In addition to the potentially low crop yield being reported in the local and national Canadian and U.S. press.<\/p>\n
Reports from the Annapolis Valley in Nova Scotia, a prime agricultural area,\u00a0note\u00a0 the\u00a0worst drought conditions in 10 years.\u00a0 I\u00a0spoke with one grower several weeks ago who incredulously noted that rain had not fallen\u00a0on his crops in two months.\u00a0\u00a0This in an area – eastern Canada, where a low pressure system goes through about every three or four days, often accompanied by rain.\u00a0 We are fond\u00a0of saying down here that the weather builds character.\u00a0 I can\u00a0just imagine what\u00a0my grower is saying now,\u00a0possibly unprintable.<\/p>\n
Forensic engineers consider the effects of drought in some investigations.<\/strong><\/p><\/blockquote>\n
Effects of drought<\/em><\/span><\/p>\n
Foundations of buildings and\u00a0civil engineering structures can settle or subside in drought conditions.\u00a0 This can happen to both new and old structures.\u00a0 This kind of settlement is\u00a0in addition to that usually predicted by geotechnical engineers during foundation design.\u00a0 That is, settlement\u00a0due to the more normal loads on a structure – see the item published\u00a0July 10, 2012\u00a0for comment\u00a0describing\u00a0more normal design\u00a0loads.<\/p>\n
(Geotechnical engineers investigate the physical properties of soils and rocks and advise design engineers of the effects of these properties on structures)<\/p>\n
Foundation settlement due to drought is not usually considered in engineering design when assessing the loads on a structure in eastern Canada.\u00a0 This is because\u00a0this type of settlement\u00a0is often slight.\u00a0 Also because there is seldom a scarcity of rain\u00a0in eastern Canada\u00a0so we seldom experience drought conditions.<\/p>\n
But, it should be considered elsewhere\u00a0in Canada and the U.S.\u00a0 And in\u00a0the future down east considering the\u00a0possible effects of global warming\u00a0on climate change as suggested by some. And maybe during the present drought.<\/p>\n
(Engineers think of loads as something than can affect a structure and most be provided for or accommodated in design.\u00a0 There is certainly an argument for considering the effects of drought as a load on a structure even if dismissed quickly\u00a0most of the timel)<\/p>\n
Is this too esoteric\u00a0a matter\u00a0for engineers to be concerned\u00a0with –\u00a0the effects of drought on conventional structures in the built environment?\u00a0 I don’t think so.<\/p>\n
Let’s not be too quick to dismiss slight foundation settlement due to drought as insignificant.\u00a0 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)<\/p>\n
These types of structures are quite sensitive to foundation settlement.\u00a0 They are\u00a0sensitive to a few millimetres settlement, like, less than 5 millimetres.\u00a0 Whereas more conventional structures –\u00a0such as\u00a0many low-rise buildings,\u00a0can tolerate in the order of 25 millimetres,\u00a0and some structures somewhat more than this.<\/p>\n
How drought causes foundation problems and damage<\/em><\/span><\/p>\n
How can drought cause foundation settlement?\u00a0 It’s quite easy and the reason will take you back to your high school physics class.<\/p>\n
A drought causes the watertable to drop.\u00a0 We all know about that – think dug wells going dry.\u00a0 When\u00a0the watertable drops\u00a0the\u00a0groundwater “drains” out\u00a0of the soil above.\u00a0 The soil is no longer submerged below the watertable.\u00a0 The recently drained, relatively dry soil above the watertable – due to the drought, is now heavier.\u00a0 Think back to your high school physics and the fact that the dry unit weight of a soil is heavier than the submerged\u00a0unit weight.<\/p>\n
Relatively dry soil will settle due to\u00a0its\u00a0(increased) weight,\u00a0due to its\u00a0increased “self weight”.\u00a0 Foundations on\u00a0soil that is settling or subsiding will also settle – go down with the subsiding soil.\u00a0 And this can be caused by drought as we saw above.<\/p>\n
For example, the gardeners amongst us\u00a0have seen the soil in a recently planted container settle over a period of days.\u00a0 This is due to the self weight of the soil causing compression or settlement within the soil.\u00a0 The increased self weight of a soil that is suddenly above the watertable due to\u00a0a drought will have the same effect –\u00a0cause the soil to compress.<\/p>\n
Another example is the depression across the road at the former location of a construction trench that has been filled in with\u00a0soil.\u00a0\u00a0A lot\u00a0of this depression is due to compression within the soil due to its self weight.<\/p>\n
The same engineering principle underlies these examples as does the effects of drought.<\/p>\n
I can easily imagine drought causing significant settlement that is greater than slight settlement, particularly\u00a0with recently designed and constructed structures in areas that have not experienced drought before.<\/p>\n
The evidence for\u00a0this lies in Mexico City.\u00a0 The\u00a0city has been settling for years,\u00a0totalling several feet in some areas and\u00a0causing much damage as you can imagine.\u00a0 This settlement is due to a lowering of the watertable.\u00a0 The watertable was lowered when water was\u00a0pumped from wells in the city over many years.<\/p>\n
Pumping is a different kind of assault on the watertable than drought.\u00a0 But the soil doesn’t care what causes the watertable to drop and its self weight to increase.\u00a0 When this occurs the soil is going to settle causing greater or lesser damage to any foundations above.<\/p>\n
The drought occurring now can conceiveably cause foundation problems.\u00a0 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.<\/p>\n
An experienced geotechnical engineer in many parts of North America will consider the effects of drought on a structure.\u00a0 They\u00a0might see it as\u00a0a load on a structure the effects of which – increased foundation settlement,\u00a0must be\u00a0provided for\u00a0by a design engineer.<\/p>\n
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.<\/p>\n
<\/p>\n
<\/p>\n","protected":false},"excerpt":{"rendered":"
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,\u00a0note\u00a0 the\u00a0worst drought conditions in 10 years.\u00a0 I\u00a0spoke with one … Continue reading