4.9 Selection of a strain rate
When testing clay specimens it is customary to perform drained tests. A drained test is one in which any porewater pressures generated by the application of the shearing load are allowed to escape from the test specimen. It does not necessarily imply zero porewater pressure in the sample. The time over which drained tests must be run is normally calculated according to a theory proposed by Gibson and Henkei (1954). In this theory, the shearing load, or rather the resultant generated porewater pressures, are inừoduceđ at a constant rate. Concurrently, water escapes from the specimen via the drains, and at the end of the loading period, only x% of the total applied porewater pressure remains in the worst place (usually the middle) of the specimen. For example, where X - 5%, this time, ÍỊ, for a doubly-drained soil sample is given by
_ 20 h2 if 5
3 Cv
Constants equivalent to the 20/ầ factor for other drainage conditions and degrees of remaining porewater pressure are given by Bishop and Henkel (1971).
A strain rate is then chosen such that the test is run for this time to achieve the estimated failure strain. With the small specimen thickness of the simple ring shear apparatus, this approach yields very short test times, for instance with a Cv of 1 m2/year and a total specimen thickness 2h of 5 mm, if is about 22 minutes. Accordingly, our standard procedure for measurement of a residual strength demands that the torque transmitted through the specimen remain sensibly constant for at least this amount of time. When it has done so, it is possible to be sure that the drainage process is complete. It will normally take about half an hour to mobilize the reaction on the proving rings of the apparatus, so that for load stages after the first, (in which the shear surface is formed initially), about an hour in total is required.
The virtually unlimited ‘strain’ capacity of a ring shear device permits a completely novel approach. When shear-induced porewater pressures dissipate, they cause changes in the shear strength of the soil. This reflects in the torque transmitted through the apparatus. Thus when a constant torque is found, for example over a period of half an hour or so, or for a time equivalent to if in the above formula,. this demonstrates the complete elimination of transient porewater-pressure effects. A similar argument may be applied to the dissipation of porewater pressures induced by normal load: these may as well be allowed to escape during the shearing stage. The use of separate consolidation stages gives little additional benefit. Indeed, our standard procedure is to start the shearing as soon as the normal load is applied.
4.10 Achieving adequate total deformation
A major source of uncertainty is in ensuring that large enough strains have been obtained to completely develop the residual strength. Bishop et al. (1971) recommend plotting the load-deformation behaviour on a semi-logarithmic base (deformation on the logarithmic axis) since this is a severe test of the data. However, for rapid determinations, for example on a commercial basis, the time requirements for this are excessive. A more satisfactory alternative is to strain the sample until it appears that the residual has been reached and then to carry on with the next normal ỉoađ stage. When a full sequence of normal loads has been applied, the total load is reduced and the strength is re-evaluated at the initial norma] load. Provided that the strength is comparable with the original measurement, it is safe to assume that die additional deformation of the ỉater load stages has not further reduced the soil strength, and by inference, residual was achieved earlier. More than one point may be so checked.
Early in the development of the simple ring shear device, it became obvious that there was a conflict between several of the requừements that the apparatus needed to meet. For instance, the specimen thickness needed to be small, so that drainage during shear was rapid, and test times could be reduced. However, the small thickness of the sample meant that the monitoring of any consolidation stage undertaken was inaccurate. The procedure for observing the torque and using that as an indữect measure of the progress of consolidation was adopted to overcome what was seen to be a problem. It was only in retrospect that it proved to be a positive feature of the technique. Similarly, the early tests were done utilizing the semi-logarithmic plot of torque V. time. It was found that the time taken in the first load stage, and the attention it demanded, ruled out the measurement of residual strength with the ease and rapidity intended in the design.
The idea of returning to the first point on the residual sừength envelope was then tried. This was principally to see if there was an over-consolidation effect on the residual strength. It was found that the results were frequently erratic but, with further strain, the earlier residual strength could be obtained. In the course of exploring the erratic behaviour, the reasons for it were discovered, and a systematic experimental procedure found which was more in line with the initial concept of a simple and rapid method of residual strength determination.
4.11 Effect of strain rate on residual strength
Since in the ring shear test it is possible to achieve full drainage at whatever strain rate is chosen, merely by extending the test, it is practical to assess the influence of strain rate on the drained residual shear strength. The effect has been explored by Lupini et al. (1981) who show that increases in Sữain rate can cause increased sứength in the soil with some brittleness becoming apparent when the strain rates are subsequently reduced. This is explained as a result of the disruption by viscous drag forces of the strongly orientated zone produced under slower shearing conditions. At strain rates below a threshold value the influence of strain rate is negligible. This strain rate threshold has been found to correspond to a speed of r/minute in a 100 mm diameter shear apparatus for most clay soils, and a much slower speed of typically 0.048Vmin provides a safety factor on this as well as allowing a convenient test programme schedule in the laboratory.
• High rates of shear may be applied deliberately or inadvertently in ring shear testing. Should the normal load be reduced, for instance, the energy stored in the torque measuring system will cause rapid deformation of the specimen with consequent changes to the nature of the shear surface. This rapid deformation takes place in the direction of shearing, and so is not related to the effect of a reversal where a change in the direction of shearing upsets the particle alignment By way of analogy, this can be visualized by thinking of stroking a cat’s back. Do it repeatedly in the same direction, and the fur becomes orientated and smooth. However, if the fur is then stroked the ‘wrong way’, it stands on end. That is the effect in a reversal test. Consider then the effect of stroking the cat the ‘right
way’, but at high speed. Static electricity is then generated so that the fur refuses to lie flat.
It was this effect that was causing the erratic behaviour of test specimens when unloaded to try to reproduce the residual strength of the first load stage. Reduction in torque is therefore essential before relieving the normal load* Notwithstanding this, some brittleness may still be imroduced into the slip surface by unloading, but not so much that the routine of about an hour’s further straining is not adequate to re-establish the drained residual strength to an appropriate experimental degree of accuracy. The mechanisms for this secondary effect are not fully understood.
A rapid assessment of the strain rate sensitivity of the soil can be made by switching off the drive. If the specimen can hold the applied torque then the test has been made at a rate to which the soil is insensitive, and may be deemed satisfactory. A significant loss of torque should give rise to concern that the sưain rate was too fast. About an hour is ample time to check this.
Typical test results from a ring shear test are given in Figure 4.6.
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