Handout26 - be based on an effective stress analysis using...

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Short tenn F ~ !. " 100 kPa .. Answer Example 13.8 1:" 60kPa = 1.7 fI' A cut slope is to be made in a clayey soil to permit construction of a new highway, as shown in Figure 13.19. A slope stability analysis is to be perfonned along the potential shear surface shown in this figure. The soils are silty clays with c' == 18 kPa, <p' = 20°, and s. == 100 kPa. If the shear stress at point A is 60 kPa, compute the short-tenn and long-tenn factors of safety at this point. ~gO"~~':.u;t!~----------- Solution ." '; .. ---_ ... - ...... --- Short-tenn stability These soils are clayey, so negative excess pore water pressures will be present at point A immediately after construction. Therefore, the short-tenn stability should be assessed using an undrained total stress analysis. Figure 13.19 Proposed highway cut for Example 13.8. Long-term stability Eventually, the excess pore water pressures will diSSipate, and the soils will attain their drained strength under the new stress conditions. The long-tenn stability analysis should
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Unformatted text preview: be based on an effective stress analysis using the drained strengths. o~ = LrH -u = (18.7 kN/m 3)(4.9 m) + (19.2 kN/m 3)(5.2 m) - (9.8 kN/m 3)(5.2 m) 140 kPa Long tenn F = !. = 69 kPa = 11 -Answer 1:" 60 kPa • .. s '" c' + 0' tancf>' " 18 kPa + (140 kPa) tan 20 0 69 k:Pa Comments Immediately after construction, the factor of safety at Point A is 1.7, which would usually be acceptable. However, once the negative excess pore water pressures have dissipated, F drops to only 1.1, which would generally not be acceptable. If the groundwater table rose, or if the actual c' and <p' values are slightly different than we think, failure could occur. The factors of safety in this example only represent the conditions at Point A, and are intended only to illustrate the effects of negative pore water pressure dissipation. Actual slope stability analyses require assessment of the factor of safety along the entire shear surface, as discussed in Chapter 14....
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This note was uploaded on 09/23/2011 for the course CIVIL ENGI CE 3400 taught by Professor Rosenblad during the Spring '11 term at Missouri (Mizzou).

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