Engineering Properties of Soils Soil Types

1. Engineering Properties of SoilsSoil Types • Engineering Properties of Soils • foundation for the project • construction material (road embankments,earth dams) • Soil Definition (Engineering) • “refers to all unconsolidated material in the earth’s crust, all material above bedrock” • mineral particles (sands, silts, clays) • organic material (topsoil, marshes) + air + water

2. Engineering Properties of Soils Soil Types • Mineral Soil Particles • weathering of rock from the crust of the earth • physical weathering and chemical weathering • Physical Weathering • action of frost, water, wind, glaciers, plant/animals, etc. breaking particles away from original bedrock • particles transported by wind, water, ice >rounding and reducing their size • soils formed are called granular soil type • “grains are similar to the original bedrock”

3. Engineering Properties of Soils Soil Types • Chemical weathering • occurs when water flows through rocks and leaches out some of the mineral components of the rock • soils formed are called clays • “clay particles are mineral crystals that have very different properties from those of the original bedrock”

4. Engineering Properties of Soils Mineral Soils

5. Engineering Properties of Soils Soil Types • Granular and Cohesive soil types • difference in engineering properties result from the large variation in size and shape of the grains • Cohesive soil type (clays) • grains are extremely small and flat • the mass of a grain as a force is negligible when compared to the forces resulting from the surface properties of the grain

6. Engineering Properties of Soils Soil Types • Water Holding Capacity of Clays • Shrinkage • evaporation of exposed clays • loading • Expansion • dry side may absorb moisture • Structure of Clays • deposited by settling out in lakes

7. Engineering Properties of Soils Soil Types • Structure of Clays • surface charges forces grains to edge to side pattern • flocculent structure as opposed to granular soils which are deposited in a denser configuration because the force of gravity on the mass of these grains is more important

8. Engineering Properties of Soils Soil Types • Clayshave surface charges due to the very large surface area per gram of material • Chemical composition results in: • negative charges along the sides of a grain • positive charges at the ends of a grain • Results of these surface properties • water holding capacity of clays surface charges attract water • structure of clay deposits clay grain

9. Engineering Properties of Soils Soil Types

10. Engineering Properties of Soils Soil Types • Clay Soils • Small flat shape • Negative/positive surface charges • Bound water on the surface • Different clay minerals are different in size • Swelling clays absorb water into the crystal lattice • Shrinkage due to evaporation or loading

11. Engineering Properties of Soils Soil Types • Granular Soils • Larger grain sizes than clays • Particles tend to be more or less spheres/cubes • Bound water is small compared to overall mass • Silt particles may not be visible to eye but tend to be gritty, have dull appearance and lack cohesion when dry

12. Engineering Properties of SoilsSoil Types • Organic Soils • Tend to be fibrous and/or amorphous • Brown to Black in color • High moisture holding capacity • Water may run out when squeezed • Dried organic soils may combust

13. Engineering Properties of SoilsSoil Types • Silts are coarser than clays and not bond tightly together • Silts are gritty, less plastic and dull when cut • Dry Strength-silts loose “apparent cohesion” when dried • Shaking test-saturated silt samples become denser water seeps to the surface - “dilantancy”

14. Engineering Properties of SoilsMass-Volume Relationships

15. Example 1-2 A soil sample has a volume of 175cm3 and a total mass of 300g. Mass when dried is 230g. Relative density of the soil solids is 2.70. Find r, rD, w, e, S and n Given: Mw = M - MD = 70g MD = 230g M = 300g V = 175cm3 Engineering Properties of SoilsMass-Volume Relationships Air Water Solids

16. Calculations: Vw = Mw/rw = 70g/(1 g/cm3) =70 cm3 VD = MD/(RD x rw) = 230g/(2.70 x 1 g/cm3) = 85 cm3 VA = V- (VD + Vw ) = 175 - 155 = 20 cm3 VA = 20 cm3 VW = 70 cm3 VD = 85 cm3 V = 175 cm3 Engineering Properties of SoilsMass-Volume Relationships

17. Engineering Properties of SoilsMass-Volume Relationships Answer: r = M/V = 300g/175cm3 = 1.71 g/cm3 rD = MD/V = 230g/175cm3 = 1.31g/cm3 w = Mw / MD = 70g/230 g = 30.4 % e = Vv / VD = 90 cm3/85 cm3 = 1.06 S = VW /Vv = 70 cm3/90 cm3 = 78 % n = Vv /V = 90 cm3/ 175 cm3 = 51 %

18. Engineering Properties of SoilsMass-Volume Relationships - Rules 1. Density is given assume total unit volume 1 cm3 or 1 m3 2. Water content is given along with total density or total mass. Use MD = M or rD = r 1 + w 1+w 3. Void Ratio is given and RD assume a unit volume of soil solids VD = 1 m3

19. Engineering Properties of SoilsMass-Volume Relationship • Density Index • Field soil condition referred to as loose or dense • Density Index is insitu soil’s density relative to the maximum and minimum for that type of soil • Assessing the stability of granular soils • Known as relative density • ID = rDRY MAX x r D - r DRY MIN r DrDRY MAX - rDRY MIN

20. Engineering Properties of Soils

21. Engineering Properties of SoilsClassification Tests • Two types of tests used in classifying soils • Grain size, measures grain sizes • Plasticity, measures grain types • Grain Size • grain size distribution curve • Sieve analysis gravel and sand • Hydrometer test for silt and clay

22. Engineering Properties of SoilsClassification Tests • Hydrometer Test • Used to find the size of smaller grains to plot a grain size distribution curve • Stokes Law • particles in suspension settle out at a rate which varies with their size • hydrometer measures the density of a soil-water mix at various times as the grain settles • The size of particle to the center of the bulb can be calculated and density of the solution indicates the percentage of the sample stillin solution

23. Engineering Properties of SoilsSieve Analysis

24. Engineering Properties of SoilsClassification Tests

25. Engineering Properties of SoilsClassification Tests • Grain Size Distribution Curve • Shape • Uniform soil is composed of mainly one size grain • Well graded soil contains a wide range of grain sizes • Effective Size • Effective size is the grain size that only 10% of the grain sizes are finer than. • The amount and type of fine grains in a soil are important in assessing the properties of that soil

26. Engineering Properties of SoilsClassification Tests • Grain Size Distribution Curve • Uniformity Coefficient Cu • indication of the shape of the curve and range of particle sizes that the soil contains • Cu = D60 / D10 • Coefficient of Curvature Cc • indication of the shape of the curve. • Cc = (D30)2 / (D60 x D10)

27. Engineering Properties of SoilsClassification Tests • Curve A - Uniform Soil • Curve B - Well Graded Soil • Soil B • Effective size = .09 mm • Cu = 7/.09=78 • Cc = 1.12/(7x.09)=1.9

28. Engineering Properties of SoilsClassification Tests • Textural Classification • based entirely on grain size • ASTM System • Gravel larger than 4.75 mm (No. 4) • Sand 4.75 mm - 0.075 mm (No.4-No. 200) • Silt 0.075 mm - 0.005 mm • Clay smaller than 0.005 mm • Identify % of the grains as classified

29. Engineering Properties of SoilsClassification Tests

30. Engineering Properties of SoilsClassification Tests • Plasticity Test (grain type) • Measures the amount of water that a soil absorbs • Plastic Limit • w/c where the soil behaves like a plastic material • Liquid Limit • w/c where the soil behaves more like a liquid • Plasticity Index (Atterberg Limits Test) • the range of w/c’s where soil is plastic

31. Engineering Properties of SoilsClassification Tests

32. Engineering Properties of SoilsClassification Tests • Atterberg Limits test measures the consistency of the soil deposit • Soil A: Wp=25 Wl=32 w/c=35 • Soil B: Wp=40 Wl=80 w/c=45 • Atterberg Limits Test • soil rolled into a thread Wp • soil flow measured in a cup Wl

33. Engineering Properties of Soils Soil Classification • Unified Soil Classification ASTM D2487 • general engineering purposes • AASHTO Soil Classification • highway engineering purposes • classifies soils to it’s suitability as a subgrade