Ground Improvement

Project Team Members

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Chadi El Mohtar, Ph.D.

Assistant Professor

Geotechnical Engineering

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Ground improvement, particularly, permeation grouting, can be a very cost-effective solution for significant infrastructure challenges. The current research on ground improvement focuses on three fronts:

 

A) Improving liquefaction resistance

B) Improving hydraulic conductivity performance

C) Containment of contaminated sediments (discussed under NonAqueous Pore Fluids)

 

A) Improving liquefaction resistance

Liquefaction includes all phenomena involving excessive deformations or movements as a result of transient or repeated disturbance of saturated cohesionless soils. In such soils, particularly fine to medium sands or even finer non-plastic soils, undrained cyclic loading, which can occur during an earthquake, may produce excess pore pressures which reduce the effective stresses; liquefaction occurs when the effective stresses approach zero. Observations in the field after earthquakes indicate that the presence of fines may increase the liquefaction resistance of cohesionless soils and much work has been done in the past years in the laboratory to explore this effect further. Particularly, the presence of plastic fines resulted in more consistent increase in cyclic resistance as compared to non-plastic fines.

 

Many soil improvement methods have been introduced to counteract liquefaction threat, the easiest of which is soil compaction by different methods. However, soil compaction (especially dynamic compaction) can often be harmful to existing structures on or nearby liquefiable soil deposits. Thus, permeation grouting or "passive site remediation" has been proposed for stabilizing liquefiable soil deposits at sensitive or developed sites. The current research approach is to use permeation grouting to deliver bentonite to the sand pore to increase its liquefaction resistance. The first figure below shows some results of cyclic triaxial testing on sand-bentonite specimens prepared through dry mixing for "wished in place" bentonite. The results show a significant increase in the number of cycles to liquefaction with increasing bentonite content and /or aging time. The second figure shows the increase in Cyclic Resistance Ratio for dry-mixed specimens versus permeated specimens. The tests on the permeated specimens are still in the preliminary stages.

Text Box: Relevant Referred Publications

El Mohtar, C., Clarke, C., Bobet, A., Santagata, M., Drnevich, V., and Johnston, C.. "Cyclic Response of Sand with Thixotropic Pore Fluid" ASCE Geotechnical Earthquake Engineering and Soil Dynamics Conference, Geotechnical Special Publication Vol. 181, May 2008. [PDF]
El Mohtar, C., Santagata, M., Bobet, A., Drnevich, V., and Johnston, C. "Effect of Plastic Fines on the Small Strain Stiffness of Sand" IS-Atlanta: Fourth International Symposium, Deformation Characteristics of Geomaterials, v.1, September 2008, p. 245-251. [PDF]
H. Hwang, J. Yoon, D. Rugg and C.S. El Mohtar. "Hydraulic Conductivity of Bentonite Grouted Sand" Geofrontiers March 2011, Dallas, Tx. [PDF]
D. A. Rugg, J. Yoon, H. Hwang, and C. S. El Mohtar. "Undrained Shearing Properties of Sand Permeated with a Bentonite Suspension for Static Liquefaction Mitigation" Geofrontiers March 2011, Dallas, Tx. [PDF]
El Mohtar, C. S. and Rugg, D. "New Three-Way Split Mold Design and Experimental Procedure for Testing Soft, Grouted Soils," ASTM Geotechnical Testing Journal, Vol 34 Issue 6, pp. 365-371, November 2011. [PDF]
C. S. El Mohtar, A. Bobet, M.C. Santagata, V.P. Drnevich, C. Johnston "Liquefaction Mitigation using Bentonite Suspensions" Journal of Geotechnical and Geoenvironmental Engineering, Vol. 139, No. 8, August 1, 2013. ASCE, p.p. 1369-1380. [PDF]
El Mohtar, C. S., Drnevich, V. P., Santagata, M. C. and Bobet, A. "Pore pressure generation in Sand with Plastic Fines" Geotechnique. Volume 64, No. 2, pp. 108-117, 2014. [PDF]

 

Static liquefaction occurs in loose sand specimens when subjected to monotonic loading. As the shearing progress, the positive pore pressures approach the initial effective stresses and the shear resistance of the sand approaches zero. While only very loose sands reach complete static liquefaction, loose and medium density sands can reach an undrained instability state and a quasi-steady state before starting to dilate and regain strength. The ability to minimize the strength loss during these intermediate "weakness zone" can provide of vital importance for stability and safety of substructures. The first figure below shows the stress path fro clean sand and permeated specimens at different initial effective stresses. The grouted specimen show a more dilative behavior and less tendency towards static liquefaction. The second figure show the calculated friction angles at the undrained instability state and at critical state. Both sands show similar friction angles indicating that the grouting improved static liquefaction resistance without compromising shear strength.

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El Mohtar et al., 2013

El Mohtar et al., 2013

Rugg et al., 2011

Rugg et al., 2011

B) Improving hydraulic conductivity performance

Bentonite grouting would be effective where seepage is the major issue or for secondary containment barrier systems by improving hydraulic performance of granular soils. Although the hydraulic conductivity of the bentonite grouted sands has not been studied extensively, previous research with Sand-Bentonite Mixtures (SBMs) showed a significant reduction in hydraulic conductivity of sands (approximately 3 to 7 orders of magnitude) with 3 to 30% bentonite contents in sands. However, the application of bentonite suspensions in permeation grouting has been limited due to low mobility of bentonite suspensions resulting in low penetration depth and little practical applications (especially for concentrated suspensions that needs to be utilized to achieve the required high bentonite contents similar to SBMs for a significant reduction in hydraulic conductivity). Current advances in Pore Fluid Engineering has allowed us to manipulate the rheology of bentonite suspension to allow for delivering high bentonite contents into the sand pores. The figure below shows an example in the reduction in hydraulic conductivity of permeated sands as a function of the delivered bentonite content.

Yoon, 2011