Interrupted Equilibrium and the Descent of Homes
For an underpinning contractor these are busy times in Santa Fe. We are getting record numbers of calls to assess and repair cracked footings and slabs and to arrest the movement of heavy, often adobe, houses sliding down hills. For most of our clients with these problems, such phenomena are mysterious events, particularly given that this has been an extremely dry year. Failed footings and massive movement are usually associated with water; whether from broken utility lines, drip irrigation systems or rainfall.
This year it's different: The extraordinarily dry weather has resulted in the desiccation of soils that, as they shrink, cease to bear the loads expected of them.
As counter-intuitive as settlement caused by the absence of moisture seems, there is another seeming paradox emerging from the pattern of failures. Amazingly, it is the newer houses that are failing far more frequently than older ones. One may logically and appropriately ask how that can be given that building standards and codes are more stringent now than they were twenty years ago (to say nothing of fifty years ago when codes, if they existed here, were not enforced). The answer to such questions is never simple, but we have some ideas.
Older homes are more flexible. Often built on a stone footing with mud mortar; adobes laid up in mud, not rigid Portland cement; pliable wooden bond beams and lintels, all lead to a structure that tends to move uniformly when called upon to respond to trauma or a change in natural conditions. They tend to get charming, not scary.
An older adobe also tends to have been built in the classic pueblo style characterized by a single story, few and typically small window and door openings, and a low roofline. The tendency over the past two decades has been to increase the height considerably and to build those sensuously thick walls broken by lots of large openings. Modestly sized vigas and beams have given way to massive (often disproportionate), green, whole trees. Concrete has replaced wood as a bondbeam. In a word, houses are heavier now. More and larger openings tend to provide more places for cracks to appear, reflecting and magnifying the affects of even minor settlement.
That is not to say that footings cannot be engineered and built to support massive structures -- of course they can. But they don't seem to be. By far the largest and most expensive underpinning jobs that we have tackled on existing adobe homes have been for houses that are less than twelve years old; most were built on non-compacted construction fill or on incompetent and unstable native soil. Sadly, the footings were often not built to the engineer's specifications.
There are several other reasons that older houses are holding together more successfully. For one thing, folks a generation or two ago tended to exercise a bit more common sense when it came to siting. The old adoberos knew to stay out of floodplains and off of hillsides. Generally, they chose flat, well-drained building sites that they could self-engineer. That approach has been subverted by excessive confidence, if not downright arrogance, instilled by the availability of modern structural building materials. Architects and engineers can certainly design a structure that will (usually) cling pretty well to a steep slope. The weakest link is the builder who looks for little ways here and there to overcome the onerous job of squeezing a profit out of the lowest bid. The coup de grace is delivered through negligence; by architects who fail to oversee the construction, and code-enforcement officials who let things slide.
Architects and builders beware! We are witnessing the dockets fill up with lawsuits, some based on errors and omissions, some on downright negligence and shortcutting by contractors whose clients' biggest investment reminds them of the Dow in its descent.