Field (Visual) Classification of Soils

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1 Field (Visual) Classification of Soils by Chuang Lin Department of Civil and Environmental Engineering University of Alaska Fairbanks June 6, Summer Transportation Institute, Fairbanks, AK

2 Outline Background and Objectives Soil types Characterization of fine-grained soils Characterization of coarse-grained soils 2

3 Background Based on Unified Soil Classification System (USCS) ASTM Standards Lab classification: ASTM D2487 Field (visual) classification: ASTM D2488 3

4 Objectives Recognition Description Classification 4

5 Recognition - Soil Types 5

6 Boulder and Cobble Boulder >12 in (>30.5 cm) 6

7 Boulder and Cobble Cobble 12 in 3 in (30.5 cm 7.6 cm) 7

8 Gravel Coarse 3 in 0.75 in (75 mm 20 mm) Fine 0.75 in 0.2 in (20 mm 4.75 mm) 8

9 Sand Coarse 0.2 in 0.08 in (4.75 mm 2 mm) Medium 0.08 in 0.02 in (2 mm 0.4 mm) Fine 0.02 in in (0.4 mm mm) 9

10 Description-Soil Properties Only for G or S Only for C or M Only for G or S Angularity Shape Color Odor Moisture Condition Consistency Cementation Range of the Particle Size Maximum Particle Size 10

11 Soil Particle Shape Bulky most cases Soil particle shape Flaky mostly clay minerals Needle shaped less common Bulky particles Angular Subangular Subrounded Rounded 11

12 Shape and Angularity Rounded or angular? Flat or elongated? 12

13 Moisture Content impermeable pores (ip) permeable pores (pp) solid (s) MC - dry MC - wet 13

14 Consistency MC - wet MC - dry What is soil consistency? A method of describing the degree of cohesion and adhesion between soil particles as related to the resistance of the soil to deform or rupture 14

15 Cementation Strong Moderate Strong If it will crumble or break with finger pressure? 15

16 Classification-Flow Charts Two Broad Categories Fine-grained: 50% or more finer than No. 200 sieve M or C, H or L, portion of coarse-grained Coarse-grained: less than 50% finer than No. 200 sieve G or S, W or P, portion of fine-grained 16

17 Types of Gradations Gap graded Uniformly graded Well graded Open graded 17 17

18 Visual Determination Well or dense graded 18

19 Visual Determination Gap graded 19

20 Visual Determination Uniformly graded 20

21 Visual Determination Remember that percent is by dry weight. Gravel Particles overpower gradation 21

22 Textual Determination When wetted thoroughly, coarse-grained soils have a distinct grittiness. Practice on known samples Easiest method to differentiate fine-grained soil from fine sand 21

23 Tests to Characterize Fine-Grained Soils Liquid Limit Evaluation (M or C) High - Greater than 50 Low - Less than 50 Plasticity Evaluation (H or L) Plastic Index (PI) above or below A-line 23

24 Liquid Limit Evaluation With small amount of soil sample that passing No. 40 sieve 24

25 Liquid Limit Evaluation Water penetration rate: fast or slow? 25

26 Liquid Limit Evaluation Any hydration heat? 26

27 Liquid Limit Evaluation How many drops of water to penetrate one tablespoon of soil? 27

28 Dilatency Evaluation When shaken, dilatent soil releases free water and becomes livery in appearance 28

29 Dilatency Evaluation When Squeezed, dilatant soil becomes dull 29

30 Toughness Evaluation Reaching plastic limit requires adding considerable dry soil. Thread can be re-rolled several times after reaching plastic limit. Thread requires pressure to form. Thread can be rolled out to lengths > 2 30

31 Ribbon Evaluation Length of ribbon Strength Related to clay content Ribbon more than 2 inches long indicates clay content is >= 40% 31

32 Shine Evaluation 32

33 Dry Strength Evaluation Very high: cannot be crushed even on hard surface High: can be crushed only against hard surface Medium: can be crushed with considerable pressure Low: crumbles with some pressure 33

34 Typical Behaviors-CH Soil High liquid limit High toughness - easily roll out thread to 1/8 inch diameter to length of several inches Zero dilatency Ribbons to more than 1-1/2 inch High dry strength High Shine 34

35 Typical CH soil Slickensides 35

36 Typical CH soil Cracks on Surface 36

37 Typical Behaviors-ML Soil Low liquid limit Slight to no toughness - if a thread can be formed to 1/8 inch diameter it can t be reformed Medium to high dilatency Ribbons only slightly if any Low to very low dry strength No Shine (watch out for mica) 37

38 Typical Behaviors-MH Soil High liquid limit Medium toughness - if a thread can be formed to 1/8 inch diameter it has low strength Low dilatency Ribbons slightly to moderately Low to Medium dry strength Low Shine (watch out for mica) 38

39 Typical Behaviors-CL Soil Low liquid limit Medium to high toughness a thread can be formed to 1/8 inch diameter with medium toughness Low to none dilatency Ribbons slightly to moderately Medium dry strength Low to Medium Shine (watch out for mica) 39

40 Distinguishing between CL and MH Difficult PI CH CL ML 50 MH PI is the same - Only difference is in LL LL 40

41 Example Problem A field sample has the following properties: Fine content 60%, gravel 18% and sand 22% Ribbon test < 1.5 in., high dilatency, water penetrates into soil relatively fast and no heat generation, low dry strength What is the Group Symbol and Group Name of this sample? Refer to the flow chart in the handout 41

42 Tests to Characterize Coarse-Grained Soils Visual Examination Determine whether G or S is predominant Determine if clean or dirty If Clean Determine if poorly or well graded If Dirty Determine if fines are plastic or not 42

43 Clean or Dirty? When wetted a clean coarse-grained soil will not leave a stain on your palm. A dirty soil will leave a stain 43

44 Clean or Dirty? If a soil sample is dropped in a beaker of water, a cloud remaining after about 30 seconds indicates more than 12 % fines 44

45 Well Graded or Poorly Graded? A well graded sample has a wide range of particle sizes that are about equally distributed by weight 45

46 Well Graded or Poorly Graded? A gap-graded sample has a range of particle sizes missing. This sample has coarse and fine but no medium size particles 46

47 Well Graded or Poorly Graded? A poorly graded sample is predominately one size of particle 47

48 Poorly Graded Sand 48

49 Well Graded Sand 49

50 Evaluation of Plasticity of Fines The Most Useful Field Tests are Toughness and Dilatency Tests 50

51 Example Problem A field sample has the following properties: Gravel 65%, sand 18% and fines17% easily roll out threads into 1/8 in. dia. to several inches, low dilatency, water can hardly penetrates into soil and heat generation, strong dry strength What is the Group Symbol and Group Name of this sample? Refer to the flow chart in the handout 51

52 Thank you! Questions? 52

Chapter 3 Soil Classification

Soil Classification - N. Sivakugan (2000) 1/11 3.1 INTROUCTION Chapter 3 Soil Classification Soils can behave quite differently depending on their geotechnical characteristics. In coarse grained soils,