2011 PE Review:

1. 2011 PE Review: Michael C. Hirschi, PhD, PE, D.WRE Professor and Assistant Dean University of Illinois mch@illinois.edu

2. Acknowledgements: Rod Huffman, PE Review coordinator Daniel Yoder (2006 presenter) Rabi Mohtar & Majdi Abu Najm (2010 presenters)

3. Topics • Principles of Soil Physics (some duplication with Management session) • Soil Strength & Mechanics • Sediment Transport • Erosion Control • Slope Stabilization

4. Sources • Environmental Soil Physics; Hillel; 1998 Hi • Essentials of Soil Mechanics & Foundations, 2nd ed.; McCarthy; 1982; M • Soil and Water Conservation Engineering • 4th ed. Schwab, Fangmeier, Elliott, Frevert: S4 • 5th ed. Fangmeier, Elliott, Workman, Huffman, Schwab: S5 • Design Hydrology & Sedimentology for Small Catchments; Haan, Barfield, Hayes: H • USLE/RUSLE Handout: HO

5. Soil Physics & Mechanics • Soil classes and particle size distributions • Soil water • Content • Potential • Flow • Soil strength & mechanics

6. Soil classes & particle sizes Hi.61

7. Soil classes & particle sizes - 2 • ISSS easiest • Sand 0.02-2.0mm (20-2000μ) • Silt 0.002-0.02mm (2-20μ) • Clay <0.002mm (<2μ) • Soil texture classes based on USDA • Sand 0.05-2.0mm • Silt 0.002-0.05mm • Clay <0.002mm

8. Soil classes & particle sizes – 3 • Texture • EX: 50% sand, 20% silt 30% clay? Hi.64

9. Soil classes & particle sizes – 4 • Particle size distribution • EX: draw in a sandy clay loam? Hi.65

10. 0 Soil water content • Mt = Ms + Mw + Ma • Vt = Vs + Vw + Va • t = total, s = solids, w = water, a = air • ρb = bulk density = Ms/Vt≈ 1.1-1.4 g/cc • ρp = particle density = Ms/Vs ≈ 2.65 g/cc • Porosity = (Vw + Va) / Vt ≈ 25-60% • ρw = water density = Mw/Vw = 1.0 g/cc

11. Soil water content – 2 • ww = Mw / (Ms + Mw) • Water content wet basis • w = mass wetness = Mw / Ms • Water content dry basis • θ = Vw/Vt = Vw / (Vs + Vw + Va) • Volumetric water content

12. Calc.: soil water content • Given: • Soil with 30% water content dry basis • Find: • Best guess at equivalent inches of water in the top foot of soil?

13. Calc.: soil water content – 2 • Mw / Ms = 0.30 • Mw = Vw * ρw • ρb = Ms / Vt; Ms = Vt * ρb

14. Calc.: soil water content – 3 • Mw / Ms = 0.30 • Mw = Vw * ρw • ρb = Ms / Vt; Ms = Vt * ρb • Mw / Ms = (Vw * ρw)/(Vt * ρb) = (Vw / Vt)(ρw / ρb) • θ = Vw/Vt • θ *(ρw / ρb) = 0.3; θ = (ρb / ρw) * 0.3 • θ = 0.3 *(1.3/1.0) = 0.39 • 0.39 * 1 ft * 12”/ft = 4.7”

15. Soil water potential Hi.157

16. Soil water potential – 2 Cuenca.58

17. Soil water potential – 3 S5.337

18. Soil water potential – 4 Hi.162

19. Calc.: soil water potential • Given: • Mercury tensiometer • SG = 13.6 • Situation as shown • Find: • Total potential at C • Above or below water table? Cuenca.64

20. Calc.: soil water potential - 2 • 1) pick datum • 2) add pressures • Suction • Water depth • Gravity • T = z + p + pos • = -86cm • Above water table

21. Soil water flow • q = A*K*H/L • K = (q*L)/(A*H) • K values S4.359; S5.261; H.430 A H L q

22. Calc.: soil water flow • Given: • Need 50000 gpd through a 1-ft thick sand filter with K = 8 ft/d, and a total driving head of 3 ft • Find : • Required diameter for circular tank?

23. Calc.: soil water flow – 2 • q = A*K*H/L; A = (q*L)/(K*H)

24. Soil strength and mechanics • M.233-237,373-379 • Soil stresses • Normal = Fn/A = σ • Shear = Ft/A = τ • Fn = normal force • Ft = tangential or shear force • As σ↑ τ to cause failure = τf↑ • tan Φ = τf / σ; Φ = angle of internal friction

25. Soil strength and mechanics – 2 M.234

26. Calc.: soil strength • Given: • Well-graded sand; density 124 lb/ft3 • Find: • Ultimate shear strength 6 ft below surface?

27. Calc.: soil strength – 2 • From table, for well-graded sand, Φ = 32-35o = 33.5o • Normal stress = (124 lb/ft3)(6 ft) = 744 lb/ft2 • tan Φ = τf / σ; τf = σ * tan Φ = τf = 744 lb/ft2 * tan(33.5o) = 492 lb/ft2

28. Footing bearing loads • M.374-379 • qult = a1*c*Nc + a2*B*γ1*Nγ + γ2*Df*Nq • c = soil cohesion beneath footer • γ1,, γ2 = effective soil unit weight above and below footer • B = footer size term • Nc, Nγ, Nq = capacity factors • Df = footing depth below surface • qdesign = qult / FS

29. Footing bearing loads – 2 M.375

30. Calc.: footing load • Given: • Strip footing 3 ft wide • Wet soil with density of 125 lb/ft3 • Angle of internal friction = 30o • Cohesive strength of 400 lb/ft2 • Use factor of safety of 3 • Find: • qdesign in lb/ft2

31. Calc.: footing load – 2 • a1 = 1.0, a2 = 0.5, B = width = 3’ • γ1 = 125/2 = 62.5 lb/ft3; γ2 = 125 lb/ft3 • c = 400 lb/ft2 • Nc = 30, Nγ = 18, Nq = 20 • qdesign = 23,700/3 = 7900 lb/ft2

32. Soil compaction and density • Soil compaction • Greater strength • Dependent on water content • Proctor test • Pack soil into mold with pounding • Modified Proctor > 56000 ft-lbs

33. The Erosion Process • Soil erosion is a multi-step process: • Soil particle/aggregate detachment • Soil particle/aggregate transport • Soil particle/aggregate deposition • There must be detachment and transport for erosion to occur • Deposition (sedimentation) will occur somewhere downstream

34. A little soils review… • Soil primary particles: • Sand, 0.05mm to 2mm, 2.65 g/cc density • Silt, 0.002mm to 0.05mm, 2.65 g/cc • Clay, <0.002mm, 2.6 g/cc • Soil aggregates, chemically/electrically bonded sets of primary particles: • Large, in the sand range, 1.6 g/cc • Small, in the large silt range, 1.8 g/cc • These aggregate sizes are approximately those used in the CREAMS model (USDA-ARS)

35. Detachment • There are many sources of force and energy required to detach soil particles & aggregates: • Raindrop impact • Shallow surface flow shear • Concentrated flow shear • Many more, at larger scales

36. Transportation • Many of the same processes contribute force and energy for soil particle & aggregate transport: • Raindrop impact • Shallow surface flow • Concentrated surface flow • Channelized flow • Others

37. Balancing act • Foster & Meyer (1972) proposed a balance between detachment and transport for flowing water: • 1 = (transport load/transport capacity) + (detachment load/detachment capacity)

38. Essentially, if the flow is using all its transport capacity transporting sediment, there’s nothing left to detach more. Likewise, if the flow is detaching new sediment at detachment capacity, there’s no capacity to transport any sediment. Natural systems balance out…

39. Example • In the 80’s and 90’s there was a successful push to conservation tillage as a method to reduce sediment in lakes and streams • In many situations, no improvement was seen, but streambank erosion became more of a problem than it was in the past • I contend that now that the streams are receiving cleaner water (because of less upland erosion), but at similar rates, from farm fields, the stream uses less of its erosive energy to transport load it receives from runoff water, so it has capacity to undercut banks and scour the streambed

40. Multi-stage erosion

41. Soil erosion and sediment yield • Hillslope erosion • Channel system erosion • Sediment delivery to streams • Sediment transport in streams • Slope stability

42. Hillslope soil erosion • S4.91-111; S5.134-156; H.238-285 • Background • Detachment • Raindrop impact • By turbulent overland flow • Runoff • Transport downslope • By runoff

43. Hillslope soil erosion background • At the top of the slope • Detachment by raindrop impact • Transport by shallow sheet flow • Sheet erosion USDA-NRCS

44. Hillslope soil erosion background - 2 • Lower on slope • Small flow concentrations • Start to cut small channels • Rills • Roughly parallel • Head straight downslope • Random formation • Flow from sheet areas between rills • Sheet and rill erosion USDA-NRCS

45. Hillslope soil erosion background - 3 • Bottom of hillslope • Ends at concentrated flow channel • Low area in macrotopography • “ephemeral gullies” USDA-NRCS

46. Hillslope erosion factors • Rainfall erosivity • Intensity • Total storm energy • Soil erodibility • Topography • Slope length • Steepness • Management • Reduce local erosion • Change runoff path • Slow and spread runoff => deposition

47. USLE/RUSLE • A = R * K * LS * C * P • A = average annual soil erosion (T/A/Y) • R = rainfall erosivity (awful units) • K = soil erodibility (awful units) • R * K gives units of T/A/Y • LS = topographic factor (dimensionless) • C = cover-management (dimensionless) • P = conservation practice (dimensionless)

48. USLE/RUSLE – background • Empirical approach • >10000 plot-years of data • International use • Unit Plot basis; LS = C = P = 1 • Near worst-case management • R from good fit rainfall-erosion • K from K = A / R • C and P from studies • Subfactors in later versions

49. USLE/RUSLE – approach • Lookup • Maps, tables, figures • Databases • Process-based calculations • Show changes over time • Where don’t have good data

50. R factor – rainfall erosivity • maps • H.251; H.Appendix 8A; S4.99(SI); S5.143(SI); HO.1-5 • R(customary SI) = 17.02 * R(customary US) S4