Slope Design with Eurocode 7

Transcription

1 Slope Design with Eurocode Slope Design with Eurocode 7 Eurocode 7 is a design document that establishes rules and standards for geotechnical engineering design across Europe (BSI, 2004). Eurocode 7 represents a major change in design philosophy. Traditionally a single, lumped factor of safety accounts for all of the uncertainties in the problem. With Eurocode 7, partial factors of safety are applied to different components of the analysis. The partial factors are applied prior to the analysis to give design values that are used in the calculation. The final result is an over-design factor, which must be greater than 1 to ensure the serviceability limit state requirement is satisfied. For more information on using Eurocode 7 in geotechnical design, see Smith (2006) and Bond and Harris (2008). This tutorial describes how to design a slope to Eurocode 7 specifications using Swedge. The focus will be on how different design combinations are applied in the computation and what they mean. The finished model can be found in the Tutorial 07 Eurocode Design.swd data file. All tutorial files installed with Swedge can be accessed from the Swedge installation folder. Topics Covered Example for Design Approach 1, Combination 2 Example for Design Approach 2 Single Source Principle

2 Slope Design with Eurocode Model Start the Swedge program. Once the program is open, maximize the view if it is not already maximized. Project Settings Open the Project Settings dialog from the Analysis menu or the toolbar. Select: Analysis Project Settings Under the General tab, keep the default units, Metric, stress as MPa. Go to the Design Standard tab. You ll see that the current Design Standard is None. We ll leave this setting as is, and come back later to assign different design standards. Click on View Partial Factors. With no design standard applied, you should see that all partial factors are set to a default of 1, which means all values used in the calculation are the exact input values. Click Cancel to exit the Partial Factors dialog. Click OK in the Project Settings dialog to finalize the settings.

3 Slope Design with Eurocode Input Data Let s use the default model for this example. The default model has a factor of safety of Let s add a bolt, using the Bolt Dialog. Select: Support Add Bolt Click on the wedge and the Bolt Properties dialog will appear. Enter the following bolt specifications: Select OK. The factor of safety is now Design Approach 1, Combination 2 Without changing the current input data, let s go back to the Project Settings dialog to apply a design standard. Under the Design Standard tab, select Eurocode 7 Design Approach 1, Combination 2 for the Design Standard.

4 Slope Design with Eurocode Let s view the Partial Factors for this Design Standard.

5 Slope Design with Eurocode Applying this set of partial factors means to enlarge unfavorable variable actions while to ignore favorable variable actions, and to reduce the shear strength parameters. If bolts are present, then their capacity or the resistance they provide to stabilize the slope is also reduced. Click OK in the Partial Factors dialog. Click OK in the Project Settings dialog. The wedge should now have a factor of safety of

6 Slope Design with Eurocode Equivalent Model We can reproduce the results from Design Standard Approach 1, Combination 2, without having to use the Design Standard. Let s first turn off the Design Standard. Go to the Project Settings dialog, and under the Design Standard tab, change the Design Standard to None. The Factor of Safety has gone back to To simulate the design approach, we need to apply a factor to the following parameters: 1. Increase variable unfavorable actions by a factor of 1.3 we do not need to anything because there are no external variable loads. 2. Reduce the effective cohesion and the friction angle (tan(φ)) by a factor of Go to the Input Data dialog, and in the Joints tab, change Phi(degrees) to Reduced Phi: Tan -1 (Tan(35 )/1.25) = Click OK to close the Input Data Dialog.

7 Slope Design with Eurocode Reduce the Tensile Capacity of all bolts. Select: Support Edit Bolt Select the bolt and the Bolt Properties dialog will appear. Change the Capacity to MN (1MN/1.1=0.9091MN) The Factor of Safety should now be , which is exactly the same as that for Design Approach 1, Combination 2. Design Approach 2 We will use the same geometry but remove the bolt. Select: Support Delete Bolt Click on the bolt to delete it. Set the friction angles back to 35, and check to make sure that the Design Standard is still set to None. We will now add an external force to the model. Select: Analysis Input Data Go to the Forces tab and enter the following data:

8 Slope Design with Eurocode Select OK. The Factor of Safety should have decreased to The direction of the applied load decreases the stability of the wedge. Let s now go to the Project Settings dialog. Under the Design Standard tab, set the Design Standard to Eurocode - Design Approach 2. Click View Partial Factors, and you should see the following dialog.

9 Slope Design with Eurocode For this design standard, a partial factor is applied to the driving force of the slope weight (permanent unfavorable action), any unfavorable external force that is set to be variable, the joint shear strength, and the tensile capacity of any bolts. Click OK in the Partial Factors dialog, and then also click OK in the Project Settings dialog. The Factor of Safety is now

10 Slope Design with Eurocode Equivalent Model We will construct a model to replicate the partial factors are applied in Eurocode 7 - Design Approach 2. Change the Design Standard to None. The Factor of Safety should revert to To simulate the design approach, we need to apply changes to the following parameters: 1. Under the Slope tab in the Input Data dialog, multiply the Rock Unit Weight by 1.35, to get MN/m 3. This has the same effect as applying a factor of 1.35 to any permanent unfavorable actions, which in this case is the driving force due to the slope weight. However, remember that the weight has both a favorable and unfavorable action component (stabilizing and driving). We have to adhere to the Single Source Principle and apply only 1 factor to any actions that comes from the same source. Click Apply and the factor of safety changes to Under the Joints tab in the Input Data dialog, we need to reduce the overall joint shear strength by a factor of 1.1. This is the equivalent to reducing the Earth Resistance by a factor of 1.1. Base Joints Friction Angle: Tan -1 (Tan(35 )/1.1) = Keep in mind that this is different than reducing individual shear strength parameters. If cohesion is present, then a factor of 1.1 is applied to the overall shear strength: Factored Shear Strength = (Normal Force x Tan(φ) + cohesion)/1.1 Click OK in the Input Data dialog. The Factor of Safety should now be We need to multiply the value of the external force by 1.5, to get 30MN. We do this because the external force is an Unfavourable Action, and is considered to be Variable. The Factor of Safety is now , the same as for Eurocode Design Approach 2. You may access the Help system for more information on Eurocode design. To do so, click the? button on the top-right corner of the Design Standard tab in the Project Settings dialog.

11 Slope Design with Eurocode References Bond, A. J. and Harris, A. J., Decoding Eurocode 7, Taylor & Francis. British Standards Institution, Eurocode 7: Geotechnical design Part 1: General rules, BS EN , London, UK. Smith, Smith s Elements of Soil Mechanics, 8 th Edition, Blackwell Publishing.