Merrill Zwanka Geotechnical Materials Engineer SCDOT Research and Materials Lab February Definitions Sampling and Testing Classification

Transcription

1 Soil as a Construction Material Merrill Zwanka Geotechnical Materials Engineer SCDOT Research and Materials Lab February 1999 Soil as a Construction Material Definitions Sampling and Testing Classification 1

2 Soil Particle Size Mixture of any or all of: Gravel - Rock Particles 5 mm-150 mm Sand - Rock Particles mm-5 mm Silt - Rock Particles mm mm Clay - Mineral Particles less than mm Soil Consistency Consistency denotes the degree of firmness - a function of particle size and moisture content Solid - like Hard Candy Semi-Solid - like Cheese Plastic - like soft butter to putty (deforms without cracks) Liquid - like pea soup to soft butter 2

3 Soil Characteristics Soil is different than any other material that we work with in construction It is non-uniform We can use it as it is (good material) We can improve the properties (mix, geosynthetics) We can modify it (cement, geosynthetics) We can remove it and replace it (poor material) Soil Properties How do we determine the soil properties such as particle size, consistency, etc.? How do you know what soils are good or poor for use in construction? 3

4 Typical Soil Profile Horizons O -Organic A - Eluviated B - Accumulated C - Parent Material Fall Line 4

5 Soil Sampling Sampled using continuous flight augers for road and bridge design purposes Sampled from road or pit for use in embankments and other earthwork Observations and measurements of the soil type is recorded Soil Testing Particle Size Analysis Mechanical analysis - separate the clay particles and vibrate the remaining particles through a nest of sieves 5

6 Soil Consistency Atterberg Limits Liquid Limit (LL) - Water content in which the soil behaves as a viscous liquid Plastic Limit (PL) - Lowest water content in which the soil behaves as a plastic material Liquid Limit Soil has moisture added to it - placed in cup, groove carved in center, cup dropped until soil contacts, repeated at different moisture contents Sample Preparation Cup Groove 6

7 Plastic Limit Soil sample is rolled between the fingers and a solid surface until a 1/8 inch diameter thread is obtained - it is re-mixed and re-rolled until it crumbles before obtaining a 1/8 inch thread - the moisture content is then determined Plasticity Index (PI) The range of moisture contents within which the material is in a plastic state PI = LL - PL 7

8 Soil Strength Tests Triaxial Shear Strength Tests drained or undrained shear strength of sample Used for bridge design, retaining wall design, embankment design, etc. CBR Triaxial California Bearing Ratio (CBR) Measures resistance to penetration (Index) Empirical Data Used for pavement design Soil Classification In order to use soil data for design and construction, we classify it according to a standard procedure Predict the behavior of each soil Check k for compliance with specifications 8

9 Soil Classification Several procedures available for classifying soil types AASHTO ASTM USDA (Agriculture) FAA (Aviation) Corps of Engineers USBR (Bureau of Reclamation) 9

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11 Example Soil Classification % Passing #10 Sieve = 83 % Passing #40 Sieve =48 % Passing #200 Sieve =31 Liquid Limit =37 Plasticity y Index =17 Given the information above, what is the soil classification? Soil Classifications A-1-a Gravel - Well Graded A-1-b Gravel - Silty A-3 Sand - Poorly Graded A-2-4 or A-2-5 Sand - Silty A-2-6 or A-2-7 Sand or Gravel - Clayey A-4 A-5 A-6 A-7 Silt - Organic or Inorganic Silt - Inorganic Clay - Inorganic Clay - Organic or Inorganic 11

12 Soil Classification Group Index Only for soils classified as A-2-6 and greater Provides an indication of the material usage for subgrade GI of 0 indicates a good subgrade material GI of 20 or greater indicates a very poor subgrade Shown in parenthesis after the classification A-2-6(5), A-7-6(25), etc. 12

13 Compaction Theory Soil is composed of three phases Air Water Solids Air Water Solids 13

14 What is compaction? Compaction p is the rapid application of force to the soil mass, resulting in the removal of air. Air Uncompacted Volume Water Solids What is compaction? Compaction p is the rapid application of force to the soil mass, resulting in the removal of air. Uncompacted Volume Air Water Solids Compacted Volume 14

15 What is compaction? During g compaction, there is no change in the volume of water and solids! Uncompacted Volume Air Water Solids Compacted Volume Why is compaction important? Compaction: p Increases strength Decreases permeability Prevents future settlement 15

16 What is moisture content? Moisture content is the weight of the water divided by the weight of the solids. Since we are only removing air from the soil, compaction does not change moisture content. Moisture content can exceed 100%. What factors control compaction? Soil type Moisture content Compactive effort We can control o moisture content t and compactive effort! 16

17 How does moisture content affect compaction? Too much water results in less compaction. Too little water results in less compaction. Water should be just right... Moisture-Density Curves Zero Air Voids Line Maximum Density Optimum Moisture Content 17

18 How does compactive effort affect compaction? Increasing compactive effort How does compactive effort typically affect compaction? As compactive effort increases Optimum moisture content necessary to achieve maximum density decreases Maximum density increases 18

19 Why is lift thickness important? Importance of Lift Thickness Compactive Effort Compactive Effort 0 0 An equal compactive effort applied to an excessively thick lift may be 1 1 substantially reduced at the lower 2 elevations. A much greater compactive force would need to be 3 applied to achieve suitable density. Variables ab Well graded materials vs. poorly graded materials Soil type and moisture condition Compaction equipment Compaction procedure Site conditions Note: Maximum lift thickness requirements are typically given in the project specifications. If requirements are not given, the project geotechnical engineer should be consulted for the recommended lift thickness. 19

20 SCDOT Compaction Specifications Overview Embankment fill 8 inches, 95% SC-T-25 or SC-T-29 Aggregate Bases Sand Clay - 10 inches, 100% SC-T-25 or SC-T- 29 Marine Limestone, Macadam, Recycled Concrete, Coquina - 10 inches, 100% SC-T-140 So, how do we determine what moisture content is just right?? The SCDOT One-Point Proctor Curve!!! 20

21 CompactionAnd Gradation Control Definitions Information Formulas & Equations DEFINITION OF COMPACTION Compaction-The densifying or Packing Together of Soil Grains or Particles while Entrapped Air is Squeezed Out. (Two Things Needed) Maximum Dry Density-Dry Unit Weight - Obtained From A Given Compactive Effort As Defined By The Peak of A Compactive Curve. Optimum Moisture Content- Amount Of Water At Which The Maximum Density of A Material Can Be Obtained Within A Given Compactive Effort. 21

22 Density Density = Weight Per Unit Volume = Weight/Volume in pounds per cu.ft. or grams per cubic centimeter Bucket Capacity-1/4 cu. ft. =22.3 lbs.when full of water. Empty bucket weighs 6.7 lbs. Wt. Water= 22.3 lbs lbs. = 15.6 lbs. Volume of Water= 1/4 cu. ft.=.25 cu. ft. Density of water= weight water/volume of water =15.6 lbs./.25 cu. ft.= 62.4 pcf As Density Increases: THESE INCREASE shear strength load bearing THESE DECREASE compressibility Permeability shrinking/swell susceptibility to erosion freeze/thaw damage 22

23 Methods Of Measuring Density LAB PROCEDURES: AASHTO T-99 moisture-density relations of soils using 5.5 lb. Rammer & 12 inch drop (method A,B,C,D). AASHTO T-180- moisture-density relations of material using a 10 lb. Rammer & 18 inch drop(method A,B,C,D). SC-T-140 same as the old AASHTO T-180. FIELD PROCEDURES: SC T-25 field method of determining moisture-density relations of soils. SC T-29 field determination of MDD & OMC of soils by onepoint proctor method. Density Determination Density which is determined by direct or indirect measurement is called the ACTUAL DENSITY. Actual Dry Density is compared to theoretical maximum dry density to determine percent compaction. % Compaction = Actual Dry Density divided by theoretical MAX Dry Density. 23

24 AGGREGATE BASE DENSITY Contract specifies 10 Macadam base. Compaction required- 100% of MDD as determined by SC- T-140. Sample submitted to lab for testing reported MDD OF pcf & 5.8 % omc You ran field density using Troxler nuclear gauge & determined the actual wet den to be pcf. >moisture content of 3.9 pcf. Does it meet compaction requirement? DD= =135.9 pcf % Compaction= 135.9/138.7 x 100= 98 % (actual/theor.) Required=100 % >>Fails Suggestions to contractor? Factors That Influence Test Results Size & shape of mold Mold support Sample prep Type,magnitude & distribution of compactive effort Layer Thickness Degradation of particles Temperature 24

25 Two Ways Of Controlling Compaction (Practical Aspects) SPECIFY A METHOD OF COMPACTION Ex. Sec of std specs (old spec) - Initial compaction of sand-clay base less than 8 thick will be done w/sheepsfoot roller and will continue until roller walks out of base coarse. Final rolling done by pneumatic(rubber tire) rollers. SPECIFY END RESULT Ex. Sec of std specs (new spec) -earth type base with total thickness greater than 8 to be compacted 100% max den. Max. den determined by SC-T-25,or SC-T-29. END RESULT USED Where there is a Specified amount of compaction to be achieved, field compaction tests required. METHODS USED BY SCDOT Actual Density- Nuclear gauge Moisture Content- speedy moisture tester, pan drying Max Dry density - Lab uses oven drying. AASHTO T-99(standard), AASHTO T-180(modified), SC-T- 140 Four Pt. Field proctor (SC- T-25) One Pt. Field proctor (SC-T-29) 25

26 FACTORS THAT INFLUENCE COMPACTION Moisture content Method of preparation Uniformity of Procedures Environmental influences Type of Roller Compactive effort Underlying layer Lift thickness Rate of Compaction Moisture Content GRADATION CONTROL All subbases & Bases in Standard Specifications except cement mod subbase have gradation requirements Gradation requirements are a means of specifying distribution of particle sizes which make up an aggregate. Normally specified as % passing a particular sieve. Ex. Macadam Base (sec ) Sieve % by wgt.pass / / # # #

27 One Point Proctor Test for Typical Soils in South Carolina (Family of Curves) Introduction Compaction is a key factor in highway construction Must know maximum dry density and optimum moisture content of soil Time consuming laboratory and field tests AASHTO T-99 - Several Moisture Contents 27

28 Procedure Development SCDOT Research and Materials Laboratory performed research for better procedure Wanted maximum dry density and optimum moisture content of soils faster and more accurately Louisiana had a successful approach using a family of curves - SCDOT would use similar method (Ohio ) Procedure Implementation Procedure discussed with FHWA and field personnel training requested (Spring 1970) Dropped supplemental sand curves Developed SC-T-29: Field Determination of Maximum Dry Density and Optimum Moisture Content of Soils by One-Point Proctor Method Report completed in 1972 Procedure conveyed to construction personnel for use on projects throughout the state 28

29 SC-T-29 Determine weight of standard proctor mold (without collar or base plate) to nearest gram. 29

30 SC-T-29 Place standard mold (with base plate and collar attached) on block of concrete. SC-T-29 Obtain approx gms of material (tested for in-place density and moisture content) and sieve through No. 4 sieve. Discard material retained on the sieve. 30

31 SC-T-29 Mix 2500 gm sample to obtain uniform moisture content. SC-T-29 Place 1/3 of sample in proctor mold and compact with 25 blows of 5.5 lb. Hammer at 12 height of drop 31

32 SC-T-29 Add another 1/3 of original sample and compact again (25 blows, etc.) Repeat above step one last time. SC-T-29 Remove extension collar and carefully trim the compacted soil with a straightedge (even with top of mold). 32

33 SC-T-29 Remove base plate from mold and weigh mold and specimen to nearest gram. SC-T-29 Remove the material from the mold and slice vertically through the center. 33

34 SC-T-22 Take a representative sample of the material from one of the cut faces and determine the moisture content by use of the speedy moisture tester. Clean all old reagent from the previous test. SC-T-22 Place 3 scoops of reagent into the speedy cavity. Place 2 steel balls into the cavity and invert the cap. 34

35 SC-T-22 Weigh the appropriate amount of material (20 g. or 26 g.) depending on the type speedy you have. SC-T-22 Place the material in the cap and turn cavity horizontal. Place the cap on the cavity without mixing the material with the reagent. Then tightened the cap and begin shaking horizontally. 35

36 SC-T-22 Shake the material for approximately 3 minutes or until needle stops rising. SC-T-22 Check moisture every minute or so. If the moisture reading looks to be over 20, then stop the procedure and add the 10g or 13g weight and run again. 36

37 SC-T-22 Read moisture to the nearest 10 th and Turn to moisture correction chart in Sec. L and convert the speedy reading to oven moisture. SC-T-22 Check for material that has not completed the chemical reaction (wet material or clay balls). If found then run test again. 37

38 SC-T-29 Determine the weight of the specimen by subtracting weight of mold. 38

39 SC-T-29 Determine wet density of soil specimen by multiplying the mold constant (stamped on the base plate) by the weight of the soils specimen. SC-T-29 Using the computed moisture content and determined wet density, plot the one point on the Family of Curves 39

40 SC-T-29 If the point falls on one of the curves, the maximum dry density and optimum moisture content may be read from table SC-T-29 If the point does not fall on one of the curves, it is necessary to interpolate between the curves and then use the table 40

41 SC-T-29 Example Calculation Weight of mold and soil = 4006 gms Weight of mold = 2074 gms Weight of soil = A - B = = 1932 gms Wet density of soil = (Mold k) * (soil wt.) = * 1932 = pcf Speedy Moisture Reading (from Speedy) = 11.3 % Moisture (Converted to Oven Moisture) = 11.8% Falls between Curves 8 and 9: Maximum dry density (Family of Curves) = pcf Optimum moisture content (Family of Curves) = 12.5% Comments May not be applicable to all soils in SC May need to conduct more detailed test SC-T-25 Decision is left to the Resident Constr. Engr. 41

42 Determine Actual Density with Nuclear Gauge Obtain Second Speedy Moisture 42

43 Determine % Compaction Compare Theoretical density from One- Point Proctor with Actual or In-Place density from Nuclear Gauge Calculate as a percentage (percent compaction) Comparison of Methods Old Way Several densities in lab Field density by balloon or sand cone methods Moisture by pan drying Time Consuming! New Way One density in field Actual density by nuclear gauge Moisture by Speedy Moisture gauge Easier and faster for inspectors 43

44 Any Questions? 44