Figure 14.10 provides a generalized case for the Rankine active pressure on a frictionless retaining wall with an inclined back and a sloping granular backfill. You are required to develop some compaction guidelines for the backfill soil when θ = 10⁸ and α = 08, 10^ο, and 20^ο. Laboratory direct shear tests on the granular soil revealed that the effective friction angle varies with the dry unit weight as follows:

The data show that the soil friction angle increases as the compacted unit weight increases. You already know from Chapter 13 that a higher friction angle means better shear strength and stability. However, according to Eq. (14.26), higher unit weight also means higher active force, P_a, on the wall, which is not desirable. 

To further investigate if a higher friction angle indeed has a beneficial effect, prepare a design chart by plotting the variations of P_a/0.5H² (which is equal to K_a(R)γ for various values of the backfill slope a and the friction angle ϕ'. Explain how this chart may aid a geotechnical engineer in developing guidelines for backfill construction for a given height (H) of the retaining wall.

Transcribed Image Text:

Dry unit weight, y (kN/m³) 16.5 17.7 19.5 Friction angle, o' (deg) 28 32 36