Equipment

PENETROMETER SENSORS

The Cone Penetration Test (CPT) consists of smoothly pushing an instrumented penetrometer into the ground at a constant rate of 4 feet per minute. Strain gage loadcells within the penetrometer are used to measure the tip and side friction resistance of the soil to penetration (ASTM 5778 and D6067). The CPT tip (end bearing) resistance is quite sensitive to grain size in complete accordance with standard geotechnical bearing capacity theory. For example, the cone tip end bearing resistance is one to two orders of magnitude higher in a medium dense sand (aquifer) than the tip resistance in a stiff clay (aquitard). The ratio of friction to tip resistance, termed friction ratio, is indicative of soil fines content. The CPT friction ratio is high in clays and low in sands, while silts and mixed soils have intermediate values. Various geotechnical parameters in addition to soil types are computer evaluated from CPT results.

Piezometric CPT (CPTU) consists of CPT with a pressure transducer to measure soil pore-water pressure response to penetration. The CPTU piezometric measurement allows for evaluation of water tables, potentiometric surfaces in confined aquifers, soil drainage and saturation characteristics, clay undrained shear strengths, soil hydraulic conductivity and consolidation characteristics. The piezometric measurement is most useful in soft to medium stiff soils. At sites with hard or very dense soils, the piezometric measurement can be much less useful as the porous filters coupling the pressure transducer to the soil can become clogged or damaged.

CPTU with soil Electrical Conductivity measurement (CPTU-EC) includes a high resolution (1 inch) electrode array mounted on the tip of the penetrometer. Soil EC depends primarily on soil pore fluid chemistry and soil clay content. Unsaturated soils or soils saturated with LNAPL and DNAPL compounds usually exhibit very low EC. Dissolved inorganic compounds, especially those contained in brines and landfill leachates, significantly increase soil EC. The EC measurement is often also useful for detecting lithologic changes.

CPTU-EC with seismic Shear wave velocity measurements (CPTU-EC-S). Vibration sensors are attached to the penetrometer to acquire seismic shear wave velocity data. CPT vibration sensors have much better coupling to the surrounding soil than borehole deployed geophones. This allows rapid, low cost performance of large numbers of shear wave tests simultaneous with CPT. STRATIGRAPHICS uses an autohammer wave source and dual triaxial vibration sensors to acquire seismic data.

CPTU-EC with MIP (CPTU-EC-MIP) A Membrane Interface Probe™ adapter can be added to CPT for soil geochemical characterization. The MIP consists of a heated membrane which volatilizes contaminants in the soil. The volatiles pass from the soil through the semi-permeable membrane and are swept to the surface through small diameter tubing by a carrier gas which passes across the back of the membrane. We provide a simple PID sensor suite for rapid screening studies. More sophisticated analytical equipment such as GC-MS (under subcontract) can also be used for analysis.

CPTU-EC with induced UV fluorescence measurement (CPTU-EC-UVF) incorporates CPTU-EC with an induced ultra-violet fluorescence (UVF) module. The UVF module consists of a sapphire window, a monochromatic LED UV excitation light source, and photodiode light detectors. UV light is transmitted through the window into the soil. If the soil contains compounds such as petroleum hydrocarbons that fluoresce, the photodiodes detect the resulting light, and provide a signal roughly proportional to the type and concentration of the photofluor.

CPTU-EC with natural Gamma measurements (CPTU-EC-G) A CPTU-EC-G penetrometer incorporating CPTU-EC and natural gamma (G) sensors can be used to detect radionuclide contamination and to enhance lithologic evaluation.

CPT-EMOD™ The standard CPT procedure is conducted as a constant rate of strain test, resulting in a continuous measurement of soil ultimate bearing and frictional strength. By conducting CPT under monotonically increasing stress conditions, soil deformation properties can be more fully evaluated. The CPT-EMOD test is conducted during short pauses in the continuous push process. Load/settlement data are analyzed using elastic theory, as is done for a plate load test for evaluation of Young's Modulus at various stress levels.