Fundamentals Of CONE PENETROMETER TEST (CPT) SOUNDINGS - Missouri S&T

Fundamentals ofCONEPENETROMETER TEST(CPT) SOUNDINGSJ. David Rogers, Ph.D., P.E., R.G.

Cone PenetrationTest CPT soundings can be veryeffective in sitecharacterization, especiallysites with discretestratigraphic horizons ordiscontinuous lenses. Toda, most of thecommercially-available CPTrigs operate electronicfriction cone and piezoconepenetrometers, whosetesting procedures areoutlined in ASTM D-5778,adopted in 1995.

ASTM adopted the CPT procedure as test D-3441 in1974. It is a valuable method of assessing subsurfacestratigraphy associated with soft materials,discontinuous lenses, organic materials (peat),potentially liquefiable materials (silt, sands andgranule gravel), and landslides.

Shape of Cone Tip and rate of advance The standardized conepenetrometer test (CPT)involves pushing a 1.41inch diameter 55o to 60ocone through theunderlying ground at arate of 1 to 2 cm/sec.

Sleeve vs Tipresistance These devices produce acomputerized log of tipand sleeve resistance, theratio between the two,induced pore pressure justbehind the cone tip, porepressure ratio (change inpore pressure divided bymeasured pressure) andlithologic interpretation ofeach 2 cm interval arecontinuously logged andcan usually be printedonsite

CPT rigs usually employ1 m long rods. They can usuallypenetrate normallyconsolidated soils andcolluvium, but have alsobeen employed tocharacterize weatheredQuaternary and Tertiaryage strata. Cemented orunweathered horizons,such as sandstone,conglomerate ormassive volcanic rockcan impedeadvancement of theprobe

The cone is able to delineate even the smallest (0.64mm/1/4-inch thick) low strength horizons, easily missedin most conventional subsurface sampling programs.

Tip Resistance The tip resistance is measured by load cellslocated just behind the tapered cone. The tip resistance is theoretically related toundrained shear strength of a saturatedcohesive material, while the sleeve friction istheoretically related to the friction of thehorizon being penetrated. The tapered cone head forces failure of thesoil about 15 inches ahead of the tip and theresistance is measured with an embeddedload cell in tons/ft2 (tsf).

The tip of the conepenetrometer sensesout ahead of itself asit induces a localbearing failure of thesoil it passesthrough. The tip resistancerecorded by theinstrument is anaverage across thistip influence zone. Therefore, cautionshould be exercisedwhen evaluatinginsitu strengthparameters forhorizons less thanabout 8 inches (20cm) thick

Local or Sleeve Friction The local friction is measured by tensionload cells embedded in the sleeve for adistance of 4 inches behind the tip. They measure the average skin frictionas the probe is advanced through thesoil. If cohesive soils are partially saturated,they may exert appreciable skin friction,negating the interpretive program.

Friction Ratio, Fr The friction ratio is given in percent. It is theratio of skin friction divided by the tipresistance (both in tsf). It is used to classify the soil, by its behavior, orreaction to the cone being forced through thesoil. High ratios generally indicate clayey materials(high c, low Ø) while lower ratios are typical ofsandy materials (or dry desiccated clays). Typical skin friction to tip friction ratios are 1%to 10%. The ratio seldom, if ever, exceeds 15to 20%. Sands are generally identified byexhibiting a ratio 1%. The exception isburied wood, which can result in Fr valuesmuch greater than 20 (and are not interpreted).

Pore Pressure Piezocones also measure insitu pore pressure (in psi), ineither dynamic (while advancing the cone) or static (holdingthe cone stationary) modes. Piezocones employ a porous plastic insert just behind thetapered head that is made of hydrophilic polypropylene,with a nominal particle size of 120 microns. The piezocell must be saturated with glycerin prior to itsemployment. The filter permeability is about 0.01 cm/sec (1x 10-2 cm/sec). When using the cone to penetrate dense layers, such ascemented siltstone, sandstone or conglomerate, the piezofilter element can become compressed, thereby inducinghigh positive pore pressures. But, the plastic filters do notexhibit this tendency, though they do become brittle withtime and may need to be replaced periodically. In stiff over-consolidated clays the pore pressure gradientaround the cone may be quite high. This pore pressuregradient often results in dissipations recorded behind theCPT tip that initially increase before decreasing to theequilibrium value.

Differential Pore Pressure The Differential Pore Pressure Ratio is used to aid insoil classification according to the Unified SoilClassification System (USCS). When the cone penetrates dense materials like sand,the sand dilates and the pore pressure drops. Inclayey materials high pore pressures may be inducedby the driving of the cone head. If transient pore pressures are being recorded thatseem non-hydrostatic, most experienced operators willask that the penetration be halted and allowed at leastfive minutes to equilibrate, so a quasi-static porepressure reading can be recorded (this can take 10 to30 minutes). In practice, experienced operators try to stop theadvance and take pore pressure measurements inrecognized aquifers and just above or adjacent toindicated aquacludes.

Temperature sensor A significant advantage of the electriccone is the temperature sensor. This has been found to be very useful inassessing the precise position of thezone, or zones, of saturation, which is ofgreat import in slope stability andconsolidation studies. A temperature shift of about 6o F iscommon at the groundwater interface,even perched horizons within landslides.

CPT log thrua complexbedrocklandslideNote porepressurepulses abovethe slipsurfacesThe tipresistancevalues aremuch higherthan themobilizedshearstrengthbecausethese lowstrengthhorizons arejust a fewinches thick

Corrected, or ‘Interpreted’ Logs Most CPT rigs are equipped with one or severalautomated interpretation programs, which classify 1 cmhorizons according to the Unified Soil ClassificationSystem. The most widely employed routine has been thatdeveloped by Robinson and Campanella (1986),available from Hogentogler & Co., or from the NaturalSciences and Engineering Research Council of Canada. The interpretation programs evaluate all of themeasured properties and classify the horizon accordingto its behavior (in lieu of petrology). For instance, whenclassifying a clayey material, the interpretive programsconsider undrained shear strength, tip resistance anddifferential pore pressure. A high differential porepressure is assumed diagnostic of more clayeymaterials.

Olsen’s Chart A more refined CPTclassification chartwas developed byOlsen (1988). It has been Rogers’experience that theOlsen chart providesslightly better resultsthan the older chartsthat preceded it.

SPt to CPTConversionChart CPT to SPTConversion Chartdeveloped by Olsen(1988). Useful whenattempting tocorrelate betweenCPT and older SPTdata

Chart relatingImpacts of Age,consolidation,and induration Normalized ConePenetrometer test(CPT) soil behaviorchart proposed byRobinson (1990). Important to considerthis when evaluatingoverconsolidatedshales or sensitiveclay materials

Conventional CPT rigs are mounted on diesel truckframes, weighing about 35,000 lbs. The entire weightof the rig is supported on hydraulic rams to increasethe normal reaction force pushing the cone downward.

Mini CPT Probe GeoProbe manufacturesmini CPT rigs, on smalltracked vehicles, vans,or pick-up trucks Note the ground screwanchors, used toincrease normal forcefor pushing the conedown into the ground Very useful for limitedaccess applications,such as mid-slopebenches

CPT rig mounted on an airboat for use inshallow water and marshes along the GulfCoast. Floating rigs must be anchoredsecurely to stabilize the platform.

Typical CPT summary logs

Typical Geologic Profile derivedfrom CPT soundings

CPT logs are extremely valuable forconstructing stratigraphic correlations, such asthe 17th Street Canal failure in new Orleans in2005, shown here.

Strengthlines employed for design

The CPT log candiscern slightvariances in density,stiffness, and shearstrength, as shownby these compactedfill lifts

Comparisons of cone penetrometer soundings inexpansive silty clay of an engineered fill embankmenttaken seven years apart. Note how the initial effects of compaction lifts wereerased by moisture absorption, desiccation and swell.

Notes of Caution - 1 Some notes of caution are advised when applying theCPT method to evaluating discrete low-strengthhorizons or partings, such as landslide slip surfaces.\ The 60o tip of the cone forces a passive failure of theground in front of the advancing tip. The instrumentedtip senses soil resistance about 21cm (8.4 in) ahead ofthe advancing tip. This means that the tip resistance reported as“undrained shear strength” is actually an averagevalue, taken over the zone within 21 cm of the cone tip.If the tip penetrates low strength horizons less than 21cm thick, the tip resistance reported on the CPT logmay not be a realistic assessment.

Notes of Caution - 2 Another problem with the CPT method is thatcone soundings advanced through desiccatedclay will often be interpreted as sand or siltmixtures (by the computerized lithologicinterpretation routine) because of recordedsleeve friction. The opposite problem occurs when reportingStandard Penetration Test (SPT) blow countsafter advancing drive samples through clayeyhorizons! The SPT test is best suited forgranular materials, and blow counts in partiallysaturated cohesive materials must be regardedwith some degree of skepticism as they mayshift dramatically upon later absorption ofmoisture.

CONCLUSIONS - 1 Engineering geologists are most often entrusted withcharacterizing difficult sites for subsequent analysis bygeotechnical engineers. Our ability to develop the mosteffective program of exploration, sampling and testing is builtupon each person’s unique pedigree of experience. The most effective means of characterizing complex sitesincludes a thorough background work-up on the area underinvestigation, followed by a well-conceived program ofsubsurface exploration that commonly includes smalldiameter borings. Cone penetrometer soundings are beingemployed with increasing regularity, especially in evaluationof soil liquefaction potential. Engineering geologists should consider employing bothtechniques whenever possible, because each has slightadvantages over the other, but are most powerful whencombined on the same sites. The SPT test allows a first-hand look at subsurface materialswhich the CPT does not, and can provide crucial “groundtruthing” as to the type of subsurface material, especially,cohensionless materials with fines.

CONCLUSIONS - 2 The CPT procedure is capable of detecting discrete horizonsthat would normally be missed using drive samples atspecific depth intervals. But, the absolute values of tip resistance skin (local) frictionand pore pressure garnered in CPT soundings must beevaluated with a great degree of judgment, because they canbe much higher than actually exists, insitu. Sites underlain by natural geologic structures should not beapproached like a foundation investigation, takingsubsurface samples at fixed intervals; they must be attackedindividually, with a focused program of exploration thatemploys a realistic working model of the site’s evolution,focused on validating the assumptions used to constructsuch models. The greatest danger we face as a profession is the inherenttendency to make the site exploration fit our pre-conceivednotions of site conditions, then employ insufficientexploration to confirm or deny such assumptions.

clay will often be interpreted as sand or silt mixtures (by the computerized lithologic interpretation routine) because of recorded sleeve friction. The opposite problem occurs when reporting Standard Penetration Test (SPT) blow counts after advancing drive samples through clayey horizons! The SPT test is best suited for