Engineering Properties of Soils Unified Soil Classification

1. Engineering Properties of SoilsUnified Soil Classification

2. Engineering Properties of SoilsUnified Soil Classification

3. Engineering Properties of SoilsUnified Soil Classification

4. Soil #1 9.5mm (100%) 4.75mm (60%) 425mm (30%) 150mm (10%) 75mm (4%) Cu = D60/D10 = 4.75/.150= 32 Cc = (D30)2/ (D60xD10 =(.425)2/(.15x4.75)=0.25 SP Soil #2 4.75mm (88%) 425mm (28%) 75mm (9%) Wp = 20 WL = 31 Ip = 31 - 20 = 11 Cu = 25 Cc= 1.6 SW-SC Engineering Properties of SoilsUnified Soil Classification - Examples

5. Engineering Properties of Soils AASHTO Soil Classification

6. Engineering Properties of SoilsAASHTO Soil Classification • Example • 38mm (100%) • 2.00mm (65%) • 425mm (45%) • 75mm (30%) WL = 35 IP = 21 A-2-6

7. Engineering Properties of SoilsSoil Water • Types of water found in soil • Free water or gravitational • found below the water table • free to flow under the forces of gravity • Capillary water • brought up through soil pores • due to surface tension and found above water table in certain soil conditions • Attached water or held water • moisture film around soil grains • quantity may be very large for clays

8. Engineering Properties of Soils Soil Water

9. Engineering Properties of SoilsSoil Water

10. Engineering Properties of SoilsSoil Water • Water Flow Through Soils • where q is the flow of water (cm3/s) • I is the hydraulic gradient causing the flow • I = H (head loss due to flow through soil) L(length of path of flow through the soil) • A is the cross sectional area of the flow path (cm2) • k is the coefficient of permeability of average velocity of water through the soil (cm/s) • Darcy’s law can also be stated as • q = k H A L

11. Engineering Properties of SoilsSoil Water

12. Engineering Properties of SoilsSoil Water

13. Engineering Properties of SoilsSoil Water • Determining Permeability of Soils • Clean uniform sands • Hazen’s formula k=(D10)2 where: • k=coefficient of permeability (cm/s) • D10 = effective size (mm) • Sands • Constant Head Permeability Test • Fine sands and silts • Falling Head Permeability Test • Clays • Consolidation Test

14. Engineering Properties of SoilsSoil Water • Coefficient of Permeability (Sands) • Darcy’s Law q = k H A L • k = qL HA q = measured flow (cm3/s) H = head loss L = length of path (cm) A = cross sectional area (cm2)

15. Engineering Properties of SoilsSoil Water - Sands

16. Engineering Properties of SoilsSoil Water - Fine sands / silts

17. K = La ln (h1/ h2) TA a = area of the standpipe A = area of sample T = time L = length of sample h1,h2 = initial and final heads For fine sands/silts, small flows Used when the quantity of flow would be too small to measure properly by a constant head permeability test Engineering Properties of SoilsSoil Water - Fine sands / silts k

18. Engineering Properties of SoilsSoil Water - Capillary Rise

19. Engineering Properties of SoilsSoil Water - Capillary Water • Water that rises in tubes or pore spaces due to surface tension • hc varies inversely with d • hc can be determined by: surface tension force = force due to gravity of the volume of water S.T. x pd = pd2/4 x hc x g x rw hc = 4 x S.T. d x g x rw

20. Engineering Properties of SoilsSoil Water - Capillary Water Cont’d Example - For Water: Using: S.T. = 0.075g/cm g rw = 1 g/cm3 hc (cm) = 0.3 d (cm)

21. Engineering Properties of SoilsSoil Water • Typical Values of capillary rise • Sands 0-1 meters • Silts 1-10 meters • Clays over 10 meters • Pore sizes in soils are similar to tubes • pore sizes vary greatly with different soils and therefore difficult to measure • Estimate by 20 % of the effective size D10

22. Engineering Properties of SoilsSoil Water • Surface Tension in Soil Water • Soil is saturated above the groundwater table • difficulty in establishing ground water table • Apparent Cohesion in sands and silts • mistakenly indicating a clay material • Frost Heaving • water in large pores freeze • water in smaller pores not frozen drawn to ice crystal freezes enlarging the ice crystal • capillary water moves up pore spaces to replace smaller water particles • continuous process

23. Engineering Properties of SoilsSoil Strength and Settlement

24. Engineering Properties of SoilsSoil Strength and Settlement • Shear strength is shear stress resisting failure along a plane • Shear stress (t ) varies with mass of the block or normal stress (s) tan f = t/s t = s tan f t = shearing resistance s = normal stress on plane of failure f = angle of internal friction

25. Engineering Properties of SoilsSoil Strength and Settlement • Clays • shear strength is due to cohesion forces between the grains t = c • Granular soils • shear strength results from friction between the grains along the shearing plane t = s tan f • Mixed Soils • shear strength is due to both cohesion and friction t = c + s tan f

26. Shear strength in soils can be measured by a number of tests At failure, ef (strain at failure) is used to correct the cross sectional area Af = Ao 1- ef Unconfined Compressive Test clays strain (change in length) and load at failure are measured Unconfined Compressive strength= qu = Max Load Af  Shear Strength (cohesion) t = qu/2 Engineering Properties of SoilsSoil Strength and Settlement

27. Engineering Properties of SoilsSoil Strength and Settlement • Shear plane develops in stiff samples at 55-60° with horizontal Note: Soft saturated clays, bulging may occur ef = 0.15

28. Engineering Properties of SoilsSoil Strength and Settlement • Direct Shear Test any soil type • Maximum value shear force is measured • Stresses at failure, ts are calculated • s = N/A t = max shear stress/ A • Cohesionless Soils • Calculate t , s f = arctan (t / s) • shear strength t = s tan f • Soft Clays • shear strength = shear stress recorded

29. Engineering Properties of SoilsSoil Strength and Settlement • Mixed Soils • two tests required • Test #1 • s1 • t1 • Test #2 • s2 • t2 t = c + s tanf t (kPa) f c s (kPa)

30. Engineering Properties of SoilsSoil Strength and Settlement

31. Engineering Properties of SoilsSoil Strength and Settlement • Triaxial Compression Testany soil type • Cell pressure applied to simulate field conditions s3 • Clays are often tested in a quick shear test without drainage or water pressure measurements • s3 is applied • axial load s1 is applied • axial load to failure (s1-s3) • Calculate A0, ef (ratio of original length) • Af =A0 qq = (s1-s3)t = qq 1-ef Af 2 Shear strength /cohesion Compressive strength

32. Engineering Properties of SoilsSoil Strength and Settlement

33. Engineering Properties of SoilsSoil Strength and Settlement • Settlement Failures • Amount Fn (compressibility of the soil) • rearrangement of soil grains to a denser thinner layer • usually involves squeezing out of water when a load is applied • Clays • may have a loose structure and a high voids and moisture content and can be compressed considerably • Due to the extremely slow movement of water in clays time for settlement could take years

34. Engineering Properties of SoilsSoil Strength and Settlement • Settlement Failures • Granular Soils • grains are usually in close contact • Settlement usually takes place as the load is applied and does not lead to long term settlement problems • Consolidation Test • measures the amount and rate of compressibility • sample placed in cell and loaded measured over time