20170908 6. BAUXITE TML TEST DEVELOPMENT...• Where moisture very freely drains from the sample at moisture content such that the test sample compaction curve cannot extend to or

BAUXITE TML TEST DEVELOPMENT

Mr. Gustavo Correia - Alcoa

Section Outline

• Introduction

• Existing IMSBC TML Tests

• Bauxite Proctor Fagerberg Test (PFT) Development

• Mould

• Hammer

• SG

• Sample Prep

• Procedure

• Bauxite PFT Results

• Bauxite PFT Reproducibility and Repeatability

• Summary

2

Introduction – Granular Material

Behaviour

• The behaviour of compacted granular assemblages at different water contents is well understood in geotechnical science;

• The compaction behaviour as a function of moisture content is typically measured using a Proctor compaction test or some modification of it;

• the typical compaction curve for a granular material such as bauxite, for a given compaction effort/energy level.


Behaviour


Behaviour


Behaviour

Existing IMSBC TML Test Methods

• Bauxites shipped worldwide exhibit a wide range of PSDs and large top sizes;

• None of the tests in Appendix 2 of IMSBC Code allow for the appropriate TML

testing of the full range of seaborne traded bauxites;

• All existing TML test methodologies present apparatus limitations regarding the

particle size of the as shipped materials;

• It should also be noted that none of the TML tests represent the same confining

stresses experienced by the actual cargo when in a ships hold.

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Existing IMSBC TML Test Methods

• Flow Table test is suitable for mineral concentrates with a maximum grain size of

7mm;

• Penetration test is suitable for materials with a top size up to 25mm;

• Proctor-Fagerberg test (PFT) is suitable for materials with a top size of 5mm;

• For some bauxites, these size limitations result in less than 50% of the as shipped

material being tested for its TML;

• In these cases the TML result does not represent a safe limit as the TML conducted

on a sample that is not representative of the cargo;

• PFT is the least subjective and most precise of the 3 methods in the IMSBC Code;

• PFT is the only test based on solid fundamentals in geotechnical science, which

allows calculation of key geotechnical parameters – such as bulk density, moisture

content, dry density, specific gravity and voids ratio – to assess the TML of the

material.

8

Bauxite Proctor/Fagerberg Test (PFT)

Development• The development of a TML test appropriate for the wide range of seaborne traded

bauxites builds on the results from the IOF TWG and Coal research findings.

• Development of a bauxite appropriate PFT involves;

• calibration of the compaction energy to the measured in-hold cargo condition

of the bauxites cargoes;

• modification of the PFT methodology to ensure the PSD of the tested material

is representative of the as-shipped cargo;

• modifications to sample preparation and procedure for determining the

compaction curve;

• modifications to required equipment.

9

PFT Mould

• In order to allow testing of coarser particles, a larger mould was required

• The mould utilised is referred to as the CBR mould based on standard equipment

refer to in:

• British Standard Methods of test for Soils for civil engineering purposes - Part

4. Compaction (BS1377: Part 4:1990),

• American Standard Test Method for CBR (California Bearing Ratio) of

Laboratory-Compacted Soils (ASTM D1883-99) and,

• Standard Test Methods for Laboratory Compaction Characteristics of Soil Using

Standard Effort (ASTM D698-2007).

10

PFT Hammer Selection

• To produce the compaction effort necessary for testing, certain size hammers

(weight and dimensions) need to be employed and these depend upon the mould

size and tamping details. The Proctor-Fagerberg hammers and respective energies

are outlined below.

11

Specific gravity (SG) determination

• To develop the PFT compaction curves, samples at various moisture contents and

bulk densities are measured for the given compaction energy.

• Knowing the moisture content and the bulk density allow the calculation of dry

density.

• To convert dry density into a void ratio, the specific gravity (SG) of the material

must be measured.

• The techniques utilised in the GBWG work included water placement methods,

glass pycnometers and helium pycnometers that are readily employed in the

minerals industry.

• All determinations of SG were done with recognised international or national

standards.

12

Sample Preparation

• Representative samples shall be obtained following ISO standards for sampling and

sample preparation

• If data points are required to be drier than the as-received condition, sub-samples

are to be dried gently (at room temperature).

• Depending on the particle size of the bauxite to be tested it may be necessary to

screen and reconstitute the bauxite sample for testing.

• Reconstitution is a process whereby the coarse particles that are too large to fit

into the test apparatus are scalped off and discarded, but the remaining sample is

enhanced in the coarser fraction that can fit the apparatus, so that the tested PSD

gives an approximation as close as possible to the full PSD of the original material

before scalping.

• For proposed Bauxite PFT, a 25mm limit has been placed on the maximum particle

size for the test apparatus.

• Reconstitution would be required for bauxites if the amount of material in the

representative sample that is coarser than 25mm is more than 10% by dry weight.

• Sample is reconstituted with an equal weight of size fraction between 25mm and

6.3mm

• The result is a sample with fines fraction similar to that of the as shipped material13

Sample Reconstitution

14


15


16

Bauxite PFT Procedure

• Screening and reconstitution: 50 to 100kg of unscreened bauxite required

• Mixing at different water contents: Compaction tests are executed for five to ten

different moisture contents

• The number of tests should be sufficient to fully define the compaction curve and

optimum moisture content

• Divide the screened bauxite into five to ten sub-samples and place each in sealed

plastic buckets

• The water content of the sub-samples are adjusted by adding a suitable amount of

water into each bucket

• Typically one test is carried out at the as-received water content.

• Typically four to nine tests are carried out at different water contents to fully define

the curve

• The sub-samples are mixed at the target water contents and then allowed to rest

and equilibrate in the sealed plastic buckets.

• The plastic buckets are remixed immediately before compaction to ensure that a

homogeneous sample is tested.17


• A compaction test is carried out on each sub-sample.

• For each subsample, the CBR mould is filled in five layers, each being uniformly

compacted with the standard Proctor-Fagerberg D hammer (150g).

• The hammer is dropped 58 times from a 0.15m drop height in a specific pattern.

• The aim is to produce a uniformly compacted sample in which the specified energy

has been put into the bauxite that occupies the mould volume.

• The performance is repeated for all five layers so that the last layer only just

overfills the mould-extension piece junction.

• If the bauxite extends more than 1cm above the mould, then the sample has been

under compacted and needs retesting

• When the fifth layer has been tamped the extension piece is removed and the

sample is carefully levelled off.

• The levelling process should be a horizontal cutting action, not a pushing action as

that would compress more material into the mould volume than had been

compacted by the specified energy input.

18


• The weight of the mould with the tamped sample is determined, the mould is

emptied, the full sample is dried and the weight is again determined.

• The test is then repeated for the other samples with different moisture contents.

• The void ratio, water content and degree of saturation are plotted for each

compaction test

• If the OMC occurs at a saturation greater than 90%, the TML is determined as the

critical water content at 80% saturation.

• If the OMC occurs at a saturation less than 90%, the TML is determined as the


• It is important to define the OMC clearly in the Proctor-Fagerberg test for bauxite.

• Where the OMC is not clearly defined by carrying out a sufficient number of tests

at water contents close to and either side of the OMC, the TML shall be based on

70% saturation to guard against possible erroneous adoption of a high OMC due to

insufficient definition of the compaction curve.

19


• The weight of the mould with the tamped sample is determined, the mould is

emptied, the full sample is dried and the weight is again determined.

• The test is then repeated for the other samples with different moisture contents.

• The void ratio, water content and degree of saturation are plotted for each

compaction test

• If the OMC occurs at a saturation greater than 90%, the TML is determined as the


• If the OMC occurs at a saturation less than 90%, the TML is determined as the


• It is important to define the OMC clearly in the Proctor-Fagerberg test for bauxite.

• Where the OMC is not clearly defined by carrying out a sufficient number of tests

at water contents close to and either side of the OMC, the TML shall be based on

70% saturation to guard against possible erroneous adoption of a high OMC due to

insufficient definition of the compaction curve.

20


• Where moisture very freely drains from the sample at moisture content such that

the test sample compaction curve cannot extend to or beyond 70% saturation, the

test is taken to indicate a cargo where water passes freely through the spaces

between particles (free draining).

• Therefore, the cargo is not liable to experience moisture instabilities including

dynamic separation or liquefaction.

21

Bauxite PFT Results

22

Bauxite PFT Results

23

Bauxite PFT Results

24

Bauxite PFT Results

25

Bauxite PFT Results

26

Bauxite PFT Results

27

Bauxite PFT Results

28

Bauxite PFT Reproducibility and

Repeatability

29

Summary

• The PFT is an appropriate test for determining a TML for all the identified moisture-controlled

instability mechanisms including liquefaction risk and dynamic separation for the bauxites

studied.

• Setting a TML that is drier than the OMC by a given buffer provides a margin of safety with

regard to the generation of pore pressures, whether by monotonic or cyclic loading.

• Setting a TML that is drier than the OMC by the said buffer also reduces the risk of moisture

migration.

• Use of a CBR mould and the recommended reconstitution method allow representative

testing of bauxites containing large particles.

• The compaction energy with the D hammer produces dry densities and void ratios that are

consistent with those measured for bauxite cargoes in vessel holds before and after voyages.

• OMC of the bauxites tested consistently occurs between 80-95% saturation.

• The proposed Bauxite PFT method provides good repeatability and reproducibility in TML

results.

• TML of tested bauxites is read at 70% or 80% saturation, depending on the OMC found, but

always applying a minimum safety margin in the degree of saturation that is greater than 10%

between TML saturation and OMC saturation;

• If bauxite is free draining such that a degree of saturation to determine the TML cannot be

reached, the material should be classified as Group C30

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20170908 6. BAUXITE TML TEST DEVELOPMENT...• Where moisture very freely drains from the sample at moisture content such that the test sample compaction curve cannot extend to or