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

Transcription

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

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

3 cubic packing: each grain touches 6 other grains

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

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

6 What causes the sediment to lose strength? overburden layer

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

8 hydrostatic grain matrix is supporting the overburden stress

9 Cubic grain immersed in water side length L )

10 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

11 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

12 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

13 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

14 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)

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

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

17 Now, assume water is flowing vertically upward

18 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

19 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

20 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

21 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

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

23

24 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

25 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

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

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

28 Earthquake motions impart shearing forces to the sediment

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

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

31 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

32 hydrostatic high water pressure hydraulic gradient sediment looses shear strength

33 hydraulic gradient F B F S

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

35 Niigata, Japan, 1964: liquefaction damage

36 hydrostatic settlement original ground surface tighter packing

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

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

39