A Parametric Study on the Effect of Drainage on Sand Liquefaction under High Overburden Pressure

The effect of high overburden pressure above 1 atm on sand liquefaction potential is typically evaluated based on cyclic undrained testing, with the overburden pressure correction factor, Kσ<1.0 and Kσ decreasing as the pressure increases. Recent centrifuge experiments of a prototype 5 m-thick clean sand layer having a permeability 1.2×10-4 m/s, with free drainage at the top and subjected to 1 and 6 atm overburden pressures, show that a high overburden pressure may increase partial drainage. As a result, the measured field overburden pressure factor, (Kσ)field was estimated to be >1.0 instead of <1.0 in these centrifuge tests. A parametric study is presented here that extends the centrifuge results for a relative density, Dr=45% and free top drainage, utilizing a high-fidelity, calibrated numerical model (P2Psand in FLAC 3D). A stepped acceleration base input is used that ensures a uniform cyclic shear stress amplitude at the elevation of maximum pore pressure ratio. The main parameters varied in the numerical study are overburden pressure, σv0′ (1-12 atm); sand permeability, k (10-6 to 10-3 m/s); and sand layer thickness, H (2-10 m). A new drainage factor, Kdr≥1.0, is proposed to separate the usual undrained Kσ from the effect of partial drainage. The recommendation is to evaluate the overburden pressure factor to be used in liquefaction charts, (Kσ)field, as the product of two factors, (Kσ)field=Kσ×Kdr. The study shows that for constant σv0′=6 atm, Kdr≈1.4-1.7 when k=10-4 to 10-5 m/s, even for a very thick sand layer of H=10 m. Still for a constant σv0′=6 atm, Kdr decreases considerably to values close to 1.0 for a low k≈10-5 m/s, when the layer thickness is H=7 or 8m or greater. And for constant k≈10-4 m/s and σv0′=12 atm, Kdr≈1.4 if H=5-10 m, increasing to Kdr≈1.8 if the layer thickness decreases to H=2 m. The parametric study provides insight on when the current State-of-Practice of using only undrained Kσ becomes too conservative and should be supplemented with consideration of partial drainage through use of factor Kdr. Ready-to-use charts are provided to evaluate Kdr and (Kσ)field for a Dr=45% sand layer with free top drainage.