Liquefaction is the sudden loss of shear strength of a saturated sediment due to earthquake shaking. Nisqually earthquake 02/28/2001: Olympia, WA

Liquefaction is the sudden loss of shear strength of a saturated sediment due to earthquake shaking Nisqually earthquake 02/28/2001: Olympia, WA

The shear strength is controlled by the degree of grain-to-grain contact friction) in the sediment

cubic packing: each grain touches 6 other grains

rhombohedron packing: each grain touches 12 other grains larger shear strength

Overburden stress increases the grain-to-grain interaction, hence the shear strength. overburden layer larger shear strength

What causes the sediment to lose strength? overburden layer

It starts with water. water table overburden layer saturated sand confining layer

hydrostatic grain matrix is supporting the overburden stress

Cubic grain immersed in water side length L )

Cubic grain immersed in water side length L ) Fluid pressure at the top = P = ρgh P = ρgl Fluid pressure at the bottom = P = ρgh +L) ρ = water density g = acceleration of gravity h = depth below the water surface

Cubic grain immersed in water side length L ) Fluid pressure at the top = P = ρgh P = ρgl Area = L 2 Fluid pressure at the bottom = P = ρgh +L) Force = pressure x area

Cubic grain immersed in water side length L ) Fluid pressure at the top = P = ρgh P = ρgl Area = L 2 Fluid pressure at the bottom = P = ρgh +L) Force = P x L 2

Cubic grain immersed in water side length L ) Fluid pressure at the top = P = ρgh P = ρgl Area = L 2 Fluid pressure at the bottom = P = ρgh +L) Force = ρgl 3 = ρgv = buoyancy force = F B

Cubic grain immersed in water F B = ρgv = the magnitude of buoyancy force is equal to the weight of the volume of fluid the grain displaces Archimedes Principle)

Cubic grain immersed in water The grain weight is reduced by F B F B

hydrostatic Grain-to-grain interaction is reduced in water because the grains are effectively lighter. lower shear strength in water

Now, assume water is flowing vertically upward

Now, assume water is flowing vertically upward h B ρg = energy at B vertical flow occurs when h A ρg> h B ρg h A ρg = energy at A

Now, assume water is flowing vertically upward h B ρg = energy at B hρg L = change in energy over the distance L h A ρg = energy at A Note: h L = hydraulic gradient

Now, assume water is flowing vertically upward h B ρg = energy at B hρg L = change in energy over the distance L h A ρg = energy at A Force = hρg L = seepage force = F s

Now, assume water is flowing vertically upward h B ρg = energy at B hρg L = change in energy over the distance L h A ρg = energy at A F s

Now, assume water is flowing vertically upward the grain will float if F S + F B > grain weight F S F B

Liquefaction occurs in saturated silts and sands which have a high void space as a result of loose packing and uniform gain size i.e., grain texture). overburden layer

cubic packing loosest possible packing) each grain touches a neighbor in 6 different places porosity = n = 47.64% void ratio = e = 90.99% e = n 1 - n

rhombohedron packing tightest possible packing) each grain touches a neighbor in 12 different places porosity = 25.95% void ratio = 35.04%

Relative Density = e max -e e max -e min X 100% e = void ratio Relative Density %) Classification 0 15 Very loose 15 35 Loose 35 65 Medium dense 65 85 Dense 85 100 Very Dense

Earthquake motions impart shearing forces to the sediment

The tendency of loose sediment is to attain a tighter packing, which compresses water in the pores spaces.

The tendency of loose sediment is to attain a tighter packing, which forces water out of pores spaces.

hydrostatic The water takes on the load of the overburden, which increases the pore pressure and decreases the stress on the grains causing a loss of shear strength. high water pressure sediment loses shear strength

hydrostatic high water pressure hydraulic gradient sediment looses shear strength

hydraulic gradient F B F S

hydrostatic high water pressure grains are suspended hydraulic gradient sediment loses all shear strength

Niigata, Japan, 1964: liquefaction damage

hydrostatic settlement original ground surface tighter packing

Nisqually earthquake 02/28/2001: near Olympia, WA sand boils

Lateral spreading lateral displacement) occurs when liquefying sediments are on a slope. Nisqually earthquake 02/28/2001: near Tumwater, WA