meters, in addition to others, are con- tinuously monitored and recorded by the onboard computer system. Typically, the computer system will automatically record the penetration speed, rotation, verticality, slurry injection, depth, cable load (an indication of drilling difficulty), energy index (an indication of drilling difficulty) and an indication of a change in soil type. The system is all interlinked and it is all computer-controlled; although, the operator in the cab has certain parameters that the operator can monitor and adjust, such as rotation speed and drilling speed. There is an ongoing real-time data exchange between the drilling rig and the control room, where the flow of grout can be adjusted by the control room. Each of the individual components and their interactions throughout the operation create a detailed trail of how an installation was performed, which is captured by the computer system. Therefore, the instal- lation operation can be verified to ensure the mixing parameters were followed, and this facilitates maintaining consistency throughout the project. Verification and Evaluation After installation, the verification process begins. The type of tests and the extent of these tests, which correlate back to the level of confidence in the installation process, can substantially affect project costs and time. Wet sampling during the production process, coring after the fact, compression load testing or permeability testing all provide a means to evaluate the possibility of when things did not go perfectly. As a secondary or a double check of the sampling results or the coring, other tests such as video logging and in-situ permeability have been also incorporated into a project. Wet samples are a good indicator of potential outcome, as they can be retrieved immediately following installation. However, a wet sample is remolded three, four or even five times before it ever actually makes it into a cylinder; so, the test results could be off considerably from those in actual in-situ conditions. Wet samples are good for testing permeability, as they are molded and do not have grooving or fracturing that is associated with coring. Coring is a common requirement to evaluate the efficiency of the soil mixing, and it provides a visual means to assess how thoroughly the mixing was accomplished. Yet, coring has its difficulties, especially with obtaining intact cores. One of the biggest difficulties to overcome is the quality of the recovered cores from a soil cement matrix. Since most soil mixing applications target strengths less than about 200 psi (1,380 kPa), coring this low strength material can be problematic, which is especially true in material with course in-situ material, such as gravels or debris from random fills. These larger particles and debris can come loose during the coring process and can destroy the soil cement matrix. Engineering Judgment Verification testing can be as much as 20% to 25% of the total cost of the project, which may seem excessive considering all of the computer-controlled monitoring used to show that the installations were performed consistently and repeatedly. This is where the engineer and owner need to apply engineering judgment to the process. For instance, video logging was an attempt to add a secondary check to determine whether it was a bad core or an improper mixing job, and then to use visual observation to make an engineering judgment. There have been other ideas proposed that facilitate the use and justification of engineering judgment for acceptance of the Damaged core from an inclusion (photo courtesy of Condon Johnson) Video logging was introduced to offset the predicament of disturbance to the soil cement matrix evidenced in the recovered core. After coring, it is possible to take a video of the core hole and to determine if the results (core and video) indicate a simply bad core or a poor mixing job. Video logging was meant as a check to verify mixing for problematic cores; unfortunately, it has turned into an added verification test that is now used to evaluate the mixing regardless of the quality of the core, which ultimately leads to another set of tests required. work, including the use of a 10-point moving average to evaluate the significance of a few poor test results and their impact on the overall project. As an example, during the evaluation of stability along a potential failure plane, it is most critical to use the shear strength in the area around the failure plane than it is to include the test results from an upper fill zone that is well above this failure plane. Furthermore, it has been well established that soil cement increases in strength over very long periods, and this understanding can be used to alleviate concerns over borderline DEEP FOUNDATIONS • JULY/AUG 2017 • 71
