Wave-Induced Liquefaction. General Lecture

1. Wave-Induced Liquefaction. General Lecture B. Mutlu Sumer Technical University of Denmark, MEK, Coastal and River Engineering (formerly ISVA) 2800-Lyngby, Denmark

2. Definition • Soft marine soils under high waves may undergo a process • in which the soil grains become completely free, and • the water-sediment mixture, as a whole, acts like a fluid! • This process is called liquefaction. • Under the liquefaction condition, obviously the soil fails!

3. Consequences. With the soil liquefied, • Buried pipelines may float to the surface of the seabed; • Pipelines laid on the seabed may sink in the soil; • Large individual blocks (like those used for scour protection) may penetrate into the seabed; • Sea mines may enter into the seabed and eventually disappear; • Or, an indirect effect: As a result of the wave motion, structures may execute cyclic motions, resulting in local liquefaction around them, which may enhance scour, thus leading to the instability of the structures; • Sometimes, we use wave-induced liquefaction to our end, to compact sand (as was done by LICengineering (Denmark), a member of LIMAS, in combination with soil replacement in an engineering exercise!)

4. Waves (for those who are not terribly familiar with waves)? • In coastal areas, Wave height = O(1-2 m) • In offshore areas, with water depth of 60-70 m, for example, wave height for 50-100 years return period = O(10-20 m) are not unusual! • Wave period = O(5-15 s)!

5. Two kinds of wave liquefaction • Liquefaction induced by the buildup of pore pressure, called the Residual Liquefaction • Liquefaction induced by the upward-directed pressure gradient, called the Momentary Liquefaction

6. Residual Liquefaction

7. Residual Liquefaction • The result of a lab experiment with a silt bottom; Two time series: • (1) Surface elevation; and (2) Pressure time series • Water depth = 42 cm • Wave height = 10 cm • Wave period = 1.6 s • Pressure measured at depth 16.5 cm in the soil

8. Residual Liquefaction • In this progressive buildup of the pore pressure, if the waves are high, the pressure may reach such levels that it will exceed the submerged weight of the soil above! • In this case, the soil grains will become unbound and completely free, and the soil will begin to act like a liquid! • This process is called the residual liquefaction!

9. Watch out: The conditions for Residual Liquefaction • Soil must be soft, like backfill in a trench hole, so that there is room for the grains to rearrange; • (A soil with a long history of wave “loading” is unlikely to liquefy because there is not much room for the grains to rearrange; this is due to compaction!); • Soil must be fine (silt, fine sand) so that all pore pressures accumulated during the wave cycle would not dissipate as rapidly as they develop • Waves must be sufficiently high

10. Residual Liquefaction. A video film • The video camera views the soil through the glass-side wall of the wave flume. • On the screen, in the upper left-hand corner, two signals in a window: One is the surface elevation, and the other the pore pressure recorded at the depth 12 cm. • The signals recorded simultaneously with the videotaping. • Soil, Silt: d50 = 0.045 mm; Water depth = 40 cm; Wave height = 17 cm, Wave period = 1.6 s • Will see a horizontal band in the middle of the screen. Do not take any account of this! It is silicon used to fill the gap between the side wall of the flume and the side wall of the silt box. • Dr. Figen Hatipoglu (She is a Post-Doct at ISVA, Tech. Univ. Denmark) made the film.

11. Momentary Liquefaction

12. Momentary Liquefaction • Pressure distributions in the soil across the depth under the trough! • For two situations: • (a) The case of a saturated soil (there is no gas/air in the soil)! • (b) The case of an unsaturated soil (there is gas/air in the soil)!

13. Momentary Liquefaction • This upward-directed pressure gradient induces a lift force on the soil under the wave trough • If the lift force exceeds the submerged weight, the soil will be liquefied! • This process is called the momentary liquefaction! • (Although there is also an upward-directed pressure-gradient force in the saturated case, this is apparently too small to cause liquefaction even under the highest waves!!)

14. Watch out: The conditions for Momentary Liquefaction • The soil must be an unsaturated soil (the soil may be liquefied even with Sr only slightly different from 1!) • Only a shallow, top layer of the soil is liquefied because of the large pressure gradient experienced (however, under extreme conditions, the liquefaction can penetrate to depths as far as O(0.5Wave height)) • Liquefaction occurs during the passage of the wave trough • Waves must be sufficiently high

15. Engineering practice • Be it the residual liquefaction or the momentary liquefaction, the question in engineering practice boils down to the following: • Given the soil; • Given the waves (50 year, 100 year,..); • Will there be any liquefaction risk for the soil supporting any structure (a pipeline, a gravity structure, a breakwater, a pier, a pile, a scour protection structure, etc.)?

16. To assess liquefaction • This has stimulated research on the topic in the area of coastal engineering over the past 20 years • Three approaches have been adopted: • Physical modelling (The ordinary physical modelling, and most recently the physical modelling involving centrifuge facilities) • Mathematical modelling • Deductions from field measurements

17. Physical modelling • The main objective of the physical modelling is to get a good understanding of the processes, simulated in the lab, under controlled conditions • It also enables systematic parametric studies • Furthermore, it provides data for the validation of mathematical models, a valuable “by-product”! • The down side, however, is that the soil response may not be properly extrapolated to the field conditions; such a lab model may be treated as an individual prototype itself! • To get around the problem (of extrapolating the results to the prototype), centrifuge wave testing on a soil bed has been tried recently (1999, 2001)!

18. Mathematical modelling • The soil is assumed to be a poro-elastic medium • The model, which governs (1) the soil deformation, and (2) the movement of pore water (including the pore pressure), is basically the Biot equations • The latter are solved under the boundary condition at the seabed

20. Mathematical modelling • The Biot equations: good enough to study the momentary liquefaction! • For the buildup of pore pressure and eventual residual liquefaction, however, we need some additional information • One such piece of information may be an empirical expression for the pressure generated by the cyclic shear (the “source term”!) such as

21. Mathematical modelling • Although this approach does a good job in engineering applications, it does not accommodate the continuous change of the soil properties, and particularly • it breaks down near liquefaction conditions! • Recently, sophisticated sand models have been developed • One such model which accounts for the contractive/dilative behavior of sand and can handle the long-term pore pressure buildup, has been adopted by HR! • HR will present early results of this approach, as applied to the wave-induced liquefaction!

22. Commercial! • Great many works have been devoted to the physical and mathematical modelling of wave-induced liquefaction. • A detailed account of these works (and its impact on scour-related problems) (with over 80 references) is given in Chapter 10 in the book by • Sumer, B.M. and Fredsøe, J. (2002). The Mechanics of Scour in the Marine Environment. World Scientific, xiv+536 p.

23. Deductions from field measurements • Field measurements: not terribly easy to interpret. This is largely because we have no control over the “test conditions”! • Yet, we can make useful deductions from field data! (We have one wave-induced-liquefaction field study in LIMAS, Université de Pau; and had one in SCARCOST, the “predecessor” of LIMAS, under EU MAST-III programme) • In this conjunction, another sensible approach would be to simulate the field conditions in the lab in a large-scale wave facility (We have one such study in LIMAS, Technische Universitat Braunschweig)

24. http://www.ce.washington.edu/~liquefaction/html/what/what1.htmlhttp://www.ce.washington.edu/~liquefaction/html/what/what1.html

25. Centrifuge facilities • Two types: • (1) Beam type (Sassa & Sekiguchi, 1999); and • (2) Drum type (pretty much the same as a washing machine!) (Mark Randolph & Liang Cheng, Univ. of Western Australia)

26. Issues in engineering practice • Marine pipelines may be buried against heavy traffic, or fishing gear. (Dredge a trench; Place the pipe in it; Backfill the trench with the excavated material)). • Question: Is the backfill material “liquefaction resistant”? (Otherwise the pipeline will float to the surface!); Or should it be replaced with a coarser material?

27. Issues in engineering practice • Marine pipelines may also be laid on the seabed. • Question: Will the seabed be liquefied under extreme storm events (100 year storm) if it is a soft soil? • (Although there will be very little room for the rearrangement of grains, and therefore for the residual liquefaction, due to a long exposure of waves. However, remember the momentary liquefaction!) • If the bed is liquefied, that will jeopardize the pipeline’s stability!

28. Issues in engineering practice • Marine structures are protected against scour by scour protection (rock, armour blocks,..). • Question: Can the supporting soil be liquefied under extreme storm events? If yes, to what depths do the elements of scour protection sink in the liquefied soil?

29. Issues in engineering practice • Marine structures (such as caisson breakwaters, gravity structures, piles) execute rocking motion under waves. • This may cause liquefaction around the structure, inducing enhancement in scour, and therefore endangering the stability of the structure • Question: What is the extent of liquefaction around the structure?

30. Residual Liquefaction

31. Large-time behavior