<p><p><div></p><p><figure id='attachment_3360' style='max-width:344px' class='caption aligncenter'><img class="wp-image-3360 size-full" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Photograph showing a drilling operation to place grout pipes." width="344" height="446" /><figcaption class='caption-text'> Drilling to place grout pipes example (Hayward Baker).</figcaption></figure></p><p></div><div><h2>Project Summary/Scope:</h2>Construction of a new subway tunnel under an existing structure including a 90-year-old brick-lined railroad tunnel. The new tunnel was to be driven 10 to15 feet below an existing railway tunnel. Underpinning with sodium silicate grout (GELOC-3) was used.</p><p>Subsurface Conditions: The new tunnel lower portion was in stiff-hard cohesive decomposed rock, the upper few feet are in Cretaceous granular materials, with 13% passing #200 sieve and an estimated permeability of 3x10^-4 cm/sec.</p><p>The predicted settlement of the existing tunnel was estimated to be a maximum of 3.5 inches. The settlement at the ground surface was estimated to be 1.5 inches.</p><p>170 3-inch diameter holes were drilled from street level around the existing tunnel to a depth of 2 feet into the cohesive material. Additional 2-inch diameter holes were drilled in a fan pattern from the bottom of the existing tunnel. The contractor was allowed variance on grout set time and concentration to allow for process optimization. 354,000 gallons of sodium silicate grout were injected at 100 to 80 psi. Initial pumping pressure of 100 psi induced heave in the existing tunnel floor up to 2 inches, pressure was reduced to 80 psi to compensate for the problem.<br><h2>Performance Monitoring:</h2><strong> </strong>Lab testing was conducted to determine material permeability and target grout concentration based on unconfined compressive strength. Two field injection tests were performed prior to construction. Pump testing with water to evaluate permeability was conducted followed by a grouting test. The first boring did not perform as desired due to installation issues. The second test was successful with grout take of 18 to 25 gpm at 10 to 40 psi with 32% and 40% sodium silicate. Boring after the test showed SPT N-values of &gt;100 and odor indicating grout presence.</p><p>Street settlement was measured to be 0.125 inches, 8% of predicted value. Instrumentation in the existing tunnel did not work properly, thus settlement was not monitored in the existing tunnel. No structural distress was noticed in the existing tunnel. Grouting continued until take was reduced to 0.5 gpm. All tunnels functioned normally.<br><h2>Cost Information:</h2>Grouting was a lower cost alternative than structural underpinning or freezing.<br><h2>Project Technical Paper:</h2>Munfakh, G.A. (1991). “Deep chemical injection for protection of an old tunnel.” ASTM Special Technical Publication 1089, ASTM International, West Conshohocken, PA, pp. 266-278.<br><h2>Date Case History Prepared:</h2>November 2012</p><p></div></p></p>