retaining wall analysis

www.oasys-software.com

Retaining Wall Analysis

Oasys Frew

February 2013


Today’s Team

Nick

Niknam

Technical

Sales

Consultant

Zeena

Farook

Geotechnical

Application

Engineer


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


Analysis Theory


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


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


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


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


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)


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


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


Frew 19.1 Features

• Model Wizard

• Automatic node creation

• Includes Limit Equilibriumstability checks

• Toe depth given for all construction stages


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


FREW Output

• Graphical and Tabular output available

• Export to Excel

• Stage by Stage output and envelope of results


Modelling Methods


Modelling methods

•Effects to be modelled:•Undrained-drained transition

•Modelling props

•Relaxation & Creep

•Modelling Sloping Ground

•Modelling Berms

•Thermal Effects


Application of EC7


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


Application of EC7

• Design Approaches:

• EC7 Partial Factors:


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


Demonstration

Cantilever Retaining WallStability Analysis Only


Geometry


Summary – Cantilever Wall Demonstration

• Input Wizard

• Graphical Input

• Tabular Input

• Exporting Tabular Outputs

• Graphical Output

• Printing Preferences


Demonstration

Propped Retaining WallStability and Soil Structure Interaction Analysis


Geometry


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


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


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


Case Study

https://www.oasys-software.com/solutions/case-studies.html


Deep Basement Design in Hong KongRedevelopment of North Point Estate


• Detailed design of the proposed 1.5m thick diaphragm wall at the boundary of the development with maximum excavation depth of 13.5m


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)


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


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….


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?......


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


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:


software.com/index.php/webinars/webinar/integral_bridge_a

nalysis_using_soil_structure_interaction/

• EC7


software.com/index.php/webinars/webinar/ec7_and_geotech

nical_analysis/

retaining wall analysis

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