare the degree of compaction vs. specs (Relative Compaction, R.C.).

5 Compaction Laboratory Tests
Standard Proctor Test Modified Proctor Test

6 Standard Proctor Test V = 1/30 ft3 (944 cc)

7 Standard Vs. Modified Proctor Compaction
A small soil sample is taken from the jobsite.  A standard weight is dropped several times on the soil. The material weighed and then oven dried for 12 hours in order to evaluate water content . The Proctor, or Modified Proctor Test, determines the maximum density of a soil needed for a specific job site.  The test first determines the maximum density achievable for the materials and uses this figure as a reference.  Secondly, it tests the effects of moisture on soil density.  The soil reference value is expressed as a percentage of density.  These values are determined before any compaction takes place to develop the compaction specifications.  Modified Proctor values are higher because they take into account higher densities needed foe certain typed of construction projects.  Test methods are similar for both tests. Modified Proctor Standard Proctor

8 Moisture-Density Curve
Zero Air Voids Line S = 100% Dry side of optimum wopt Wet side of optimum dry side of optimum As the water content increases, the particles develop larger and larger water films around them, which tend to “lubricate” the particles and make them easier to be moved about and reoriented into a denser configuration. At wopt: The density is at the maximum, and it does not increase any further. Above wopt (wet side of optimum): Water starts to replace soil particles in the mold, and since w << s the dry density starts to decrease. Test 1 2 3 4 w gt gd gZAV

9 Allowable Moisture = (OMC – 3%) to (OMC + 2%)

10 Factors affecting Compaction
Compactive Effort Moisture Content Soil Type

11 Compactive Effort gd max wopt Zero air void Water content w (%)
Dry density gd (Mg/m3) Dry density d (lb/ft3) Line of optimums Modified Proctor Standard Proctor Holtz and Kovacs, 1981 gd max wopt Optimum water content is typically slightly less than the plastic limit. The line of optimum moisture contents is usually around 85% saturation

12 Soil Type Grain size distribution. Shape of soil grains.
Specific gravity of soil solids. Amount and type of clay minerals.

13 Constant compaction energy
Soil Type (cont’d) Dry Density Sand, some fines Zero air voids line gd max OMC Sand double peak moisture density curves because of capillary rise Clay Moisture content Constant compaction energy

14 Check Point Method Check Point Method wopt Line of optimums
100% saturation 1 point Proctor test Known compaction curves A, B, C Field check point X (it should be on the dry side of optimum) A Y (No) Dry density, gd gd max B X X M M C wopt Holtz and Kovacs, 1981 Water content w %

15 Relative Compaction Correlation between relative compaction (R.C.) and the relative density Dr Typical required R.C. = 90% ~ 100%

16 Field Compaction

17 Elephant and Compaction
Question? The compaction result is not good. Why? He He! I’m smart. Heavy Weight

18 Static or Vibratory Vibration Impact Kneading Pressure
Types of Compaction Vibration Impact Kneading Pressure Static or Vibratory 

19 Compaction Equipments and Techniques
Smooth Wheel rollers Sheepsfoot Rollers Pad Rollers Pneumatic (Rubber-Tired) Rollers Grid Rollers Vibratory Rollers

20 Smooth-wheel roller (drum)
100% coverage under the wheel All soil types except for rocky soils. Contact pressure up to 380 kPa Compactive effort: static weight Most common use is for proof-rolling subgrades and compacting asphalt pavement. Holtz and Kovacs, 1981

21 Sheepsfoot Rollers 8% ~ 12 % coverage Best for clayey soils.
Contact pressure from 1400 to 7000 kPa Compactive effort: static weight and kneading.

22 Pad Roller About 40% coverage
Best for compacting fine-grained soils (silt and clay). Contact pressure is from 1400 to 8400 kPa Compactive effort: static weight and kneading.

23 Pneumatic Rollers 80% coverage under the wheel.
Bet for Granular and fine-grained soils. Contact pressure up to 700 kPa. Compactive effort: static weight and kneading.

24 Grid Rollers About 40% coverage
Contact pressure is from 1400 to 8400 kPa Best for compacting fine-grained soils (silt and clay). Compactive effort: static weight and kneading.

25 Vibratory Compactors Compactive effort: static weight and vibration.
Suitable for granular soils

26 Compaction Type Vs. Soil Type
Materials Vibrating Sheepsfoot Rollers Static Sheepsfoot Grid Roller Scraper Vibrating Plate Compactor Vibrating Roller Vibrating Sheepsfoot Scraper Rubber-tired Roller Loader Grid Roller Lift Thickness Impact Pressure (with kneading) Vibration Kneading (with pressure) Gravel 12+ Poor No Good Very Good Sand 10+/- Excellent Silt 6+/- Clay

27 Compaction Difficulty Vs. Soil Type
Fill Materials Permeability Foundation Support Pavement Subgrade Expansive Compaction Difficulty Gravel Very High Excellent No Very Easy Sand Medium Good Easy Silt Medium Low Poor Some Clay None+ Moderate Difficult Very Difficult Organic Low Very Poor Not Acceptable

28 Nondestructive Testing:
Field Density Destructive Testing: Sand Cone Core Cutter Rubber Balloon Nondestructive Testing: Nuclear Density

29 Standard Sand with known Gs
Sand Cone Test W1 Standard Sand with known Gs W2 W1= mass of sand cone before test W2 = mass of sand cone after test W3 = mass of sand filling cone and hole (W3 = W1-W2) W4 = mass of sand filling the cone = gsand*Vcone W5 = mass of sand filling the hole = W3-W4 Vhole = W5 / gsand W6 = mass of soil extracted from the whole w = moisture content of soil W4 W5

30 Sand Cone Test Procedure
A small hole (6" x 6" deep) is dug in the compacted material to be tested.  The soil is removed and weighed, then dried and weighed again to determine its moisture content.  The specific volume of the hole is determined by filling it with calibrated dry sand from a jar and cone device.  The dry weight of the soil removed is divided by the volume of sand needed to fill the hole.  This gives the density of the compacted soil. 

31 Suitable for cohesive soils only
Core Cutter Static Load Suitable for cohesive soils only 5 in 4 in

32 Rubber Balloon

33 Nuclear Density Nuclear Density meters are a quick and fairly accurate way of determining density and moisture content.  The meter uses a radioactive isotope source (Cesium 137) at the soil surface (backscatter) or from a probe placed into the soil (direct transmission).  The isotope source gives off photons (usually Gamma rays) which radiate back to the mater's detectors on the bottom of the unit.  Dense soil absorbs more radiation than loose soil and the readings reflect overall density.  Water content can also be read, all within a few minutes. 

34 Nuclear Density

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