Time-Rate Settlement in Normally Consolidated Clays and Remedial Measures

1. Time-Rate Settlement in Normally Consolidated Clays and Remedial Measures Justin Phalen December 3, 2001

2. Saturated silts, mostly clays Ability to transport fluid through pore space is small Can take months to hundreds of years for water to escape Low-Permeable Soils

3. Increase in stress from external surface loads, or groundwater fluctuations Water cannot escape pores, bears majority of stress (excess pore pressure) Eventually soil settles Consolidation Settlement

4. H: thickness of clay layer Cc: Compression Index eo: Initial void ratio svo’: initial effective stress Ds: Applied stress increment Estimation of Settlement (Terzaghi & Peck, 1969)

5. More important than how much is how long Pore pressure dissipates with time Depends on intrinsic properties of soil Drainage path distance Time-Rate Settlement Kansai International Airport Osaka Bay, Japan

6. Ue function of t, z and clay properties When Ue = 0, no longer consolidating Dimensionless time factor, T Longest drainage path, Hd Rate of Consolidation (estimation)(Terzaghi 1925) Coeff. of Consolidation

7. Isochrone Map (Single Drainage)

8. Isochone Map (Double Drainage)

9. Practical Consolidation Remediation • Excessive Overburden Stress • Improving drainage conditions • Vertical Drains • Drilled wells • Stone/sand columns or walls • Prefabricated drains (wick drains)

11. An application

12. Ds = 500 psf Ultimate Effective Stress At t = 1 Ultimate EffectiveStress U Initial Effective Stress (Before loading) Initial Effective Stress (Before loading) At t = 10 At 1 Year At 10 Years

13. At 1 Year Ultimate Effective Stress Ultimate Effective Stress At t = 1 At t = 1 U Ue Initial Effective Stress (Before loading) Initial Effective Stress (Before Loading) Ds = 500 psf Ds = 1000 psf

14. Enter wick drains • Triangular distribution • 5-ft spacing between drains • Extend to bottom of clay layer • Reduces max distance water must travel to exit soil

15. AMAZING!

16. Thank You