anchored (tie back) retaining walls and soil nailing in brazil
TRANSCRIPT
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Anchored (Tie Back) Retaining Walls and
Soil Nailing in Brazil
Summer Term 2015 Hochschule Munchen
Fakultat Bauingenieurwesen
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LAYOUT
Details and Analysis of Anchored Walls
Details and Analysis of Soil Nailing
Examples of Executive Projects
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ANCHORED “CURTAIN” WALLS
(Tie Back Walls)
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Introduction Details:
• Earth retaining structures with active anchors
• A.J. Costa Nunes pioneer work in 1957
• 20 – 30 cm thick concrete wall face tied back
• Ascending or descending construction methods
• Niche excavation
• ACTIVE anchor
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Excavation Procedure
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Molding Joints 7/60
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Stability Analysis
Verification of failure modes:
• Toe bearing capacity
(NSPT < 10)
• Bottom failure
• Wedge or generalized failure: limit equilibrium analyses
• Excessive deformations
• Anchor stability and punching
• Structural failure
• Construction failures (e.g. during excavation)
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Stability Analysis Methodologies
Wedge Method:
• Kranz (1953) is the pioneer
• One or two wedges
• Ranke and Ostermeyer (1968) German Method
• Nunes and Velloso (1963) Brazilian Method
• Hoek and Bray (1981)
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Clayton et al (2001)
Kranz (1953) Method:
•FS in relation to each anchor
•FS= max allowable / actual anchor load
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Hoek and Bray (1981) Method:
•Simple geometries
•Homogeneous soils
•FS by vertical and horizontal equilibrium
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Pre-design Charts:
• Safety Factor = 1.5
• Surcharge q = 20 kPa
• Unit Weight = 18 kN/m3
• Preliminary analyses
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TECNOSOLO (1964)
Original Report 3310
Nunes and Velloso (1963) Method:
•FS for an existing Culmann wedge
But modified to have
•FS in relation to cohesion vector
esStableForcInstable
Cohesion
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Stability Analysis Methodologies Complex Cases:
• Numerical or analytical tool
• Limit equilibrium approach
• Non homogeneous soils
• Complex load and geometries
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Bishop (1955)
Geoslope Slopew
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Anchor Spacing:
•Counterbalance Instability x Stability Forces
•Anchor force to yield general FS > 1.5
•Length > “critical” plane
Micropiles
•Whenever there is low capacity soils at wall base
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Stresses and Deformation Analysis
Tools:
• User friendly numerical FEM programs
• Distinctive models
• Laboratory parameters
• Pre and post processors
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Example:
•Águas Claras Site – Fed. District, Brazil
•Porous clay over soft soil
•Close to train rail
•15 m height and 4 anchor layers
•Staged analyses
•Laboratory parameters
•Mohr Coulomb model
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SOIL NAILING
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Introduction Basics of Design:
• Reinforcement of soil with thin elements: nails
• Pre-bored sub horizontal hole, with grout
• Originated from shotcrete flexible support in tunnels
• Active zone is formed around excavation
• Started in Brazil in 1970 and France 1972 (sol cloué)
• PASSIVE anchors = “nails”
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Experience and Construction Method
Experience:
• High and successful experience in Brazil
• Use for man made, residual and saprolitic slopes in Hong Kong
• Not suitable for very loose sands or soft clays
Construction:
• Similar as tieback walls: top – down excavation stages (1-2 m)
• Vertical or inclined slopes – depends on geology
• Installation of nails, mesh, drains and shotcrete
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Installation of Nails:
• After driving or drilling
• Short nails (3 m) by hand hammers
• Corrosion protection aspects
• Driving is not adequate with boulders
• Common drilling with 50-100mm ´s
• 20-32 mm steel bars
• > 100 kPa lateral friction
• Pneumatic drill rigs are used
• Light drill rigs are desired
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Construction Details Nail Head:
• With or without steel plate and wrenches
• Small torque of 5 kN is incorporated as residual load
• Inclinations of 10-20 degrees
• Embeddement in a cast-in-place concrete niche
• Grounting with or without (gravity head) pressures
Geocompany (2009)
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Souza et al. (2005)
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Slope Facing:
• Shotcrete is applied through dry or wet mix
• Thickness of 50-150 mm
• One or two steel meshes
• Steel reinforced shotcrete (SFRS) is also used:
fibers 30-50 mm lingth, 0.5 mm dia.
dosage 35-60 kg/m3
good for slope irregularities
• Vegetation combined with nails
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Details:
• Wall
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Details:
• Nail
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Details:
• Injection
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Details:
• Frontal Spacing
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Comparisons
With Tieback Walls:
• Generally do not use prestressed active anchors
• Uses passive low prestressed nails (5-10 kN)
• Load transference by friction along entire length
• Very low loads on shotcrete facing compared to tieback walls
• Inclined or vertical facings
• Length of nails 60-120% of height (shorter than walls)
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With Reinforced Walls:
• Top-down versus upwards construction sequence
• Distinct displacement patterns (0.1 - 0.3 % of height)
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Advantages Economy: • Cost effective technique, as low as 50% of a tieback wall Rate of Construction: • Fast rate specially with SFRS shotcrete
Deformation: • 0.1 – 0.3% of height at top of wall for well designed structures
Flexibility: • Deformation can be controlled with combined use of anchors
Reliability: • Already proved in residual and saprolitic soils in Brazil • Increases stability in unsupported slopes with weak surfaces
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Limitations Displacements: • May render unacceptable deformations close to structures Construction: • Needs temporary stability of excavated face
Geology: • Risky solution for weak materials or very height walls
Durability : • Corrosion protection of nail is fundamental
Testing and post-execution intervention: • Generally not possible with nails. • Post execution corrective injection is still not widely used
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Analysis of Nailed Structures
Theoretical Methods:
• Several approaches and simplifications
• Active and passive zones
• Global Limit Eq. (slice) analysis with nail effects
• Circular, bilinear, linear surfaces
• Tension only or with bending effects in nails
• Constant or variable soil-nail interface friction
• Winkler type analysis for nail or force vectors
• Single or multiple surfaces – FS optimization
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Effect of Injection Phases
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Modified after
Souza et al. (2005)
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Computer Programs
Benchmark Tests:
• Comparative comparisons are made
• Talren is the most widely used
• Prosper is a research tool
• Clouage and Nixesc are french softwares
• Rstabl adopts Bishop and Janbu´s method
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Results:
• Influence of bending is rather small
• Janbu´s method tends to yield lower SF´s
• Few differences between methodologies
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Nailing Software (Czech Republic):
• Good experience and successful results in Brasília porous clay
• Nice research and design tool
• User friendly
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0 4.00[m]
Length of structure = 3.20mGeometry of structure Structure load
21.08
25.00[kPa]
0
Cut1
0.50
1.00
1.00
0.70
Max. M = 4.32kNm/mBending moment
3.43
-3.43
4.32
-5.00 5.00[kNm/m]
0
Max. Q = 11.14kN/mShear force
-6.87
-6.87 6.87
6.87-6.81
-10.71 11.14
-25.00 25.00[kN/m]
0
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Examples
Icaraí Beach, Niteroi-RJ:
• 25mm bars in 90 mm holes – 150 mm shotcrete, inclined 75° – 1.5 m spacings (H:V) and two steel meshes
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Railway, São Paulo-SP:
• 25mm bars in 75 mm holes – 50 mm shotcrete, inclined 75° – 2.5m x 2.0m (H:V)
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Ortigão et al. (1993)
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Cindacta Project – Friburgo-RJ
Executive Design Project 57/60
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Cindacta Project – Friburgo-RJ
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Soil Nailing
Tie Back Wall
Active Anchor
Passive Anchor
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Cindacta Project – Friburgo-RJ
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Tie Back Wall
Soil Nailing
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REFERENCES
• Ortigão & Sayão (2004). Handbook of Slope Stabilisation, Springer, New York, 478 p.
• Hunt, R. E. (1986). Geotechnical Engineering Techniques and Practices, McGraw Hill, New York, 729 p.
• Personal pictures.
• Internet pages.
• Executive Design projects from ACRosa Engenharia de Consultoria Ltda., Rio de Janeiro, Brazil.
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