Closure to “Design and Construction Guidelines for Deep Soil Mixing to Stabilize Expansive Soils” by Raja S. Madhyannapu and Anand J. Puppala

Abstract

The writers would like to express their appreciation to the discusser for the comments provided on the original paper. Following are the responses to the comments by the discusser. As noted by the discusser in Item 1a of the discussion, predrilling of deep soil mixing (DSM) locations prior to soil mixing was necessary and achieved using a flight auger of diameter similar to the one used for the construction of DSM columns [0.6-m (2-ft) diameter]. Due to the stiff to very stiff nature of unsaturated in situ soils, spoil produced was minimal during the predrilling process. During retrieval of flight auger, few of the loosened soil lumps were reintroduced into the predrilled DSM hole prior to soil mixing step. The extra material shown in Fig. 12 in the original paper was produced during actual soil mixing process and it shows the low amounts of spoil produced during the field mixing operations. The primary objective of this research was to understand the behavior of a treated composite expansive soil test section as a whole system subjected to seasonal climatic changes. Though the writers did not specifically study the zone of influence of soil mixing beyond the DSM columns, the following explanation on the potential zone of influence of DSM columns in the expansive soils is offered. Previous studies based on the laboratory tests on soft soils noted that the zone of influence was close to four times the diameter of the DSM columns. The present expansive soils, on the other hand, are stiff to very stiff in nature and also highly impervious. Hence, the writers opine that the zone of influence due to soil mixing in expansive soils will be less than the ones observed in soft soils. The discusser mentioned the potential heave-related protrusion from isolated rigid inclusions into the soil mass. The writers do not agree with this assessment and offer the following reasons explaining that the heave movements less than the allowable magnitude are anticipated within the treated composite section. Differential soil movements are more prevalent in soft soils than in expansive soils. In present design methodology, the treated composite section is designed based on the treatment area ratio determined using an allowable heave of less than 25 mm for the composite section. Thus, the differential swelling anticipated within the treated area is expected to be less than 25 mm and potentially insignificant. The uplift resistance of DSM columns primarily depends on the depth of treatment, strength and stiffness characteristics of subsoils, and zone of influence beyond the diameter of the DSM column. Chemical modification is the primary method of treatment for expansive soils because alteration of physicochemical properties from chemical reactions result in reduced or insignificant swelling. Also, the placement of a geosynthetic layer over the treated area was to provide additional mechanical reinforcement to transfer any induced swell pressures from untreated soils to treated columns through the inverse of arching behavior. All these factors contribute to minimizing the potential heave across the treated composite area. The writers agree with the discusser in extending the anchoring system into the pavement shoulder area. Further research studies are also recommended on deep soil mixing treatments of expansive soils, which would help in addressing issues such as zone of influence, load transfer mechanism within the composite section, and others.