Unforeseen, or differing site, conditions can fall into three categories:
Type I- the conditions encountered at the site differ materially from those depicted in the contract. Included in this type is when actual conditions were reasonably unforeseeable based upon all the information available to the contractor at the time of bidding.
Type II- the conditions encountered at the site differ materially from those typically encountered in similar situations. In other words, the contractor could not have reasonably anticipated the actual site condition from an eyes-on inspection or past experience. It may be an unusual condition for that locality, but does not need to be a geological anomaly.
Type III- the condition encountered at the site is a hazardous and/or toxic waste that was not reported or identified in any site conditions report provided during the bidding period. The American Institute of Architects Document A201 – 2007, General Conditions of the Contract for Construction, §10.3.1, Hazardous
Materials, defines a Type III condition as “… a hazardous material or substance not addressed in the Contract Documents and if reasonable precautions will be inadequate to prevent foreseeable bodily injury or death to persons resulting from a material or substance, including but not limited to asbestos or polychlorinated biphenyl…".
Remember, a contractor has a duty to perform a reasonable inspection of the site to determine the existing conditions. In addition, in contracts I draft or review I ensure a "site inspection" clause is included. This requires the contractor to conduct a reasonable inspection of the site. "Reasonableness" is determined by looking at what a rational, experienced, prudent and intelligent contractor in the same field of work could discover.
A common example of an unforeseen condition in an existing building is the discovery of asbestos that must be abated before the work proceeds. For this reason as much exploratory investigation should be performed as possible prior to putting the contract out to bid. A few dollars spent up front on cutting, testing and patching will save hundreds of dollars, and numerous headaches, later. Notes on the plans detailing where the samples were taken and what the results were are very important.
In my part of the country ledge/bedrock is a common unforeseen condition where earthwork is involved. I always recommend test borings in locations where foundations, piping and subsurface drainage will be required. I then have the depths of soil shown on the plans at the respective locations. While this does not depict where all ledge is located, it does provide some indication of what the material is below the surface. I have still had instances where one location underneath a proposed road was clear of obstructions 6 feet below the surface and less than 4 feet away ledge was just 6" below the surface. Fortunately, during the initial design phase we scouted the area on either side of the proposed road location and it appeared a ridge of ledge could possibly be running under the road. If an on-site investigation and test borings did not take place, that section would have required a change order to remove the ledge to the design depth of the road bed. Typically, the cost of ledge removal included in a bid is less expensive than the cost of a change order upon discovery.
However, no matter how much investigation and research is performed, some surprises still occur, as can be seen here.