Geotechnical and civil/site engineering services to support the design and construction for the addition of a new cooling towner at the Logan Airport Central Heating Plant

Nobis was the geotechnical and civil subconsultant for design and construction services for the addition of a new cooling tower to the Logan Airport Central Heating Plant. The project consisted of the addition of a new cooling tower to supplement the four existing towers that were in place. The subsurface conditions at the site consisted of approximately 15 to 20 feet of unsuitable urban fill and organics overlying 15 feet of natural silt/sand over roughly 150 feet of Boston Blue Clay which got softer with depth. Bedrock was located at a depth of about 180 feet.

The existing cooling towers were founded on 14-inch-square pre-cast prestressed concrete piles; however, use of driven piles for the new cooling tower could generate vibrations that could damage the existing towers. As a result, Nobis recommended the cooling tower be supported on drilled micropile foundations embedded in the Boston Blue Clay crust.


  • Poor soils at the project site
  • Presence of extensive existing utilities within the proposed tower footprint which could not be relocated.
  • Protection of the existing adjacent cooling towers and extremely tight settlement tolerances.


  • Adjacent existing cooling towers were sensitive to disturbance and vibrations limiting potential driven pile types. Nobis determined that a drilled micropile foundation system would limit impacts to the existing cooling tower foundations (i.e., vibrations and heave) and provide the required capacity beneath the poor soils encountered at the site.
  • The micropiles extended to a depth of approximately 60 feet below the ground surface consisting of steel casing in the upper 30 feet and a bonded zone within the lower 30 feet in the stiff clay crust.
  • A mat foundation was recommended with full below-grade waterproofing. This consisted of a bathtub-type structure design that ensures the below-grade vivarium space will be completely dry without the need for a permanent dewatering system with pumping.


Nobis Group provided both civil and geotechnical services on this Logan Airport project. The additional cooling tower was necessary due to the recent expansions at the airport. In addition to design, Nobis also performed micropile load testing and full-time observation of the production pile installations during construction. The main design challenges consisted of the extremely poor subsurface conditions and proximity of the adjacent cooling tower. The generalized soil conditions consisted of 30 feet of contaminated urban fill and organics overlying 140 feet of Boston Blue Clay (stiff at the top and very soft at bottom) and bedrock. Nobis was able to use the stiff clay crust to our advantage and design a micropile to utilize the resistance of the stiff clay without affecting existing structures. This saved the client hundreds of thousands of dollars from not having to extend the piles to the extremely deep bedrock.