Testing was done using GEOCOMP Corporation’s triaxial system. The GEOCOMP system is fully automated. The triaxial test cell used was rated to 1000 kPa. The system consists of 4 parts. The first part of the system is a PC computer that has the control software installed. This PC communicates with the other components of the system to run the test. The second part of the system is the load frame. This device holds the triaxial cell, applies the vertical load to the specimen, and monitors the force and displacements of the load piston. The GECOMP load frame is shown in Figure 6. The load frame interfaces with the rest of the triaxial compression system using a control box with an LCD display. This control box can be used to operate the load frame independently of the software if needed. The control box for the GEOCOMP load frame is shown in Figure 5.
Note the numeric keypad used to operate the load frame. The control box also interfaces the load frame with the control software in the PC.
Figure 5 - GEOCOMP Control Box
Figure 6 - GEOCOMP LLoad FFrame with assmebledAssembled SSpecimen
LVDT
S-Type Load Cell
Assembled Triaxial Cell with Specimen in Latex Membranes
The third and fourth components are the FLOWTRACK II flow pumps that apply the pressures for the triaxial cell. Flow lines connect the two pumps to the triaxial cell. One pump is for the cell pressure and the other pump is for the specimen. Each pump has a 250cc bladder that is pressurized as controlled by the computer. The use of two pumps is needed so that the specimen can be pressurized to a different pore pressure than the total stress applied from the cell. Each flow pump contains a calibrated pressure gauge to monitor the pressures continuously throughout the test. The pumps also adjust automatically to keep the pressures constant as required by the user. To keep the pressures constant, a volume of water flows in or out of the pump. This change in volume is monitored and allows for volumetric strain measurements on the specimen. The force applied to the specimen from the load piston is monitors using an S-type load cell. An LVDT measures displacements of the specimen in the vertical direction. The load cell and LVDT can be seen in Figure WW on the top of the GEOCOMP Load frame. Drainage of the specimen is allowed through the top and bottom pore stones. The GEOCOMP FLOWTRACK II flow pumps are shown in Figure 7.
Figure 7 - GEOCOMP FLOWTRACK II Flow Pumps
Procedures
No ASTM test method exists for the consolidated drained triaxial test of a soil. ASTM standards D4746 and D2850 are test standards for undrained tests. A working standard is being developed and is designated as standard WK3821. The standard D4746 was followed as closely as possible for specimen preparation.
Care was taken to minimize any sample disturbance during specimen preparation. For details on careful sampling and specimen preparation of Bonneville clays, see Bay et.al (2003). Each specimen was extruded from the Shelby tube after the tube had been trimmed to length using an electric ban saw. The Ccut Shelby tube ready for extraction of specimen is shown in Figure 8.
Figure 8 - Cut Shelby tube ready for specimen extraction
Specimens were extruded slowly from the Shelby tube, using a nearly constant rate. The specimens tested in the triaxial device were trimmed to 120 mm to 150 mm in length to meet a height to diameter ratio of 2 to 2.5. Each specimen was weighed, measured, and a moisture sample was taken from the cuttings to aid in the data reduction. After the specimen was measured, it was placed on the triaxial device base cap, with its saturated pore stone and filter paper. One or two latex membranes were place gently over the specimen to separate the specimen from the cell water. A top cap, and its pore stone and filter paper, was place on the top of each specimen. The triaxial cell was then assembled, filled with distilled water, and placed in the loading frame. Figures 9 to 11 show the assemblage of the triaxial cell. Figure 9 shows the base of the triaxial cell. Connections for flow lines are mounted on the bottom of the cell. The plastic bottom cap is also shown.
Figure 9 - Bottom of triaxial cell
Plastic Bottom Cap for Specimen
Connections for flow lines to the flow pumps.
Figure 10 - Assembled triaxial cell
After the cell is assembled (Figure 10), it is placed on the load frame as shown in Figure 3. All flow lines were purged of air, connected to the triaxial cell, and the cell was lifted into place. The instrumentation was zeroed out and a very small vertical load was applied to seat the load piston. Flow lines are then attached as shown in Figure 11.
Internal Flow Lines to allow top and bottom drainage of the specimen
Clay Specimen in latex membranes O-rings to isolate
specimen and cell
Figure 11 - Flow Line Connections
To begin the triaxial test, an initialization stage was first undergone. The initialization phase consists of application of a nearly equal vertical stress, horizontal stress, and pore pressure to the specimen to check for leaks and compliance issues before the test proceeded. After the initialization pressure was held for a short time, the saturation phase was begun. All specimens were backpressure saturated to a Skempton’s porewater pressure B value greater than 0.95 (Holtz and Kovacs, 1981( (Skempton, 19XX). The GEOCOMP system automatically measures the pore pressure changes during the saturation phase and calculates the Skempton B parameter continuously. Pore pressures for the backpressure saturation ranged from 200kPa to 500kPa depending on the specimen, its hydraulic conductivity, and the initial level of saturation. Air in the system goes into solution at around 200kPa (Black and Lee, 1973).
Connection to the cell
Specimen Bleed-off
Connection to the Specimen
After the specimen has reached a B value greater than 0.95 (which does not guarantee full saturation), a phase of consolidation is undertaken. The cell and vertical stresses are increased incrementally, while keeping the pore pressure constant to achieve an effective stress condition chosen for consolidation. Consolidation can be either isotropic or to an anisotropic Ko condition. Isotropic consolidation is always recommended in the literature for drained tests. Isotropic or Ko consolidation can be used for undrained test. This consolidation phase lasts until at least 95% consolidation has been achieved using the square root of time method. The GEOCOMP system monitors this automatically. After the specimen has consolidated, the test is paused for a short time to age the specimen at the stress level. This aging is recommended by several researchers (Bay et.al, 2005), especially for consolidation pressures that exceed the in-situ field conditions the specimen was taken from conditions for the specimen. Once the aging is complete, the specimen is then sheared.
The shearing phase of the triaxial test is done either drained or undrained. Undrained tests a conducted at higher strain rates, with no drainage of the specimen allowed. Drained tests are conducted at slow strain rates based on the consolidation properties of the specimen. The slow strain rates allow for drainage. Several authors have noted that despite being drained, increasingly slow strain rates of loading lead to higher strengths in triaxial testing (Reference, 19MM). Strain rates were determined using the method recommended by Bishop and Henkel (1962) and Gibson and Henkel (1954) which is based on the coefficient of consolidation of the soil (Cv or t90).
The GEOCOMP software monitors the force, displacement, stresses, and strains continuously thought the shear phase of the test. The specimen is sheared from 20% to 30% axial strain to assure that the critical state has been reached before end of shear.
After the shear phase is completed, the specimen is removed from the triaxial cell and a final moisture specimen is obtained.