Cone Penetrating Testing (2007)

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21 In this section, field testing procedures for CPT are reviewed, including calibration, assembly, filter element preparation, baseline readings, pushing, and withdrawal, as well as special testing practices. Procedures for calibrating, maintaining, and preparing the penetrometer and field advancement of CPT are well established through ASTM D 5778, Lunne et al. (1997), International Reference Test Proceedings (1999), and other guidelines. In the retraction of the cone penetrometer and com- pletion of the sounding, however, procedures are quite differ- ent and vary across the United States and Canada, depending on hole closure requirements established by the state or province. In many cases, the closure criteria depend on the regional groundwater regime and aquifer characteristics. CALIBRATION AND MAINTENANCE OF PENETROMETER The penetrometer requires calibration and maintenance on a regular basis; the frequency of which depends on the amount of use and care taken during storage between soundings. For most CPT operators, it appears that the penetrometers and/or field computers are returned to their respective manufactur- ers to confirm the equipment is within calibration and toler- ances. However, calibrations can be conducted in-house to check for load cell compliance using a compression machine. A sealed and pressurized triaxial apparatus can be used to check for pressure transducer calibrations, as well as the net area ratio (an). Full details concerning the calibration of cone and piezocone penetrometers are given elsewhere (e.g., Mulabdic´ et al. 1990; Chen and Mayne 1994; Lunne et al. 1997). The tip and sleeve should be replaced if damaged or if excessively worn. For a typical CPT rate of 60 m/day, used 4 days/week, an annual production of 12 000 m/year would likely require tips and sleeves being replaced once to twice per annum. The rate will depend on soils tested, as sands are considerably more abrasive than clays. FILTER ELEMENTS The filter elements used for piezocone testing are usually constructed of porous plastic, ceramic, or sintered metal. The plastic versions are common because they are disposable and can be replaced after each sounding to avoid any possible clogging problems particularly those associated with plastic clays. For face elements, a ceramic filter is preferred because it offers better rigidity and is less prone to abrasion when compared with plastic filters. The protocol for environmen- tal soundings recommends that sintered stainless steel filters be used, because polypropylene types are from petroleum- based manufacturer and may cross contaminate readings. Sintered elements are not to be used for face filters however because of smearing problems. The sintered metal and ceramic filters are reusable and can be cleaned using an ultra- sonics bath after each sounding. Saturation of the filter elements should be accomplished using a glycerine bath under vacuum for a period of 24 h. An alternative would be the use of silicone oil as the saturation fluid. It is also possible to use water or a 50–50 mix of glyc- erine and water; however, those fluids require much more care during cone assemblage. It is normal practice to presat- urate 10 to 15 elements overnight for use on the next day’s project. The DOT survey indicated that 39% use glycerine, 18% silcone oil, 18% water, and 7% a half–half mix of glyc- erine and water (Note: 18% responded not applicable). In the field, the filter elements must be installed so that a continuity of fluid is maintained from the filter face through the ports in the penetrometer and cavity housing the pressure transducer. These ports and cavities must also be fluid-filled at all times. This is best accomplished using a penetrometer having a male plug in the tip section to promote positive fluid displacement when the tip is screwed onto the chassis. The fluid should be 100% glycerine (or silicone oil) that is easily applied using a plastic syringe. Otherwise, if a female plug is provided on the tip unit, the penetrometer must be carefully assembled while submerged in the saturating fluid, usually accomplished with a special cylindrical chamber designed for such purposes. Considerably more effort is expended with this procedure than the aforementioned approach with a positive displacement plug on the tip. Once assembled, it is common practice to tightly place a prophylactic containing saturation fluid over the front end of the penetrometer. Several rubber bands are used to secure the rubber covering and help maintain the saturated condition. During the initial push into the ground this light rubber mem- brane will rupture automatically. In new developments, in lieu of a filter element and satu- ration procedure, it is possible to use a very thin (0.3 mm) CHAPTER FOUR TESTING PROCEDURES AND SOUNDING CLOSURE

grease-filled slot to record porewater pressures (Elmgren 1995; Larsson 1995). This avoids problems associated with vacuum presaturation of elements, assembly difficulties in the field, and desaturation of elements in the unsaturated vadose zone, however, at the expense of a more sluggish transducer response and less detailing in the um profiling. BASELINE READINGS Before each sounding, electronic baselines or “zero readings” of the various channels of the penetrometer are recorded. It is also recommended that a set of baseline readings be secured after the sounding has been completed and the penetrometer withdrawn to the surface. These baselines should be recorded in a field log booklet and checked periodically to forewarn of any mechanical or electronic shifts in their values, as possible damage or calibration errors may occur. ADVANCING THE PENETROMETER The standard rate of push for CPT soundings is 20 mm/s, usu- ally applied in one-meter increments (standard cone rod length). With dedicated CPT rigs, the hydraulic system is automatically established to adjust the pressures accordingly to maintain this constant rate. Using a rotary drill rig, however, the driller must be attentive in manually adjusting pressures to seek a rate of approximately 20 mm/s (0.8 in./s). Therefore, in those cases, it would be desirable to measure time as well as depth so that the actual rate can be ascertained. TESTS AT INTERMITTENT DEPTHS At each one-meter rod break, there is an opportunity to con- duct intermittent testing before the next succession of push- ing as the next rod is added. Two common procedures include: (1) dissipation testing, and (2) downhole shear wave velocity measurements. Porewater Dissipation Tests Dissipation testing involves the monitoring of porewater pressures as they decay with time. The installation of a full- displacement device such as a cone penetrometer results in the generation of excess porewater pressures (Δu) locally around the axis of perturbation. In clean sands, the Δu will dissipate almost immediately because of the high permeabil- ity of sands, whereas in clays and silts of low permeability the measured Δu will require a considerable time to equili- brate. Given sufficient time in all soils, the penetrometer porewater channel will eventually record the ambient hydro- static condition corresponding to u0. Thus, the measured porewater pressures (um) are a combination of transient and hydrostatic pressures, such that: um  Δu  u0 (2) 22 During the temporary stop for a rod addition at one-meter breaks, the rate at which Δu decays with time can be moni- tored and used to interpret the coefficient of consolidation and hydraulic conductivity of the soil media. Dissipation readings are normally plotted on log scales; therefore, in clays with low permeability it becomes impractical to wait for full equilibrium that corresponds to Δu 0 and um  u0. A standard of practice is to record the time to achieve 50% dissipation, designated t50. Shear Wave Testing A convenient means to measure the profile of shear wave velocity (Vs) with depth is through the seismic cone penetra- tion test (SCPT). At the one-meter rod breaks, a surface shear wave is generated using a horizontal plank or autoseis unit. The shear wave arrival time can be recorded at the test ele- vation by incorporating one or more geophones within the penetrometer. The simplest and most common is the use of a single geophone that provides a pseudo-interval downhole Vs (Campanella et al. 1986), as depicted in Figure 20. This approach is sufficient in accuracy as long as the geophone axis is kept parallel to the source alignment (no rotation of rods or cone) and a repeatable shear wave source is generated at each successive one-meter interval. A more reliable Vs is achieved by true-interval downhole testing; however, this requires two or more geophones at two elevations in the penetrometer [usually 0.5 or 1.0 m vertically apart (1.5 to 3.0 ft)]. Provision of a biaxial arrangement of two geophones at each elevation allows correction for possi- ble cone rod rotation, because the resultant wave can be used (Rv2 x2 y2). For downhole testing, incorporation of a tri- axial geophone with vertical component offers no benefit, because shear waves only have movement in their direction of motion and direction of polarization (only two of three Cartesian coordinate directions). The vertical component could be used in a crosshole test arrangement (e.g., Baldi et al. 1988). HOLE CLOSURE After the sounding is completed, a number of possible paths may be followed during or after extraction: • CPT hole is left open. • Hole is backfilled using native soils or pea gravel or sand. • Cavity is grouted during withdrawal using a special “loss tip” or retractable portal. • After withdrawal, hole is reentered using a separate grouting system. The need for grouting or sealing of holes is usually estab- lished by the state or province, or by local and specific con- ditions related to the particular project. For instance, for CPTs

23 FIGURE 20 Setup and procedure for pseudo-interval seismic cone penetration testing (SCPT). FIGURE 21 Hole closure methods: (left) reentry techniques; (right) retraction with expendable tip (Lutenegger and DeGroot 1995). advanced through asphalt pavements, sealing of the hole would be warranted to prevent water infiltration and/or long- term damage. Most often, the state or province will deem the need or requirement for hole closure by grouting or sealing in specific geologic settings where the groundwater aquifer(s) needs to be protected against vertical cross talk, contamina- tion, or water transmission. The requirement of borehole closure can significantly reduce CPT production rates. Hole sealing can be accomplished using either a bentonite slurry or a lean grout made from portland cement, gypsum, or a bentonite–cement mix. Pozzolan-based grouts can also be adequate, but they tend to setup more slowly (Lee et al. 1998). The grout or slurry sealants can be placed using sur- face pour methods, flexible or rigid tremie pipes, or special CPT systems that provide grouting during advancement or during withdrawal, as depicted in Figures 21 and 22. A full discussion of these systems and their advantages and disad- vantages is given by Lutenegger and DeGroot (1995) and Lutenegger et al. (1995). Results of the questionnaire on the subject of CPT hole closure indicated that 43% allow the hole to remain open, 20% backfill with soil, 18% grout during retraction, and 18% grout using a secondary deployment system (e.g., a GeoProbe).

24 FIGURE 22 Hole closure methods: (left) temporary casing; (right) grouting through ports in friction reducer (Lutenegger and DeGroot 1995).