The first step is construction of the floor slab. Workers also place a 4-inch-high concrete curb either at the time of slab placement or afterwards but before setting the forms. The curb is as wide as the wall thickness and serves to position the bottom of the forms and stop the flow of concrete when the tunnel form is jacked up a couple inches to its final position.

Workers then place steel reinforcement for walls and rough electrical before the tunnel forms are positioned, while there is room to work. Then each half of the tunnel form is lifted by crane and set in place. The two halves of the deck are then locked together. Door and window bucks are mounted on one wall of the form, making their installation very easy. A trick that workers often use is to establish the final elevation for the forms by using a laser level to mark a chalk line 2 inches above the floor on the curb. Then, when the form is being set, they can jack the tunnel form into position so that the bottom of the form is on the chalk line. This allows the elevation of the form to be quickly established, time after time. Taper ties are placed on 4-foot centers horizontally and in two or three rows vertically, depending on ceiling heights.

With the forms in place, setting steel on the deck is the last step before placing concrete. Contractors usually place concrete that gains strength quickly so that the following morning the forms can be removed and the cycle can be repeated.

To remove forms, workers unlock the two halves at the deck, remove the taper ties, and lower the jacks so that the built-in wheels touch the floor. Each form is then rolled out through the open wall, one third of its length, exposing a “lifting hole” located on the top of the form. A crane lifts the form and moves it to the next location, starting the cycle over again. The cycle can be repeated to construct another shell either on top of or alongside the work just completed.

Joe Nasvik Shown here is a completed tunnel form, the curbs they rest against, and the pre-tied wall reinforcement with rough electrical (the blue tubes) in position.

When the tunnel forms are removed, two or three walls and the deck are complete. What remains is the one open exterior wall and any interior walls. These can be constructed with steel studs or unit masonry. Sometimes these walls are cast in place using conventional wall forming systems. With this approach, a block-out placed in the deck during the tunnel form pour, over the position of the wall forms, makes it possible to fill the forms with concrete.

In the Florida housing market typical costs range from $12 to $16 per square foot. Construction costs should at least be equal to wood-frame construction. Although, as one industry member commented, “Why would you choose to compare a high-quality home building system to a low-quality system?”

There are other hidden cost benefits. Within a couple days the shell and the primary interior walls, the rough electrical, the insulation, and block-outs for plumbing and HVAC are complete. This means that other trades don't waste time cutting holes or preparing to do the work they are hired to do. Construction cycles are therefore much shorter than with other building systems so the duration of construction loans is shorter, and homes can be turned over to their owners more quickly, enhancing cash flow.

The blessing and the curse of tunnel form construction is to be found in the details and planning required to fully take advantage of the system. With the opportunity to strip, set forms, and place concrete every day, shell contractors can easily produce more than other trades can keep up with, so they too must be involved in the planning.