Sample preparation

In order to avoid premature development of non-homogeneous deformation of the sample, some special techniques were employed such as enlarged platens, free ends, together with a moist tamping sample preparation method.

3.3.3.1 Enlarge platen with free ends

The frictional force at the contact between the end platen and the soil specimen is one of the reasons of premature development of non-homogeneous deformation during shearing. Therefore, Rowe and Barden (1964) and Lo (1985) proposed to use enlarged platens with free ends to reduce the frictional force and to delay the onset of non-homogeneous deformation significantly .

20 21 22 23 24 25 26 27 28 29 30 -10 0 10 20 30 40 50 0 200000 400000 600000 T e m p e ra tu re (C e lc iu s ) V o lu m e c h a n g e (m L /1 0 0 ) Time(s)

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Initially, a pattern for the latex membrane (at the platens) was prepared by cutting a transparent sheet with a diameter slightly bigger than the sample diameter so that cutting the latex from this pattern would prepare a slightly larger end membrane than the sample diameter. This was done to ensure that soil particles would not come in contact directly with the platen at the periphery of the sample. One side of the membrane was lubricated with a thin film of high vacuum grease and placed on the enlarged platen. An enlarged platen was used to allow lateral expansion of soil during shearing. Air trapped between the membrane and platens, if any, was removed carefully.

3.3.3.2 Sample reconstitution

It was very difficult to mix sand with fines thoroughly at natural moisture content because fines form lumps even with a little moisture. The fines were first dried in an oven at 1050c for 24 hours. The materials were then cooled down to room temperature in tightly sealed desiccators. Finally sand with fines was obtained by mixing the sand and fines in a dry state.

Several methods for preparing reconstituted samples in the laboratory have been reported in the literature. The choice of the sample preparation method depends on the objective of the study and achievability of testing requirements. The variation in achievable ranges of density based upon the sample preparation method greatly affects the deformational soil behavior. It is reported in the literature that different stress-strain responses can be found from different sample preparation methods, even at the same density (Ishihara, 1993; Vaid et al., 1999). Therefore, it is most appropriate to compare soil behavior based on the same sample preparation method (Yamamuro & Covert, 2001). Considering the achievable range of densities required for this study (loose and dense), moist tamping, a method proposed by (Ladd, 1978), was mostly used in this study. For this method a predetermined amount of water (10%) was added to control the moisture content. Then the moist soil is put into the split mould, layer by layer (5 layers used in this study). For each layer the soil is then compacted to a specific height. Moist tamping produces a

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consistent preparation method and avoids segregation problems (particularly for silty sand as used in this study). It is also found to give a good control over the global specimen density. While moist tamping does not replicate natural deposition, maintaining a consistent preparation method throughout testing ensured that all results were equally affected by the preparation method. Nevertheless, few samples were prepared later on by dry funnel pluviation (Bahadori et al., 2008; Ishihara, 1993; Sitharam & Dash, 2008; Yamamuro & Wood, 2004). Dry funnel deposition involves deposition of dry material via a funnel into the bottom of a split mould. The material is placed in the funnel, which is raised along its axis of symmetry. The specimen’s sedimentation was formed by placing the soil in a low energy state without any drop height. In order to achieve higher densities, the split mould can be tapped uniformly.

3.3.4 Sample set up

First, line marks were drawn on the rubber membrane to indicate the position for the local axial and radial pads. The specimen was reconstituted, confined inside the three split moulds along with the ‘free end’ enlarged base platen. The specimen was then locked at the base of the triaxial chamber. After that, the top cap including the plastic sleeve was slid carefully within the space between the top of the specimen and the extension cap which connects to the load cell. The specimen was moved so the top of the specimen just touched the top lubricated end membrane (e.g. load cell reading less than 0.002 kN). Drainage lines were then connected to the specimen. While targeting load cell to 0 kN, 15kPa of vacuum was applied to the top drainage line so the split mould could be removed. Local axial pads (i.e. top and bottom) including the gauge rods were then assembled for each side. These sets of local axial LVDTs were glued on the specimen by instant loctite glue at their positions. Care was taken such that the pair of local LVDTs was located diametrically so the radial belt could be installed with relative ease. Figure 3-12 shows the whole set up before the triaxial chamber was filled. After the chamber was filled the vacuum pressure on specimen was replaced by confining pressure, still targeting 0kN on the load cell. CO2 was then percolated through the specimen

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slowly enough (targeting 2-3 air bubbles/second) for about 1 hour in order to help the saturation process. Water was then percolated through the sample until no air bubbles were observed in the drainage line (about two hours in this study) and/or a minimal 1.5 times the volume of voids water was percolated. Some trials were performed to find the suitable different head between the de-aired water and glass container at the exit. It was decided that a head of about 0.3m was slow enough to prevent/minimize loss of fines during water percolation (collected water at the exit was relatively clear). The triaxial chamber was then filled with water. All the local LVDT readings and GDS controllers were then zeroed. To compensate the use of two separate data logging systems, both of them were started at the same time.

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3.3.5 Saturation

The saturation was done by increasing the cell pressure and back pressure simultaneously while targeting 0kPa deviator stress. The B value was checked every 100kPa increment of back pressure. This is done using GDS Advance Loading Module. The final back pressure used in this study was 500kPa, as recommended by the GDS handbook (2000). The B value at this back pressure was found to be satisfactory i.e. ≥0.98. This relatively high back pressure was found later to be helpful during the K0 consolidation.

3.3.6 Consolidation

The K0 module program developed by GDS was used to perform K0

consolidation. There are two methods to maintain zero lateral strain, either direct reading of the specimen diameter or using the volume change calculation. K0 using

direct reading is preferable; however the connection/splitter to tie the GDS data acquisition system and local data could not be made until the end of the study. Thus throughout this study K0 consolidation were done based on volume change

measurement. During K0 consolidation, external axial displacement is adjusted

slowly thus ensuring the diameter of the specimen remains constant, where the specimen diameter change is theoretically calculated from the back pressure volume change. The required inputs of this program module were the target confining and back pressures and the time to reach the target. The module increases them simultaneously at a certain loading rate. Trials were performed to choose a suitable loading rate which, in this study, was 15kPa/hour. This rate is considered reasonably slow enough to allow the sample to fully consolidate during loading, and no significant change in pore water pressure was observed during K0 loading. In

this study, consolidation is considered to be finish if no excess pore pressure was generated when the target consolidation stress is reached. Similar consideration was used by previous studies (e.g. Bowman, 2002; Lam, 2003)

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The effective confining pressures used in this study were 30kPa, 60kPa and 120kPa, given that the focus was on creep at shallow depth. 30kPa was selected as the minimum confining pressure to avoid the influence of over consolidation due to 15kPa of vacuum pressure being applied during sample preparation. Using a high back pressure (i.e. 500kPa) proved to be useful in achieving accurate volume change measurement, thus maintaining practically zero radial strain of local and external measurement as exemplified in Figure 3-13. It can be seen that direct radial strain reading is well within the requirement e.g. Japanese Geotechnical Standard (JGS) requires that the lateral strain accuracy during K0 consolidation is ±0.05%

(JGS, 2000).

The deviator stress at the end of consolidation cannot be controlled using the K0 module program and was found to be sensitive to the sample preparation. Thus

careful attention was needed in to the sample preparation so that the mean effective stress at the end of consolidation between samples was reasonably close (± 2kPa). Figure 3-14 shows the relationship between K0 values and mean effective stress for

K0 consolidated undrained tests in this study. It can be seen in Figure 3-14 that

measured K0 values decrease and converge to a certain ultimate value with the

increase in effective mean pressure. The same trend was observed by Okochi and Tatsuoka (1984). This chart is used to check the repeatability of the sample preparation.

Figure 3-13 Radial strain control during K0 consolidation (a) local (b)

external 0 1 2 3 x 104 −2 −1 0 1 2x 10 −3 (a) time(s) ε a (%) local 0 1 2 3 x 104 −2 0 2 4x 10 −3 (b) time(s) external

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Figure 3-14 Relationship between K0 and mean effective stress

3.3.7 Creep

Following the consolidation stage, a creep stage was performed by targeting constant cell pressure, back pressure and deviator stress for certain periods of creep – i.e. mainly one hour, one day and one week. It was important that these stresses during creep were reasonably stable i.e. within ±1kPa of the target value (see Appendix C).

3.3.8 Shear

Undrained monotonic loading was performed in a strain controlled manner using a strain rate of 0.01mm/s. This strain rate is commonly used for undrained tests on silty soil (JGS, 2000; Lade et al., 2009). As the focus of this study is on small strain range, the majority of the tests were sheared up to about 1% strain; some tests were sheared beyond that for a repeatability check. For undrained cyclic loading, the specimens were sheared under stress control at a loading rate of 2 min per loading cycles. This loading rate was chosen considering the logging rate of the GDS data acquisition system (Rees, 2010).

0 50 100 150 200 0.2 0.4 0.6 0.8 1 1.2 p’(kPa) K 0 Dense Loose

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