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71 This section discusses a variety of other applications for CPT, including slope stability investigations and landslide forensics, pavement investigations, sinkholes, and environ- mental investigations. In certain circumstances, cone penetrometer technology has been employed to assist in delineating and detecting anomalous conditions or unusual features in the ground. Because traditional drilling and sampling is intermittent at say 5-ft-depth increments (1.5 m), the continuous nature of CPT helps provide detailed logging with three or more chan- nels. Although downhole probes (e.g., geophysical tools or video cameras) can be lowered down a predrilled borehole, the process often requires casing of the hole and is much more destructive than CPT invasion (i.e., an augured 8-in. or 200-mm-diameter hole versus a 1.4-in. or 36-mm pushed- place hole). A listing of select special applications by CPT is presented in Table 12, with a cited reference source given should additional details be desired. CPT has enjoyed particular use on geo-environmental site investigations because the test produces no samples, no cut- tings, and no spoil, thereby minimizing the generation of above-ground cleanup in sensitive areas and contaminated ground. Of well-known acclaim, the conductivity cone is commercially available from manufacturers and CPT service firms as an expedient means to map contaminant plumes and detect the presence of underground pollutants (Campanella and Weemees 1990). Resistivity (ohm-meter) is the recipro- cal of electrical conductivity; therefore, the device is also referred to as the resistivity cone (see Figure 87). The elec- trodes are provided as an array of either four axial rings at set vertical spacings or with a button array (positioned diametri- cally). The special membrane interface probe offers a single button electrode for an index determination of in situ resistiv- ity penetration and gas sampling. An example resistivity piezocone sounding (RCPTu1) from downtown Memphis, Tennessee, is presented in Figure 88. Electrical conductivity can be used to identify soil types. It is also employed in coastal areas to distinguish the upper freshwater table from the lower salt water regime. Whereas resistivity induces a direct current electrical cur- rent into the ground, a similar approach can be provided using alternating current and thus established to obtain dielectric measurements (permittivity). These dielectric readings can be interpreted to provide direct real-time profiles of volumetric water content. For portions of the sounding that extend below the groundwater table, the gravimetric water content can be mapped. Figure 87 (cen- ter) shows a special dielectric CPT penetrometer developed for this purpose (Shinn et al. 1998). New developments in sensors and testing procedures for CPT have been introduced to enhance the capabilities of direct-push technologies. Selected instruments and innova- tions are listed in Table 13. Illustrative examples of these CPT technologies include the use of cableless systems to transmit or store data, as shown in Figure 89, including: (left) memocone and (right) audio-signal cone. Another cableless system utilizes special glass-lined rods to allow transmission by infrared signals. These systems are advantageous in the following situations: (1) when conducting CPTu with drill rigs where the crew is not sensitive to working with elec- tronic cables, (2) offshore deployment, and (3) wireline sys- tems and deep soundings. In the case of the memocone, the data are stored downhole until the penetrometer is retrieved back at the ground surface and the readings of time t, qt, fs, and u2 are matched with the depth logger readings of time t and depth z. In the audio-signal cone, the data are transmit- ted by sound waves up through the center of the rods in real time and a special microphone used to capture the sounds that are digitally decoded for the data logger. A similar approach is used for infrared signals. With standard analog systems, the basic logging was restricted to depth (z), cone tip stress (qt), sleeve friction (fs), porewater pressures (u), and inclination (i), often because the electronic cable was of the 10-pin type (10 wires). Although 12-, 16-, 24-, and even 32-pin wires have been available, they are fragile with short lives because of the restrictive inner diameter of the cone rods that the cable must be threaded through. A few analog systems could circumvent the 10-wire limitations by either forgoing the inclinometer or friction readings. The multi-piezo- elements shown in Figure 90 are all analog penetrometers that allow simultaneous porewater pressure readings. In other novel analog systems, wiring is shared during differ- ent portions of testing (such as the Fugro true-interval seis- mic cone). CHAPTER ELEVEN MISCELLANEOUS USES OF CONE PENETRATION TESTING AND SPECIALIZED CONE PENETRATION TESTING EQUIPMENT
72 FIGURE 88 Example of conductivity piezocone test at Mud Island, Memphis, Tennessee. CPT Application Reference Source Environmental Site Investigation and Detection of Soil Contamination Campanella and Weemees (1990) Auxt and Wright (1995) Bratton and Timian (1995) Campanella et al. (1998) Lambson and Jacobs (1995) Lightner and Purdy (1995) Mlynarek et al. (1995) Pluimgraaff et al. (1995) Robertson et al. (1998) Shinn and Bratton (1995) Landslide Forensics and Slope Stability Collotta et al. (1989) Leroueil et al. (1995) Romani et al. (1995) Hight and Leroueil (2003) Pavement Investigations Badu-Tweneboah et al. (1988) Newcomb and Birgisson (1999) Sinkhole Detection in Limestone Terrain Foshee and Bixler (1994) TABLE 12 SPECIAL APPLICATIONS OF CONE PENETROMETER TECHNOLOGY FIGURE 87 Electrical conductivity measurements: (left) Fugro conductivity cones, (center) Vertek Dielectric and Hogentogler resistivity cones, and (right) Diametric (Button) Electrode Array for Resisitivity. Most recently, electronic digital cones can now process the readings downhole and the data can be transmitted uphole in series (rather than parallel with analog). Thus, the restric- tion on the numbers of simultaneous channels has been lifted. Figure 91 (left) shows a multi-friction sleeve penetrometer that uses several sleeves of different roughness and textures to quantify soil-pile interface response (DeJong and Frost 2002). Other developments include vibrocone penetrometers [Figure 91 (center)] for site-specific evaluation of soil liquefaction potential (without the use of empirical CRR curves) and T-bar testing [Figure 91 (right)] for defining shear strengths of very soft clays and silts (Long and Gudjonsson 2004). The T-bar is actually a penetrometer with a larger 100 cm2 hammerhead that replaces the
73 Specialized CPT System Reference Source Notes/Remarks Houlsby and Ruck (1998) Indicator of soil type Acoustic Emission CPT Menge and Van Impe (1995) Delineate soil type and lenses AutoSeis Generator Casey and Mayne (2002) Portable remote shear wave source dedeen nehw elpmas lios sniatbO relpmaS lioS TPC Dielectric CPT* Shinn et al. (1998) Maps volumetric water contents Horizontal CPT Broere and Van Tol (2001) Towards tunnel investigations Takesue and Isano (2001) Measures total horizontal stress Lateral Stress Cone Campanella et al. (1990) Total lateral stress during penetration Juran and Tumay (1989) Dual-element piezocone Skomedal and Bayne (1988) Triple-element piezocone Multi-Element Piezocones Danzinger et al. (1997) Quad-element piezocone Multi-Friction Sleeve Penetrometer DeJong and Frost (2002) Hebeler et al. (2004) Four friction sleeves of different roughness for pile interface studies Radio-Isotope CPT Shrivastava and Mimura (1998) Dasari et al. (2006) Measures density and water content in real time T-Bar Penetrometer Randolph (2004) Penetrometer with 100 cm2 head to increase load cell resolution in soft soils Lunne et al. (1997) Kurfurst and Woeller (1988) Measures thermal changesTemperature Lunne et al. (2005) McGillivray et al. (2000) Evaluate site-specific soil liquefaction Vibro-Piezocone Bonita et al. (2000) Vibration to locally cause liquefaction Hryciw et al. (1998) Real-time videocam of soil profile Vision Cone Penetrometer sesnel dna sreyal niht fo noitceteD )4002( nihS dna wicyrH )TPCsiV( *Also termed âsoil moisture probe.â TABLE 13 SPECIALIZED SENSORS OR MODIFICATIONS TO CONE PENETROMETER TECHNOLOGY FIGURE 89 Cableless CPT systems: (left) Memocone (ENVI) and (right) audio-signal unit (Geotech AB). FIGURE 90 Multi-piezo-element penetrometers: (left) dual-element type with midface and shoulder filters (van den Berg type), (center) Fugro triple-element cone, and (right) quad element (Oxford University).
standard 10 cm2 cone tip to increase resolution on the force gauge. If soil samples are deemed absolutely critical, then special CPT samplers have been developed that can obtain a disturbed pushed sample from specified depth (Figure 92). In lieu of sampling, several vision or video cone systems have been built that allow a real-time digital cam- era viewing of the soils by means of a small window port (Figure 93). The VisCPT has been used with digital image analysis processing to better define soil type and particle characterization, as well as show clear evidence of soil contamination. For seismic cone testing, automatic seismic sources have been constructed to produce repeatable transient shear waves that can be detected by the geophone(s). When the SCPT was devised, the recording of analog wavelet signals required the paired matching of left and right strikes to define the first crossover point that was used in the pseudo- interval downhole procedure (Campanella et al. 1986). With the advent of autoseis units, the downhole testing offers a quicker field testing time and only left (or right) series of strikes are needed because computers can easily post-process the consecutive waveforms and match them using cross-correlation. A selection of autoseis sources is shown in Figure 94, including portable electric, pneumatic, and electro-mechanical units. Heavy-duty hydraulic units for generating deep (60 m) waves are also available that are mounted to the truck belly (Figure 95). 74 FIGURE 91 CPT modifications: (left) multi-friction sleeve penetrometer, (center) vibro-piezocone, (right) T-bar. FIGURE 92 CPT sampling devices for necessary retrieval of soil samples. FIGURE 93 Vision penetrometer system for real-time videocam soil viewing (after Hryciw and Shin 2004).
75 FIGURE 94 Autoseis units for surface shear wave generation during seismic cone testing. FIGURE 95 Rig with mounted hydraulic autoseis unit for deep downhole testing.
