Liquefaction

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This cargo of Iron Ore Fines was suspected of undergoing liquefaction. Luckily, in this case the bulk carrier was assumed to be unaffected (Source: Gard - Liquefaction of solid bulk cargoes).
Padang Hawk’s Cargo of nickel ore, which underwent liquefaction (two views)[1].

Liquefaction, in regards to transportation of solid bulk cargoes on board bulk carriers, is a phenomenon where a cargo loses a large percentage of its shear strength when subjected to monotonic, cyclic or shock loading, and flows in a manner resembling a liquid[2]. Liquefaction is more commonly seen during earthquakes where the shear strength of a soil is reduced under cyclic loading resulting in subsidence and moisture migration to the surface.

See Also: Incidents - List of incidents involving bulk carriers carrying solid bulk cargoes which may have possibly undergone liquefaction.

Other Definitions[edit]

There are any number of scientific definitions of Liquefaction:

The 1997 annual of the American Society for Testing and Materials defines ‘spontaneous’ liquefaction as; …the sudden large decrease of the shearing resistance of a cohesionless soil. It is caused by the collapse of a structure by shock or other type of strain and is associated with a sudden but temporary increase of the prefluid pressure. It involves a temporary transformation of the material into a fluid mass.[1].

Causes[edit]

Solid bulk cargoes such as iron ore fines or nickel ore normally contain a degree of moisture within the particles[3]. The moisture within the solid bulk cargoes can come from processes including extraction, transportation and stockpiling, when if not monitored can increase significantly, especially in regions with high humidity and high amounts of rainfall.

When solid bulk cargoes are subjected to recurring cyclic forces, such as the movement of the bulk carriers (rolling/pitching/slamming), the volume of spaces between the particles reduces[3]. When the spaces between the particles reduces (increase in density, reduction in void ratio) the degree of saturation is increased. With the reduction in air voids and considering water is in-compressible, the pore water pressure within the solid bulk cargoes can increase resulting in the reduction in the effective stress and therefore shear strength of the solid bulk cargo, resulting in it acting like a liquid.

Although fully saturated materials are considered to have more of a potential to liquefy, partially saturated materials are also considered to be potentially liquefiable under similar conditions [4] [5] [6] [7] [8] [9].

Effects[edit]

MV Black Rose's cargo of Iron Ore Fines was suspected of undergoing liquefaction causing the bulk carrier to capsize (Source: Top News).
MV Asian Forest's cargo of Iron Ore Fines was suspected of undergoing liquefaction causing the bulk carrier to list then capsize (Source: Mangalorean News).

Since the holds of bulk carriers are not designed to carry liquid, if liquefaction of solid bulk cargoes occur while being transported, it can cause the bulk carrier to list or even capsize[10]. This is due to the cargo shifting from one side of the hold to the other and causing the bulk carrier to become unbalanced and does not necessarily return to the center[10][3]. If a bulk carrier begins to list, ballast is used to try to correct the problem. Due to the weight of solid bulk cargoes this can sometimes be very difficult to correct. If the bulk carrier continues unbalanced the list can cause an increase in the amount of shifted cargo and the bulk carrier can capsize[10][3].

Theory[edit]

Dynamic liquefaction occurs when a cyclic loading is applied to a material causing parameters within the cargo to change and therefore liquefy [10][11]. Cyclic liquefaction propagates throughout the cargo from a single point to cause total liquefaction. This phenomenon can occur in saturated or unsaturated cargoes[10].

Referring to basic soil mechanics, the effective stress (σ') directly affects the resulting shear stress (τ)[12]. This can be seen in the following equation by Terzaghi (1942) [13] where the shear stress (τ) of a soil can be seen to be a function of the effective stress (σ'), friction angle (Φ'), and cohesion factor (c'). The expression shows that a reduction in the effective stress of a cohesionless soil results in a reduction in the shear stress.

Shearstress.gif

The following equation shows the relationship between the effective stress (σ'), normal stress (σ) and pore water pressure (uw) of a saturated material [10][14], where increasing the pore water pressure reduces the effective stress. The pore water pressure can be suddenly increased by applying a cyclic load. If the effective stress of a cohesionless material is reduced to zero, the shear stress is also reduced to zero and the material has the potential to liquefy[10].

Bishopeffectivestress.gif

Likewise, in partially saturated cohesionless materials, when the effective stress is reduced to zero, the shear stress is also reduced to zero. The difference between the resulting effective stress of a saturated verses partially saturated material is the presence of pore air pressure (ua) and a factor of the degree of saturation (X), as seen in the following equation[15][10].

Bishop1959effectivestress.gif

When static, the pore water pressure is negative and this suction force has a tendency to hold the particles together, but when cyclic loading is applied changes in the pore air pressure and pore water pressure will force the particles apart. Since air compresses and water does not, the pores filled with air will reduce causing a decrease in the void ratio and an increase in the degree of saturation. Under certain conditions this can cause the effective stress of the cargo to reduce to zero causing it to become potentially liquefiable[10].

Prevention[edit]

Currently the only parameter used to determine the liquefaction potential of solid bulk cargoes is the Transportable Moisture Limit (TML). The Transportable Moisture Limit (TML) is the maximum Gross Water Content (GWC) by weight that a Group A or liquefiable solid bulk cargo may contain while being transported on a bulk carrier without it being at risk of liquefying. The TML is determined using one of three methods stated in Appendix 2 of the International Maritime Solid Bulk Cargoes Code (IMSBC Code)[16] or, as in the case for Iron Ore Fines (IOF) and Coal, the modified method as stated in DSC.1/Circ.71[17] and Modified Proctor/Fagerberg Method for Coal[18], respectively.

The TML test methods are the:

  1. Flow Table Test,
  2. Proctor/Fagerberg Test,
  3. Penetration Test,
  4. Modified Proctor/Fagerberg Test for Iron Ore Fines and
  5. Modified Proctor/Fagerberg Test for Coal.

References[edit]

  1. 1.0 1.1 Australian Transport Safety Bureau, Report #148, "Investigation into the shift of cargo on board the Singapore flag bulk carrier Padang Hawk in the Coral Sea on 26 and 27 July 1999", September 2000. (Download PDF)
  2. Sladen, J.A., R.D. D’Hollander, and J. Krahn, The liquefaction of sands, a collapse surface approach. Canadian Geotechnical Journal, 1985. 22: p. 564-578.
  3. 3.0 3.1 3.2 3.3 http://www.standard-club.com/media/23989/StandardCargoLiquefactionFeb2011.pdf
  4. Baker, R. and S. Frydman, Unsaturated soil mechanics - Critical review of physical foundations. Engineering Geology, 2009. 106: p. 26–39.
  5. Fredlund, D.G., N.R. Morgenstern, and R.A. Widger, The Shear Strength of Unsaturated Soils.
  6. Md.Noor, M.J. and W.F. Anderson. A comprehensive shear strength model for saturated and unsaturated soils. in Proceedings of the 4th International Conference on Unsaturated Soils - ASCE Geotechnical Publication No. 147. 2006. Carefree, Arizona, USA.
  7. Toll, D.G., B.H. Ong, and H. Raharjo. Triaxial testing of unsaturated samples of undisturbed residual soil from Singapore. in Proceedings of the Conference on Unsaturated Soils for Asia. 2000. Singapore, Balkeema.
  8. Khalili, N. and M.H. Khabbaz, A unique relationship for χ for the determination of the shear strength of unsaturated soils. Geotechnique, 1998. 48(5): p. 681-687.
  9. Geiser, F. Applicability of a general effective stress concept to unsaturated soils. in Proceedings of the Conference on Unsaturated Soils for Asia. 2000. Singapore, Balkeema.
  10. 10.0 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 Munro, M. and A. Mohajerani, Determination of Transportable Moisture Limit of Iron Ore Fines for the Prevention of Liquefaction in Bulk Carriers. Marine Structures, 2015. 40(1): p. 193-224.
  11. Davies, M., E. McRoberts, and T. Martin, Static Liquefaction of Tailings – Fundamentals and Case Histories. AMEC Earth & Environmental, 2002.
  12. Munro, M. and A. Mohajerani, Determination of Transportable Moisture Limit of Iron Ore Fines for the Prevention of Liquefaction in Bulk Carriers. Marine Structures, 2015. 40(1): p. 193-224.
  13. Terzaghi, K.v., Theoretical Soil Mechanics. 1942, New York: John Wiley & Sons. 528.
  14. Bishop, A.W. and G. Eldin, Undrained Triaxial Tests on Saturated Soils and their Significance in the General Theory of Shear Strength. Geotechnique, 1950. 2: p. 13-32.
  15. Bishop, A.W., The Principle of Effective Stress. Teknisk Ukeblad, 1959. 106 (39): p. 859–863.
  16. International Maritime Organisation, International Maritime Solid Bulk Cargoes Code, 2013 Edition, London: International Maritime Organization.
  17. International Maritime Organisation, DSC.1/Circ.71 - Early Implementation of Draft Amendments to the IMSBC Code Related to the Carriage and Testing of Iron Ore Fines, 15 November 2013, London. (Download PDF)
  18. Modified Proctor/Fagerberg Method for Coal, 24 November 2014. (Download PDF)

See Also[edit]

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