I was remiss last week in not commenting on how the situation in sinkhole country relates to the interests of many of you in civil litigation and insurance.\u00a0 I remember a vague feeling at the time that something was missing from my blog – I neglected to refer to the standard of care<\/em>. (Ref. 1)<\/p>\n Last week’s blog commented on what an experienced engineer would do in an area susceptible to the formation of sinkholes – see below.<\/p>\n You’re certain to want to know for a failure in the built environment or an insurance claim that the standard of care was observed at a construction site.\u00a0 We check this during a forensic investigation.<\/p>\n The standard of care for an undeveloped<\/em> building site in an area with known foundation problems – like the risk of sinkholes – requires, at the very least, carrying out a geotechnical investigation.\u00a0 Something similar applies to a developed<\/em> site.<\/p>\n The investigation can be fairly routine for a compact site like a single building, bridge, wind turbine, etc. and the results can be quite accurate.\u00a0 The investigation can also be inexpensive considering the cost of these structures.<\/p>\n Reference<\/strong><\/p>\n ***<\/p>\n (Originally posted last week, March 31, 2019)<\/p>\n That was quite a picture in the newspaper a few days ago of a sinkhole that appeared last August in Oxford, Nova Scotia. (Ref. 1)\u00a0 And the pictures of the sinkholes in British Columbia a few weeks ago (Ref. 2) and before that – back to Nova Scotia – some time ago in Falmouth..<\/p>\n Striking pictures but sad too when homes are lost, and scary at the thought of potential injuries and death.<\/p>\n But it’s time to stop<\/strong>.\u00a0 There’s no need for the formation of sinkholes to surprise anyone nor pose a risk..\u00a0 Nor be a “…a money pit”. (Ref. 1)\u00a0 There would be no news and no pictures if experienced engineers were involved before these areas were developed.\u00a0 And every chance some of these areas would have been undeveloped.\u00a0 Engineers would carry out a standard geotechnical investigation of the ground at a site proposed for construction.\u00a0 Simple as that.<\/p>\n The news story mentioned a geophysical survey.\u00a0 That’s an engineering technique that’s been around a long time.\u00a0 The fact that this is sinkhole country – Karst terrain – has been known a long time too.<\/p>\n Karst is an irregular limestone area with sinkholes, underground streams and caverns. (Ref. 3)<\/p>\n Remote sensing geophysical surveys – sort of like MRIs in medicine – detect features in the ground of interest to land owners.\u00a0 Features like voids or conditions conducive to the formation of voids.\u00a0 Sinkholes start as voids in the ground.<\/p>\n Limestone bedrock, gypsum and salt dissolve in the water in the ground to form voids, caverns and underground streams.\u00a0 The voids get larger with time and eventually the top, or roof of the void, appears at the ground surface – a sinkhole. The voids are said to\u00a0migrate<\/em>\u00a0to the ground surface.<\/p>\n The most recent picture shows red soil around the perimeter of the sinkhole.\u00a0 This is glacial till – soil deposited by glaciers 1,000s of years ago.\u00a0 The soil is heavy and would cause the top of a void to break and appear at the ground surface sooner rather than later.<\/p>\n Investigating\u00a0undeveloped<\/em>\u00a0Karst country for sinkholes<\/strong><\/p>\n What would an engineer do if asked to investigate the foundation conditions at a proposed construction site?\u00a0 For example, the site of a building, or any of the structures in an urban area, or a residential subdivision or a strip mall.\u00a0 This would be an engineer experienced in geotechnical or geological work.<\/p>\n They’re not always asked – and it appears not always in Karst country – but what would he do if he were?<\/p>\n He would first check.the published geology maps and aerial photographs of the area available to all of us.\u00a0 It’s called terrain analysis in engineering.\u00a0 He would see in the maps that\u00a0a large area<\/strong>\u00a0of Nova Scotia – many square kilometres – is underlain by Karst terrain.\u00a0 He would also see in the photographs evidence of large sinkholes like the one in the picture.\u00a0 He would tell you that the area is susceptible to the formation of sinkholes.\u00a0 He would also tell you that he can’t predict where the sinkholes will appear in a large area.<\/p>\n But\u00a0tell him approximately\u00a0where you want to construct\u00a0a subdivision or strip mall\u00a0and he’ll give you a pretty good idea of the risk of sinkhole development in\u00a0a small\u00a0<\/strong>area<\/strong>\u00a0like that.\u00a0 Not the location of all future sinkholes but the location of some, and the risk of others.\u00a0 He would do this after carrying out a geophysical survey.\u00a0 The survey could be expensive for an area proposed for a subdivision or strip mall.<\/p>\n I did a geophysical survey of an airport runway on South Andros Island in the Bahamas one time.\u00a0 I was looking for voids that might form sinkholes.\u00a0 A runway is not unlike a residential street or a strip mall.\u00a0 You can be sure I ran a lot of closely spaced survey lines down the runway looking for voids.<\/p>\n Tell the engineer precisely\u00a0where you want to build a house, a multistory building, a bridge, a road, etc.,\u00a0a tiny area<\/strong>, and he will tell you if and where sinkholes will form and undermine your structure.\u00a0 He would do a geophysical survey.\u00a0 This wouldn’t be that expensive for a single structure and a good investment considering construction costs.\u00a0 You might consider relocating the structure after the survey..<\/p>\n If the risk of sinkholes forming is low and you still want to build there then he would drill boreholes at the location of your structure and any features of interest found during the geophysical survey.\u00a0 Boreholes retrieve samples of bedrock like limestone or indicate when the borehole passed through a void.\u00a0 It’s called\u00a0ground proofing<\/em>\u00a0in engineering work that relies on non invasive, non destructive geophysical surveying and terrain analysis.<\/p>\n Summarizing, this is what the experienced engineer might do depending on what you need:<\/p>\n It wouldn’t take a “money pit” of money to investigate for sinkhole-forming voids if you know the precise location of your proposed structure.<\/p>\n Investigating\u00a0developed<\/em>\u00a0Karst country for sinkholes<\/strong><\/p>\n If you are concerned about the stability of a developed area, particularly foundation stability, then,\u00a0depending on the size of the area and the preciseness of the information wanted,\u00a0the engineer would go through the above steps.\u00a0 The emphasis would be on Step #2 if you are concerned about a small area.\u00a0 Or Step #3 if you are concerned about a single structure and wanted precise information.<\/p>\n ***<\/p>\n The approach for\u00a0developed<\/em>\u00a0or\u00a0undeveloped<\/em>\u00a0land would be much the same – that of a fairly routine engineering investigation by an experienced person.\u00a0\u00a0It wouldn’t be so expensive for a single structure, a tiny area – a few 1,000s of dollars rather than many 10,000s.\u00a0 But more, of course, for a larger area, particularly if a lot of precise information was wanted.<\/p>\n It wouldn’t be so newsworthy either\u00a0particularly if undeveloped land was being investigated.\u00a0 The reporter’s eyes would glaze over at the thought of covering an engineering story like that.<\/p>\n That’s where we need to get to in Karst country – investigate before at reasonable cost not after when newsworthy sinkholes and problems develop.<\/p>\n References<\/strong><\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" Blog Update I was remiss last week in not commenting on how the situation in sinkhole country relates to the interests of many of you in civil litigation and insurance.\u00a0 I remember a vague feeling at the time that something … Continue reading \n
Sinkhole news highlights a problem that can be fixed<\/h1>\n
\n
\n