Sinkholes, like the one that\u00a0undermined the house in Falmouth, Nova Scotia last\u00a0week, are a\u00a0serious issue for subdivision and lot developers.\u00a0 As reported in the newspapers, the risk of\u00a0sinkholes\u00a0forming in an area can be known from published maps.\u00a0(Refs 1, 2)\u00a0 And the cavities in the ground associated with sinkholes can be found – before construction.<\/p>\n
Sinkholes start their life<\/strong> as roofed-over cavities at some depth in the limestone, gypsum and salt\u00a0deposits beneath an area.\u00a0 The cavities get bigger and the roof gets thinner as rock like this\u00a0dissolves in the ground water.\u00a0 In a sense, the cavities “migrate” to the ground surface and eventually break through as sinkholes. \u00a0The\u00a0fairly precise size, location and depth of cavities can be determined beforehand.<\/p>\n The technology for locating cavities in rock is well developed and has been around a long time.\u00a0 It’s called g<\/strong>round p<\/strong>enetrating r<\/strong>adar, GPR for short,<\/em> and it’s well known to experienced geotechnical engineers.\u00a0 I used\u00a0GPR years ago<\/strong> to locate\u00a0cavities beneath an airport runway on South Andros Island in the Bahamas.\u00a0 The same technique is used for locating unmarked graves.<\/p>\n It sounds technical but all it involves is sending radio waves into the ground and analysing what is reflected back.\u00a0 There’s nothing too exciting in the data that comes from uniform soil or intact rock.\u00a0 But lots of excitement in the\u00a0data from a cavity, a well-recognized\u00a0anomaly to the GPR operator – and\u00a0a potential sinkhole beneath a building. The remote sensing, non-intrusive technique is not unlike CAT scans and MRIs in medicine.<\/p>\n The type of potential sinkhole I was looking for\u00a0at the airport\u00a0is called a banana hole. So named because banana plants grow in<\/strong> the sinkholes<\/strong> once\u00a0the cavities break through the surface.\u00a0\u00a0A banana hole\/sinkhole can be seen in the runway\u00a0by an inbound pilot but not the roofed-over cavity just before it breaks through the surface.<\/p>\n Geo engineers\u00a0investigate the adequacy of the foundation soil and rock conditions beneath a site where someone wants to build something – like a house, for example, or a\u00a0bridge, dam,\u00a0or airport runway.\u00a0 The conditions would not be adequate if there was a cavity beneath the building site.<\/p>\n GPR\u00a0would not normally\u00a0be used on a building site in the Atlantic provinces, nor in Canada for that matter.\u00a0 But it\u00a0certainly should if deposits of limestone, gypsum and salt are noted when the engineer checks the published geology of the site – an important and\u00a0standard task in a geo\u00a0investigation.\u00a0 These types of rock with\u00a0their inherent risk\u00a0are not so common but\u00a0they do exist\u00a0as noted in the\u00a0news report\u00a0on the\u00a0Falmouth sinkhole.<\/p>\n If there’s a risk<\/strong> and the technology is available to quantify it, and it’s not used, then it seems to me it’s a potentially\u00a0litigious matter<\/strong>.<\/span><\/p>\n References<\/strong><\/p>\n Sinkholes, like the one that\u00a0undermined the house in Falmouth, Nova Scotia last\u00a0week, are a\u00a0serious issue for subdivision and lot developers.\u00a0 As reported in the newspapers, the risk of\u00a0sinkholes\u00a0forming in an area can be known from published maps.\u00a0(Refs 1, 2)\u00a0 And … Continue reading \n