Slope Stability in Harris...

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Slope Stability in Harris County

Transcript of Slope Stability in Harris...

Page 1: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Slope Stability in HarrisCounty

Page 2: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Slope Stability in Harris County

Page 3: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Slope Stability in Harris County

• Overview of slope stability.

• Conditions, causes, and types of slope failures.

• HCFCD geotechnical investigationrequirements.

• Variables in analyzing slope stability.

• HCFCD Research.

Page 4: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Harris County and Its Channels

• Harris County’s population of 3.7 million isthe third largest in the United States.

• The drainage and flood controlinfrastructure of Harris County are extensiveand include more than 1,500 channels andabout 2,500 miles of channels (about thedistance from New York to Los Angeles).

• The HCFCD spends $7M to $8M each year torepair these 2,500 miles of channels, most ofwhich are earthen channels.

Page 5: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Conditions and Causes of Slope Failures

Page 6: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Types of Slope Failures

• Deep rotational failure.

• Shallow rotational failure – toe failure.

• Shallow sloughing failure.

• Wedge or block failure.

• Erosion failure.

• Failure due to presence of dispersive soil.

Page 7: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Types of Slope Failures

Page 8: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Types of Slope Failures – Failures Due to SoilWeathering

Page 9: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Peak vs. Residual Shear Strength

Page 10: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Types of Slope Failures – Failures Dueto Dispersive Clays

Page 11: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Deep Rotational Failure

Page 12: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Shallow Rotational Failure

Page 13: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Progressive Shallow Rotational Failure

Page 14: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Seepage Through Sandy Soil Slope

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Wedge or Block Failure

Page 16: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Channel Erosion

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Slope Failure - Dispersive Clays

Page 18: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,
Page 19: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

• Adopted on October 5, 2004.

• Updates will be posted on www.hcfcd.org.

• It is not a cookbook.

• Good engineering practice and judgment arestill necessary.

Background

Page 20: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Manual applies for all flood controlfeatures such as channels and basins thatthe HCFCD will maintain. This includes:

• New HCFCD Facilities• Modification of Existing HCFCD

Facilities

Manual Applies When …

Page 21: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Geotechnical Investigation Requirements• For HCFCD maintained facilities, a geotechnical

investigation must be performed.• Geotechnical investigation must follow guidelines in

Appendix D of manual.• Appendix D requires minimum numbers and

minimum depths of borings; lab tests to include CUtriaxial tests and pinhole dispersion tests; and,stability analyses must be performed for the shortterm, rapid drawdown, and long term conditions.

• Deviation from HCFCD design criteria requires avariance.

Page 22: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

HCFCD Design Requirements forChannels and Basins

• Grass-lined earthen slopes of 4(H):1(V) orflatter for channels.

• Concrete lined slopes of 2(H):1(V) or flatterfor channels.

• Grass-lined earthen slopes of 3(H):1(V) orflatter for detention basins.

Page 23: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Why Do We Require Channel Slopes4(H):1(V) or Flatter?

• Stability analysis results vs. observations.

• Weathered soil shear strength.

• Weathered soil shear strength and rapiddrawdown.

• Back-calculated weathered soil shear strengthfor failed slopes.

Page 24: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Back-calculated Weathered Soil ShearStrength for Failed Slope

Page 25: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Why Do We Allow Concrete LinedChannel Slopes 2(H):1(V) or Flatter?

• Very few failures of 2(H):1(1) concrete linedslopes.

• Soil weathering inhibited.

• Toe erosion precluded.

• Rapid drawdown condition precluded.

Page 26: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Why Do We Require Detention BasinGrass-Lined Earthen Slopes 3(H):1(V) or

Flatter?• Basin slope toe erosion not as prevalent as with

channels.

• Wetting and drying of basin slope toe not asfrequent as with channels.

• Basin slope failures may not be as critical as withchannels.

• Observations of performance of basin channels with3(H):1(V) slopes.

Page 27: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Back Slope Drainage and DispersiveClays

• Mechanism for dispersive soil collapse.

• Goal is to keep water from ponding and infiltratinginto dispersive clays.

• Decrease interceptor structure spacing andincrease backslope swale gradient.

• Lime treatment or clay lining backslope swaleand/or maintenance berm.

Page 28: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,
Page 29: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Variables• Slope inclination.• Soil types, soil strengths, soil plasticity, and

layer thicknesses.• Extent of strength loss due to weathering.• Duration of periods of dry weathering.• Ground water conditions.• Surface water conditions.• Shrinkage crack patterns, crack depths.• Degree and rate of erosion – channel

hydraulics.

Page 30: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Mesri and Abdel-Ghaffar (August 1993Geotechnical Journal)

Page 31: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Friction Angle vs PI

Page 32: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Correction Factor for Mobilized(“Weathered”) Shear Strength (c’)

Page 33: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Research for Highway Embankments

Page 34: Slope Stability in Harris Countyfoundationperformance.org/pastpresentations/RonLangstonPres-13Jul05.pdfSlope Stability in Harris County •Overview of slope stability. •Conditions,

Residual Secant Friction Angle vs. Effective Pressure and LL

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Questions?