Efficiency Evaluation of Deep Soil Mixing Method in Stiff Clayey Soils: A Comprehensive Field Study

The quality of deep soil mixing (DSM) columns in fine-grained soils remains challenging and needs to be addressed unequivocally to solve the existing practical problems. Breakthroughs can be achieved through field investigations; hence in the present study, the results of a field study on 16 DSM elements with various mix designs in cohesive clayey soils are presented. The rig data, continuous full-depth coring, and unconfined compressive strength (UCS) tests were used to evaluate the strength and uniformity of DSM columns. The results shed light on the fact that the occurrence of entrained mixing phenomena, an issue in which cohesive soils stick the blades, could be estimated using rig data, which might be solved by implementing a water predrilling phase. Furthermore, by following the fluctuations in the pull-down, rotary, and hoist pressures of the drilling rig data, the uniformity of DSM elements could be assessed indirectly. Based on the finding, it is noticeable that a conspicuous discrepancy has been observed between the Federal Highway Administration (FHWA) equation for predicting the UCS of DSM columns and the captured results. Furthermore, the correlations between elastic modulus and UCS of DSM elements were about the half of values by FHWA. The implementation of offshore mega projects, such as refinery sites, can pose significant challenges, particularly when dealing with cohesive clayey soils with inadequate strength. In such circumstances, conducting deep soil mixing as a soil improvement method can emerge as a viable solution. The present study illustrates the detailed process of selecting the best mix design using the results of a comprehensive field validation program in Hormozgan. Drilling in stiff clayey soils is highly prone to sticking soils to the drilling auger, which hinders the proper functions of auger blades. The problem can be remarkably controlled by adding a predrilling phase to the mix design. Moreover, continuous monitoring of the critical drilling machine parameters, such as induced pressures, can shed light on the quality of deep soil mixing columns, without conducting extra field tests that play a key role in the project costs. The findings of the study can serve as a benchmark for engineers, contractors, and clients who are involved in soil improvement projects that utilize the deep soil mixing method.