Seepage Through Earthen Dams: Agenda

Day One - June 12, 2018

8:00 - 8:30 Introduction

  • Instructors
  • Course content/Agenda
  • Introductions and participant expectations
  • Contents of Workbook and CD

8:30 – 10:00 Seepage and Embankment Dams

  • Cite dam failure statistics highlighting prominence of seepage incidents and failures for embankment dams
  • Review seepage failure modes with graphic illustrations
    • Backward erosion piping
      • Through the embankment
      • Through the foundation
      • Blowout or heave
      • From embankment into foundation
    • Concentrated Leakage
      • Flaws (many types)
      • Contact erosion (Embankment at foundation)
      • Along penetrations (e.g. conduits or structure walls)
  • Illustration of failure modes with photographs and information from failures and incidents
    • Teton Dam – multiple explanations [Contact erosion; Hydraulic fracturing; Settlement (Flaw); Cross trench arching)
    • Anita Dam
    • A.V. Watkins - BEP of foundation
  • Truckee Canal - Rodents

10:00 – 10:15 Morning Break

10:15 – 11:15 Permeability (Hydraulic Conductivity)

  • Definition of coefficient of permeability (hydraulic conductivity)
    • Darcy’s law
    • Difference between Darcy velocity and water flow velocity through soil
  • Estimating coefficient of permeability
    • Published tables – by soil type and gradation
      • Casagrande and Fadum (SCS)
      • Freeze and Cherry
      • NRCS
      • Bureau of Reclamation
    • Empirical equations (is this too many? Pros and cons? What’s the most common/recent? Many are similar to each other. Look to see if we can skim down.)
      • Hazen original one
      • Modified Hazen. Trying to improve
      • Schlicter
      • Chapuis
      • Kozeny – Carman most complicated but probably a little better.
      • NRCS filter research equations
      • Justin, Hinds, and Creager
      • Comparison of results with different equations – typically within about an order of magnitude
  • Heterogeneity and Anisotropy
    • Heterogeneity – k = f (position)
    • Anisotropy – k = f (direction)
  • Anisotropy of layered system
    • Equivalent coefficients of permeability of layered systems
      • Derive equivalent vertical k
      • Equivalent horizontal k
  • Anisotropy ratios of different materials (published tables)
    • Natural deposits
    • Embankment materials
  • Effects of fines on coefficient of permeability
  • Effects of clay type on coefficient of permeability
  • Effects of angularity and density
  • Coefficient of permeability tests
    • Laboratory tests
      • Constant head
      • Falling head
      • Flexible wall permeameter
    • Field tests
      • Borehole soil permeability tests
      • Rock packer tests

11:15 – 12:00 Exercise No. 1

  • Given soil descriptions, estimate permeabilities
  • Given soil gradation data, estimate permeability using two of the empirical equations

12:00 – 1:00 Lunch

1:00 – 1:15 Exercise No. 1 Solution and Review

1:15 – 2:00 Seepage Analysis – Part I

  • Types of flow
    • Through porous media (soil)
    • Through fracture systems (foundation rock). Don’t usually analyze just recognize it’s there and design
    • Laminar vs. turbulent
    • Steady vs. non-steady (transient)
    • Saturated vs. unsaturated
    • Case most common in embankment analysis/design: laminar, steady, saturated flow through porous media
  • Governing equations
    • Darcy’s law
      • History and formulation – Q = kiAt or q = kiA
      • Limitations of Darcy’s law
    • Continuity (LaPlace) equation
      • Present equation and explain what it means
  • Solutions of the equations
    • Closed form – only for very limited cases
    • Graphical – flow nets
    • Numerical
      • Relaxation
      • Finite element
      • Finite difference
      • Boundary element
    • Analogs
      • Electrical analog models
      • Viscous fluid models
    • Method of fragments – limited used; older method
  • Flow nets
    • Advantages
      • Easy to draw for many cases
      • Relatively fast
      • Inexpensive
      • Provide insights
    • Flow net introduction
      • Simple example – homogeneous embankment with toe drain
      • Rules and guides
    • Anisotropy and transformed sections
    • Flow Net Errors

2:00 – 3:00 Seepage Analysis – Part II

  • Applications of Flow Nets
    • Seepage quantities
    • Pore water pressures
    • Gradients
    • Uplift pressures
    • Design of seepage reduction or collection measures
    • Design of filters and drains
  • Flow net illustrations
    • Seepage quantities - Q = K x h x (Nf/Np)
    • Pore water pressures - ui = (np / Np) x h x w
    • Gradients - i = Δh/L
    • Uplift pressures – flow net illustrations of effect of partial cut off location beneath a spillway structure
    • Flow net illustrations – flow through embankment and foundation under different conditions including anisotropy
    • Flow net illustrations – partially penetrating cutoffs
    • Flow net illustrations – effects of upstream blankets
    • Flow net illustrations – effects of anisotropy and chimney drains on phreatic surface and pore water pressures
    • Flow net illustrations – core thickness and “escape” gradients
    • Flow net illustrations – plan flow nets: abutment flow, flow around the end of a cutoff, etc.
  • Numerical analysis
    • Considerations
      • Can be used for same applications as flow nets
      • Easier for complicated cases
      • Not necessarily as easy to visualize results – need to use multiple output formats
      • Need to properly characterize permeability in the unsaturated zone
      • Need to calibrate mode
      • Easy to do parametric studies
  • Simplified solutions – available for a number of practical cases
    • Blanketing and partially penetrating cut off
    • “Impervious” blankets
    • “Semi-pervious” blankets
    • Efficiency of blankets and cut offs

3:00 – 3:15 Afternoon Break

3:15 – 3:45 Heave, Blowout, and Critical Gradients

  • Factor of safety calculations
  • Relationship to piping

3:45 – 4:45 Exercise No. 2 – Seepage Analysis

  • Given a flow net, calculate flow quantities, pore water pressures, and gradients
  • Evaluate a given case based on simplified solutions

4:45 – 5:00 Exercise No. 2 – Solution and Review

 

Day Two - June 13, 2018

8:00 – 9:30 Seepage Control Design – Filters and Drains

  • Explanation of internal erosion and piping
  • Filters – to prevent internal erosion (movement of soil particles under flow)
  • Drains – to collect and convey seepage water
  • Filter design
    • Agency guidelines – NRCS, Bureau of Reclamation, Corps of Engineers, FEMA
    • Foster/Fell
    • Review elements of filter design
      • Mathematical regrading of base soils
      • Requirements for particle retention – filter function
      • Requirements for permeability – drainage function
      • Requirements to prevent internal instability
      • Requirements to prevent segregation
    • Design and construction considerations
      • Natural deposits vs. processed materials
      • Advantages of using commonly available commercial aggregates
      • One-stage vs. two-stage chimney filters
      • Importance of limiting fines content
      • Filter/drain zone dimensions controlled by constructability
      • End-product vs. method specifications
      • Benefits of two-stage envelopes around drain pipes
      • Material types for drain pipes
    • Geotextiles
      • Federal agency guidance
  • Foster and Fell (2001)
  • Industry practice

9:30 – 9:45 Morning Break

9:45 – 10:30 Seepage Control Methods – Part I

  • Two general categories of seepage control methods
    • Seepage collection (filtered)
    • Seepage reduction/cut off
  • Seepage collection
    • Objectives
      • Prevent piping and internal erosion
      • Limit pore pressures, uplift, and seepage forces
      • Prevent slope instability and surface sloughing
      • Prevent “wet spots” and surface erosion
      • Collection may be a reasonable second line of defense, even with cutoff
    • Considerations
      • Can be constructed where seepage has been observed
      • Often can directly observe placement of constructed elements
      • May require reservoir lowering
      • May require dewatering
    • Collection and control tools
      • Filters to prevent internal erosion
      • Drains to collect seepage
      • Relief wells to reduce uplift
    • Remediation configurations with illustrations
      • Filter diaphragms
      • Filter/drain blankets
      • Filter/drain blankets and berm
      • Shallow toe drain
      • Deep toe drain
      • Toe drain and berm
      • Narrow trench toe drain and berm
      • Chimney filter overlay
      • Internal chimney filter
      • Benefits of full-height chimney filters and current thinking on filter top elevations
      • Relief wells
    • Combinations of above

10:30 – 11:15 Exercise No. 3 – Filter Design

  • Given gradations for an embankment core, identify filter requirements
  • Given gradations for a chimney filter, identify filter requirements for a two-stage drain

11:15 – 11:30 Exercise No. 3 – Filter Design Solution and Review

11:30 – 12:30 Lunch

12:30 – 1:30 Seepage Control Methods – Part II

  • Seepage reduction/cutoff
    • Objectives
      • Reduce or eliminate seepage
      • Reduce gradients
      • Intercept flaws
      • Provide non-erosive barrier
    • Considerations
      • Will cut off be positive?
      • What technology will be used?
    • Seepage reduction/cutoff tools
      • Cores
      • Cut off trenches
      • Blankets/Geomembranes
      • Cutoff walls
      • Grouting
    • Remediation configurations with illustrations
      • Cores
      • Cut off trenches
      • Upstream blankets
      • Diaphragm walls
        • Continuous Walls
        • Element Walls
        • Concrete secant pile walls
        • Deep soil mix (DSM) walls
        • Jet grouting
        • Grout curtains

1:30 – 2:30 Seepage Detection and Monitoring

  • Importance of Visual Monitoring
  • Weirs
    • Shapes
    • Example installations
  • Flumes
    • Example installations
  • Toe drain flow meters
  • Water property measurements (turbidity, chemical and temperature)
  • Geophysical methods
  • Piezometers
  • Automated data collection
  • Sample data plots illustrating different behaviors

2:30 – 2:45 Afternoon Break

2:45 – 3:45 Case Studies and Examples

  • Collection and control
    • Washakie Dam
    • New Creek Dam
  • Seepage reduction and cutoff
    • Wolf Creek Dam
    • Guanella Dam

3:45 – 4:30 Exercise No. 4 – Seepage Remediation Design

  • Divide students into small work groups
  • Provide information on an embankment dam with a seepage problem and have teams develop remediation alternatives
  • Teams report back to the group

4:30 – 5:00 Exercise No. 4 - Review Actual Remediation

5:00 – 5:15 Course Wrap-Up and Final Questions

 

Day Three - June 14, 2018

8:00 – 12:00 Optional Computing Session – Hands-On Session Working with SEEP/W to Analyze Embankment Examples

  • SEEP/W Operational Fundamentals
  • Example Problem 1
  • Example Problem 2
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