I compacted unbound aggregates at optimum moisture content and then let it dry in the oven for one day. The repeated load triaxial test was carried out on this sample in which I faced a weird behavior (please find the attached figures). The permanent deformation initially increases with a high rate and then its rate decrease slowly and finally it has a negative displacement (like expansion).
This behavior only happened in dried samples and the lower stress conditions (1.59 KN and 1.767 KN deviatoric stresses and 120 and 41.7 confining pressures). These samples have normal behavior, the permanent deformation keep increasing with load cycles, at higher loading environment.
I appreciate your time.
Hi Tatjana Rukavina,
Thank you for your attention to my question.
These are base-course aggregates. The porosity of aggregates is not that high (the water absorption is less than 1%) and the compacted sample at the optimum moisture content possess low air voids.
I think the drying process may affect the density of compacted aggregates. I mean the dry-back process may increase the volume of the sample as a result of reduction in the suction pressure. It may also lead to re-orientation of aggregates (I have no evidence!!) due to reduction in the friction.
I was thinking that even an increase in the volume of the sample should result in a rise in permanent deformation, not a distinct reduction.
Please let me know if you need more information.
Kind regards,
Ebrahim
Cohesionless soils when sheared at low confining stress will dilate and generate negative pore pressures. The particles are interlocked but at some point of continued axial stain they must lift to ride across one another. The perfect example can be seen in the direct shear device with vertical axial stain measurements. Taught in basic soil mechanics. At high confining stress soil is contractive. Dense soil is dilative and loose soil is contractive and depsnds on confining stress.
Hi Jeffrey A. Farrar
Many thanks for your straight answer. I do believe that the same thing happened to my materials. The suction pressure has been disappeared during drying and makes it easier for granular materials to reoriented during the test.
Kind regards,
Hi Ebrahim,
I think that suction pressure disappearing during drying can explain the second part of permanent deformation graph where the rate of permanent deformation decrease slowly (granular material material easier reorient during the test). The last part of graph where the negative displacement appear, result from some other reason which is not connected with material itself. Maybe the humidity of the sample change duo to some other reason.
Maybe it seems you impossible but on your place I will check.
Kind regards, Tanja
@Ebrahim Sangsefidi
This is due to the soil property called dilation (volume increase) that occurs in overconsolidated soil samples. An overconsolidated sample has undergone higher pressure in the past than it is carryng now. When the load on it increases initially it will undergo volume decrease but at a certain stage it wll start expanding.
Dear Amaechi J. Anyaegbunam
Good explanation. But, do you think that over-consolidation may happen in granular materials? If that is the case, why the dilation happens only in the oven-dried samples?
Cheers,
Dear @Ebrahim Sangsefidi
Thank you very much for your inquiry. An oven dried sample will develop high suction (negative pore pressure) which will impart a high effective stress on the sample seemingly giving the sample an apparent overconsolidation.
Hello Ebrahim
If I interpret your question correctly, I do not think dilation can occur because as you said you dried the samples completely in oven. Furthermore I believe you kept your valves open during your impulse loading which would not lead to excess pore pressures within the samples. Looking at your results the decrease in permanent strain is very small probably of the order of 0.001%. I believe the answer lies in mechanical re-orientation of particles which resulted from initial loading cycles and resulted in a compacted mass at a later stage that the material showed resistance in subsequent loading cycles. Because the magnitude of rebound is so small I dont think you can explain it in terms of dilation (and conditions of the sample would not allow that to happen).
- Badges
- Science topic
- Similar topics
- Agriculture
- Soil Science
- Soil