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Retaining Wall Analysis
Oasys Frew
February 2013
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Today’s Team
Nick
Niknam
Technical
Sales
Consultant
Zeena
Farook
Geotechnical
Application
Engineer
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Objectives
1. Presentation
• Analysis Methods
• Case Studies
2. Live Demonstration
• Stability and full Soil Structure Interaction Analysis
• Using the modelling wizard to set up quick
analyses
• Useful shortcuts and tips to enable proficient
modelling
• Simple checks that the user can utilize for outputs
Other webinars and videos cover:
• Setting up models in Frew
• Advanced Analysis Methods in Frew
• Integral Bridge Analysis using Frew
• EC7 Design Methods
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Analysis Theory
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Frew
• Embedded retaining wall analysis over the complete construction sequence
• Stability checks for cantilever and propped walls
• Earth and water pressure calculations at every stage
• Suitable for sheet pile, secant, contiguous or diaphragm walls
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Analysis Theory - Development
•The analysis method adopted in FREW and relevant assumptions:
•Pappin J W, Simpson B, Felton P J and Raison C.
Numerical analysis of flexible retaining walls. Proc
NUMETA ’85, University College, Swansea, pp 789-802
•The program has undergone continuous development over the years
•Latest Windows version is 19.1
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Analysis Theory - Numerical Representation
• Wall modelled as series of elastic beam elements
• Soil to each side of the wall connected to nodes
• Only horizontal forces are transmitted
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Analysis Theory - Soil Stiffness
• stiffness matrix computed by applying loads to the edge of a finite element mesh
• FE Flexibility Model (SAFE Model) or Mindlin Flexibility Model
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Analysis Theory - Is it conservative?
•Number of assumptions:
•Simplifying likely effects of wall friction
•Simplification of the action of structural supports
•Linear-elastic soil stiffness
BUT compares reasonably well with FE analysis
•Various illustrations can be found in literature:
•Ciria C580
•Rocoe, H. (2003), Retaining Wall Movements, CTRL
Contract 430 – Ashford Tunnels, BGA International
Conferenve on Foundations
•Hong Kong Cut and Cover Tunnels (Oasys Case Study)
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FREW Input
•Overview of key inputs
•Soil strength and stiffness – short and long term
•Groundwater / water pressures
•Wall stiffness (EI) – short and long term
•Support stiffness (props, anchors, berms, slabs)
•Surcharges and applied loads
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Inputs - Geometry and Soil Parameters
• Plan geometry in advance – soil levels, dig levels, prop positions. May depend upon design approach (EC7)
• Automatic Node Generation vs. Manual Node Generation:
•Keep nodes approx. equally spaced and place nodes below the wall
•Generally:• Props/loads applied at nodes
• Soil switches mid-node
• Fill/Excavations are at mid-nodes
• Toe of wall extends to node below that specified
• Passive softening can be specified via Analysis data at appropriate stage
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Frew 19.1 Features
• Model Wizard
• Automatic node creation
• Includes Limit Equilibriumstability checks
• Toe depth given for all construction stages
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Frew 19.1 Advanced Features
• Manual Node Override
• Batch Testing
1. Override toe level
2. Specify range of levels
3. Create multiple FREW files
for each toe level
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FREW Output
• Graphical and Tabular output available
• Export to Excel
• Stage by Stage output and envelope of results
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Modelling Methods
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Modelling methods
•Effects to be modelled:•Undrained-drained transition
•Modelling props
•Relaxation & Creep
•Modelling Sloping Ground
•Modelling Berms
•Thermal Effects
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Application of EC7
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Application of EC7
• Limit State Design – Considers ULS and SLS
• Partial factors applied to• Characteristic actions
• Characteristic material properties
• Characteristic resistances
• Tolerances applied to characteristic geometries
• Strength and stability must be verified
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Application of EC7
• Design Approaches:
• EC7 Partial Factors:
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Frew –Flexible Retaining Wall Analysis
• Analyse the wall and ground
• Throughout excavation & construction stages
• In the final condition
• Check horizontal equilibrium failure modes
• Fixed earth rotation
• Free-earth rotation
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Demonstration
Cantilever Retaining WallStability Analysis Only
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Geometry
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Summary – Cantilever Wall Demonstration
• Input Wizard
• Graphical Input
• Tabular Input
• Exporting Tabular Outputs
• Graphical Output
• Printing Preferences
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Demonstration
Propped Retaining WallStability and Soil Structure Interaction Analysis
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Geometry
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Frew Inputs - Preferences and defaults
• Tabbing past a table cell with a default will fill the cell with the default value
• Defaults can also be edited – click in the grey cell at the top
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Frew Inputs - Table tips
• = in a cell copies the cell above
• == in a cell copies the rest of the line above
• In long tables, ctrl-G lets you type in the line to go to
• Right-click brings up a whole list of options – modify probably the most useful
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Summary – Propped Wall Demonstration
• Input wizard – Limit Equilibrium and SSI analysis
• Graphical input – staged construction
• Tabular input – copy and past from excel
• Minimum toe depth calculation
• Automatic node generation
• Graphical output – staged construction
• Checking outputs
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Case Study
https://www.oasys-software.com/solutions/case-studies.html
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Deep Basement Design in Hong KongRedevelopment of North Point Estate
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• Detailed design of the proposed 1.5m thick diaphragm wall at the boundary of the development with maximum excavation depth of 13.5m
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Geotechnical Constraints
•Poor ground conditions
• generally underlain by 10m thick reclamation fill
followed by 5m thick marine deposits (small shear
strengths and SPT N of 10 on average)
•Stringent movement control
• site is encompassed by many movement sensitive
structures and utilities in close proximity
• The government has imposed very stringent ground
movement criteria (allowable ground movement of
25mm to cater for all activities)
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Proposed Solution
Tender Design:
• Proposed diaphragm wall is a 1.5m thick reinforced concrete structure
• Will be supported by at most five layers of struts, and three layers of permanent floor slabs
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Analysis
• FREW version 19.0 (BD ref. no. G0164)
• The design of the diaphragm wall includes:
• modelling of the excavation sequence
• determination of lateral deflection, moment and shear of the
diaphragm wall
• The temporary and permanent condition were analysed
Example analysis….
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Objectives
1. Presentation
• Analysis Methods
• Case Studies
2. Live Demonstration
• Stability and full Soil Structure Interaction Analysis
• Using the modelling wizard to set up quick
analyses
• Useful shortcuts and tips to enable proficient
modelling
• Simple checks that the user can utilize for outputs
What next?......
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What next?
• Support:
• Try step by step examples
http://www.oasys-software.com/frew-tutorials.html
• Web site and technical FAQs
• mailto:[email protected]
• Online training movies
• Telephone support at +44 (0) 191 238 7559
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More advanced Analysishttps://www.oasys-software.com/support/webinars.html
• Advanced Methods:
• https://www.oasys-
software.com/index.php/webinars/webinar/advanced_retaini
ng_wall_analysis/
• Integral Bridge Analysis:
• https://www.oasys-
software.com/index.php/webinars/webinar/integral_bridge_a
nalysis_using_soil_structure_interaction/
• EC7
• https://www.oasys-
software.com/index.php/webinars/webinar/ec7_and_geotech
nical_analysis/
