Topic #4 Soil Compaction

Transcription

1 Topic #4 Soil Compaction

2 What is Compaction? A simple ground improvement technique, where the soil is densified through external compactive effort. Compactive Effort + Water = 2

3 Compaction and Phase Diagram Phase diagrams showing the changes in soil as it moves from its natural location to a compacted fill. Note that the volume of solids does not change during the process. 3

4 Volumetric Change During Excavation and Compaction Changes in volume as soil is excavated, transported, and compacted. The numerical values are examples and would be different for each soil. 4

5 Definition Soil compaction is defined as the method of mechanically increasing the density of soil by reducing volume of air. Load g soil (2) > g soil (1) Air Air Soil Matrix Water Solids Compressed soil Water Solids g soil (1) = W T1 V T1 g soil (2) = W T1 V T2 5

6 Compaction of Soils in the Laboratory 6

7 Compaction in the Laboratory Compaction test is performed on graded aggregate soils in the laboratory to obtain the compaction curve. The compaction curve is used to determine the optimum water content (w opt ) and the maximum dry density (g dry ) of the soil sample for a specific compactive effort. Standard Proctor 3 layers 25 blows per layer 2.49 kg (5.5 lb.) hammer 305 mm (12 in.) drop Modified Proctor 5 layers 25 blows per layer 4.54 kg (10 lb.) hammer 457 mm (18 in.) drop 7

8 Mold and Hammer for a Proctor Compaction Test In the standard test we compact the soil in three layers, (as shown), while in the modified test we compact the soil in five layers. 8

9 Dry Density (g d ) Compaction Curve - Soil grains densely packed - Good strength and stiffness g d (max) - Low permeability Optimum Moisture Content (OMC) Water content (w%) 9

10 Role of the Water at the Two Sides of the MD Curve Dry Side: Water acts as a lubricating agent and replaces the air in the void structure, therefore results in higher density Beyond OMC, excess water results in lower density. The water content-density relationship indicating the increased density resulting from the addition of water and applied compaction effort. Soil is a silty clay, LL = 37, PI = 14, standard Proctor compaction (after Johnson and Sallberg, 1960). 10

11

12 Soil Compaction in the Laboratory Standard Proctor Test 5.5 pound hammer g dry g 1 wet w g ZAV Gsg w Gsw 1 S Zero Air Void Curve S r =100% Dry Density H = 12 in g d max blows per layer w c1 w c2 w c3 w c4 w c g d1 g d2 g d3 g d4 g d5 Increasing Water Content 4 inch diameter compaction mold (V = 1/30 cubic foot) g dry G s g 1 e w Dry to Optimum OMC Wet to Optimum Optimum Moisture Content Compaction Curve Water Content 12

13 Dry Density ( d ) Zero Air Void (ZAV) Curve corresponds to 100% saturation S<100% Zero air void curve (S=100%) d Gs w 1 wg S>100% (impossible) s All compaction points should lie to the left of ZAV curve Water Content (w) 13

14 Summary of Proctor Compaction Test Methods: 1- Standard Proctor Test 2- Modified Proctor Test Compaction Energy: 12,400 ft.- Ib/ft 3 Compaction Energy: 56,200 ft.- Ib/ft 3 Standard Proctor Test Modified Proctor Test Test

15 Comparison between Standard and Modified Proctor Compaction Tests 15

16 Dry density ( d ) Effect of Compaction Energy on Moisture-Density Curve Increasing compactive effort results in: E 2 (>E 1 ) Lower optimum water content Higher maximum dry density E 1 Water content 16

17 Compaction Energy Proctor Tests 1- Standard Proctor Test ASTM D-698 or AASHTO T-99 Energy = 12,375 foot-pounds per cubic foot Dry Density g d max Line of optimum (LOO) g d max Zero Air Void (ZAV) Curve S r = 100% Compaction Curve for Modified Proctor test 2- Modified Proctor Test ASTM D-1557 or AASHTO T-180 Energy = 56,520 foot-pounds per cubic foot Compaction Curve for Standard Proctor test w p w s Moisture Content Energy = Number of blows per layer x Number of layers x Weight of hammer x Height of drop hammer Volume of mold 17

18 Dry Density Effect of Energy on Soil Compaction Increasing compaction energy Lower OMC and higher dry density Higher Energy In the lab increasing compaction energy = increasing number of blows (assuming weight of the hammer and the drop height stays the same) or by dividing the soil in thinner lifts. In the field increasing compaction energy = increasing number of passes or reducing the thickness of the lift. Water Content 18

19 Comparison of Standard Proctor Test and Modified Proctor Test Results for the Same Soil 19

20 20

21 more dispersed fabric Dry density ( d ) Dry Density Compaction and Clay Fabric Higher water content or higher compactive effort results in more oriented fabric. more dispersed fabric Moisture Content 21

22 Standard and Modified Proctor Compaction Curves (Summary) g dry g 1 wet w g ZAV Gsg w Gsw 1 S 22

23 Improvements in Particle Contacts by Mechanical Compaction 23

24 Laboratory Compaction Test Example The laboratory test for a standard proctor is shown below. Determine the optimum water content and maximum dry density. If the G s of the soil is 2.70, draw the ZAV curve. Volume of Proctor Mold (ft 3 ) Weight of Wet Soil (lb.) Water Content (%) 1/ / / / / /

25 Laboratory Compaction Test Solution g wet W V wet mold g dry g 1 wet w Volume of Proctor Mold (ft 3 ) Weight of Wet Soil (lb.) Wet Unit Weight (lb/ft 3 ) Water Content (%) Dry Unit Weight (lb/ft 3 ) 1/ / / / / /

26 Dry Density (lb/ft 3 ) g dry g 1 wet w 111 g dry max Optimum Water Content (%) g ZAV Gsg w Gsw 1 S 26

27 Typical Compaction Specifications Relative Compaction (RC) in percent: g d RC g ( Field ) d ( Lab)

28 Field Compaction Curvess The most efficient or economical water content is between w opt and b. Because of the differences between lab and field compaction methods, the maximum dry density in the field may reach 90% to 95%. 28

29 Relative Density-Definitions In practice, its often useful to know how dense or how loose the soil is with respect to its densest and loosest conditions. That s when we use the relative density concept. Relative Density or Index Density based on Void Ratio (e) Relative Density or Index Density based on Dry Density (g d ) e max : Void ratio of the soil in its loosest condition e min : Void ratio of the soil in its densest condition e : Current void ratio of the soil in the field g d (max) : Dry unit weight of the soil in its densest condition g d (min) : Dry unit weight of the soil in its loosest condition g d : Current unit weight of the soil in the field D r I d e e g max g max d d (max) e e g min d (min) g d (min) 29

30 Filed Compaction Different types of rollers (clockwise from top right): - Smooth-wheel roller - Vibratory plate - Pneumatic rubber tired roller - Sheepsfoot roller 30

31 Smooth Wheeled Roller Field Compaction Compacts effectively only to mm; therefore, place the soil in shallow layers (lifts) 31

32 Field Compaction Vibrating Plates Commonly used for compacting relatively small areas, effective for granular soils. 32

33 Sheepsfoot Roller Field Compaction Provides kneading action; walks out after compaction, very effective on clays. 33

34 Field Compaction Impact Roller Provides deeper (2-3 m) compaction. e.g., airfield 34

35 Recent Modifications to the Impact Rollers

36 Grid Roller 36

37 Selection of Rollers Based on Soil Type Soil types best suited for various kinds of compaction equipment. (Adapted from Caterpillar, 1993.) 37

38 Compaction Control A quality control measure where you check at regular intervals whether the compaction was done according to specifications. e.g. one core per 1000 m 2 of compacted soil in the field Minimum dry density Range of water content Field measurements of d obtained using: Non-Destructive Testing, such as Nuclear Density Gauge Destructive Testing, such as Sand Cone Test 38

39 Sand Cone Test 39

40 Determination of the Soil Density in the Field Sand Cone Test 1- Sand Cone (ASTM D1556) 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 us the density of the compacted soil in lb. per cubic foot. This density is compared to the maximum Proctor density obtained in the lab, which gives us the relative density of the compacted soil in the field. 40

41 Performing Sand Cone Test in the Field 41

42 Performing a Nuclear Density Test in the Field 42

43 Determination of the Soil Density in the Field Nuclear Density Gauge 2- Nuclear Density (ASTM D2292) Nuclear Density meters are a quick 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 detectors at the bottom of the unit. Dense soil absorbs more radiation than loose soil and the readings reflect overall density. Water content (ASTM D3017) can also be read, all within a few minutes. 43

44 Intelligent Compaction Intelligent Compaction (IC) measures stiffness (the ability of a material to resist deformation under a load) rather than density of the compacted soil. A strong correlation exists between stiffness and bearing capacity of foundations. A machine equipped with an intelligent compaction system provides four basic functions: 1) Measures the stiffness of the soil. 2) Controls or guides the compaction effort in response to the measured stiffness. 3) Displays the stiffness measurement to the operator. 4) Maps and records the compaction results. Such a system would enable the user to produce detailed plots of the soil stiffness levels, the number of roller passes, as well as the location and time of the application. 44

45 Dynamic Compaction Pounding the ground by a heavy weight Suitable for granular soils, land fills and karst terrain with sink holes. Pounder (Tamper) solution cavities in limestone Crater created by the impact (to be backfilled) 45

46 Dynamic (or Impact) Compaction 46

47 Dynamic Compaction Pounder (Tamper) Mass = 5-30 ton Drop = m D = n (W H) 0.5 D: Influence depth (improvement)in meters W: Mass of tamper in mega grams H: Drop height in meters n: model parameter, typically less than 1 47

48 Construction Equipment Large Dump Truck 48

49 Construction Equipment Grader for spreading soil 49

50 Construction Equipment Bulldozer for evenly spreading soil 50

51 Construction Equipment Loader 51

52 Construction Equipment Backhoe 52

53 Construction Equipment Crawler mounted Hydraulic Excavator 53

54 Construction Equipment Rock Breaker 54

55 Construction Equipment Water Truck 55