Massive Excavation Project Required No Vibrations From Construction

By Jeff Griffin, Senior Editor | January 2013, Vol. 68, No. 1

HP14 by 117 pound I-beams were used as walers, hauled into the excavation and held in place horizontally by the excavator while a welder tacked them to the vertical beams in the caissons. Four, 20-foot long I-beams were cut as braces in the corners and welded to the horizontal waler.

The second stage of the excavation began as I-beams continued to be welded into place, with the Komatsu excavating to a depth of 17 feet. As the depth was finished in one section, the five sets of Mega Brace struts were pressurized with a gasoline-powered pump. United Rentals Trench Safety imported the Mega Brace system from England, where it was originally developed as a time- and space-saving solution for urban excavations in close quarters.

In total, United Rentals Trench Safety provided the steel I-beams, steel plates, hydraulic Mega Brace struts and engineering to build a 90-foot long by 35-foot wide by 16-foot deep beam and plate shoring system with the ability to pressurize, de-pressurize and re-pressurize hydraulic Mega Brace struts to facilitate the installation of precast concrete tank sections.

With the shoring system in place, concrete crews constructed forms for the two-foot thick floor to support the holding tank. The concrete floor was poured and cured. After the required curing time, the forms were stripped and concrete seal applied, and it was time for the precast concrete tank sections to be set.

Ten pre-cast tank sections with a maximum weight of 25.6 tons per section were set in place by a 220-ton crane. As sections were added, the Mega Brace struts were repositioned in five pre-engineered sequences to create space for new tank sections, while at the same time keeping the shoring system safe. The total capacity of the storage tank is 10,940 cubic feet.

With the tank in place, crews sealed joints of the sections and the concrete floor.

The excavation then was backfilled to the level of the horizontal walers. Next the Mega Brace was removed using pumps to de-pressurize the struts, which were lifted out with a bridle. The welds from the horizontal walers to vertical I-beams were cut. Finally, the shoring system’s 20 by eight-foot steel plates were lifted out by a Komatsu excavator. After the removal of all the steel shoring I-beam and steel plates, and hydraulic rams, backfilling was completed and access hatches installed.

To make the excavation, install the shoring system, pour the concrete floor, construct the large holding tank, fill the excavation and remove shoring components required an amazingly short period of time.