Figure A.2 in Appendix A provides a generalized case for Rankine active pressure on a frictionless retaining wall with inclined back and a sloping granular backfill. You are required to develop some compaction guidelines for the backfill soil when θ = 10°, and a = 0°, 10°, and 20°. Laboratory direct shear tests on the granular soil revealed that the effective friction angle varies with the dry unit weight as follows:
Dry unit weight γ, Friction angle, ϕ'
(kN/m3) (deg)
16.5........................................28
18.7........................................32
19.5........................................36
The data show that the soil friction angle increases as the compacted unit weight increases. You already know from Chapter 12 that higher friction angle means better shear strength and stability. However, according to Eq. (13.24), higher unit weight also means higher active force, Pa, on the wall, which is not desirable. To further investigate if higher friction angle indeed has a beneficial effect, prepare a design chart by plotting the variations of Pa / 0.5H2 (which is equal to Ka(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.