AUTHOR'S REPLY A Device For Extracting Large Intact Soil Samples Developed and used in a remote region of Bolivia by Ian Coker and Roger Flores, EJGE, Vol. 5, 2000 Reply by Ian Coker to a discussion by Mirata

Roger Flores and I would like to express our thanks to Professor Mirata for taking an interest in our work. We apologise to him for the delay in responding to the points that he raised.

Below is a response which I hope answers the Professor adequately. I have used the email text format since this seems to be the most convenient.

1) Steel nails or box walls for isolating the soil sample

Discussions with other researchers (Dr. Andrew Collinson, of Kings College London { personal communication 1997}; Dr Jane Clark, Junior Forestry Advisor, DFID, {personal communication 1992} ) lead us to believe that to attempt to isolate a soil plinth in the manner described by Prof Mirata would have been extremely difficult and time consuming and would have involved the risk that excessive disturbance might have been caused. The physical constraints involved: very steep slopes, high altitude etc demanded that a quick and "systematic" solution should be sought. The correspondent is correct to say that the hammering in of nails would have resulted in inevitable densification but it is felt that this would have been localised, relatively minor and, above all, a systematic error common to all samples. The design of the Nail Cage Soil Sampler (NCSS) reflects that a common garden fork can penetrate soil many times more easily than can the edge of a spade. The points of the fork prongs (and indeed the nails used in this case) can "deflect" stones aside (a "direct hit" on a large stone would have required an alternative site) but it is felt that this would still have been preferable to the use of a small, reconstituted soil sample in a laboraory shear test. Once the nails were in place, forming a protective cage around the soil sample it was very easy to cut beneath the sample and to extract the soil intact. The alternative, attempting to excavate a cube of soil, whilst at the same time fitting a shear box over it, would have been difficult, we were told, even in a university soil bin, let alone on an Andean hillside.

2) Saturation or soaking?
Professor Mirata writes "Immersing an unsaturated sample of soil in water, in the way described, can never cause full saturation of the soil". I believe that perhaps we had not described the procedure sufficiently well and that this has led to a misunderstanding. We stated:

The shear test apparatus included a large "split" shear box. Intact soil samples were extracted from the site and then carefully contained in the split box and saturated. The split box and the now saturated soil sample were placed in a bath of water and then both were fixed to the base of the shear test apparatus. It was not practicable to immerse the whole apparatus into a bath since this would obstruct the shearing mechanism during testing. However, by immersing to just a few millimitres below the shear zone it was thought that the shear zone would remain saturated due to capilliary action (Maurico Salinas, Director of UMSS Geotechnical Laboratory, personal communication, 1998). With the sample in place the normal load was applied directly to the soil surface via a wooden pressure plate. The whole was allowed to settle and drain and achieve equilibrium"

Having read this through again I realise now that it was not clear that we had in fact used two baths. The first was a disused oil drum filled with water. The sample was immersed in this for 90 minutes (see Photo 8). Once saturated, the sample was placed onto the shear test mechanism and immersed in the second bath which only extended up to the top of the lower section of the split shear box. The objective of the second box was to minimise the effects of drainage so that the shear plane would remain saturated during the shear test. I believe that the correspondent has been under the impression that we attempted to saturate the soil by capilliary action only, which would clearly not be possible.

However if this is not the case then I would refer to two definitions of saturation (which were conveniently accessible on the internet.)

Definitions Saturation can be defined in two different ways.
a. Water Content Definition--a soil layer is saturated when all its pores are filled with water, except for those pores that contain entrapped air.
b. Water Pressure Definition--a soil layer is saturated when its water has a pressure that is equal to or greater than atmospheric pressure.

We feel that the prolonged immersion in the large bath would have saturated the soil adequately.

3) Strain rate
We have to admit to a typographic error here. We reported "4mm/min" but I have checked and in truth the actual value was 0.25 mm/min. The cause of this error was that it was quoted in a slightly unorthodox way in Ing. Roger Flores undergraduate thesis "1mm/4min" (Flores, L.R 1998). The data is reported on p60 (Section D.2, "Prueba de Corte", /16).

4) Apparatus developed by Mirata 1974 (Iswest)
The apparatus and methodology described by the correspondent does seem to be an excellent system. However, again, we were working in an environment which would have made in-situ testing extremely difficult to carry out. Another reason that we wished to shear a sample on level ground was to enable us to saturate the sample so that we could assume that the the pore pressures would have been zero thus guaranteeing zero soil suction. We did not feel able to measure in-situ soil suction, and felt that if we calculated soil suction at the moisture content encountered, in order to be able to derive the soil strength under saturated conditions, it would have introduced another source of possible error.

However, with a rooted soil I think that the apparatus described by the correspondent would be an excellent means for incorporating the increased soil strength due to soil roots.

Other points

I would be very interested to consider the possibility of combining the two procedures. For instance in theory it should be possible to extract a much larger sample. Say 400mm long by 300mm wide by 300m deep. It might then be possible to saturate this sample and shear the sample whilst saturated using the Iswest system. Only a part of the soil would be sheared. Perhaps this would produce a very good hybrid system. Would the correspondent like to comment on this.

References

Flores, R (1998) Efecto de las barreras vivas en la estabilidad de laderas utilizando modelos de simulacion ("The use of simulation models to assess the effect of live barriers on slope stability" -undergraduate thesis), Tesis de grado presentada para obtener el titulo de Ingeniero Agronomo, Universidad Mayor de San Simon, Facultad de Ciencias Agricolas, Pecuarias, Forestales y Veterinarias. Cochabamba, Bolivia.

Here is the response to the request (further discussion)