cone shaped piece of metal into the ground and recording the resistance (i.e., the so- called mechanical CPT). In the 1970s, the electronic cones became the standard, allowing the user to measure and record the tip resistance, sleeve resistance and pore pressure continuously as the cone is pushed into the ground. The CPT method was standardized in Split spoon sampler The method is so simple that every combination of rod, rod diameter, impact setup, weight of the impact hammer and drop height can be used to obtain an N- value. Although the specifications (e.g., ASTM D1586, Standard Test Method for Standard Penetration Test (SPT) and Split- Barrel Sampling of Soils, and EuroCode 7 Part 2, EN 1997-2 Ground Investigation and Testing) that control how the SPT is performed try to “standardize” the test by clearly specifying the hammer weight and drop height, the test results can still vary widely. Section 4.1 of the recently revised ASTM D1586 standard provides a good overview of this: N-values are affected by many variables allowed in the design and execution of the test. Investigations of energy transmission in SPT testing began in the 1970s and showed that differing drop hammer systems provide different energies to the sampler at depth. There are so many different hammer designs that it is important to obtain the energy transfer ratio (ETR) for the hammer system being (...) Since the N-value is inversely proportional to the energy delivered, resulting N-values from different systems are far from standard. It is now common practice to correct N-values to an energy level of 60 % of total (PE), or N60 (...). If ETR of the hammer/ anvil/rod system is known, the hammer PE can still vary after calibration, thus it is essential that hammer drop heights/rates be monitored to confirm consistent performance. (…) Using previous ETR data for a hammer system does not assure that it will perform the same on the current project. If onsite ETR is not obtained, be sure to check hammer drop height/rates to assure the hammer is operating the same as when previously checked. 98 • DEEP FOUNDATIONS • MAY/JUNE 2019 But even when the ETR is recorded on site for every blow, translating N-values into actual soil behavior is still far from straightforward, in part because other operational variables, such as drilling methods, are used and result in various disturbances. The Alternative: Characterization with CPT Cone penetration testing (CPT) is the second most common in-si tu soi l investigation test, following only the SPT. This test method originated in the Netherlands for soft ground engineering in the 1930s, and originally involved pushing a the early 1980s. A cone with narrowly defined dimensions is pushed into ground at specified and constant speed (0.8 in/sec or 2 cm/sec), thereby separating the influence of the test operator (i.e., independent) from the procedure. Measurements are taken in the cone itself and these measurements are then transferred to the field computer, allowing the operator to print the soil investigation report as soon as the cone is retrieved. In addition, provided internet access is available, the operator can even send the report to the client from the test site. Finally, it is important to recognize that with the increased pushing capacity this method is no longer just suitable for soft soils. As matter of fact, using a cone with a 2.33 sq in (15 sq cm) cone base and applying 20 tons of pushing force, both of which are common practice currently, the CPT can be performed in the same stiff and hard soils where an SPT is performed. CPT rig
