Cored soil cement (photo courtesy of Malcolm Drilling) strength and/or permeability). When developing a mix design, one must carefully evaluate the specifications and determine what are the requirements for permeability, strength, etc. This is where experience matters — past knowledge of how those materials should behave when mixing. These factors will produce the basis for design mixes, and should be based, when possible, on representative soil samples and groundwater from the jobsite. Then, the mix design should be determined for the worst-case soil conditions. That is, if permeability is the key parameter, look at the sand and gravel layers more than the clay layers; however, if strength is the key parameter, look at the clay layers more than the sand and gravel layers. For the preliminary mix, it is essential to evaluate a number of trial batches in the lab to establish a range of mix variables that can be implemented in the field. Test Program After choosing preliminary mixes, it is important to evaluate those mix designs onsite in test sections in the actual field conditions where the production work will be performed. Each mix design is programmed into the batching system and the test section is constructed using the means and methods expected for the production work. On occasion, a full-scale test program is not possible due to a number of reasons, including time constraints or that the job is so small that it doesn’t warrant waiting for results of a test program. In the event of no test sections, it is justifiable to proceed using a more conservative mix 70 • DEEP FOUNDATIONS • JULY/AUG 2017 design, with the understanding that the mix design may need to change based on actual conditions encountered during mixing. However, a lot of information can be garnered from a formal test program, where the mix design can be optimized based on those test results. During the Soil Mixing Operation On site, the entire operation from the batch plant through the drilling system should be monitored. Secondary checks can be handled with conventional methods, such as a mud balance (a density measurement) to ensure the computer is accurate. Through the interaction between and i n n o v a t i o n o f c o n t r a c t o r s a n d manufacturers/suppliers, the industry has developed very accurate computer controls for soil mixing operations. In the early stages of the technology (e.g., at Jackson Lake Dam), it was sufficient to report how much cement was used during the day and how deep the columns were drilled; end of report. Now, computer controls can produce amazing amounts of data about the drilling parameters, grout mix, grout flow, etc. The installation is somewhat operator dependent, where the operator controls drilling speeds and rotation rates, but limits are established within the test program or by the calculation of mixing efficiency, which is based on the soil type and mix design. The onboard computer system can monitor drilling variables to maintain the mixing energy necessary to produce, at least, the minimum required parameters. The type of mixing energy varies among equipment, blade rotation numbers or energy index. However, the computer algorithms evaluate the rotation speed and the amount of energy being supplied into the column, which, combined with the drilling speed, ensure a good, thorough blend of the materials is being created. The key principle is to ensure there are sufficient revolutions of the blades to ensure thorough mixing. These drilling para- Core drilling operation (photo courtesy of Condon Johnson)
