cell is illustrated in Fig. A3. During sedimentation the cell was clamped to the base of the column as shown in Fig. A2. It could be removed when the column was drained.
1.2 Monitoring and pressure equipment
The consolidation phase of the experiment was monitored for both settlement and pressure. Displacement and pressure transducer readings were recorded automatically under instruction from a computer program written by the author. The program is presented in Appendix VI.
Consolidation pressure was maintained by an oil and water constant pressure apparatus. The monitoring and pressure equipment are shown in Figs. A4 and A5.
2.0 Experimental procedure
The procedure for obtaining consolidated samples of laboratory sediraented clay was as follows:
1.
prepare water medium2. add clay powder and mix for 15 mins 3. leave mixture for 48 hrs
4a. remix for 15 mins
4b. add organic compounds (optional) 5. fill sedimentation column
6
.
draw off samples 7. drain column9. allow to stand for 48 hrs
10. remove surface water and pre-consolidate (cell open)
11. consolidate sample (cell closed)
10. release pressure
11. determine moisture content and shear strength 12. cut sediment samples for SEM analysis.
The preparation tank was filled with fresh water and if saline conditions were required an amount of ’instant ocean’ crystals were added to give the desired salinity. Clay minerals in powdered form were then added slowly to a concentration of 8 g/1, appropriate for natural settling. The clay suspension was thoroughly mixed by recirculation and then left to settle. This allowed time for the clay powder to absorb water and achieve natural equilibrium.
Before delivery to the sedimentation column the suspension was remixed'. At this stage the chosen organic compounds could be added if required. The column was filled and the clay particles allowed to settle into the consolidation cell at the base. Periodic checks were made of the stage of settlement by drawing off samples from the drain tubes and checking the turbidity value of the fluid. When it was confirmed that most of the clay had settled the column was drained. The drainage points were spaced so that any turbulence created was minimised.
The consolidation cell was unclamped from the column base and transferred to the workbench. The clay sediment at this stage was too ’thin’ to place under pressure from the constant pressure
equipment. Pre-consolidation was necessary and this was achieved by placing a perspex disc on the sediment surface with drainage allowed through ’weep holes’ in the disc. The pressure was raised from 0.1 kPa to 10 kPa by the addition of more discs. At this pressure the clay sediment had achieved sufficient stiffness for consolidation to be carried out by the pressure equipment itself.
With some modification the lowest attainable pressure provided by the constant pressure apparatus was approximately 31 kPa. By adding further weights this was slowly raised to the nominal minimum pressure of 100 kPa. Thereafter the sediment was consolidated in stages to the maximum of 1000 kPa, each increment being held for 24 hours. Full monitoring of displacement, supply pressure and pore- water pressure (pwp) was carried out with data being recorded by the computer at pre-set time intervals.
On completion of the final increment the pressure was released and the sediment allowed to swell. Moisture content determination of the sediment enabled the void ratio to be calculated for the various steps of the consolidation process.
Before the sediment was removed from the cell the shear strength was determined using the falling cone method of Hansbo (1957) .
Samples were obtained from undisturbed parts of the sediment and prepared for electron microscopy work as outlined in Appendix IV.
3.0 Additional experimental work
Part of this study was conducted away from the sponsoring establishment and this required the fabrication of new equipment. The new set-up was based around the standard 7 5 mm diameter oedometer cell of the Casagrande type. Although not as elaborate the system consisted of the same basic units of mixing tank, sedimentation column and consolidation cell.
There was no facility with the Casagrande oedometer to measure pore water pressure which meant that only settlement could be monitored. This was not such a drawback since pwp measurement in the Rowe Cell had proved difficult.
The experimental procedure was similar to that explained in Section
2 above, except that pressure was provided by the normal loading arrangement of the Casagrande-type oedometer.
4.0 Experimental calculations
The consolidation and strength properties of the sedimented clays have been presented in Chapter 6. The following example illustrates the calculation procedure used to determine those properties.
4.1 Settlement-time record
Test No 3 : Kaolinite sedimented in salt water Consolidation pressure = 1 0 0 kPa
Initial sample thickness = 31.19 mm Final sample thickness = 28.15 mm
/ time (mins) 0 2 4 7 11 15 19 24 29 38 settlement (mm) 0 .58 1.34 2.28 2.74 2.84 2.86 2.88 2.91 3.04
The coefficient of consolidation (cy) is given by the equation
where time factor T = 0.848 at 90% consolidation d is the mean drainage path length
t^Q is the time taken for 90% consolidation.
The value of t^Q is obtained from the graph of settlement against time (Fig. A6) using Taylor and Merchant’s method. This is shown to be 84.6 mins.
The mean drainage path length is given by
d = 1(31.19 + 28.15) = 29.67 mm. Substituting values into the equation gives
0.848 x 29.672 „ „„ 2, . c = --- v 84.6 = 8.82 mm / m m
2
= 4.64 m /year.
4.2 Void ratio - effective stress record Test No 3: kaolinite sedimented in salt water pressure range 0 - 1000 kPa
initial sample thickness = 116.0 mm final sample thickness (Hq) = 24.25 mm final moisture content (w ) = 38.1%o