Assignment 3-2020 PDF

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Assignment 3...

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DEPARTMENT OF INFRASTRUCTURE ENGINEERING CVEN90050 Geotechnical Engineering

Retaining walls – Assignment 3 Group Submission

This assignment is based on a slightly modified version of Example 3 from AS5100.3 S1-2008 as worked through in the Tutorial. In the tutorial the toe depth was assumed. The task is to write a spreadsheet that will enable calculation of the factor of safety against rotation about a specified prop level for varying toe levels and the resulting prop load, shear force diagram and moment diagram. Several methods of determining these diagrams are presented in the AS5100.3 example. The method you are to employ is to assume: • •

A free earth support limiting equilibrium method where the strength factor is applied to the passive resistance in front of the wall only (i.e. as given in the Tutorial and Lecture). Mobilised passive resistance is to be assumed to be the reduced by the actual factor of safety calculated for the selected toe level.

A propped cantilever sheet pile is installed to retain a 7.9 m deep excavation (Figure 1). The ground water level was found to be 1 m below the ground surface. A pump station is set up at the bottom the excavation to make sure the water level is kept 1.1 m below the proposed excavation level. A surcharge of 20 kPa is applied at ground level. Soil properties are derived from advanced laboratory testing and are shown in Table 1. The sheet pile can be considered as a temporary retaining structure with a maximum service life of 1 year.

Table 1 Soil Layer Layer I Layer II

Unit Weight Drained Friction (kN/m 3) Angle (°) 20 30 22 28

Drained Cohesion (kPa) 0 10

1

prop

D?

Figure 1 A) If a prop is installed at 2 m below the ground surface (Figure 1), calculate the required depth of embedment and prop force, and demonstrate the shear force and bending moment diagram over the length of the sheet pile. Analysis must be done according to the Limit Equilibrium method Example 3 in AS5100.3 S1 2008.

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Layer II

Figure 2

B) If you were required to design the simpler case with a single layer 1 with no groundwater as a cantilever wall without a prop as shown in Figure 2, what would be the required depth of embedment for a factor of safety of 2.0 on the net restoring forces with zero wall friction? Comparing your results with Part A, comment on whether this is a realistic design and what the likely impact of introducing groundwater to this case might mean. Note that you will need a different spreadsheet for this calculation and will need to use an iterative solver (see note 4).

Note #1: This is a group assignment. Note #2: Example 3 in Tutorial no. 8 (Flexible retaining walls) can be used as a guideline for Part A. The procedure in the tutorial example should be followed in terms of over-excavation etc. Note #3: Assume water pressures are balanced at the toe and vary linearly. Treat the net water pressure on the wall as a driving force. Note #4: For Part B an excel file needs to be created and solved iteratively in order to find the hinge depth and toe depth according to Flexible Retaining Walls lectures (Week 9, lecture no. 1- page 32) using solver parameters (Figure 3). A combined excel file with solutions to Part A and B should be uploaded to the LMS in addition to assignment file.

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Note #5: Diagrams of shear force and bending moment need to be presented professionally. The choice of software (or even hand presentation) is up to you, but what is required is a professional presentation. Assume this is a design job and you are handing over a design proposal to a council. Note #6: References used need to be cited accordingly. Note #7: Brief explanation of each step is required. Look at Flexible Wall Tutorial question as a reference. Note #8: Use LMS to submit your work. PDF format is required for submission to avoid change of format etc! with the Excel work, the excel file needs to be submitted.

Figure 3

Marking Guideline (0) The cover page of your assignment report must include names, surnames and student numbers of your group members. Failure to do so will result in a 5 mark deduction. (-5 marks if not done) (a) Find the appropriate lateral active and passive earth pressure coefficients include copies of the marked charts or equations you use. (5 marks) (b) Calculate and draw water pressure distribution on both sides of the wall. (5 marks) 4

(c) Calculate the lateral earth pressure distribution from the soil and surcharge, and the corresponding resultant forces acting on the wall for Part A and B. Draw the pressure distribution diagram and resultant forces over the length of the wall with the moment arms. (10 marks) (d) Use excel software to perform stability analysis and then present stability analyses with references to all required tables and factors according to AS5100.3 and find the required embedment depths for Part A and B. Comparing your Part B results with Part A, comment on whether this is a realistic design and what the likely impact of introducing groundwater to this case might mean. (30 marks) (e) Calculate the design anchor force per unit length of wall (Part A only). (5 marks) (f) Calculate and draw the shear force and bending moment diagrams over the length of the sheet pile wall. (15 marks) (g) Define ultimate structural actions and determine free length of anchor if anchors’ spacing is at 3 m centres (Part A only). (10 marks)

Total: 80 marks

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