By Andrew I. Strutynsky, Bruce J. Douglas, A.M.ASCE, Larry J. Mahar, A.M.ASCE, George F. Edmonds, and Eugene Hencey1
ABSTRACT: Geotechnical exploration in offshore arctic regions relies upon the electric Cone Penetrometer Test (CPT) because of its speed, economy, data continuity, and general reliability. By adding additional sensors to the basic CPT, simultaneous multichannel data acquisition including pore pressure, temperature, and electrical conductivity, has allowed more detailed characterization of in situ conditions than is usually possible. Descriptions of some of these new sensors and typical data obtained with them are presented in this paper.
Soil exploration in the offshore arctic environment imposes severe and unique demands upon the exploration methods. The constant threat of weather shut down requires rapid methods. The high cost of arctic programs additionally requires methods that provide uniformly high quality data. Finally, because of the complex thermal regime, sampling invariably introduces change in thermal state prior to any laboratory testing.
For these reasons, rapidly deployable, multichannel in situ penetration testing systems have been developed based upon the electric Cone Penetrometer Test (CPT). The multichannel penetration testing systems include piezometric, thermal and electrical conductivity sensors in addition to cone end bearing and friction sleeve measurements. Data collected with these systems have allowed for fuller characterization of the complex dependence of mechanical properties on thermal and physiochemical states of saline saturated permafrost soils.
The electric CPT instrument (subtraction type) used as the base module of the multichannel system has a 15 sq cm end area and 200 sq cm friction sleeve as shown in Figure 1.
FIGURE 1 CONE PENETROMETER
This particular design has proven robust enough for general applications, and with suitable choice of load cell configuration, provides good resolution in soft soils. The oversized end area, in addition to providing increased load resolution, acts as particularly efficient friction reducer, thus increasing penetration depth for a given total pushing force. Biaxial inclinometers are included inside this instrument for monitoring instrument drift from vertical.
The two load transducers are composed of 120 ohm foil strain gages connected as an active 240 ohm four arm Wheatstone bridge. Two opposing legs are in axial strain and the other two are Poisson gages. The bridge is temperature compensated to .02%/C°, from -20°C to +50°C. The sensitivity is a nominal 2.2 mv/v with a tolerance of +/-10%. Excitation is 10v with remote load sensing. Power density is 3.65 Watt/sq inch and the thermal time constant of the bridge is 90 sec.
The biaxial inclinometers consist of two strain gage accelerometers, each forming an active half bridge, with two units in quadrature forming a full bridge. Accelerometers are air damped.
The piezometric sensor added to the basic instrument to allow measurement of generated fluid pressures is shown in Figure 2. Typical data collected using this sensor are also shown in Figure 2. This particular design has the transducer ported through the mid-point of the cone tip to a circular porous stone filter. This design allows for measurement of pore pressures at their maximum level. The effects of filter location on measurements must be accounted for when comparing different piezocone data bases (1).
FIGURE 2. CONE PENETROMETER TEST WITH PIEZOMETRIC SENSOR
The piezometric transducer is a piezoresistive, gage pressure type. It consists of an active four arm bridge deposited on a silicon diaphragm. The transducer dead volume is .0003 in-3. The compensated temperature range is -16°C to +93°C. Sensitivity is 0.63+/- 0.18 mv/psi at 10v excitation. The thermal time constant is 4 sec.
The addition of a thermal sensor to the CPT allows for measurement of dynamic temperatures generated during cone penetration. By ceasing penetration and allowing thermal dissipations to occur (analogous to pore pressure dissipations) an equilibrium thermal profile may be obtained.
Two different thermal sensors were used in the arctic explorations. The first design has a PTAT transducer mounted behind a copper alloy cone tip. The transducer is thermally coupled (but stress isolated) through thermal grease to the copper alloy tip. The sensor and data collected are presented in Figure 3.
FIGURE 3. CONE PENETROMETER TEST WITH THERMAL SENSOR
This PTAT transducer consists of a hybrid bridge semiconductor chip, with current output proportional to absolute temperature. It is scaled to provide 1 mv/°K at 10v excitation.
This first tip sensor design allows for rapid dissipation of generated heat to obtain ambient temperatures, but precludes using the piezometric sensor for simultaneous pore pressure measurement. Thus, a second thermal sensor used in arctic explorations has a PTAT transducer, bonded to the inside of the cone instrument. While the thermal mass coupled to the transducer with this design is greater than that with the first design, this configuration allows for simultaneous pore pressure measurement. However, thermal equilibration requires longer time periods.
The second PTAT transducer consists of a semiconductor chip with current output proportional to absolute temperature. The Input is 5 to 30 volts, with output of 1 ? amp/°K. Output may be scaled as desired with one resistor.
Electrical conductivity measurements were added to the CPT to investigate their potential for assessment of physiochemical properties of penetrated soils. This sensor was mounted behind the cone instrument and allowed the measurement of soil electrical conductivity as shown in Figure 4. Data collected with this instrument are also shown in Figure 4.
FIGURE 4. CONE PENETROMETER TEST WITH ELECTRICAL CONDUCTIVITY SENSOR
The downhole portion of the conductivity sensor consists of a four wire-two electrode configuration excited at 2000 Hz. Alternating current is used to control polarization. The voltage across the electrodes is measured and compared with an inphase current through a reference resistor. This comparison is performed digitally. Scale can be changed by selecting different reference resistors. The two brass electrodes are insulated from the instrument housing using Teflon rings.
IN SITU PORE FLUID CONDUCTIVITY/THERMAL PROBE
To rapidly obtain measurements of in situ pore fluid salinity, without the thermal and mechanical disturbance caused by drilling fluids, a push-in probe was developed. This In Situ Pore Fluid Conductivity/Thermal Probe is shown In Figure 5. The probe is pushed into the soil while maintaining a positive pressure inside the probe. At a selected sampling depth, the internal pressure is released at the ground surface, and soil pore fluid flows into the internal conductivity cell due to the positive fluid gradient between the soil and the inside of the probe.
FIGURE 5. IN SITU PORE FLUID CONDUCTIVITY/THERMAL PROBE
A two ring electrode conductivity cell mounted inside the probe is used to measure pore fluid conductivity. The electrode configuration is similar to that as described previously. A PTAT transducer is bonded to one of the electrodes in order to provide a measurement of pore fluid temperature. Both temperature and conductivity are required to define fluid salinity.
Another PTAT transducer was mounted to a copper ring located above the filter on the probe exterior. This ring is in good thermal contact with the surrounding soil. Thermal equilibrium measurements were obtained during the time period required to fill the internal conductivity cell.
Multichannel data acquisition during a CPT is a reliable and economical method of obtaining valuable in situ data during the fast paced field explorations common to remote arctic frontier areas. Proven systems have been utilized with up to six channels of active data acquisition cone end bearing and friction resistance, piezometric pressures, thermal response, electrical conductivity, and biaxial inclination. Data obtained with these multichannel In situ penetration testing systems have provided:
It is important to note that the entire penetration test system (including uphole electronics) was calibrated under expected thermal conditions in the Earth Technology cold room laboratory this process is of importance in any arctic field program to preclude thermally induced changes in sensor performance between laboratory calibration and actual field testing.
Further research is being supported by Earth Technology for development of other push-in probes, Including a dielectric CPT for ice content assessment, a CPT push-in pressuremeter, and BAT in situ fluid samplers (2) for permeability measurements or long term pore fluid/dissolved gas sampling.
The authors wish to thank Sohio Petroleum Company, and especially Dr. Jeff Weaver of the Sohio Technology Center, for their support of many of these activities.