J.-H. YIN Department of Civil & Structural Engineering, The Hong Kong Polytechnic University Kowloon, Hong Kong, China

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1 AN INNOVATIVE DOUBLE CELL TRIAXIAL SYSTEM FOR CONTINUOUS MEASUREMENT OF THE VOLUME CHANGE OF GASEOUS OR UNSATURATED MARINE SOILS AND TWO OTHER ADVANCED SOIL TESTING SYSTEMS J.-H. YIN Department of Civil & Structural Engineering, The Hong Kong Polytechnic University Kowloon, Hong Kong, China Abstract This paper introduces three advanced laboratory testing systems for measuring the stress-strain-strength behavior of soils. The three systems are: (a) a new Double Cell Triaxial System (DCTS) for continuous measurement of the volume change of a gaseous (unsaturated or saturated) soil specimen in triaxial testing (this system is developed by the author). (b) a Hollow Cylinder Apparatus (HCA) for measuring the behavior of a hollow soil specimen under conditions of pure shearing, plain strain, rotation of the middle principle stress, etc., and (c) a Truly Triaxial System (TTS) for measuring the behavior of a brick-shape soil specimen under independent control of the three principle stresses. Calibration results on the DCTS are presented and discussed. Keywords: Double Cell Triaxial System, Volume Change, Gaseous soils, Unsaturated soil, Marine Soils 1. Introduction The measurement of the stress-strain-strength behavior of soils in laboratory is important for geotechnical applications and research, including the stability and deformation analysis of submarine mass movements. The measurement can provide soil parameters, such as strength and deformation parameters, for design analysis and quality control of new geotechnical structures and performance analysis of existing projects. The measurement also provides test data for better understanding of the fundamental behavior of soils and developing improved correlations, models, and theories. The conditions of a soil in the field are complicated. For example, the soil may be saturated under water table or unsaturated above water table. Gaseous marine soils in the seedbed under water table are unsaturated. Generally speaking, the stress state of a soil under loading is three-dimensional. The measurement of the behavior under the real field conditions is necessary. This paper introduces three advanced laboratory testing systems for measuring the stress-strain-strength behavior of soils, that is, (a) a new Double Cell Triaxial System (DCTS) for continuous measurement of the volume change of an unsaturated or saturated soil specimen in triaxial testing; (b) a Hollow Cylinder Apparatus (HCA) for measuring the behavior of a hollow soil specimen under 171

2 172 Yin conditions of pure shearing, plan strain, rotation of middle principle stress, etc., and a Truly Triaxial System (TTS) for measuring the behavior of a brick-shape soil specimen under independent control of the three principle stresses. 2. A New Double Cell Triaxial System (DCTS) 2.1. EXISTING SYSTEMS The volume change of a soil specimen is an essential parameter to be measured in triaxial testing. For a 100% saturated soil triaxial specimen, the volume change of the specimen (enclosed with rubber membrane and top and bottom caps) during consolidation or compression is equal to the volume of water coming out of the specimen (from inside). Therefore, the common measuring method for 100% saturated specimen is measuring the volume of water coming out of the specimen. However, for an unsaturated soil specimen, the water volume coming out is no longer equal to the volume change of the specimen. Alternative methods have been used for unsaturated soil specimens. But all have significant limitations. Bishop and Donald (1961) firstly used a modified cell similar to that in Fig.1 for measuring volume changes of partly saturated soils. An open-top inner cylindrical container was used inside a conventional cell. The inner container was filled with mercury (Bishop and Donald 1961). Outside the inner container was filled with water. Volume changes of the partly saturated soil specimen were measured by monitoring the vertical position of a stainless steel ball floating on the surface of the mercury using a cathetometer. Following Bishop and Donald s work (1961), Yin (1998) put an open-top cylindrical container inside a conventional cell. The inner container was filled with distilled water to the position as shown in Fig.1. Outer Pressure Cell Inner Cylindrical Container with Outer Cell Air Pressure Supply Tube (Pressure Measurement) Soil Specimen Top Drainage Tube (Pressure or Fill up to the Top Vertical Loading Piston Soil Specimen Inner Cell Pressure Soil Specimen Bottom Wate Outside the inner container was filled with air. When the specimen volume is changed, for example, decreased under axial loading, the water table in the inner container will come down. At this time, water was supplied into the inner container through the Inner Cell Pressure Supply Tune as shown in Fig.1 to maintain the water table in the Air Figure1. An existing old cell triaxial system with open inner cylindrical container for measurement of volume change of a soil specimen a schematic diagram (after Yin 1998).

3 An innovative double cell system 173 inner container at the previous position. In this way, the water volume (measured by burette) entering the inner container is equal to the compression volume change of the specimen. This process is tedious and cannot be done automatically. The accuracy is questionable since the water table position was judged by naked eye readings. Wheeler (1988) extended the idea of Bishop and Donald (1961) and developed a double cell triaxial system for testing soils with large gas bubbles. The schematic diagram of Wheeler s double cell system (Wheeler 1988) is shown in Fig.2(a). (a) Outer Pressure Cell Inner Pressure Cell Outer Cell Pressure Soil Specimen Top (b) Outer Pressure Cell Inner Pressure Cell Outer Cell Pressure Soil Specimen Top O-ring Seals Shaft Leaking Shaft σ i_cell σi_cell σo_cell Vertical Loading Piston Rolling Diaphragn Soil Specimen Inner Cell Pressure Soil Specimen Bottom Vertical Loading Piston σo_cell Soil Specimen Inner Cell Pressure Soil Specimen Bottom Figure 2. Two existing double cell triaxial systems for measurement of volume change of a soil specimen (a) after Wheeler (1988) and (b) after Chen, et. al. (2001). In particular, the volume of the inner cell was measured by using water burette. The vertical axial load was measured outside the cell using a local proving ring. As shown in Fig.2(a), the shaft with the loading piston extends up and comes out of the outer cell. Leaking could be a problem and this was minimized by using a rolling diaphragm between the loading piston and inner cell top shaft (Fig.2a). Wheeler (1988) reported limited calibration results and found that the relationship of volume changes of the inner cell with cell pressure up to 400 kpa was non-linear and the largest inner cell volume change was 0.7 cm 3 at cell pressure of 400kPa. Chen et al. (2001) showed a double wall triaxial system in Fig.2(b). A few limitations of the arrangement in Fig.2(a) and (b) are : (a) Leaking is a potential problem even though a rolling diaphragm or the shaft is used. This is because the rolling diaphragm or the shaft is subject to differential pressure of σ i_cell (i.e. the

4 174 Yin inner cell pressure σ i_cell at point 1 minus the outside atmospheric pressure of zero at point 2). (b) The rolling diaphragm in Fig.2(a) or the top cap in Fig.2(b) will deform under the differential pressure of σ i_cell. This will affect the volume changes of the inner cell. (c) The shaft is subject to an extension under the differential pressure of σ i_cell. This will cause volume changes of the inner cell and errors in measuring volume changes of the soil specimen. (d) The vertical load was measured externally and the frictional force between the piston and the shaft is included. This will cause an error in the measurement of the actual vertical load on the soil specimen. (e) The water volume changes were all measured using water burette. This is not convenient for computer control and automatic data acquisition. 2.2 AN IMPROVED DOUBLE CELL TRIAXIAL SYSTEM Based the pioneering work above, the author has developed an improved Double Cell Triaxial System (DCTS) for continuous measurement of the volume change of an unsaturated or saturated soil specimen in triaxial testing as shown in Fig.3. The main new features of the DCTS, which are different from the modified cells proposed by Bishop and Donald (1961), Yin (1998), Wheeler (1988), and Chen et al, (2001) are, as shown in Fig.3 : (a) The inner cell is totally enclosed within the outer cell. Vertical Loading Piston O-ring Seals De-aired water is used to fill Outer Pressure Cell both the inner cell and the σo_cell outer cell. Load Transducer Cell Inner Pressure Cell A Outer Cell Pressure Soil Specimen Top Outer Perspex Cell Wall Inner Perspex Cell Wall 2 1 σi_cell A-A Section Soil Specimen A Inner Cell Pressure Supply Tube (Pressure an Soil Specimen Bottom Wa Drainage Tube (Pressure o Soil Specimen Figure 3. A new double cell triaxial system for measurement of volume change of a soil specimen a schematic diagram. (b) Both outside and inside the inner cell are subject to the same magnitude of the cell pressure σ i_cell (inside)= σ o_cell (outside). (c) Because of the same water pressure σ i_cell, both the wall and top cap of the inner cell will have negligible deformation. This will avoid errors caused by the inner cell deformations. (d) Since the inner cell water pressure σ i_cell is equal to outside cell water pressure σ o_cell, the hydraulic gradient along the piston at the inner cell top cap (from Pint 1 to

5 An innovative double cell system 175 Point 2 as shown Fig.3) is zero. Thus no water flow will occur along the gap between the piston and the inner cell top cap. This will avoid errors due to water flow/leaking at the gap. In fact, an O-ring is used at the gap. (e) A submersible electric load cell is placed inside the inner cell and used to measure the vertical load on the soil specimen directly. This will avoid the error due to the friction between the piston and the cell caps (inner and outer cell caps). (f) All water volumes are measured by electric volumemeter. All data, such as vertical load, pore water pressure, and volume changes are collected automatically by a PC computer. The DCTS has been made and set-up at the soil laboratory as shown in Fig.4. The inner cell with a soil specimen and the submersible load cell is shown in Fig.5a. The outer cell with the inner cell inside is shown in Fig.5b. For the DCTS in Fig.5, the outer cell has an internal diameter D of 230mm, height H of 425mm and the wall thickness T of 8mm. The inner cell has an internal diameter d of 90mm, height h of 235mm and the wall thickness t of 6mm. The load cell has dimensions of thickness of 30mm and diameter of 65mm. The axial load piston has a diameter of 20mm. The standard size for a soil specimen is diameter of 50mm and height of 100mm. 2.3 CALIBRATION OF THE DCTS A solid copper specimen with diameter of 50mm and height of 100mm was first used to calibrate the Double Cell Triaxial System (DCTS). Since the solid copper specimen is considered incompressible under a pressure, say, up Outer Pressure Cell (Perspex Wall) Inner Pressure Cell (Perspex Wall - inside) Soil Specimen Top Drainage Tube (Pressure or Volume Measurement) Vertical Loading Piston Loading Frame Soil Specimen Bottom Drainage Tube (Pressure or Volume Measurement) Automatic Volumemeter Data-Logger Figure 4. The real set-up of the Double Cell Triaxial System (DCTS) in the Soil Mechanics Laboratory. to 600kPa, therefore, the copper specimen can be used to assess the volume changes of the inner cell and the outer cell under increasing pressure. All tubes and holes were de-aired. The copper specimen was installed in the inner cell. A standard rubber membrane was put on the copper specimen. The vertical filter stone and nylon cap were put on the top of the specimen. The specimen was then sealed by using double O- rings on the rubber membrane. In order to make the water inside the rubber membrane fully saturated, a back-pressure of 100 kpa was applied with the cell pressure in the inner cell and the outer cell increased to 105 kpa accordingly. The B-value was checked and found to be 0.99.

6 176 Yin (a) Load Cell Soil Specimen After the back-pressure saturation, the top drainage tube valve of the specimen was closed to make the copper specimen undrained. Since the purpose of the calibration was to assess the volume changes of the inner cell and the outer cell. Keeping the specimen undrained (or all drainage tube valves closed) would avoid the volume changes (errors) due to the water squeezed out of the gag between the rubber membrane and the copper specimen and between the cap/filter stone and the specimen. (b) Figure 5 A close-up view of the inner cell (top) and a close-up view of the outer cell (bottom). The measured volume changes of the inner cell and the outer cell with effective cell pressure (cell pressure minus the initial cell pressure of 105 kpa) are shown in Fig.6. It is seen that the relationship of the outer cell volume change and the effective cell pressure is non-linear; while the relationship of the inner cell volume change and the effective cell pressure is linear. The volume changes of the outer cell is 3 to 4 times of the volume changes of the inner cell. For example, the accumulated volume change under the effective pressure of 400 kpa is cm 3 for the outer cell and 0.40 cm 3 for the inner cell.

7 An innovative double cell system 177 Compared to the results Effective inner/outer cell presure increase (kpa) reported by Wheeler (1988), the volume change 0.0 of the inner cell was 0.7 cm 3 under effective 0.5 pressure 400 kpa. Using the DCTS, the inner cell volume change is only y = x cm 3, that is, 0.57% of 0.7 R 2 = cm 3 using the design by Wheeler (1988). Furthermore, the relationship of 1.5 the inner cell volume Inner cell volume change change and the effective Outer cell volume change cell pressure using the 2.0 DCTS is linear; while that Figure 6. Volume changes of a copper specimen under isotropic reported by Wheeler (1988) compression measured by the changes of (a) water volume of the inner was non-linear. A straight cell and (b) the water volume of the outer cell with back pressure equal to 100kPa and the specimen drainage valve closed. line has been used to fit the relationship of the inner cell volume change and the effective cell pressure as shown in Fig.6. The fitting equation is: V i _ cell = σ e _ cell (1) Inner/outer cell volume change (cm 3 ) where V i _ cell is the accumulated volume change of the inner cell and σ e _ cell is the effective cell pressure. The volume of the copper specimen is cm 3. The inner cell volume change of 0.4 cm 3 is only 0.20% of the copper specimen volume and negligible for the volume strain calculation of a soil specimen. Time (min) The marine clay used in the testing was taken from 0 depth 1m to 2m at a volume change coming out of (or into) soil specimen marine site in Hong 5 volume change of inner cell Kong s waters. The marine clay was in dark grey color 10 and were a mixture of clay, silt and fine sand with 15 occasionally shells and coarse particles. In order to 20 obtain uniform and consistent soil samples, the 25 marine deposits were sieved in wet condition Figure 7. Volume changes of a saturated marine clay during through a sieve with an consolidation measured by the changes of (a) the water coming out of opening size of 150 mm. (or into) the clay specimen and (b) water volume of the inner cell. The marine clay after wet sieving had a composition of silt and clay with some fine sand. The composition is 27.5% of clay, 58.4% of silt Volume change (cm 3 )

8 178 Yin and 14.1% of fine sand. The marine clay was re-consolidated in cylindrical mould with 300mm in diameter and 450mm high. The basic properties of the marine clay are specific gravity G s =2.664, liquid limit w L =60.0%, plastic limit w P =28.5%, plasticity index I P =31.5%, and initial water content w=57.4% (after re-consolidation but before odometer testing). A thin-wall plastic tube with 50mm internal diameter was pushed into the reconsolidated marine clay in the mould. The clay sample in the tube was then extruded out and trimmed to form a specimen of 50mm in diameter and 100m in height. The clay specimen was installed in the inner cell following the code of BS 1377 (1990). A backpressure of 200 kpa with a cell pressure of 205 kpa was applied to ensure near 100% saturation. B-value measured was Fig.7 shows the results of volume changes with time under effective cell pressure of 50kPa. Two methods of volume measurement were used: (a) Method A by measuring the volume of water coming out of the specimen and (b) Method B by measuring the water volume change of the inner cell. Fig.7 shows that the curves of accumulated volume change with time using the two methods are very close. The volume measured using the inner cell (Method B) is slightly larger than that measured using Method A. In particular, at the end of the consolidation i.e. time of 1560 mins, the V i _ cell using Method B is 21.1 cm 3 ; while V w _ cell using Method A is 20.7 cm 3. The relative difference is V i _ cell Vw _ cell / Vw _ cell = / = 1.93%. The total volume of the clay specimen is V o = cm 3. The volume strain error is V V / V = / %. This error may be negligible. i _ cell w _ cell o = 3. A Hollow Cylinder Apparatus (HCA) A fully computer controlled Hollow Cylinder Apparatus (HCA) has been installed in Soil Mechanics Laboratory for soil testing. The soil specimen is hollow with an internal diameter of 50mm, outer diameter of 100mm and height of 100mm. Cyclic loading can be applied at a frequency of up to 20Hz. Unsaturated soil testing functions have also been incorporated with the HCA. The HCA is being used to measure the timedependent stress-strain behavior of Hong Kong Marine Clays. The stress states in HCA are shown in Fig.10.

9 An innovative double cell system A Truly Triaxial System (TTS) A new Truly Triaxial System has been installed in the Soil Mechanics Laboratory for soil testing as shown in Fig.9. The soil specimen has a brick-shape with a height of 150mm and a cross-section of 70mm by 70mm. The vertical stress and lateral stress are applied using oil jacks and are the major principle stress and the middle principle stress respective. The minor principle stress is applied by oil pressure in the chamber when the door is closed (see the outside view in Fig.9). The stress states in the TTS are shown in Fig Remarks From the work presented above, the following remarks are presented: (a) The new Double Cell Triaxial System (DCTS) has advantages over the existing modified cells. The DCTS is more accurate and reliable and has continuous measurement of the volume changes of unsaturated and saturated soils in consolidation and compression. Figure 8. A new Hollow Cylinder System for soil testing (static and cyclic loading up to 20Hz). (b) The calibration data using a solid copper specimen show that the outer cell volume change is 3 to 4 times of that of the inner cell volume change. For effective pressure of 400 kpa, the inner cell volume change is only 0.4 cm 3 and 0.20% of the copper specimen volume. The error of 0.2% may be negligible for the volume strain calculation of a soil specimen. (c) In drained compression, the relative difference of volume changes measured using Method A and Method B is 5.62%. The volume strain error in terms of V i _ cell Vw _ cell / Vo is only 0.21%. This error is small and considered acceptable for most soils.

10 180 Yin (a) (b Figure 9. A Truly Triaxial System for soil testing (a) inside view and (b) outside view. (d) The new HCA and TTS are needed to study the stress-strain-strength behavior of soils in more complicated stress states, which are closer to those in the field. Figure 10. Stress states in HCA and TTS.

11 An innovative double cell system Acknowledgements Financial supports from a RGC grant (PolyU 5041/01E and PolyU Account No.Q414) of the University Grant Council of the Government of Hong Kong Special Administrative Region and from the Hong Kong Polytechnic University are acknowledged. 7. References BS 1377, (1990). Methods of Test for Soils for Civil Engineering Purposes, British Standards Institute, London, Bishop, A.W. and Donald, I.B. (1961). The experimental study of partly saturated soils in the triaxial apparatus. Procc. 5th Int. Conf. Soil Mech. and Found. Engineering, Paris, Vol1, pp Chen, Z-H, Lu, Z-H., PU, Y-B. (2001). Unsaturated soil triaxial apparatus CT system and application, Chinese Journal of Geotechnical Engineering Vol.23, No.4, pp Wheeler, S.J. (1988). The undrained shear strength of soils containing large gas bubbles. Geotechnique, Vol.28, No.3, pp Yin, Z.Z. (Editor) (1998). Settlement and Consolidation of Soil Mass, China Electric Publication House (in Chinese).