Preferred Design Procedure
There is no FHWA published design procedure for mass mixing, which includes shallow soil mixing (SSM) and mass stabilization (MS). However, there are several publications that provide design guidelines or outlines. The design for SSM and MS is primarily composed of: (1) determining the extent and properties of treatment necessary to achieve performance objectives of controlling settlement and stability and (2) mix design to determine the mixture proportions necessary to achieve the required mixture properties. The mix design varies depending on the soil type, binder type, and application. The objective of SSM or MS design is to improve strength, settlement, or permeability characteristics of the in situ soil. Mix design is often determined in the laboratory but may be refined in field test mixing. Target performance values for strength, settlement, etc. should be established by traditional geotechnical means. The procedures summarized in this document have been used in a number of applications:
- Roadway/railway embankment foundation soil stabilization
- Stabilization of foundation soil below storage tanks and structures
- Temporary excavation support
- Contaminant fixation
- Dredged fill stabilization
- Peat and organic soil stabilization
Table 1 lists typical inputs and outputs for SSM and MS design.
Note that detailed FHWA guidance for deep mixing design is available in the Bruce, M.E.C., Berg, R.R., Collin, J.G., Filz, G.M., Terashi, M. and Yang, D.S. (2013). FHWA Design Manual: Deep Mixing for Embankment and Foundation Support, (Bruce et al., 2013). SSM and MS, like deep mixing, produce a product with variable properties. Some guidance for addressing variability is also in the FHWA deep mixing design manual.
Table 1. Typical inputs and outputs for design and analysis procedures.
Performance Criteria/Indicators
Shear strength/unconfined compressive strength
Bearing capacity
Factor of safety against instability
Settlement
Permeability
Volume change
Density
Subsurface Conditions
Delineation of Stratigraphy
Soil Classification
Obstructions
Groundwater elevations
Undrained Shear Strength, su
Compression index/compression ratio
Organic Content
Soil pH
Chemical Composition of Soils/Groundwater
Water Content
Effective Internal friction angle, ϕ'
Secondary Compression Index/Secondary Compression Ratio
Young's Modulus
Preconsolidation Stress/Maximum Past Pressure
Recompression index/Recompression ratio
Void Ratio
Loading Conditions
Construction loading
Embankment loading
Traffic loading
Structural loading
Seismic loading
Material Characteristics
Binder type
Shear strength of stabilized soil
Compressibility of stabilized soil
Homogeneity of binder and stabilized mix
Calcium content
pH of stabilized soil
Permeability of stabilized soil
Construction Techniques
Binder quantity/rate of application
Mixing tool rotation rate
Number/configuration of mixing blades
Mixing time
Geometry
Column size
Column depth
Column spacing
Treatment area/volume
Block size
Treatment depth
In the United States, design-bid-build and design-build are the primary contracting vehicles. While design-build contracts (single party is responsible for the design and construction tasks) are becoming more popular, design-bid-build contracting (separate parties are responsible for design [engineer] and construction [contractor]) remains the most popular vehicle. Under mass mixing method design-bid-build contracts, the engineer is responsible for specifying the required mixture properties and the contractor is responsible for developing the mix design and procedures necessary to achieve the specified properties. The engineer and/or contractor may also perform pre-construction laboratory evaluations to determine suitable candidate binders and the effectiveness of each. The following summaries present, without distinction of party roles, design considerations for mass mixing methods.
References
Aldridge, C.W. and Naguib, A. (1992). “In situ mixing of dry and slurried reagents in soil and sludge using shallow soil mixing.” 85th Annual Meeting & Exhibition, Air and Waste Management Association, Kansas City. 14p.
ALLU. (2007). Mass Stabilisation Manual, ALLU Finland Oy, Orimatilla. 57p. http://www.allu.net/products/stabilisation-system (June 24, 2014).
Andersson, R., Carlsson, T., and Leppanen, M. (2001). “Hydraulic Cement Based Binder for Mass Stabilization of Organic Soils.” Proc., United Engineering Foundation / ASCE GeoInstitute Soft Ground Technology Conference, ASCE, Netherlands, pp. 158-169.
ASTM D2166 - 13 “Standard Test Method for Unconfined Compressive Strength of Cohesive Soil,” ASTM International, West Conshohocken, PA, 2013. 7p.
Broomhead, D., and Jasperse, B.H. (1992). “Shallow Soil Mixing – A Case History.” ASCE Geotechnical Division Specialty Conference Grouting, Ground Improvement and Geosynthetics, ASCE, New Orleans. 12p.
Bruce, M.E.C., Berg, R.R., Collin, J.G., Filz, G.M., Terashi, M. and Yang, D.S. (2013). Federal Highway Administration Design Manual: Deep Mixing for Embankment and Foundation Support, FHWA-HRT-13-046, U.S. DOT, Federal Highway Administration, Washington, D.C., 244p. https://www.fhwa.dot.gov/engineering/geotech/pubs/nhi14007.pdf
EuroSoilStab (2002). Design Guide Soft Soil Stabilisation. EuroSoilStab, Development of design and construction methods to stabilise soft organic soil. European Community CT97-0351, BE 96-3177. Project report. EP60. Published by BRE Press (www.ihsbrepress.com). 94p.
Garbin, E., Mann, J.A., McIntosh, K.A., Dasai, K.R. (2011). “Mass Stabilization for Settlement Control of Shallow Foundations on Soft Organic Clayey Soils.” Geotechnical Special Publication 211, ASCE, pp. 758 – 767.
Hannigan, P.J., Goble, G.G., Likins, G.E. and Rausche, F. (2006). Design and Construction of Driven Pile Foundations, FHWA-NHI-05-042 (Vol I) and FHWA-NHI-05-043 (Vol II), Federal Highway Administration, Washington, D.C., 968p. (Vol I) and 486p. (Vol II).
Harris, P., Harvey, O., Puppala, A., Sebesta, S., Chikyala, S.R., and Saride, S. (2009). Mitigating the Effect of Organics in Stabilized Soils: Technical Report. FHWA/TX-09/0-5540-1. 136p.
Hodges, D.K., Filz, G.M., and Weatherby, D.E. (2008). “Laboratory mixing, curing and strength testing of soil-cement specimens applicable to the wet method of deep mixing.” Center for Geotechnical Practice and Research, Virginia Polytechnic Institute and State University. 132p.
Jasperse, B.H. (undated). In-Situ Stabilization Using Shallow Soil Mixing and Deep Soil Mixing. Geo-Con, Inc. 20p.
Lahtinen, P. and Niutanen, V. (undated). “Development of In-Situ Mass Stabilization Technique in Finland.” 6p.
Sleep, M., Duncan, J.M., Hickerson, H., and Ritter, K. (2009). “Geotechnical considerations for organic soils and peats.” Center for Geotechnical Practice and Research, Virginia Polytechnic Institute and State University. 24p.
Tielaitos. (1993). Deep Stabilization at Veittostensuo. Research Report (in Finnish). Tielaitoksen selvityksi~i 81/1993. TIEL 3200205. 87p.