In addition to the potentially low crop yield being reported in the local and national Canadian and U.S. press.
Reports from the Annapolis Valley in Nova Scotia, a prime agricultural area, note the worst drought conditions in 10 years. I spoke with one grower several weeks ago who incredulously noted that rain had not fallen on his crops in two months. This in an area – eastern Canada, where a low pressure system goes through about every three or four days, often accompanied by rain. We are fond of saying down here that the weather builds character. I can just imagine what my grower is saying now, possibly unprintable.
Forensic engineers consider the effects of drought in some investigations.
Effects of drought
Foundations of buildings and civil engineering structures can settle or subside in drought conditions. This can happen to both new and old structures. This kind of settlement is in addition to that usually predicted by geotechnical engineers during foundation design. That is, settlement due to the more normal loads on a structure – see the item published July 10, 2012 for comment describing more normal design loads.
(Geotechnical engineers investigate the physical properties of soils and rocks and advise design engineers of the effects of these properties on structures)
Foundation settlement due to drought is not usually considered in engineering design when assessing the loads on a structure in eastern Canada. This is because this type of settlement is often slight. Also because there is seldom a scarcity of rain in eastern Canada so we seldom experience drought conditions.
But, it should be considered elsewhere in Canada and the U.S. And in the future down east considering the possible effects of global warming on climate change as suggested by some. And maybe during the present drought.
(Engineers think of loads as something than can affect a structure and most be provided for or accommodated in design. There is certainly an argument for considering the effects of drought as a load on a structure even if dismissed quickly most of the timel)
Is this too esoteric a matter for engineers to be concerned with – the effects of drought on conventional structures in the built environment? I don’t think so.
Let’s not be too quick to dismiss slight foundation settlement due to drought as insignificant. It can be very significant to transformer foundations in substations and to the foundations of free standing, un-guyed towers and to wind turbines. (Substations are components of an electrical power distribution system)
These types of structures are quite sensitive to foundation settlement. They are sensitive to a few millimetres settlement, like, less than 5 millimetres. Whereas more conventional structures – such as many low-rise buildings, can tolerate in the order of 25 millimetres, and some structures somewhat more than this.
How drought causes foundation problems and damage
How can drought cause foundation settlement? It’s quite easy and the reason will take you back to your high school physics class.
A drought causes the watertable to drop. We all know about that – think dug wells going dry. When the watertable drops the groundwater “drains” out of the soil above. The soil is no longer submerged below the watertable. The recently drained, relatively dry soil above the watertable – due to the drought, is now heavier. Think back to your high school physics and the fact that the dry unit weight of a soil is heavier than the submerged unit weight.
Relatively dry soil will settle due to its (increased) weight, due to its increased “self weight”. Foundations on soil that is settling or subsiding will also settle – go down with the subsiding soil. And this can be caused by drought as we saw above.
For example, the gardeners amongst us have seen the soil in a recently planted container settle over a period of days. This is due to the self weight of the soil causing compression or settlement within the soil. The increased self weight of a soil that is suddenly above the watertable due to a drought will have the same effect – cause the soil to compress.
Another example is the depression across the road at the former location of a construction trench that has been filled in with soil. A lot of this depression is due to compression within the soil due to its self weight.
The same engineering principle underlies these examples as does the effects of drought.
I can easily imagine drought causing significant settlement that is greater than slight settlement, particularly with recently designed and constructed structures in areas that have not experienced drought before.
The evidence for this lies in Mexico City. The city has been settling for years, totalling several feet in some areas and causing much damage as you can imagine. This settlement is due to a lowering of the watertable. The watertable was lowered when water was pumped from wells in the city over many years.
Pumping is a different kind of assault on the watertable than drought. But the soil doesn’t care what causes the watertable to drop and its self weight to increase. When this occurs the soil is going to settle causing greater or lesser damage to any foundations above.
The drought occurring now can conceiveably cause foundation problems. But a forensic engineer investigating a specific problem would need to look at factors like the sensitivity of the foundation to settlement, the magnitude of the drop in the watertable, the soil, and historical records of drought in an area.
An experienced geotechnical engineer in many parts of North America will consider the effects of drought on a structure. They might see it as a load on a structure the effects of which – increased foundation settlement, must be provided for by a design engineer.
An experienced forensic engineer will consider the effects of drought when investigating the cause of the poor performance or complete collapse/failure of a structure.