www.geotecnia.unb.br/gpfees Summer Term 2015 Hochschule Munchen Fakultat Bauingenieurwesen Anchored (Tie Back) Retaining Walls and Soil Nailing in Brazil www.geotecnia.unb.br/gpfees 2/60 LAYOUT Details and Analysis of Anchored Walls Details and Analysis of Soil Nailing Examples of Executive Projects www.geotecnia.unb.br/gpfees 3/60 ANCHORED “CURTAIN” WALLS (Tie Back Walls) www.geotecnia.unb.br/gpfees 4/60 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 4 www.geotecnia.unb.br/gpfees 5/60 Excavation Procedure www.geotecnia.unb.br/gpfees 6/60 www.geotecnia.unb.br/gpfees 7/60 Molding Joints www.geotecnia.unb.br/gpfees 8/60 www.geotecnia.unb.br/gpfees 9/60 www.geotecnia.unb.br/gpfees 10/60 www.geotecnia.unb.br/gpfees 11/60 www.geotecnia.unb.br/gpfees 12/60 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) www.geotecnia.unb.br/gpfees 13/60 www.geotecnia.unb.br/gpfees 14/60 www.geotecnia.unb.br/gpfees 15/60 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) www.geotecnia.unb.br/gpfees 16/60 Kranz (1953) Method: •FS in relation to each anchor •FS= max allowable / actual anchor load Clayton et al (2001) www.geotecnia.unb.br/gpfees 17/60 Hoek and Bray (1981) Method: •Simple geometries •Homogeneous soils •FS by vertical and horizontal equilibrium www.geotecnia.unb.br/gpfees 18/60 www.geotecnia.unb.br/gpfees 19/60 Pre-design Charts: • Safety Factor = 1.5 • Surcharge q = 20 kPa • Unit Weight = 18 kN/m3 • Preliminary analyses www.geotecnia.unb.br/gpfees 20/60 Nunes and Velloso (1963) Method: •FS for an existing Culmann wedge But modified to have •FS in relation to cohesion vector Cohesion Instable StableForces TECNOSOLO (1964) Original Report 3310 20 www.geotecnia.unb.br/gpfees 21/60 www.geotecnia.unb.br/gpfees 22/60 22 Example www.geotecnia.unb.br/gpfees 23/60 Stability Analysis Methodologies Complex Cases: • Numerical or analytical tool • Limit equilibrium approach • Non homogeneous soils • Complex load and geometries www.geotecnia.unb.br/gpfees 24/60 Bishop (1955) Geoslope Slopew www.geotecnia.unb.br/gpfees 25/60 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 www.geotecnia.unb.br/gpfees 26/60 Surcharge www.geotecnia.unb.br/gpfees 27/60 Stresses and Deformation Analysis Tools: • User friendly numerical FEM programs • Distinctive models • Laboratory parameters • Pre and post processors www.geotecnia.unb.br/gpfees 28/60 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 www.geotecnia.unb.br/gpfees 29/60 SOIL NAILING www.geotecnia.unb.br/gpfees 30/60 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” www.geotecnia.unb.br/gpfees 31/60 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 www.geotecnia.unb.br/gpfees 32/60 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 www.geotecnia.unb.br/gpfees 33/60 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) www.geotecnia.unb.br/gpfees 34/60 Souza et al. (2005) www.geotecnia.unb.br/gpfees 35/60 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 35 www.geotecnia.unb.br/gpfees 36/60 Details: • Wall www.geotecnia.unb.br/gpfees 37/60 Details: • Nail www.geotecnia.unb.br/gpfees 38/60 Details: • Injection www.geotecnia.unb.br/gpfees 39/60 Details: • Frontal Spacing www.geotecnia.unb.br/gpfees 40/60 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) 40 www.geotecnia.unb.br/gpfees 41/60 With Reinforced Walls: • Top-down versus upwards construction sequence • Distinct displacement patterns (0.1 - 0.3 % of height) www.geotecnia.unb.br/gpfees 42/60 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 42 www.geotecnia.unb.br/gpfees 43/60 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 43 www.geotecnia.unb.br/gpfees 44/60 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 www.geotecnia.unb.br/gpfees 45/60 www.geotecnia.unb.br/gpfees 46/60 www.geotecnia.unb.br/gpfees 47/60 Effect of Injection Phases www.geotecnia.unb.br/gpfees 48/60 Modified after Souza et al. (2005) www.geotecnia.unb.br/gpfees 49/60 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 49 www.geotecnia.unb.br/gpfees 50/60 Results: • Influence of bending is rather small • Janbu´s method tends to yield lower SF´s • Few differences between methodologies www.geotecnia.unb.br/gpfees 51/60 Nailing Software (Czech Republic): • Good experience and successful results in Brasília porous clay • Nice research and design tool • User friendly www.geotecnia.unb.br/gpfees 52/60 www.geotecnia.unb.br/gpfees 53/60 Geometry of structure Structure load Bending moment Shear force Length of structure = 3.20m Max. M = 4.32kNm/m Max. Q = 11.14kN/m -6.87 0.50 3.43 -6.87 6.87 1.00 -3.43 -6.81 6.87 1.00 Cut1 4.32 -10.71 11.14 0.70 21.08 0 4.00 0 25.00 -5.00 0 5.00 -25.00 0 25.00 [m] [kPa] [kNm/m] [kN/m] www.geotecnia.unb.br/gpfees 54/60 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 54 www.geotecnia.unb.br/gpfees 55/60 Railway, São Paulo-SP: • 25mm bars in 75 mm holes – 50 mm shotcrete, inclined 75° – 2.5m x 2.0m (H:V) 55 www.geotecnia.unb.br/gpfees 56/60 Ortigão et al. (1993) www.geotecnia.unb.br/gpfees 57/60 Executive Design Project Cindacta Project – Friburgo-RJ www.geotecnia.unb.br/gpfees 58/60 Tie Back Wall Active Anchor Passive Anchor Soil Nailing Cindacta Project – Friburgo-RJ www.geotecnia.unb.br/gpfees 59/60 Tie Back Wall Soil Nailing Cindacta Project – Friburgo-RJ www.geotecnia.unb.br/gpfees 60/60 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. .