<p><p><figure id='attachment_3398' style='max-width:1058px' class='caption alignnone'><img class="wp-image-3398 size-full" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Schematic representation of Deep soil mix plan of Fort Point Channel." width="1058" height="606" /><figcaption class='caption-text'> Deep soil mix plan of Fort Point Channel (Maswoswe 2001; With permission from ASCE).</figcaption></figure></p><p><div><h2>Project Summary/Scope:</h2>In the largest single application of deep soil mixing in the US to date, more than 420,000 cubic meters of Boston blue clay was stabilized at the I-90/I-93NB interchange as part of the Fort Point Channel (FPC) crossing. Construction through the FPC area involved deep open excavations, immersed tube tunnels, and jacked tunnels where thick organic layers and deep deposits of marine clay are located. In conventional DMM, grout is injected on both the downstroke and upstroke cycles. When this type of installation was attempted at FPC, the clay was not properly fluidized and the soil cement set too quickly. The procedure adopted at FPC injected only water during the downstroke (increasing the water content to 70%) and grout on the upstroke.</p><p>Subsurface Conditions: Boston blue clay varying in thickness from 10.7 to 33.5 meters; at some locations, the clay was so soft that penetration of some borings occurred solely under the weight of either the drilling rods or drilling rods and hammer.</p><p>Design unconfined compressive strength of DMM columns was 2.0 mPa. Stability was analyzed using 1) free-body static analysis for mass sliding, overturning, and bearing, 2) the finite element program GTSTRUDL, and 3) the finite difference program FLAC. The DMM columns were used as a buttress and foundation for tunnels.</p><p>Two M250 rigs were used to install the majority of the soil cement at the site both on land and from a barge. The cutting and mixing assembly consisted of three auger cutting heads, with five levels of paddles located within a 4.5-meter distance above the cutting heads. The 608 rig was used where depth limitations, site constraints, and obstructions prevented effective deployment of the M250 rig. The mixing assembly of the 608 rig consisted of discontinuous, counter-rotating augers and alternating paddles. The connections between the DMM panels and underlying till were developed by penetrating 0.3 meters into the till. Using a three-auger rig, the DMM panels were installed using both a full overlap and a partial overlap pattern.<br><h2>Complementary Technologies Used:</h2><strong> </strong>Jet Grouting</p><p><figure id='attachment_3400' style='max-width:692px' class='caption aligncenter'><img class="wp-image-3400 size-full" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Schematic diagrams of the installation and construction sequence for DMM walls in the Fort Point Channel portion of the Boston Central Artery Project." width="692" height="474" /><figcaption class='caption-text'> DMM wall installation (O’Rourke and McGinn 2006; With permission from ASCE).</figcaption></figure><h2>Performance Monitoring:</h2>Core drilling and a soil-cement hardness identification procedure using the scratch test derived from soft rock identification procedures was employed in the field to quickly categorize the soil-cement core for strength and consistency or degree of heterogeneity. Unconfined compressive strength testing was completed on wet grab samples. Continuous cores were taken at 40 locations and unconfined compressive strength tests were performed. No more than 2% horizontal deviation of deep mixed wall panels was allowed.<br><h2>Project Technical Papers:</h2>Lambrechts, J., Roy P., and Wishart, E. (1998) "Design Conditions and Analysis Methods for Soil-Cement in Fort Point Channel." <em>Design and Construction of Earth Retaining Structures, Proceedings of Sessions of Geo-Congress '98</em>, ASCE Geotechnical Special Publication No. 83, pp. 153-74. <a href="https://cedb.asce.org/CEDBsearch/record.jsp?dockey=0113641">https://ced…, J.R. and Nagel, S. (2003). “Coring soil-cement installed by deep mixing at Boston’s CA/T project.” ASCE, <em>Grouting 2003</em>, pp. 670-680. https://ascelibrary.org/doi/abs/10.1061/40663%282003%2938?src=recsys</p…, J.R., and Roy, P. (1997), “Deep soil-cement mixing for tunnel support at Boston’s I-93NB/I-90 interchange.” <em>Geotechnical Special Publication No. 69: Proceedings of the sessions sponsored by the Committee on Soil Improvement and Geosynthetics of the Geo-Institute of the American Society of Civil Engineers in conjunction with Geo-Logan ’97</em>. Logan, Utah, pp. 579-603. <a href="https://cedb.asce.org/CEDBsearch/record.jsp?dockey=0106183">https://ced…, J. (2001). "QA/QC for CA/T Deep Soil-Cement." <em>Foundations and Ground Improvement, Proceedings of Specialty Conference</em>, ASCE GSP No. 113, pp. 610. <a href="http://cedb.asce.org/cgi/WWWdisplay.cgi?126026">http://cedb.asce.org/cg…’Rourke, T.D. and McGinn, A.J. (2006). “Lessons learned for ground movements and soil stabilization from the Boston Central Artery.” <em>Journal of Geotechnical and Geoenvironmental Engineering</em>. 132(8), pp. 966-989. <a href="http://ascelibrary.org/doi/abs/10.1061/%28ASCE%291090-0241%282006%29132… Case History Prepared:</h2>November 2012, updated December 2014</p><p>&nbsp;</p><p></div></p></p>