An Alarmingly Thin Cavern Roof


Crane Danger

We don’t work on many commercial development projects, and when we do it’s typically something unusual. We were on a project in Pearl Harbor when Healy Tibbitts (HTBI) called asking for help on the King Street Student Housing Tower they were building near the University of Hawaii on O’ahu. A large patch of ground had caved in earlier that day while they were mobilizing their crane. It’s not safe to drive heavy construction equipment on a site that caves in unexpectedly.

Skating on Thin…Limestone

It turns out that the site is located over the Moilili Cavern, a karst formation formed by flowing groundwater dissolving the limestone of an ancient near-surface reef. The cavern underlies most of several city blocks around the intersection of King Street and University Avenue in the McCully neighborhood near the UH campus. It’s well-known locally; sometimes divers explore it. But the cavern roof was much thinner than expected, so thin that it was unsafe to operate construction equipment. It was not clear how the equipment would be able to get to the foundation locations.

X Marks the Solution

The whole team pulled together to collaborate on a solution. While we computed the strength of the cavern roof, HTBI took stock of available materials like steel beams and crane mats and the Owner’s engineer performed additional probing to separate the “alarmingly thin” roof zones from the just “concerning” zones.

A grillage of heavy steel beams (like a giant tic-tac-toe board) covered with crane mats to make a giant snowshoe that spreads out the track loads and reduces the stresses on the cavern roof.

We combined this lower grillage pressure, the cavern roof thicknesses measured by probing, and our stability computations to make a map of where the drill rig should be positioned when installing drilled shafts through the water-filled cavern. With everyone working together, the foundations were built almost exactly as initially planned.

Project Type

  • Building
  • Deep Foundation
  • Dispute
  • Excavation

Project Details

Honolulu, Hawaii


Healy Tibbitts Builders, Inc.




November 2018