I suspect this is where the pit will get deeper, boards will be inserted between the I-Beams to make a retention wall, after the beams are trimmed down. Chicago | Sales Force Tower by Harry Carmichael, on Flickr
Trimming - note the ends on the horizontal bars - these screw on pieces are to give the bar more grab in the concrete mat - the steel is for tension (pulling) Chicago | Sales Force Tower by Harry Carmichael, on Flickr