Continuous CPT(ASTM D5778) HistoryHistory

In 1932, the Dutch developed a simple device to measure continuously soil properties in situ. At the end of steel rods was a penetrometer consisting of a cone tip. Over the years, the rods were advanced by driving them with a hammer (dynamic force), applying dead

TABLE 1.12 Reconnaissance Methods of Exploration MethodApplicationsProcedureLimitations Test pit or trench ●Detailed examination of soil strata●Excavation by backhoe, bulldozer, ●Usually limited in depth by water excavation●Observation of groundwater seepageor by handtable (GWL), rock depth, or ●Identification of GWL●Can be extended below GWLby reach of equipment ●Recovery of disturbed or undisturbed samples sheeting and pumping if soils have ●Can be dangerous if left above GWLand in situdensity tests at least some cohesionunsheeted and depths are above 4 to 5 ft ●Examination of fault zones ●Examination of miscellaneous and rubble fills ●Identification of rock surface and evaluation of rippability ●Borrow material investigations Large-diameter holes●Detailed examination of strong ●Holes 60 to 100 cm in diameter ●Strong cohesive soils with no cohesive soil strata and location ofexcavated by rotating large augerdanger of collapse slickensides and other details affecting bucket (Figure 1.52), or excavated ●Rock penetration limited except stability and seepageby handby calyx drilling (Section 1.4.5) Adits and tunnels●Used in rock masses for preparation of detailed ●Excavation by rock tunneling ●Very costly geological sections and in situtesting; methods●Rock masses that do not require lining for primarily for large dams and tunnelssmall-diameter tunnels are left open for relatively short time intervals Bar soundings ●To determine thickness of shallow ●Ametal bar is driven or pushed into ●No samples obtained stratum of soft soilsground●Penetration limited to relatively weak soils such as organics or soft clays Hand auger or ●Recovery of disturbed samples and ●Rotation of a small-diameter auger ●Above GWLin clay or granular posthole diggerdetermination of soil profile to shallow depths into the ground by handsoils with at least apparent cohesion ●Locate GWL(hole usually collapses in ●Below GWLin cohesive soils with soils with little to slight cohesion)adequate strength to prevent collapse ●Penetration in dense sands and gravels or slightly plastic clays can be very difficult One-inch retractable-●Blows from driving give qualitative ●Small-diameter casing is driven into ●The entire rod string must be plug samplermeasure of penetration resistance to depths ground by 30 lb slip hammer removed to recover sample of 30 m in soft claysdropped 12 in.●Penetration depth in strong soils limited ●One-inch diameter samples can be ●Samples are obtained by retracting ●Small-diameter samples retrieved up to 1 m in lengthdriving plug and driving or pressing the casing forward Continuous cone ●Continuous penetration resistance ●Probe is jacked against a reaction for ●No samples are recovered penetrometer including side friction and point continuous penetration (CPT)(see Sections resistance for all but very strong soils on 1.3.4 and 2.4.5)land or water Standard penetration ●Recovery of disturbed samples and ●Split-barrel sampler driven into ●Penetration limited to soils and test (SPT)(see Section determination of soil profileground by 140 lb hammer soft rocks. Not suitable for 2.4.5)●Locate GWLdropped 30 in.boulders and hard rocks

weights (static force), or pushing hydraulically. Initially, only penetration resistance at the cone tip was measured (q_cor q_t). Later, a cone was developed with a sleeve to measure shaft (side) friction f_s(Begemann cone) in addition to tip resistance. The Begemann cone was termed a subtraction cone. It measures the total sleeve plus tip force on the cone and the tip resistance when pushed into the ground. Sleeve friction is calculated by subtract-ing the tip resistance from the total resistance.

Fugro, ca. 1965, developed an electric cone (the compression cone) that measured and recorded both tip resistance and shaft friction separately. Some electric cones have a max-imum value for sleeve friction of the order of 20 tons. The subtraction cone has no sleeve friction limit; the only limit is the total penetrometer force. Subtraction cones can be used where sleeve friction is high, such as in very stiff clay, and the limit of the electric cone is exceeded.

CPT Operations

Modern cones are pushed continuously into the ground by a hydraulic-force apparatus reacting against a machine. The apparatus can be mounted on a variety of platforms, including truck or track mounts, small portable units, and barges or drill ships. The inte-rior of a truck-mounted CPT is shown in Figure 1.38a. Large modern rigs have capacities of up to 30 metric tons. CPT rigs are often mounted with test boring drill rigs, but the reac-tion force is limited.

Advanced by hydraulic thrust, the electric cones employ load cells and strain gages that measure electronically both tip resistance and local sleeve friction simultaneously. The results are recorded digitally at the surface with an accuracy of measurement of usually better than 1%. Readings are usually taken at 5 cm intervals. Cones vary in size with areas of 10 and 15 cm² the most common because ASTM criteria apply. The 15 cm²cones can push well in loose gravels, cemented sands, and very stiff fine-grained soils and weath-ered rock. Various cone sizes are shown in Figure 1.38b.

The CPT method permits rapid and economical exploration of thick deposits of weak to moderately strong soils and provides detailed information on soil stratification. There have been many modifications to cone penetrometers in the past 20 years. The test can measure in situ many important soil properties applicable to geotechnical and environ-mental studies as summarized below. The interpretation of strength and compressibility properties are covered in Section 2.4.5.

Although soil sampling is possible with a special tool, soil samples are normally not obtained. CPT data are usually confirmed with test borings and soil sampling, but the number of borings is significantly reduced.

See also ASTM D5778, Sanglerat (1972), Schmertmann (1977), and Robertson et al. (1998).

Engineering Applications

Standard CPT. The common application of the CPT is to obtain measurements of engi-neering strength properties. In relatively permeable soils, such as fine and coarser sands, pore pressure effects during penetration at standard rates often have negligible influence, and the CPT measures approximately fully drained behavior. In homogeneous, plastic clays, the CPT measures approximately fully undrained behavior. Mixed soils produce in-between behavior.

Piezocones are currently in common use (CPTU). They have a porous element near the tip and a built-in electric transducer to measure pore water pressure in addition to tip resist-ance and shaft friction. Information on stratification and soil type is more reliable than the standard CPT. The interpretation of material strength properties is improved, and data are obtained on deformation characteristics. To obtain pore-pressure data, penetration is

stopped at the desired depth and readings are taken until the pore pressure generated by the penetration has dissipated. The coefficient of consolidation (c_h) and the coefficient for horizontal hydraulic conductivity (k_h) are determined. Hole inclination is also recorded with the piezocone illustrated as in Figure 1.39a. A plot of a CPTU log showing point and shaft friction, friction ratio, pore pressures, and a soil log is given in Figure 1.40. CPT plots now normally include the friction ratio and, with the CPTU, pore pressure measurements.

FIGURE 1.38 (a)

Cone penetrometer test equipment: (a) interior of CPT truck showing hydraulic force apparatus. (Courtesy of Fugro.)

The seismic cone penetration test (SCPT) (Figure 1.39a) combines the piezocone with the measurement of small strain shear wave velocities (P and S waves). A small geophone or accelerometer is placed inside a standard cone, and seismic wave velocities are measured during pauses in cone penetration. A hammer blow to a static load on the surface can pro-vide the shear wave source. Explosives can be used offshore or the “downhole seismic test” onshore. The results have been used for evaluating liquefaction potential (Robertson, 1990).

The active gamma penetrometer (GCPT) (Figure 1.39b) measures in situ soil density. The test is particularly important in clean sands, that are difficult to sample in the undis-turbed state.

FIGURE 1.38 (b)

Cone penetrometer test equipment: (b) various cone sizes. (Courtesy of ConeTec.)

A vision cone penetrometer (VisCPT) has been developed recently to overcome the prob-lem of no recovered samples (Hryciw et al., 2002). Miniature cameras are installed in the CPT probe, and continuous images of the soil’s stratigraphy are obtained through syn-thetic sapphire windows mounted in the side of the probe.

Triaxial geophones

Friction sleeve (F_s)

(a) (b) (c)

FIGURE 1.39

Various types of cone penetrometers. (a) Piezo cone penetrometer (CPTU): measures tip resistance, shaft friction, pore pressures, temperature, inclination, and shear wave velocities. (b) Active gamma penetrometer:

GCPT measures in situ soil density, particularly useful in sands which are difficult to sample undisturbed.

(c) Electrical resistivity cone (RCPT): provides measures of relative soil resistivity. (Courtesy of ConeTec.)

Site: 2001 CPTBT

Max. depth: 45.75 m Depth Inc: 0.05 m

SBT: Soil behavior type (Robertson et al., 1990) Estimated phreatic surface

Drilled Out Drilled Out Drilled Out

250 0.0 2.5 0.0 5.0 0 120

Sample of CPTU log. Data plots are provided in real time. (Courtesy of ConeTec.)

Environmental Applications

Cone penetrometers have been modified for environmental studies and the identifica-tion of contaminants (Robertson et al., 1998). Sensors have been developed to measure temperature, pH, radioactivity (gamma), resistivity, ultraviolet-induced fluorescence, and total petroleum hydrocarbons and other contaminants. SCAPS (site characterization and analysis penetrometer system) is a program in use by the government agencies in which the cone penetrometer is used for hazardous waste site characterization. An elec-trical resistivity cone (RCPT) is shown in Figure 1.39c.

Classification of Materials Correlations

CPT values are influenced by soil type and gradation, compactness, and consistency, which also affect the relationship of q_cwith f_s. Correlations have been developed between cone-tip resistance (q_cis also referred to as cone-bearing capacity) and the friction ratio R_f( fs/q_c) to provide a guide to soil classification as given in Figure 1.41. These charts do not provide a guide to soil classification based on grain size, but rather on soil behavior type. Figure 1.41 is based on data obtained at predominantly less than 30 m, and overlap between zones should be expected (Robertson, 1990).

It has been found that cones of slightly different designs will give slightly different val-ues for q_cand f_s, especially in soft clays and silts. This apparently is due to the effect that water pressures have on measured penetration resistance and sleeve friction because of unequal end areas. It has also been recognized that overburden pressure increases with depth also affecting strength values, as it does with the Standard Penetration Test results.

Cone resistance, qc (MPa) Cone resistance qc (bars) (1 bar = 100kPa = 1.04tsf)

11 very stiff fine-grained 12-11 over consolidated

or cemented

Simplified soil behavior type classification for standard electric friction cone. (Adapted from Robertson, P.K., et al., Proceedings, In situ ’86, Blacksburg, VA, 1986.)

For these reasons, correction factors have been proposed for the strength parameters and cone geometry. For equal end area cones only q_tis normalized. It is noted that cone resist-ance q_cis corrected to total cone resistance q_tas follows (Robertson, 1990):

q_tqc(1-a)u (1.4)

where u is the pore pressure measured between the cone tip and the friction sleeve and a is the net area ratio.

New classification charts have been proposed based on normalized data (Robertson, 1990).

Operations Offshore General

Offshore shallow-water investigations, such as for ports and harbors, normally involve water depths of 3 to 30 m. Jack-up rigs or spud barges are used and the cone is pushed from the vessel by conventional methods. A drill casing is lowered to the seafloor to pro-vide lateral support for the CPT rods.

For offshore deep-water exploration, such as for oil-production platforms, the CPT is usually operated in conjunction with wire-line drilling techniques (Section 1.3.5), with equipment mounted on large vessels such as shown in Figure 1.42. The major problem, maintaining adequate thrust reaction from a vessel subjected to sea swells, can be over-come by a motion compensator and the drill string. Thrust reaction can be provided by weighted frames set on the seafloor as shown in Figure 1.42.

Seafloor Reaction Systems

Underwater cone penetrometer rigs that operate from the seafloor have been developed by several firms. The Fugro-McClelland system, called “Seaclam,” operates in water depths up to 300 m. A hydraulic jacking system, mounted in a ballasted frame with a reentry funnel, is lowered to the seabed (Figure 1.43). Drilling proceeds through the Seaclam and when sampling or testing is desired, a hydraulic pipe clamp grips the drill string to provide a reaction force of up to 20 tons. A string of steel rods, on which the electric friction cone is mounted, is pushed hydraulically at a constant rate of penetration. Data are transmitted digitally to the drill ship.

ConeTec have developed an underwater CPT that can operate in water depths up to 2500 ft that can penetrate to 30 ft below the mudline. It is lowered over the side of a steel vessel and set on the seafloor. Fugro also has an underwater ground surface CPT which presently has a penetration of about 6 m.

Sampling and In Situ Testing

The various underwater sampling and in situ tools using the Fugro Seaclam are illustrated in Figure 1.43. Some sampling and testing is obtained by free-falling down the drill pipe.

Fugro have developed the “Dolphin” system for piston sampling, in situ CPT, and vane shear testing to water depths of at least 3000 m. Tools that require controlled thrust for operation, such as the cone penetrometer and piston sampler, employ a mud-powered thruster assembly at the base of the drill string. Vane testing and push sampling do not require the use of the thruster.

The Dolphin cone penetrometer is illustrated in Figure 1.44, and a plot of cone resistance data is shown in Figure 1.45. Penetration distances are 3 m or may refuse penetration at less than 3 m, and tip resistance and side friction are recorded. Drilling advances the hole to the next test depth and the CPT repeated. The remote vane shear device is shown in

Figure 1.46 and a data plot is shown in Figure 1.47. The vane data, compared with undrained laboratory test results from piston samples, are shown in the figure.

1.3.5 Test and Core Borings