Soil Stabilization With Grout

By Jeff Griffin, Senior Editor | August 2011, Vol. 66 No. 8

Stabilization of soil to ensure the ground will support foundations and the weight of buildings, highways, bridges and other improvements is a critical element in construction projects.

Soil or ground stabilization also comes into play for underground utility construction projects. The two basic stabilization methods -- mechanic and additive – are applicable to utility as well as civil and commercial projects. The type of stabilization process used depends on the level of stabilization required and conditions on the project site.

Ground stabilization in excavation and tunneling often is called “support of excavation,” said Guy Dickes, president, Constellation Group LLC.

“Because ground cannot stand up by itself and tends to collapse, trench boxes are used in cut and cover projects and sheet piling is used for shafts and excavations,” Dickes said. “Grouting is used to control water and stiffen the surrounding soils.”

The Constellation Group recently provided ground stabilization consulting services to Northeast Remsco for a project to replace a sewer outfall for the Somerset Raritan Valley (New Jersey) Sewer Authority.

A railroad line crosses the route of the pipeline, requiring a 201-foot-long, 108-inch diameter liner-plate tunnel under three railroad tracks. Cut and cover was not viable as the railroad tracks were in daily operation. No interference with railroad operations would be allowed.

Northeast Remsco performed the tunnel operation. Once the liner plate tunnel was complete and grouted, NRC then poured a pad to slipline the 72-inch PCCP carrier pipe inside the newly constructed 108-inch LP tunnel. The annular space was then pumped/filled with a low density cellular concrete fill by NRC. The invert of 108-inch diameter steel pipe was set approximately 13-25 feet below the surface. The top of the pipe was approximately four to 16 feet below the railroad tracks and ground.

The 25-foot-deep launch pit was set up on the north side of the railroad tracks with the receiving pit at the south side.

Soil stabilization
To protect the tracks on the surface, soil stabilization was required. A previous attempt by another contractor had failed, and Dickes said the railroad was extremely concerned that the installation of the tunnel not causes settling of soil which would have halted vital rail traffic.

The contract permitted a maximum of 0.5 settlement of the railroad tracks, said Dickes. Anything more would cause the project to be shut down.

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