Consolidometer Test Results The normalized data set results from the bench test and all four research sites (Sites A-D) reveal an apparent increase in unconsolidated strength (UCS) with confinement. The soil type (defined by ASTM International’s Unified Soil Classifi- cation System) also appears to have an influence in the magnitude of the strength increase with confinement. Sites with higher percentages of fine-grain soils, such as Sites C and D, reveal significantly larger strength gains with in situ confinement than sites that are predominately granular soils (Sites A and B). well to the individual data sets, with R values greater than 0.6. The coefficient of variability (C ) between the individual data v sets also showed good agreement to what is typically demonstrated for cement-treated soil. The C values for the data sets ranged v between 0.43 and 0.74 and averaged 0.60, which is consistent with a C value of 0.56 v presented in the literature for cement- treated soil (FHWA 2013). Cross-sample Comparisons The DMM core strength data ( Δq %u ) collected from four sites (Sites 1-4) were normalized using the method described by the percent change in strength increase equation described previously. The data from the bench test and the data collected at the research test sites (Sites A-D) were then compared to the previously obtained core samples. Analyzing the entirety of the research data sets, to date, shows how the strength increases compare to each other when exposed to confinement. As with bench tests alone, an increase in strength with confinement is noted. A differentiation in the amount of increase in strength with confinement relative to the soil type is also apparent, as fine-grain soils appear to develop a greater strength increase with confinement relative to purely granular soils. Soils that contain a mixture of fine and coarse-grain particles appear to have strength increases greater than pure granular soils, but less than predominately fine-grain soils. Linear regression curves fit reasonably 2 Approximate boundaries of the zones separating the behavior in the different soil types have been included on the plot. These boundaries are presented solely for the purpose of illustrating the general behavior of the different soil types. Preliminary Conclusions Thanks to considerable interest within the professional community on the effect of curing stresses on the properties of soil- cement mixes, a substantial amount of data was collected. Over 10 industry col- laborators contributed data, equipment, field support, laboratory and/or consulting services towards this research. Our finding that the unconsolidated strength (UCS) of soil-mix samples is influenced by confinement results from data obtained at four different active sites across North America, from preparation of a field consolidometer and testing during a control bench study using sand, and from core sampling data collected from four different sites. The research data indicate that the UCS generally increases when a soil-mix material is subjected to confinement. The effect of confining pressure during curing on the UCS of the specimens can be Organizations that offered technical, specialized equipment or collaborative contributions include: ConeTec Fugro Deep Foundations Institute, Soil Mixing Committee GEI Consultants Geo-Solutions GEOKON Geotechnics Keller Foundations Kiewit Foundations Company Malcolm Drilling Company Nicholson Construction Company Remedial Construction Services South Jersey Gas Industries Virginia Polytechnic Institute and State University (Virginia Tech) significant. The research data also indicate that the type of soil has an impact on the magnitude of increase in the strength of the mix. Finer-grained soils appear to exhibit a larger relative strength increase with confining stress as compared to pre- dominately granular soils. The results of this research are encour- aging, since sites with fine-grain soils typically exhibit lower unconfined com- pressive strengths and are perceived to be more difficult to improve. Given our understanding of the soil types (mixture of fine sands to clay) and the natural variability at these research sites, the data appear to support this theory reasonably well. The results of the testing also indicate that wet grab samples tested without confinement may result in significant underestimation of the actual in situ UCS. This has the potential to have a tremendous impact on the material cost, schedule and level of effort needed to achieve the desired UCS results. The initial research report is available at www.dfi.org/commhome.asp?SLMX. Acknowledgements The DFI Committee Project Fund provided financial support through the Soil Mixing Committee, and GEI Consultants provided matching support. Added support came from A.J. Metz, P.E., Roberto Nevarez, Asher Peltz, P.E., Brain Wilson, P.Eng., Nathan Malero, and Professor George Filz, P.E. George Onorato, P.E., is a senior engineer at GEI Consultants’ Denver office. He has over 25 years’ experience in consulting and design for specialty ground improvements projects across North America. Onorato is GEI’s director of research into the study of geomechanics of soil-mixing properties and provides construction engineering support to clients across North America. Giovanni Bonita, Ph.D., P.E., P.G., is senior vice president of GEI Consultants, and covers projects nationwide from the Washington, D.C., office. He has over 23 years of experience in geotechnical and geostructural engineering relating to deep founda- tions, cutoff walls, permanent and temporary retaining systems, and ground improvement. DEEP FOUNDATIONS • JULY/AUG 2020 • 101
