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Discussion |
1 17 Tithe Court, Glebelands Road, Wokingham, Berkshire, RG40 1DS, UK
2 Geotechnical Engineering Research Centre, City University, London EC1V 0HB, UK
3 'Lafonia', 11A Edgar Road, Winchester, Hampshire, SO23 9SJ, UK
4 Ove Arup and Partners, 13 Fitzroy Street, London, W1P 6BQ, UK
5 83, Langdale Avenue, Mitcham, Surrey, CR4 4AJ, UK
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Introduction |
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 TOP Introduction Breakdown of coarse aggregationsStability of fine aggregationsClay mineralogyReferences |
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During the 1960 s much of the new motorway construction in southern Britain took place over the outcrop of the Mercia Mudstone (Dumbleton & West 1966a). Generally, the material encountered was a weak mudstone, but it was found that after excavation, transport, spreading and compaction on site it broke down to the condition of a clay. Because of this the then Road Research Laboratory carried out an investigation of the mineralogy and physical properties of the material.
The conditions of working of the Mercia Mudstone by a tunnel boring machine in a tunnel site are probably more aggressive than by earthmoving plant on a motorway site.
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Breakdown of coarse aggregations |
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TOPIntroductionBreakdown of coarse aggregationsStability of fine aggregationsClay mineralogyReferences |
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Stability of fine aggregations |
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TOPIntroductionBreakdown of coarse aggregationsStability of fine aggregationsClay mineralogyReferences |
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In both the earlier and the later studies the British Standard clay contents (percentage of material finer than 0.002 mm) were measured by sedimentation, and the clay mineral contents were measured by quantitative X-ray diffraction analysis; Table 1 summarizes theresults. It is at once apparent that in both studies the clay mineral contents measured by X-ray diffraction are much greater than the clay contents measured by sedimentation. There are two possible explanations for this: firstly that the clay mineral particles are larger than 0.002 mm (i.e. of silt size), and secondly that the clay mineral particles are cemented, or joined together by physical forces as suggested by Lees (1965), to form aggregations of silt size. The British Standard sedimentation analysis test involves first pre-treating the soil to remove organic matter and carbonates, and then treating the soil with sodium hexametaphosphate in a milk-shake mixer to disperse the clay particles, and this is effective for most normal soils. Sherwood (1967), by a process of elimination, deduced that if a chemicalcement was present in the Mercia Mudstone the most likely cement was silica. In the 1960 s the possibility of clay mineral particles being larger than 0.002 mm could not be investigated and remained a conjecture. However, at the present time, with the ready availability of the electron microscope, it should be possible to make an examination of silt-sized particles taken from the sedimentation analysis test and see if they consist of large discrete clay mineral particles or of aggregations of clay particles, thus settling this uncertainty. I would like to ask Atkinson et al. if they would consider doing this for their samples of Mercia Mudstone. There is also the question of whether another, more vigorous, treatment may be better in dispersing the clay in the Mercia Mudstone, and I would like to ask Atkinson et al. if they had considered using some other method of dispersion which might be more effective than the milk-shake mixer used in the British Standard procedure.
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The ‘activity’ of a soil (Skempton 1953) is the plasticity index divided by the percentage of clay-sized particles (0.002 mm) as determined in the sedimentation test.An activity of 0.75 to 1.20 indicates a normal clay, an activity of greater than 1.20 an active clay, andan activity of less than 0.75 an inactive clay. The mean activity of the thirteen samples of Mercia Mudstone studied in the 1960 s was 0.75 indicating that thematerial was on the borderline between a normal clay and an inactive clay. However, for the six samples of Mercia Mudstone presently studied, the mean activity is the much higher value of 2.78 indicating a very active clay; this is puzzling - but can be explained as a consequence of the extremely low sedimentation clay contents reported by Atkinson et al. (see Table 1).
To sum up, in spite of some differences discussed above, both studies lead to the same conclusion, namely that in both the liquid limit test and the sedimentation test the clay aggregations in the samples of Mercia Mudstone are highly resistant to breakdown and therefore their cementing or bonding must be relatively strong.
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Clay mineralogy |
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TOPIntroductionBreakdown of coarse aggregationsStability of fine aggregationsClay mineralogyReferences |
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Atkinson et al. reply:
The Authors are grateful to Dr Graham West for his contribution. He refers to very early work on the Mercia Mudstone carried out by himself and others which pre-dates CIRIA 47 (Chandler & Davis 1973). These and other studies established that Mercia Mudstone breaks down under the influence of modest mechanical disturbance and in the presence of water behaves like a slurry. Modest mechanical disturbance includes cycles of wetting and drying, mixing with a palette knife and site construction processes of excavation, transport, spreading and compaction. It has been clearly established that this happens becuase Mercia Mudstone consists of mainly silt sized particles bonded with relatively weak cement which is broken by modest mechanical disturbance. This process we called destructuring. It is the process which gives rise to the well known problems which arise when Mercia Mudstone reduces to a slurry in earthworks in wet weather.
The particles released by destructuring consist partly of silt sized grains and partly of silt sized aggregations of clay mineral particles. These silt sized aggregations are bonded with relatively strong cement which can be broken only by severe mechanical disturbance. This process we called disaggregation. As noted by Graham West it has been known for a long time that clay mineral contents in Mercia Mudstone measured by quantitative X-ray diffraction are much greater than the clay contents measured by sedimentation. The difference is the quantity of clay strongly bonded into silt sized aggregations. The issues are to determine how much mechanical disturbance is required to break the relatively strong bonds and disaggregate the silt sized clay aggregations and to determine what clay minerals are then released. In practice it is also necessary to determine whether proposed construction activities may cause disaggregation and the influence of the released clay on the works.
Our work showed that, at least for the samples of Mercia Mudstone we tested which were from theEdwalton and Ibstock formations, disaggregation and release clay depends on both the degree of mechanical work applied and on the water content. We recorded increases of liquid limit as an indication of disaggregation and we found that the increases were much greater for the samples from Edwalton Hill than for the samples from Ibstock. The liquid limits of all samples increased after very aggressive working of dry material in a Tema disc mill. There were increases in liquid limit after prolonged mixing at water contents close to the plastic limit in the samples from Edwalton Hill but not in the samples from Ibstock. We found that after mincing in a meat mincer changes of liquid limit depended on the water content during mincing. At low and high water content there was no change and the maximum changes occurred when samples were minced at water contents close to their plastic limits. The changes of liquid limit after mincing at the plastic limit were significantly greater than those which occurred after prolongedmixing in a food mixer at the plastic limit.
Mixing in a milk-shake mixer would have to be done at relatively high water content. From our results we would not expect this to be effective in disaggregating Mercia Mudstone to release clay. We used the meat mincer as it most nearly represented the cutting action of the tunnel boring machine (TBM) cutters and the conditions in the screw conveyor in the TBM in closed mode and it was readily available. A grease-worker, as used by Sherwood (1967), might represent conditions during other construction operations but it would be necessary to investigate the influence of working at different water contents, especially close to the plastic limit, and to compare the changes of liquid limit with those due to very aggressive dry milling.
Graham West notes the unexpected presence of smectite clay in the samples from the Edwalton Hill formation although this was not found in the samples from Ibstock. This was discussed by French et al. (2000). He also considers application of electron microscopy and other investigative techniques in our studies. These were in fact done and the findings are being prepared for publication.
The early work by Graham West and others summarized in CIRIA 47 established that Mercia Mudstone could destructure in standard laboratory classification tests and during construction activities releasing silt sized material which form a slurry in wet weather. Our work has shown importantly that Mercia Mudstone can also disaggregate during some construction activities releasing clay particles. The samples we tested from the Edwalton Formation contained substantial quantities of smectite clay which appears to be unusual for the Mercia Mudstone.
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References |
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TOPIntroductionBreakdown of coarse aggregationsStability of fine aggregationsClay mineralogyReferences |
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Chandler, R.J., Davis, A.G., Further Work on the Engineering Properties of Keuper Marl. CIRIA Report No. 47 1973.
Dumbleton, M.J., West, G., Studies of the Keuper Marl: geology and geography. Report No. 39 1966a. Road Research Laboratory, Crowthorne.
Dumbleton, M.J., West, G., Studies of the Keuper Marl: mineralogy. Report No. 40 1966b. Road Research Laboratory, Crowthorne.
Dumbleton, M.J., West, G., Studies of the Keuper Marl: stability of aggregation under weathering. Report LR 85 1967. Road Research Laboratory, Crowthorne.
Lees, G.Geology of the Keuper Marl. Proceedings of the Geological Society of London, No., 1621 1965. 46.
French, W.J., Pettifer, G.S., Fookes, P.G., The occurrence of smectite in the Mercia mudstone ofLeicestershire. Proc. Conf. on the Engineering Properties of the Mercia Mustone Group., 2000. 137–141 CIRIA Conf. Record 2000/1.
Sherwood, P.T.Classification tests on African red clays and Keuper Marl. Quarterly Journal of Engineering Geology, 1 1967. 47–55.
Sherwood, P.T., Hollis, B.G., Studies of the Keuper Marl: chemical properties and classification tests. Report No. 41 1966. Road Research Laboratory, Crowthorne.
Skempton, A.W.The colloidal ‘Activity’ of clays. Proceedings of the Third International Conference on Soil Mechanics.1 1953. 57–60.
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