ASCE Metropolitan Section - Deep Soil Mixing Methods and Applications

Deep Soil Mixing Methods and Applications

Left to right: Dr. David Yang, P.E. of Raito, Inc.,
Hiren Shah, P.E., Chair of the Geotechnical Group,
and Dr. Allen Sehn, P.E. of Hayward Baker, Inc.

The Geotechnical Group began the New Year with a lecture on January 11, 2007 that included a pair of presentations by Dr. Allen Sehn and Dr. David Yang, distinguished engineers with experience applying deep soil mixing methods to a variety of geotechnical design challenges. Dr. Sehn kicked off the lecture with a general overview of soil mixing methods, particularly emphasizing the use of single axis mixing. Deep soil mixing involves mechanically incorporating a cementitious binder into soft in-situ soils for the purpose of improving the strength and stability of the soil. The result creates soil-crete columns which are typically three feet in diameter, but which can be as large as seven feet across. These columns are constructed one after another to create configurations within the ground such as walls or grid patterns. The primary factors which most affect the overall performance of mixed soils are the initial soil conditions, the mixing energy applied to the soil by the cutting tool and the content of binder material added during mixing.

Soil mixing involves either the addition of dry cementitious binder or a premixed grout made from water and cement. These alternatives each offer unique benefits and should be individually evaluated depending upon the specific site conditions. For example, in sites with soils having naturally high water contents, typically in excess of 40 percent, it is often possible to use the dry mixing method. This method involves pneumatically blowing dry cement binder into the soil from the cutting tool without requiring the use of additional water. A benefit of this method it that it produces very little spoils compared to the wet mixing method where as much as one third of the volume of the soil column can be displaced as the slurry grout is added to the soil. While soil mixing can provide increased strength and stiffness, a challenge exists due to the inevitable variability in physical properties within the soil-crete created during the mixing process. To minimize and quantify this uncertainty it is necessary to apply QA/QC procedures during construction which must include appropriate laboratory or in-situ strength testing.

Dr. Sehn shared several examples of projects where these methods have been employed including in the repair of levees in Louisiana, highway construction in the Florida Keys and to create foundations for industrial storage structures. Dr. Yang's presentation emphasized the use of multiple shaft deep mixing method (DMM), a variation of the previously described methods involving special equipment capable of simultaneously mixing with as many as six interlocking cutting tools working together to improve a larger area of ground. This method is particularly well suited for large projects which benefit from the added speed and control offered by mixing multiple soil shafts at the same time. In his presentation, Dr. Yang described several recent projects which used DMM in applications including the stabilization of liquefiable soils at depths of up to 34 m and for the creation of a cutoff wall to allow the dewatering of a cast basin used in the construction of large marine structures.

Last Updated ( Saturday, 09 February 2008 )