May-June 2017. Grand Portage National Monument

This is the gatehouse structure at the Grand Portage National Monument. It is a reproduction the old trading post built and run by the North West Company in the late 18th century.The current structure was built in the early 1960’s and is the oldest of the museum, fire having destroyed the Great Hall.

We were called in to address shifting foundations that were putting severe stress on to the hybrid timber and log structure.

Existing post bottoms showed signs of decay which needed addressing.

The lower end of this brace shows that there has been significant leaning of the structure to the left. The gap on the right side of the logs where they tenon into the post also exhibits somewhat alarming movement.

After a thorough assessment of the current state of the structure we stabilized it with tension straps and picked it up using a combination of cribbing and screw jacks.

With the weight taken off the posts the next task was to devise a way to rack the building back to square and level as best as we could, taking care to preserve the patina and not add any drill holes for fasteners.

The existing foundations turned out to be significant at 21″ square and up to 6 feet deep. Deep enough not to be lifted, though they had been slowly tilted by freeze-thaw cycles.

The wet spring would show us quite clearly how much water was funneled to this area by the lay of the land. It is not surprising that a combination of environmental factors including cold winters, very little snow cover and soil conditions and would lead to forces sufficient to move even such substantial pieces of concrete.

Removal proved to be a formidable task, greatly aided by the power of hydraulics in the form of Greg Olsons crane.

Next we could addess the post bottoms. They had been replaced previously, so we decided to use the existing simple scarf joints, again with a view to preserve as much of the original material as possible.

The posts were replaced with white spruce timbers sourced at Hedstroms Lumber Company in Grand Marais, MN. The were sawn an inch oversized and proceeded to hew the faces off with a broad axe.

One adaptation we suggested and got approval for was the manner in which the posts are tied to the foundation:

The post bottoms are scribed to locally sourced slate rocks. A stainless steel threaded rod goes through this rod, is pinned to the post, a nut and washer secure the stone.

This allowed us to attach the post at the scarf joint, suspending it in the freshly poured footing.

Once backfilled this makes for a nice clean look and dispenses with the need to hide unsightly angle iron.In addition to big 3’x3′ bases at the bottom of the footers we buried 2″ of rigid insulation a foot below grade to help minimize frost penetration.

All digging was done by hand due to the historical nature of the site. The monuments archeologigal staff was kept informed of our (re-)discoveries of old foundations and other findings.Where possible the old braces were reused. In some instances the straightened out structure required replacement timbers, which we scribed to fit.

Two proud craftsmen that have done more digging than they expected to.

Newly tight joinery.

It turned out that this is a replacement brace that had been inserted years ago to take up a gap that had formed. We were able to cut it back to have it fir into a fully plumb and square frame.