The proposed route of a new railway line requires a 10 m deep cutting at an...

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The proposed route of a new railway line requires a 10 m deep cutting at an angle of 60° to be made near the toe of an existing 8° slope, which extends 150 m upwards from the edge of the cutting (see Figure Q1.1). The phreatic surface was found to be 1 m below the slope surface and running parallel to it, but its position is considered to vary seasonally. A site investigation borehole near the toe of the slope revealed stiff silty clay to a depth of about 20 m below the slope surface, underlain by chalk. Undisturbed samples of the clay from this borehole were sent for laboratory testing, which determined that the saturated unit weight was (19.1+X/10) kN/m³, and the average undrained shear strength over the height of the proposed cutting was cu = (50+X) kPa. However, a 'discontinuity' running parallel to the slope surface at a depth of 3 m (also shown in Figure Q1.1) was completely missed in the initial site investigation. 150 m to top of slope 8°( Proposed cutting .A 60° 10 m Discontinuity CLAY 1m HAV Phreatic surface (Not to scale) Figure Q1.1 (a) Ignoring the discontinuity, sketch the most likely short term failure mechanism for the cutting, and use Taylor's (1937) method to calculate the short term global factor of safety. Justify any assumptions made and comment on your answer. [7] (b) Ignoring the discontinuity, sketch the changes over time in total stress, pore water pressure and effective stress at point A caused by the excavation of this cutting, indicating clearly (i) before, (ii) immediately after and (iii) a long time after excavation. Identify the key features of your sketch and explain the implications for the design of the cutting. [7] (c) During top-down excavation of the cutting, the discontinuity shown in Figure Q1.1 was exposed in the face, causing work to be halted. Further exploration identified the discontinuity as a pre-existing failure surface running parallel to the slope surface and extending to the top of the slope. An undisturbed block sample incorporating the failure surface was taken, and laboratory testing determined residual shear strength parameters c₂' = 0 kPa and or= (14.5 + X/10)°. With this new information, check the long term stability of the main slope (not the cutting) in accordance with EC7 Design Approach 1. Identify the likely failure mechanism, assume reasonable values for any parameters that are not given, justify your methodology where appropriate, and comment on your answer. [16] (d) In the light of your answers to parts (a) - (c) above, what recommendations might you make for the design of this railway cutting? [4] The proposed route of a new railway line requires a 10 m deep cutting at an angle of 60° to be made near the toe of an existing 8° slope, which extends 150 m upwards from the edge of the cutting (see Figure Q1.1). The phreatic surface was found to be 1 m below the slope surface and running parallel to it, but its position is considered to vary seasonally. A site investigation borehole near the toe of the slope revealed stiff silty clay to a depth of about 20 m below the slope surface, underlain by chalk. Undisturbed samples of the clay from this borehole were sent for laboratory testing, which determined that the saturated unit weight was (19.1+X/10) kN/m³, and the average undrained shear strength over the height of the proposed cutting was cu = (50+X) kPa. However, a 'discontinuity' running parallel to the slope surface at a depth of 3 m (also shown in Figure Q1.1) was completely missed in the initial site investigation. 150 m to top of slope 8°( Proposed cutting .A 60° 10 m Discontinuity CLAY 1m HAV Phreatic surface (Not to scale) Figure Q1.1 (a) Ignoring the discontinuity, sketch the most likely short term failure mechanism for the cutting, and use Taylor's (1937) method to calculate the short term global factor of safety. Justify any assumptions made and comment on your answer. [7] (b) Ignoring the discontinuity, sketch the changes over time in total stress, pore water pressure and effective stress at point A caused by the excavation of this cutting, indicating clearly (i) before, (ii) immediately after and (iii) a long time after excavation. Identify the key features of your sketch and explain the implications for the design of the cutting. [7] (c) During top-down excavation of the cutting, the discontinuity shown in Figure Q1.1 was exposed in the face, causing work to be halted. Further exploration identified the discontinuity as a pre-existing failure surface running parallel to the slope surface and extending to the top of the slope. An undisturbed block sample incorporating the failure surface was taken, and laboratory testing determined residual shear strength parameters c₂' = 0 kPa and or= (14.5 + X/10)°. With this new information, check the long term stability of the main slope (not the cutting) in accordance with EC7 Design Approach 1. Identify the likely failure mechanism, assume reasonable values for any parameters that are not given, justify your methodology where appropriate, and comment on your answer. [16] (d) In the light of your answers to parts (a) - (c) above, what recommendations might you make for the design of this railway cutting? [4]

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a Ignoring the discontinuity the most likely short term failure mechanism for the cutting is a circular slip failure To calculate the short term global factor of safety using Taylors 1937 method we ne... View the full answer