Unknown Conditions

Construction unless its in the middle of a flat desert is guarantied to have unknown conditions. Your goal should be to minimize these as much as possible prior to putting any pen to paper, much less start demo or put a shovel in the ground; but always expect a surprise boulder right where you least want it.

When it comes to your geotechnical report, you can't do enough borings to know the true underground landscape of rock, soil, water, boulders, but you should have enough to at least have a good estimate. I suggest a minimum of 5 borings, four at the perimeters and one at the location of your deepest foundation, usually the elevator core. Obviously this recommendation is for a small site larger sites should have more. Even if there is an existing building on site you have to get in with a small rig and take your cores through the cellar slab. If your site is an old brown field you should have a ground penetrating survey completed, even in Manhattan.

Images above of unknown abandoned oil tank and old doorways in adjacent building.

Before starting a demolition project I suggest wall probes through the intended demolition subject. These should be done every floor at an interval that makes sense. The probes will give you insight into your own building and how to best demolish it but even more important it will let you know the condition of the adjacent property walls. Nothing is worse than starting to demo a building to realize the adjacent wall is not straight and you have to stop work to shore up the walls that had been being relying on the one you are tearing down.

Always try and attain access to your the neighboring properties to have existing conditions surveys taken. Many a neighbor will blame your new work for old pre-existing problems with their own property. If you can't gain access take as many exterior pictures and even set up cameras. In some locations such as New York City you are required to seismically monitor adjacent properties this is good but nothing beats a picture with written report by a professional.

Know your sites history. Be ahead of the curve and take initiative, don't wait for some local history buff blogger to create a stir that something important might be being destroyed. Make an effort to be transparent through your public relations, acknowledge it and have a clear plan. While excavating a site with historical connections dating back to the American Revolution we discovered an old well that had been filled with trash, it was still there even after two previous structures. We already had an archeologist monitoring the excavators knowing there may be something on site so it was easy to transition to a quick archeological survey in the location. If this had caught us by surprise it may have stopped excavation totally and taken weeks to arrange for the surveys and negotiate with landmarks.

Unknown conditions will always add time and/or costs but if you follow these recommendations you shouldn't get completely rolled over by the unexpected boulder.