Saffer and his group maintain a sediment mechanics laboratory, which hosts a high-pressure consolidation and permeability system, as well as several computers for laboratory data reduction. It is housed in a large shared rock/sediment mechanics laboratory at PSU with a biaxial shearing apparatus and true triaxial system developed and maintained by C. Marone. The facility is equipped for uniaxial sediment consolidation studies at loads up to 50 kN
(corresponding to stresses up to 100 MPa on 25 mm diameter samples), for high-precision constant-flow rate permeability measurements on 25 mm, 38 mm and 50 mm diameter core samples, and for standard triaxial deformation and permeability measurements at confining pressures up to 14 MPa. The systems are optimized for high-precision, long-term measurement of hydraulic and mechanical properties, in which long duration stability of pressure/volume and load frame control are essential (for example, flow-through permeability and/or consolidation experiments on tight samples that typically require several days to a few weeks to complete).
Each system consists of a load frame, ram, load cells and displacement transducers (LVDT), sample cell, and high-precision external fluid pressure/volume controllers. The pressure/volume controllers and load frames can operate under manual or computer control (typical tests are run in computer control). For strain-controlled tests, strain rate and fluid pressure (or flow rate) are controlled directly from a dedicated PC via software; stress and volume change are recorded but not actively controlled. For stress-controlled tests, stress is applied by relative physical movement of the ram against the sample. In this case, stress is measured continuously and controlled via a feedback by adjusting the ram movement to maintain constant stress. In all cases, volume change is measured to +/- 1 mm 3 (1 µl), deformation is measured to +/- 1 µm, ram advance rate is controlled to a precision of +/- 10 -2 µm/min, and stress is measured to +/- 1 kPa.
For uniaxial deformation experiments, the sample cell is a high-pressure fixed-ring oedometer vessel designed jointly by Saffer and GDS Instruments Ltd (Figure 1). It consists of a cell base and top, which bolt together at a mid-point allowing for easy sample insertion and access. The vessel includes two fluid ports at the top, and two at the base. One pressure/volume controller (GDS design) is connected at the top and the other at the base – in order to conduct singly or doubly drained consolidation experiments and flow-through permeability measurements. The sample cell is rated to 10 MPa internal pore pressure. Sintered porous discs are emplaced in machined recesses in the ram cap and cell base. The ram inserts through a guide tube and two seals (viton low-friction o-ring, and M3 variseal) that hold pore pressure and fluid volume inside the vessel. Recent modifications to the system for permeability measurement on hard-rock samples include top and base gaskets and design of sample rings specifically machined to match the sample diameter and which include a central o-ring seal at the sample mid-point; these adaptations have worked well on outcrop and SAFOD drill hole samples (see proposal text for discussion of these data). Such measures are unnecessary for typically more compressible sedimentary rocks or fault zone materials, which self-seal against the sample ring under axial load [e.g., Saffer and McKiernan, 2005].
For triaxial experiments, two cells are available: a low-pressure cell rated to 3 MPa confining pressure and a high-pressure cell rated to 14 MPa (Figure 2). The vessels are interchangeable within the load frame, but differ slightly in their construction. The low-pressure vessel is constructed from plexiglass, and is designed for use with water or oil as a confining fluid. It has 4 ports for fluid or electrical lines to pass into the vessel. These are typically used for confining pressure and pore pressure, which can be controlled separately at the sample base and top. The high pressure vessel is constructed of 3-16 stainless steel, and is designed for use with oil as a confining fluid. It has 8 ports for fluid or electrical lines, which allow the use of additional measurement elements, including an internal load cell for measurement of the axial stress without interference from the seal friction (although this can be calibrated and is typically small) and a radial strain gauge. In our experiments, samples are typically jacketed in latex with o-ring seals at the top and base of the sample, and silicon oil is used as a confining fluid.
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Each cell contains top and base caps (stainless steel platens) that are internally plumbed to deliver fluid to the sample faces, and which are machined to be the same dimension as the desired sample diameter. The base cap is a platform that bolts to the vessel bottom, and fluid lines connect from a fitting at the vessel exterior through the platform. The top cap is plumbed with a fitting for a fluid line. Porous discs are placed between each cap and the sample, as with the oedometer system, to distribute fluid evenly to the sample faces. Sample diameters up to ~60 mm can be accommodated. For the low-pressure vessel, the maximum sample length is 120 mm; the maximum sample length for the high-pressure vessel is 80 mm. For both vessels, longer samples can be accommodated if desired by fabricating shorter end caps.
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In typical configuration, both the top and base of the sample are plumbed to pressure/volume controllers, to maintain sample pore pressure and to allow permeability testing. A third pressure/volume controller can be used to maintain confining pressure, and to monitor volume changes of the confining fluid related to sample dilation or consolidation. We also measure the volumetric strain using the external LVDT for axial deformation and the radial strain caliper equipped with a submersible LVDT. Any desired stress path within the apparatus maximum pressure and stress capability can be achieved under (1) stress feedback control (controlled mean and differential stress, resulting strain is monitored), (2) strain control (controlled shortening rate and/or rate of volume change, resulting stresses are monitored), or (3) manual control.
Figure 4: Left: Annotated photograph of the low-pressure (3 MPa) triaxial vessel and load frame. The high-pressure (14 MPa) system is similar but is steel-walled rather than plexiglass.
Right: Schematic of triaxial configuration. All measurements of load and sample deformation are conducted inside of vessel, and are corrected for platen and porous frit stiffness.
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Friction experiments are performed in a biaxial load frame using the double-direct shear geometry. Load axes are precisely perpendicular, and surfaces of frictional shear are oriented such that normal and shear tractions are controlled independently. Each load frame of the biaxial apparatus consists of stiff steel platens connected by precision cut steel rods. The vertical ram is fitted with a 10-inch bore, 6-inch stroke hydraulic ram capable of producing forces up to 1 MN.
The horizontal ram is capable of forces up to 670 kN. The apparatus is capable of accepting samples with nominal frictional contact dimensions up to 30 cm x 30 cm. Steel blocks are used to support samples at a position precisely aligned with the center of the horizontal and vertical load axes. Steel platens also buttress the side blocks of the friction sample to eliminate rotational motion induced by direct shear. All steel work is precision ground flat, parallel, and square.
The apparatus was designed to be stiff so that details of friction and fracture can be measured without violent unstable failure. The nominal stiffness of each frame is measured by replacing the friction sample with solid steel. For forces in the range 0 to 500 kN, stiffnesses of the vertical and horizontal frames are 0.45 kN/µm and 0.33 kN/µm, respectively. Stiffness increases nonlinearly at low loads, and these values are 10-15% larger at the upper end of this force range. For friction experiments, precise calibrations are carried out using a relaxation technique and transducers mounted directly on the friction sample). For results discussed in this paper, stiffnesses are 0.50 and 0.37 ±0.02 kN/µm for the vertical and horizontal frames, respectively.
Expressed as stresses on surfaces with nominal contact area of 10 x 10 cm 2 , and accounting for the factor of two vertical force needed to shear two surfaces, the stiffnesses are 250 MPa/cm and 370
MPa/cm for shear and normal stresses, respectively.
67 3 mm
1 MN Ram
10 " bore , 6" stroke
Fri cti on forci ng b locks
Goug e la ye rs
3"
Lo ad cell
80 mm
DCDTs
10 0 mm
57 2 mm
Lu bricated , coppe r shi m
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The true-triaxial apparatus consists of 10,000-PSI pressure vessel (Figure 3) fitted in a servocontrolled biaxial load frame shown above. Measurements of along-fault and across-fault permeability can be carried out in the double-direct shear and single direct shear geometry. The machine is designed for friction, fracture, flow, and deformation experiments under fullycontrolled conditions. Each axis of triaxial loading can be applied with force or displacement boundary conditions. Porosity, permeability, and pore fluid volume changes are measured during deformation. Samples can be deformed under drained or undrained loading conditions.
Capabilities include double-direct shear and single-direct shear of granular and clay rich simulated fault gouge, natural fault gouge, bare rock surfaces in frictional contact, and in-situ shear fracture of intact samples.
Figure 3. Pressure vessel for true-triaxial deformation. Left panel: single direct shear geometry.
A frictionless roller-way bearing replaces one of the shear surfaces of the double-direct shear configuration. Right panel: photo of the pressure vessel with the sample assembly installed.
Note deformable latex jacket that separates pore fluids from the confining pressure medium.
The pressure vessel has removable doors to allow sample access and set-up of electrical and fluid connections (Figure 3). Fluids enter through the platens and access the sample via porous frits (Figure 4). A flexible latex jackets separates pore and confining fluids. Pore-pressure (Pp) and confining pressure (Pc) intensifiers are designed for pressures to 10,000 PSI with a volume of
155 cc. The system is capable of resolving volume changes as small as 1x10 -4 cc. Biaxial applied stresses (heavy arrows in Figure 4a) of up to 300 MPa can be applied.
Figure 4. (a) Left panel shows schematic diagram of double direct shear geometry with crossfault fluid flow during shear. Blue lines show fluid distribution in the central block. Green lines show fluid distribution in the side forcing blocks. Grey areas are porous frits. (b) Right panel shows jacketing arrangement and internal geometry for double direct shear configuration.
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This apparatus allows deformation experiments over a broad range of conditions relevant to tectonic- sedimentological- and hydrologic problems, including consolidation, porositypermeability relationships, and lithification. The apparatus includes independent control of confining and pore pressure and sampling of pore fluids for aqueous chemistry.
For shear of granular layers the left- and right-side platens deliver fluid to the layer. For shear of rock samples, a rollerway bearing is used (Figure 3a) so that a single surface can be sheared.
Jackets are made using a commercial dip-mold service or an injection mold system constructed in-house. Fluids enter through the platens and access the sample via porous frits at the back of each block. The pore-pressure (Pp) intensifier and confining pressure (Pc) intensifier are designed for pressures to 10,000 PSI. Each intensifier is based on a highpressure piston-cylinder and a servo-controlled hydraulic ram driven by a double-acting, 3,000-PSI hydraulic cylinder.
The hydraulic cylinders are 2.5-in bore, 12-in stroke and conform to NFPA standards. The Pc system uses a noncombustible, refined and hydrogenated, paraffinic white oil and a 1-in diameter piston for a total volume of 155 cc. For this piston size the system is capable of routinely resolving volume changes as small as 1x10 -4 cc. The pore-pressure system has an adaptable closure nut and dynamic seal so that it can accommodate a 1-in diameter piston or a 5/8-in diameter piston where higher resolution measurements of volume change are needed.
Latex jacket used for the double direct shear configuration in the true-triaxial testing apparatus
Photo showing biaxial load frame with pressure vessel installed.
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