The liquefaction potential of bulk cargoes is a topic that has gained increased attention and awareness over the past few years in the shipping industry.
Most readers will be familiar with the International Maritime Solid Bulk Cargoes (IMSBC) Code definitions of cargo groups A and C: group A cargo is likely to liquefy at excessively high moisture contents and group C cargo is not likely to liquefy.
It is also widely understood that the moisture content and the amount of fine particles within a representative sample can affect the potential for a material to lose friction and exhibit flow behaviour under a dynamic or cyclic external force (such as the forces vessels are subjected to in rough seas). However some bulk cargoes, such as sand, have inadvertently raised concern regarding their liquefaction potential as they can be both group A and C.
The onset of liquefaction is defined as the point at which flow is initiated due to a sudden loss of shear strength and increase of pore pressures. The flow moisture point (FMP), when increased pore pressures exceed the upper limit of moisture absorption for a material and shear strength decreases, is determined by comparing samples’ deformation response to external cyclic forces according to a range of moisture con- tents. The moisture content (MC), FMP and the transportable moisture limit (TML) are established in the laboratories of Competent Authorities and reported in the test certificates for group A cargoes. Historically many group A cargoes have been wrongly declared; either by the shipping documents wrongly reporting the cargo as group C (not likely to liquefy) in the absence of proper test certification or by improper sampling and testing procedures leading to invalid certificates. Although group C cargoes do not flow and therefore do not require the laboratory testing and certification that group A cargoes re- quire, samples are sometimes taken from ships during loading and sent for testing in order to validate a cargo declaration or test certificate. Investigation can lead to delays for shipments and the additional costs can quickly increase if parties disagree on the sampling and/or testing methods Confusion can exist for shipments that appear as both group A and group C in the IMSBC Code.
For example, group C sand (included in the schedule as foundry, silica, potassium feldspar, soda feldspar and quartz sands in the Code) are described as “usually fine particles” with particle sizes of 0.1mm to 5mm, whilst group A heavy mineral sand is “characterised by their heavy bulk density and relatively fine grain size” of up to 5mm.
Two schedules are provided for two cargoes of similar grain sizes, yet their distinguishing characteristics require laboratory analysis. Furthermore, if a cargo is suspected to have been misdeclared, determining its group is not always a straightforward process. A sound knowledge of the test methods is essential.
When group A samples have a moisture content greater than the certified transportable moisture limit (TML) value, the cargo is not suitable for transport by sea. The potential for liquefaction can have devastating implications on the ship’s stability due to cargo shift, so alarm is raised when a cargo appears wet at the time of loading and especially when free water appears in the hold.
Which laboratory test is suitable?
Appendix 2 of the IMSBC Code describes the test for the determination of moisture content and three test procedures for the determination of a TML value. These tests were originally developed to observe the correlations of moisture content, density and other soil parameters to the liquefaction resistance of materials in geotechnical engineering laboratories. A sound understanding of the expected soil behaviour under the different test methods is required in order to understand the liquefaction potential of a cargo.
The IMSBC Code Appendix 2 tests for the determination of the TML value for a cargo comprise the flow table test, the penetration test and the Proctor-Fagerberg test. The tests all use different forces to initiate material deformation, therefore the scale of the material to the test apparatus is a governing factor for this deformation. The flow table test is generally suitable for mineral concentrates and other fine material with a maximum grain size of 1mm and may be applicable to materials with a maximum grain size up to 7mm. This test uses the cyclic forces of the flow table apparatus to cause a sample to deform. The penetration test is suitable for mineral concentrates and similar materials with a maximum grain size of 25mm.
This test uses vibration to initiate the settlement of a weight (a penetration bit) within a sample. The Proctor-Fagerberg test described in Appendix 2 is suitable for both fine and relatively coarse-grained ore concentrates and similar materials up to a maximum grain size of 5mm. This test uses dynamic compaction by hand to determine the degree of saturation of a sample. Larger test equipment are used for penetration and Proctor-Fagerberg tests on larger grain sizes. Understanding what behaviour the tests are actually measuring is imperative for the test method selection. The Sub-committee on the Carriage of Cargoes and Containers regularly meet at the IMO to review the latest proposed amendments to the IMSBC Code. Modifications to the Proctor-Fagerberg and penetration tests have been studied to account for scale effects for testing fine materials in order to avoid misjudgment of the acceptable TML value (please refer to the IMSBC Code for further information). A modified Proctor-Fagerberg test has been proposed to specifically identify the TML value for group A iron ore fines.
The size of the test apparatus and the saturation degree have both been reduced in the modified test compared to the original version of the test. The Proctor-Fagerberg compaction tests in Appendix 2 do not determine the flow point or flow potential of a material, so these particular Appendix 2 tests are only suitable for the determination of the TML value (with respect to degree of saturation) for granular group A materials. Specific modifications have been applied to the penetration test in studies undertaken for technical review at the IMO in order to determine the flow point for group A nickel ores with a fine particle size. The penetration test was chosen as the preferred test for this cargo as nickel ore is a more clay-like material and as such does not exhibit deformation behaviour in the same way as granular ores. The penetration test method determines the FMP for a moisture content value when the bit settles into a vibrating sample under its own self-weight greater than 50mm in depth. As the friction between the particles reduces, the bit settles into the sample surface due to the reduced shear strength of the sample. The FMP is determined according to measured settlement depth of the test bit and reported according to the sample moisture content. In the case of the modified penetration test, further investigation is required before a new test method that can be universally adopted will be included in the Code.
There have not been any modifications proposed at the IMO for the flow table test. Perhaps this is because this test is best suited to determining the flow point (or lack thereof) for fine particles as the material is not contained within a mould during the vibration. Nonetheless, this test requires a technician who is familiar with the deformation behaviour in order to properly identify the FMP.
Once the preliminary FMP is observed, the tests are repeated with the moisture contents of samples adjusted by ± 1% to determine the final FMP value.
Photograph 2 shows a sand sample that has deformed due to the vibration of the test plate. The deformation appears as the loss of shear strength, but there is neither “flow” behaviour nor free water on the plate.
A cone of group C cargo on the flow table may lose shape and crumble under the cyclic forces, but that is not the same as plastic flow which is a non-recoverable phase change from solid to liquid behaviour, and as such does not indicate liquefaction potential. As stated in the Code: “In certain conditions, the diameter of the cone [sample on the table] may increase before the flow moisture point is reached, due to low friction between the grains rather than plastic flow. This must not be mistaken for a flow state.”
Emerging amendments to the IMSBC code
Proposals for new cargo schedules and modified test methods for mineral ores (such as iron, nickel, bauxite, coal, etc.) are regularly reviewed based on technical and editorial content for acceptance into the IMSBC Code by experts at the IMO. New schedules are drafted for different types of mineral ores and other naturally-occurring soils as they have inherently unique mechanical properties. Because the soil’s moisture con- tent can greatly affect the strength parameters, understanding the changes in volume and pore pressures that soils experience under cyclic stress is fundamental to determining their failure mechanisms.
To quote Suzanne Lacasse of the Norwegian Geotechnical Institute in the 2015 Rankine Lecture, an annual geotechnical lecture named after an early contributor to the theory of soil mechanics, the geotechnical engineer “is called upon to identify and define situations that are potentially hazardous and to at least initiate a decision process as to whether the hazards are acceptable or not.” Experts in the mechanical behaviour of soils are best placed to define and explain the deformation behaviour of concentrates, silts, sands, ores, clays and gravels under the Appendix 2 test methods in the IMSBC Code and to what ex- tent the material is safe for sea transport.
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This article first appeared in International Bulk Journal