Specimen preservation

For quantification of soil microstructure using image analysis techniques, soil specimens reconstituted in the laboratory have to be preserved. Preservation of non- cohesive soil specimens can be accomplished by impregnating a curing agent into them. In this study, a low viscosity curing agent called EPO-THIN, a two part epoxy resin, manufactured by Buehler Inc. was impregnated into the specimen. Prior the impregnation of epoxy resin, a moist soil specimen had to be completely dried because the epoxy resins do not cure well in a moist environment.

An apparatus to preserve the soil fabric using resin developed by Massad (1998) which was adapted by Bowman (2002) was also used in this study. The resin impregnation chamber was constructed to include a pedestal and cap end-plates with a 50mm diameter by 70mm high split steel cylinder in between. The split cylinder was held together by three bulldog clips. Rubber O-rings at the bottom formed a seal between the cylinder and the lower pedestal. A central hole through the base pedestal, leading to a 1/4 inch BSP to 4mm push-fit connection, allowed the steady introduction of the resin up through the cylinder after assembly. Grooves were cut diametrically from the central hole, 5mm deep by 5mm wide, in a cruciform manner to facilitate the dispersion of resin sideways at the base during its application. The upper cap was free to move vertically as a piston within the cylinder as loads were applied via a ball bearing and weight hanger. Displacement at the surface of the sand was measured using a dial indicator placed on the hanger.

An interface chamber purchased from Geo-Test Instrument Corp. USA, with an in-built rubber membrane was used to separate the hydraulic control panel and the resin. Contact with resin after some tests could damage the manufacturer’s Plexiglas made top cap thus it was replaced with in house made aluminium top cap. Resin was placed in the upper part of the interface chamber with water filling the lower section. Pressure was applied using a GDS Pressure Controller or, later, an

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air-water interface to inject the resin into the base of the impregnation chamber via the rubber membrane. The air-water interface was connected to an automatic system for volume change. Modification of existing Visual Basic software for undergraduate triaxial testing was made to allow monitoring of the resin volume infiltrated. Precaution was taken to ensure air-bubbles were expelled from both sides of the interface chamber. This enabled an accurate assessment of the quantity of resin being used as the infiltration was carried out.

The sequence of operation used in the specimen preservation procedure consists of the following:

• Wipe the split steel cylinder/impregnation chamber, pedestal, and top cap with mould- release agent to facilitate removal of the sample after the test.

• Apply silicon sealant to the edges and base of the steel cylinder before assembly with the pedestal and lower O-ring. Apply also silicon grease at the edge of the stainless steel cylinder of the interface chamber to prevent leakage of resin during impregnation.

• Clamp the impregnation chamber using the three bulldog clips and seal the assembly

• Place fine metal gauze in the assembly base over the cruciform

• Pour soil into the impregnation chamber and reconstitute in four layers.

• Place gently the top cap on top of the levelled silty sand and apply loads for the required times before resin infiltration.

Monitor strain due to initial top cap loading and during aging up to one week using a dial gauge. Samples of the same density which show any discrepancy of strain during top cap loading, before the impregnation, are discarded and a new sample is made.

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Initially the mix proportion (by weight) used was 100: 35: 12 for resin: hardener: acetone. The ratio was slightly modified from the ratio used by Masad (1998) and Bowman (2002), as particles were somewhat finer. As the same product was no longer produced, later on a new product of EPO-THIN (mixing ratio by weight is 5: 1.95) was mainly used for impregnation. Before mixing, the epoxy was heated for 10 minutes under 500C, thus eliminating the use of acetone. An attempt was also made to aid fabric visualisation by adding about 1% fluorescence epoxy dye. As silty sand is more compressible than clean sand at the same relative density (Lade et al., 2009), a maximum pressure of 15kPa was used in this study for both dense and loose samples. The impregnation process typically took up to two hours. Samples were then left in the mould at least 48 hours before removal. The axial dial gauge measurement indicated that the typical average axial strain during epoxy intrusion and curing was 0.15%. This suggests that the disturbance to the microstructure is relatively small. This amount of strain is comparable to Yamamuro’s et al findings (2008). After the mould had been removed (see Figure 3-16), the sample was left for two weeks to ensure that the resin was completely hard before cutting.

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