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I.B Soil Conservation Systems. Rabi H. Mohtar Professor, Environmental and Natural Resources Engineering Executive Director, Strategic Projects, Research & Development Qatar Foundation [email protected] or [email protected] July 2013. Materials To Be Covered .

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i b soil conservation systems
I.B Soil Conservation Systems

Rabi H. Mohtar

Professor, Environmental and Natural Resources Engineering

Executive Director, Strategic Projects, Research & Development

Qatar Foundation

[email protected] or [email protected]

July 2013

materials to be covered
Materials To Be Covered
  • Principles of Soil Physics
  • Sediment Transport
  • Erosion Control
  • Soil Mechanics
  • Slope Stabilization

This review will provide you with an overall understanding and not necessarily makes you an expert!

I.B Mohtar

sources
Sources
  • Environmental Soil Physics; Hillel; 1998 Hillel (1998)
  • Essentials of Soil Mechanics & Foundations, 7th ed.; McCarthy; 2007; McCarthy (2007)
  • Soil and Water Conservation Engineering
    • 4th ed. Schwab, Fangmeier, Elliott, Frevert: Schwab et al (1993)
    • 5th ed. Fangmeier, Elliott, Workman, Huffman, Schwab: Fangmeier et al (2006)
  • Design Hydrology & Sedimentology for Small Catchments; Haan, Barfield, Hayes: Haan et al (1994)
  • USLE/RUSLE: USDA Agricultural Handbook No. 537 (1978)
  • Cuenca, R. H. 1989. Irrigation System Design - An Engineering Approach. Prentice-Hall, Inc., Englewood Cliffs, NJ. 552 pp. Cuenca (1989).
  • Ward, Elliot 1995 (Environmental Hydrology, Lewis Publishers).
  • http://cobweb.ecn.purdue.edu/~abe325/: Mohtar soil and water resources conservation course.

I.B Mohtar

soil physics mechanics
Soil Physics & Mechanics
  • Soil classes and particle size distributions
  • Basics of soil water
    • Water Content
    • Water Potential
    • Water Flow
  • Soil strength & mechanics

I.B Mohtar

soil classes particle sizes
Soil Classes & Particle Sizes

Hillel (1998) page 61

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soil classes particle sizes 2
Soil Classes & Particle Sizes - 2

ISSS classification is easiest

  • Sand 0.02-2.0mm (20-2000μ)
  • Silt 0.002-0.02mm (2-20μ)
  • Clay <0.002mm (<2μ)

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soil classes particle sizes 3
Soil Classes & Particle Sizes – 3

Soil Textural Triangle

Example 1:

Find the soil texture for this soil:

  • 50% sand,
  • 20% silt

I.B Mohtar

Hillel (1998) page 64

soil classes particle sizes 4
Soil Classes & Particle Sizes – 4

Particle size distribution

Example 2

Draw in a

sandy clay

loam?

Hillel (1998) page 65

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soil structure and functionality

Pedon

Soil Structure and Functionality

Clay particles

Primary

peds

Inter-ped

pore space

Mineral

grains

Primary soil

mapping unit

Clay pore space

Primary soil mapping

unit

Soil type

REV

Horizon

=

Pedostructure

=

Primary ped

=

Geomorphological

unit

Clay plasma porosity

(micro-porosity)

Vertical porosity

(cracks, fissures)

Interpedal porosity

(macro-porosity)

Pedostructure, primary peds, primary particles, are functionally defined and quantitatively determined using the shrinkage and potential curve measurement

+

Pedostructure

+

Primay particles

and pedological features

+

Primary peds

and free

mineral grains

I.B Mohtar

Mohtar (2008)

soil water content

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 (why dry basis?)
  • ρp = particle density = Ms/Vs ≈ 2.65 g/cc
  • Porosity = (Vw + Va) / Vt ≈ 25-60%
  • ρw = water density = Mw/Vw = 1.0 g/cc

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soil water content 2
Soil Water Content – 2
  • Water content wet basis:

Ww = Mw / (Ms + Mw)

  • Water content dry basis:

W = mass wetness = Mw / Ms

  • Volumetric water content:

θ = Vw/Vt = Vw / (Vs + Vw + Va)

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calc soil water content
Calc.: Soil Water Content

Soil Water

Example 3.

Given:

  • Soil with 30% water content dry basis

Find?

  • Best guess at equivalent inches of water in the top foot of soil?

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

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calc soil water content 3
Calc.: Soil Water Content – 3
  • 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”

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soil water potential soil characteristic curve
Soil Water Potential soil characteristic curve

Hillel (1998) page 157

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soil water potential 2
Soil Water Potential – 2

Cuenca (1989) page 58

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slide17

Soil Water Management

Ward, Elliot 1995 (Environmental Hydrology, Lewis Publishers)

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soil water potential 3
Soil Water Potential – 3

Fangmeier et al (2006) page 337

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soil water potential 4
Soil Water Potential – 4

Hillel (1998) page 162

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calc soil water potential
Calc.: Soil Water Potential

Soil Water Potential

Example 4.

  • Given:
    • Mercury tensiometer
      • SG = 13.6
    • Situation as shown
  • Find:
    • Total potential at point C
    • Is point C above or below the current water table?

Cuenca, (1989) page 64

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calc soil water potential 2
Calc.: Soil Water Potential - 2
  • Pick datum
  • Add pressures
    • Suction
    • Water depth
    • Gravity
  • T = z + p + pos
    • z = + 80 cm
    • p = ?
    • T = -86cm
    • Point C is above water table. Why?

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soil water flow
Soil Water Flow
  • q = A*K*H/L
    • K = (q*L)/(A*H)
  • K values

A

H

L

q

Fangmeier et al (2006) page 261; Schwab et al (1993) page 359; Haan et al (1994) page 430

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calc soil water f low
Calc.: Soil Water Flow

Darcy Law Application

Example 5.

  • 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?

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calc soil water flow 2
Calc.: Soil Water Flow – 2

q = A*K*H/L; A = (q*L)/(K*H)

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soil erosion and sediment yield
Soil Erosion and Sediment Yield
  • Hillslope erosion
  • Channel system erosion
  • Sediment delivery to streams
  • Sediment transport in streams
  • Slope stability

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hillslope soil erosion
Hillslope soil erosion
  • Background
    • Detachment
      • Raindrop impact
      • By turbulent overland flow
        • Runoff
    • Transport downslope
      • By runoff

Schwab et al (1993) pp:91-111; Fangmeier et al (2006) pp:134-156; Haan et al (1994) pp:238-285

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hillslope soil erosion background
Hillslope Soil Erosion Background

At the top of the slope

  • Detachment by raindrop impact
  • Transport by shallow sheet flow
  • Sheet erosion

USDA-NRCS

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hillslope soil erosion background 2
HillslopeSoil 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

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hillslope soil erosion background 3
Hillslope Soil Erosion Background - 3
  • Bottom of hillslope
    • Ends at concentrated flow channel
    • Low area in macrotopography
    • “ephemeral gullies”

USDA-NRCS

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hillslope erosion factors
HillslopeErosion 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

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usle rusle
USLE/RUSLE
  • A = R * K * LS * C * P
    • A = average annual soil erosion (T/A/Y)
    • R = rainfall erosivity (long empirical units)
    • K = soil erodibility (long empirical units)
      • R * K gives units of T/A/Y
    • LS = topographic factor (dimensionless, 0-1)
    • C = cover-management (dimensionless, 0-1)
    • P = conservation practice (dimensionless, 0-1)

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usle rusle background
USLE/RUSLE – background
  • Empirical approach been in use since 1960
    • >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
    • Sub-factors in later versions

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

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r factor rainfall erosivity
R factor – rainfall erosivity

Haan et al (1994) pp:251; Haan et al (1994) Appendix 8A; Schwab et al (1993) 99(SI); Fangmeier et al (2006) pp:143(SI); USDA (1978) pp:1-5

  • Maps
    • R(customary SI) = 17.02 * R(customary US)

S4

I.B Mohtar

k factor soil erodibility
K factor – soil erodibility
  • Soil surveys, NASIS, Haan et al (1994) 261-262; USDA 6
  • Erodibilitynomograph: Haan et al (1994) 255; Schwab et al (1993) 101; Fangmeier et al (2006) pp144; USDA (1978) pp: 7
    • No short-term OM

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ls topography factor
LS – Topography Factor
  • New tables & figures
    • Haan et al (1994) 264; USDA (1978) 8
  • Know susceptibility to rilling
    • High for highly disturbed soils
    • Low for consolidated soils

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c cover management factor
C – cover-management factor
  • Part of normal management scheme
    • Lookup: Schwab (1993) 102; Fangmeier et al (2006) pp: 146; Haan et al (1994) 266; Hillel (1998) Appendix 8; USDA (1978) 9
  • It Changes over time

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c cover management factor 2
C – Cover-Management Factor - 2
  • Subfactor approach (RUSLE)
    • C = PLU * CC * SC * SR * SM; all 0-1
      • PLU = prior land use
        • roots, buried biomass, soil consolidation
      • CC = canopy cover; % cover & fall height
      • SC = exp(-b * % cover)
        • b = 0.05 if rills dominant; 0.035 typical; 0.025 interrill
      • SR = roughness; set by tillage, reduces over time
      • SM = soil moisture; used only in NWRR

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p conservation practice factor
P – Conservation Practice Factor
  • Common practices
    • Contouring, strip cropping, terraces
  • Change flow patterns or cause deposition
  • Lookup tables
    • Schwab (1993) pp:103; Fangmeier et al (2006) pp:146; Haan et al (1994) pp: 281; USDA (1978) pp:10

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calc usle rusle
Calc.: USLE/RUSLE

Example 9:

  • Given:
    • Materials in handout
    • 3-Acre construction site near Chicago
    • Straw mulch applied at 4 T/A
    • Average 20% slope, 100’ length
    • Loamy sand subsoil
    • Fill (loose soil)
  • Find:
    • Erosion rate in T/A/Y

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calc usle rusle 2
Calc: USLE/RUSLE – 2
  • R = 150 (HO.1)
  • K = 0.24 (HO.7)
  • LS = 4 (HO.8-high rilling)
  • C = 0.02 (HO.9)
  • P = 1.0
  • A = R * K * LS * C * P = 2.9 T/A/Y

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calc usle rusle 2 1
Calc: USLE/RUSLE – 2.1

Example 10:

  • Given:
    • Materials in handout
    • 16-A site near Dallas, TX
    • Silty clay loam subsoil
    • Average 50% slope, 75’ length
    • Cut soil
  • Find:
    • By what percentage will the erosion be reduced if we increase our straw mulch cover from 40% cover to 80% cover?

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calc usle rusle 2 2
Calc: USLE/RUSLE – 2.2
  • Only thing different is C
    • Only subfactor different is SC
  • SC = exp(-b * %cover)
    • For consolidated soil, b = 0.025
  • SC1 = exp(-0.025 * 40%) = 0.368
  • SC2 = exp(-0.025 * 80%) = 0.135
  • Reduction = (0.368 – 0.135)/0.368 = 63%

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sediment delivery
Sediment Delivery
  • USLE/RUSLE for hillslopes
    • Erosion
    • Delivery
  • Erosion critical for soil resource conservation
  • Delivery critical for water quality
    • Movement through channel system

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sediment delivery 3
Sediment Delivery – 3
  • SDR (Sediment Delivery Ratio)
    • Hillslope erosion
    • Empirical fit for watershed delivery
  • Channel erosion/deposition modeling
    • Erosion
    • Transport
    • Deposition

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sediment delivery ratio
Sediment Delivery Ratio
  • Haan et al (1994) pp:293-299
  • SDR = SY / HE
    • SDR = sediment delivery ratio
    • SY = sediment yield at watershed exit
    • HE = hillslope erosion over watershed

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sediment delivery ratio 2
Sediment Delivery Ratio – 2
  • Area-delivery relationship

Haan et al (1994) pp:294

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sediment delivery ratio 3
Sediment Delivery Ratio – 3
  • Relief-length ratio
    • Relief = elev change along main branch
    • Length = length along main branch

Haan et al (1994) page .294

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sediment delivery ratio 4
Sediment Delivery Ratio – 4
  • Forest Service Delivery Index Method

Haan et al (1994) pp:295

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sediment delivery ratio 5
Sediment Delivery Ratio – 5
  • MUSLE (Haan et al (1994) pp: 298 and 298
    • Y = 95(Q * qp)0.56 (Ka)(LSa)(Ca)(Pa)
      • Y = storm yield in tons
      • Q = storm runoff volume in acre-in
      • q = peak runoff rate in cfs
      • K, LS, C, P = area=weighted watershed values
    • SDR = 95(Q * qp)0.56/(R * area)
      • R = storm erosivity in US units
    • Routing for channel delivery

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calc sdr
Calc: SDR

Example 11:

  • Given:
    • Flow path length in watershed = 4000ft
    • Elevation difference = 115ft
  • Find?
    • SDR

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calc sdr 2
Calc.: SDR – 2
  • R/L = 115/4000 = 0.029
  • From figure SDR = 0.45

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channel erosion deposition modeling
Channel Erosion-Deposition Modeling
  • Process-based small channel models
    • Foster-Lane model
      • Haan et al (1994) pp285-289
      • Complicated and process-based
    • Ephemeral Gully Erosion Model
      • EGEM
      • Fit to Foster-Lane Model results

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channel erosion deposition modeling 2
Channel Erosion-Deposition Modeling – 2
  • Large-channel models
    • Sediment transport
    • Channel morphology

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sediment transport
Sediment Transport
  • Settling (Haan et al (1994) pp:204-209)
    • Stokes’ Law
      • Vs = settling velocity
      • d = particle diameter
      • g = accel due to gravity
      • SG = particle specific gravity
      • ν = kinematic viscosity
    • Simplified Stokes’ Law
      • SG = 2.65
      • Quiescent water at 68oF
      • d in mm, Vs in fps

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calc stokes law settling
Calc.: Stokes’ Law Settling

Example 12:

  • Given:
    • ISSS soil particle size classification
  • Find:
    • Settling velocities of largest sand, silt, and clay particles

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calc stokes law settling 2
Calc.: Stokes’ Law Settling – 2
  • ISSS classification
    • Largest particles size
      • Clay = 0.002mm
      • Silt = 0.2mm
      • Sand = 2mm
    • Vs,clay = 1.12*10-4 fps = 0.04 ft/hr = 0.97 ft/day
    • Vs,silt = 0.11 fps = 405 ft/hr = 1.83 mi/day
    • Vs,sand = 11.24 fps = 7.66 mph = 184 mi/day

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calc stokes law settling1
Calc.: Stokes’ Law Settling

Example 13:

  • Given:
    • Stokes’ Law settling
  • Find: particles larger than what size can be assumed to settle 1 ft in one hour?

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calc stokes law settling 21
Calc.: Stokes’ Law Settling – 2
  • Vs = [(1 ft)/(1 hr)](1 hr/3600s) = 2.778*10-4 fps
  • d = (Vs/2.81)1/2 = 0.00994mm

I.B Mohtar

break
BREAK

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soil strength and mechanics
Soil Strength and Mechanics

From McCarthy (1982) pages 233-237,373-379

  • Soil stresses
    • Normal Stress = Fn/A = σ
    • Shear Stress = Ft/A = τ
      • Fn = normal force
      • Ft = tangential or shear force
    • As normal stress (σ) ↑, sheer stress (τ) to cause failure (τf)↑ i.e. shear strength ↑
    • tan Φ = τf / σ; where Φ = angle of internal friction

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soil strength and mechanics 2
Soil strength and mechanics – 2

McCarthy (1982) page 234

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calc soil strength
Calc.: Soil strength

Example 6:

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

I.B Mohtar

calc soil strength 2
Calc.: Soil Strength – 2
  • From table 10-1, 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

I.B Mohtar

footing bearing loads
Footing bearing loads

qult = a1*c*Nc + a2*B*γ1*Nγ + γ2*Df*Nq

total support=soil cohesiveness+ below footing +soil bearing

    • 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

McCarthy (1982) page 374-379

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footing bearing loads 2
Footing Bearing Loads – 2

McCarthy (1982) page 375

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calc footing load
Calc.: Footing Load

Example 7:

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

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calc footing load 2
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

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soil compaction and density
Soil Compaction and Density
  • Soil compaction
    • Greater strength and reduced permeability
    • Dependent on water content dry soil cannot be compacted well
  • Proctor test
    • Pack soil into mold with pounding at various moistures. Find soil moisture for maximum compaction and density.
    • Modified Proctor > 56000 ft-lbs of energy exerted.

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slope stability failure possible forms of failure
Slope Stability & FailurePossible Forms of Failure

McCarthy (1982) page 437-455

McCarthy (1982) page 440

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slope stability failure 2
Slope stability & failure – 2
  • Terms
    • β=max. slope angle before sliding
    • Φ=angle of internal friction
  • Cohesionless soil
    • tan(β) = tan(Φ)
    • Saturated: tan(β) = (1/2)tan(Φ)

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slope stability failure 3
Slope Stability & Failure – 3
  • Cohesive soil
    • γ*z*sin(β)*cos(β) = c + σ*tan(Φ)
      • z = assumed depth
      • c = cohesive force
      • σ = effective compressive stress
    • Rotational or sliding block

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slope stability failure 4
Slope Stability & Failure – 4

For clay soil

For soil with cohesion and internal friction > 0

McCarthy (1982) page 474

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slope stability failure 5
Slope Stability & Failure – 5
  • Ns = c / (γ * Hmax)
    • c = cohesion force
    • γ = soil unit weight
    • Hmax = max depth without sliding

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calc slope stability
Calc.: Slope Stability

Example 8:

  • Given:
    • Cohesion strength = 500 lb/ft2
    • Unit weight = 110 lb/ft3
    • Slope steepness = 50o
    • Internal friction angle = 15o
  • Find:
    • Max. slope height

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calc slope stability 2
Calc.: Slope Stability – 2
  • Fig. b, φ = 15o, i = 50o
  • Hmax = c / (γ * Ns) =

(500 lb/ft2)(1ft3/ 10)(1/ 0.095) = 48 ft

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materials covered
Materials Covered
  • Principles of Soil Physics
  • Sediment Transport
  • Erosion Control
  • Soil Mechanics
  • Slope Stabilization

I.B Mohtar

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