Denver, CO-based Coggins & Sons combines its experience with the latest in high-capacity, duty-cycle cranes technology to drill caissons in the daunting and beautiful Rocky Mountains. The caisson project the company is working on is the Veterans’ Hospital new parking garage project in downtown Denver.

Coggins drilled 100 shafts for the installation of the deep-foundation piers. The multitude of piers is necessary because of the load that will be incurred on the ground from the new multi-level parking garage. To tackle the job after drilling 12 caisson and three tiebacks, the contractor added two, Link-Belt LS-208H II 72-metric-ton (80 ton) capacity lattice boom crawler cranes to its fleet. The Link-Belt crawler cranes are necessary for carrying out various types of geotechnical projects, including the new parking garage, the company has under contract.

Since the 1990s responsibility has been placed on drilling contractors to supply design-engineering expertise for improved geotechnical methods that enable the project to be completed more cost-efficiently. “We were willing to meet the need for engineering services because it has enabled us to expand our business,” said Larry Coggins, president, Coggins Construction. “This has been especially true with clients in the private sector.”

The various geotechnical engineering and contracting projects Coggins performs includes deep-foundations projects, earth retention systems, and tieback anchoring in sub-surface walls, micropile installation and large diameter deep well drilling.

Active in deep-foundation systems, the addition of two new duty-cycle cranes was a necessity. Most of the deep foundation contracts call for large-diameter, deep-well drilling to an acceptable stratum (level) in the earth.

Caisson foundation (or non-displacement pile) drilling is accomplished by using a strong and reliable duty cycle crane with a drilling attachment. To reach competent ground (i.e. ground capable to support the load it is intended), the shafts are being drilled an average of 15 meters (50 ft.) deep. The shafts’ diameters vary from 60 to 360 centimeters (1 to 6 ft.). These variances are determined on the changing soil competency found where the individual pier bearing point is to be located. The soil conditions are classified as clay, sandstone or sandstone-bedrock.

Outfitted with a Stewart-Stevenson drill auger on a 72-ton (80 ton) capacity Link-Belt LS-208H II crane equipped with angle boom, the shafts were drilled with little problem. Upon completion of the drilled shaft and using the same crane, a site fabricated, steel reinforcing bar cage is lowered into the shaft. Concrete is poured into the shaft until it is completely filled and the steel cage is embedded.

The process in an eight-hour shift ranged from three to five shafts. For tougher soil conditions with rock, a carbide-tipped auger was used in the drilling. The crane and auger system bored the holes very efficiently, according to Coggins. That includes incompetent rock with an unconfined compression strength value of up to 12,000 pounds per square inch (psi).

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