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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.

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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

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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

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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

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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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I b soil conservation systems

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

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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 2

Sediment Delivery – 2

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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

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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?

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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

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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


    Thank you and best luck

    Thank You and Best Luck

    I.B Mohtar


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