The Chicago Transit Authority began construction of four new relay stations near the elevated train tracks in downtown Chicago. Limited job site access and poor soil conditions necessitated a specialized approach for the deep foundation work that was required.

Restricted space overhead and narrow alleys ruled out the use of caissons and auger cast piles. Helical piles were the logical solution due to the lack of spoils that are generated and the smaller sized installation equipment.

Soil borings indicated a very stiff to dense 3’ layer of clay soils at a depth of 57 feet would need to be penetrated in order to seat the helical flights in competent deeper soils.

The helical pile was designed to allow penetration through the 3’ layer and into the hard clay bearing stratum at a depth of approximately 70’. This was accomplished by using a square 2 ¼” bar “stinger” lead section with 10”/12”/14”/14” helical flight arrangement. The 4 ½” round pier section was used for the remainder of the pile.

The design meant lower torque resistance was encountered during installation through the lower strength clays. An aggressive flight design allowed deeper penetration into the hard clay bearing stratum. The tubular shaft was grout filled and allowed to cure prior to performing load tests. Based on the stratigraphy identified on the site boring, the helical flights are bearing in cohesive soils with an average cohesion of 6500 psf.