How Steep Is Too Steep? - Smith Gardner, Inc.€¦ · Example 1 - 20 Acre Landfill With 3.5:1...
Transcript of How Steep Is Too Steep? - Smith Gardner, Inc.€¦ · Example 1 - 20 Acre Landfill With 3.5:1...
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Design and Construction of Final Cover Systems
How Steep Is Too Steep?
Thomas Maier, P.E.
What’s Right for Your Site?
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Vulnerabilities of Caps
Airspace vs. Maintenance
Case Studies
Rain Gutters (4H:1V)
Tack-On Diversions – Soil Cap (3H:1V)
Tack-On Diversions – Geosynthetic Cap (3H:1V)
Cut-In Diversions – Delamination (2.75H:1V)
Reinforced Cap (2H:1V)
Selection Guidance and Costs
Outline
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Cap Failures
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Cap Failures
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Remember When…
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Ride The Love-Hate Rollercoaster
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Example 1 - 20 Acre Landfill
With 3.5:1 benched slopes, 80 ft high: 1,470,000 CY
With 3:1 slopes, 100 ft high: 1,740,000 CY
Airspace vs. Maintenance Costs
Airspace increase of about 20% (say $3 million revenue over 10 yrs)
Would you be willing to spend 10% to 20% of that on maintenance?
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Example 2 - 100 Acre Landfill
With 3.5:1 benched slopes, 200 ft high: 16.3million CY
With 3:1 slopes, 250 ft high: 19.1 million CY
Airspace vs. Maintenance Costs
Airspace increase of about 20% (say $28 million revenue over 40 yrs)
Would you be willing to spend 10% to 20% of that on maintenance?
Actual maintenance costs will vary.
Factors: waste stream, leachate recirculation, soil type, vegetation type.
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Case Study 1: Rain Gutters
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Case Study 1: Rain Gutters
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Case Study 1: Rain Gutters
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Case Study 1: Rain GuttersLessons Learned
A cap on 4:1 slopes is very low maintenance.
Rain gutters are a proven stormwater management method.
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Case Study 2: Soil Cap with Tack Ons
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Case Study 2: Soil Cap with Tack Ons
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Case Study 2: Soil Cap with Tack Ons
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Case Study 2: Soil Cap with Tack Ons
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Case Study 2: Soil Cap with Tack Ons
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Case Study 2: Soil Cap with Tack Ons
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Case Study 2: Soil Cap with Tack OnsLessons Learned
Site specific soil testing is necessary for steep-sided tack-on berms.
With tack-on berms built of erodible soil, vegetative stabilization is of utmost importance.
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MSW Landfill with 3:1 Slopes and No Benches
Case Study 3: GM Cap with Tack Ons
Slope Stability
Veneer Stability vs. Berm Stability
Gas Pressure Uplift
Draintube
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Draintube or Geonet Drainage Composite
Case Study 3: GM Cap with Tack Ons
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Draintube
Case Study 3: GM Cap with Tack Ons
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Case Study 3: GM Cap with Tack Ons
0.0
0.5
1.0
1.5
2.0
2.5
3.0
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0.90 1.00 1.10 1.20 1.30 1.40 1.50 1.60 1.70
Soil Th
ickn
ess (feet)
Factor of Safety
Soil Thickness vs. Factor of Safety
3H:1V SlopesSoil Unit Weight = 110 PCFInterface Friction = 30 degrees
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Case Study 3: GM Cap with Tack Ons
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0.80 0.90 1.00 1.10 1.20 1.30 1.40 1.50 1.60 1.70 1.80
LFG Up
lift Pressure (in.‐w
.c.)
Factor of Safety
Gas Pressure vs. Factor of Safety
3H:1V SlopesSoil UnitWeight = 110 PCFInterface Friction = 30 degreesCohession = 0 psfNo Seepage, butLFG uplift pressure varies.
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Case Study 3: GM Cap with Tack Ons
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Case Study 3: GM Cap with Tack Ons
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Case Study 3: GM Cap with Tack Ons
Flatter berm slope may reduce the factor of safety against veneer failure.
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Lessons Learned
Case Study 3: GM Cap with Tack Ons
For tack-on berms over a geomembrane, cohesive soil is ideal.
The effect of some variables on slope stability is not obvious (i.e., flatter berm slopes may result in lower FS).
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Progressive Delamination of Drainage Geocomposite During Cover Soil Placement
Case Study 4: Delamination
Source: Thiel, R. and Narejo, D. "Lamination Strength Requirements for Geonet Drainage Geocomposites,"
Proceedings of the Geo-Frontiers 2005 Conference, Austin, Texas, January 24-26, 2005.
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Case Study 4: Delamination
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Case Study 4: Delamination
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Case Study 4: Delamination
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Case Study 4: Delamination
The peel strength was 2 to 5 times greater than specified.The material was tested and found to comply with specifications.
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Lessons Learned
Case Study 4: Delamination
Specifications that work for 3:1 slopes must be re-evaluated for steeper slopes.
Drainage geocomposite specifications need to address unbonded geotextile width.
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Case Study 5: Reinforced Cover
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Case Study 5: Reinforced Cover
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Lessons Learned
Case Study 5: Reinforced Cover
Steep cover systems are possible with the use of reinforcement.
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Selection Guide
0
100
200
300
400
500
600
10 15 20 25 30 35
Slop
e Height (Feet)
Slope Angle (Degrees)
NOREINFORCEMENT
REQUIRED
REINFORCEMENT REQUIRED
(=32°, =30°)
2.6:13:1
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Selection Guide
0
100
200
300
400
500
600
10 15 20 25 30 35
Slop
e Height (Feet)
Slope Angle (Degrees)
REINFORCEMENT REQUIRED
(=32°, =30°)
TACK‐ON BERMS FEASIBLE ON
SLOPES UP TO 3:1
3:1 2.6:1
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Selection Guide
0
100
200
300
400
500
600
10 15 20 25 30 35
Slop
e Height (Feet)
Slope Angle (Degrees)
REINFORCEMENT REQUIRED
(=32°, =30°)
TACK‐ON BERMS FEASIBLE ON
SLOPES UP TO 3:1
CUT –IN SWALES
3:1 2.6:1
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Selection Guide
0
100
200
300
400
500
600
10 15 20 25 30 35
Slop
e Height (Feet)
Slope Angle (Degrees)
REINFORCEMENT REQUIRED
(=32°, =30°)
TACK‐ON BERMS FEASIBLE ON
SLOPES UP TO 3:1
CUT –IN SWALES
3:1 2.6:1
RAIN GUTTERS
3.5:1
BENCHES (3.5:1 AVG.)
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Cost Comparison
Drainage Feature Type $/LF Comments
Rain Gutters $40 to $50 Pipe, Stone, Etc.
Tack-On Berms (2:1) $25 to $35 TRM and FGM Included
Tack-On Berms (1.5:1) $15 to $25
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Cost Comparison
Closure System Type $/AC Comments
Rain Gutters $140K - $150K
Costs are inside the anchor trench
excluding LFG work.
Tack-On Berms (2:1) $130K
Tack-On Berms (1.5:1) $120K
Costs are approximate and are based on a limited number of recent projects.
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ConclusionRemember the airspace.
Embrace the maintenance.
No one size fits all.
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Thank You
Thomas Maier, P.E.