Innovative Construction Techniques Used at North Shore Construction Project-Article

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The North Shore Connector project extended Pittsburgh’s light-rail system beneath the Allegheny River through twin tunnels. Nicholson Construction performed ground improvement and excavation support in challenging, permeable river-valley soils and a congested urban setting.<br /><br />The work used jet grouting, Cutter Soil Mixing (CSM), cement-bentonite walls, and concrete diaphragm walls. Jet grouting strengthened and reduced the permeability of soil around tunnel launch and receiving areas, and helped limit settlement near buildings and retaining walls. A field test program established jetting parameters. The project also used Cyljet, an electrical-resistivity method, to estimate jet-grout column diameters at depth.<br /><br />CSM formed temporary, overlapping soil-mix walls below the water table. Its cutter wheels mixed cementitious binder into the ground, creating stiff, watertight support while limiting spoil, noise, and vibration. Cement-bentonite walls provided support in low-headroom areas, while diaphragm walls served as both temporary excavation support and permanent walls for the new station.<br /><br />Real-time monitoring and material testing supported quality control across the systems. The methods generally met project needs, but coarse gravel and cobbles created difficulties: jet-grout strengths had to be increased substantially to permit successful core sampling, and cobbles reduced CSM production rates and increased equipment wear. The authors note that the required jet-grout strength was ultimately governed by the coring process rather than the original design requirement.<br /><br />Overall, the project demonstrated how several ground-improvement and wall systems could be adapted to variable ground, restricted access, and urban construction demands.

Keywords

North Shore Connector

Pittsburgh light rail

Allegheny River tunnel

jet grouting

Cutter Soil Mixing

cement-bentonite walls

diaphragm walls

ground improvement

excavation support

urban construction