I was surprised at the news last week about the evacuation of 14 homes in Sechelt, on the Sunshine Coast near Vancouver because of sink holes and unstable ground.<\/p>\n
What\u2019s wrong with the picture is why such unstable ground was developed and built on in the first place if the risk was known.\u00a0 Sink holes as large as an estimated 15 feet across and 3 to 4 feet deep in one news picture if typical are an obvious sign of unstable ground.<\/p>\n
A geotechnical investigation – a well developed applied science – had been carried out and the risk identified according to news reports.<\/p>\n
Sink holes not unusual and easy to investigate<\/em><\/span><\/p>\n Sink-hole-prone ground is not unusual in nature.\u00a0 We got unstable ground like this in the Atlantic provinces.\u00a0 There’s lots of sinkholes in the Bahamas where they’re called banana holes because banana plants grow in them.<\/p>\n A neighbourhood of 14 homes is about the size of my neighbourhood, and compact and the ground easy to investigation.<\/p>\n Geotechnical investigation identifies ground that is susceptible to sink holes like this.\u00a0 The ground can be natural or due to the works of man.\u00a0 It also identifies the different layers of soil and rock beneath the ground \u2013 the stratigraphy<\/em> in geology \u2013 and the areal extent of the different layers.\u00a0 It determines the physical properties of the materials forming the layers and their susceptibility to conditions of interest \u2013 sink hole development in this case.\u00a0 Finally, it checks out the depth to the ground water and where it\u2019s flowing.<\/p>\n Was the risk of building a house high or low?<\/em><\/span><\/p>\n Did the geotechnical work really conclude that the risk was 10% – possibly a low number to some?\u00a0 Or did the work miss something which seems likely as evident by the evacuation?\u00a0 How was the 10% calculated?\u00a0 A probability analysis was not mentioned in the news.<\/p>\n It occurs to me that I wouldn’t build a house in an area where there was a 1 in 10 chance of my house being undermined by a sink hole.<\/p>\n Easy to investigate and improve the ground<\/em><\/span><\/p>\n Ground terrain like this can be improved.\u00a0 It\u2019s called ground improvement<\/em> in engineering and is a well developed technique.\u00a0 But it can be expensive involving lots of geotechnical work and construction work.<\/p>\n These kinds of investigative and improvement techniques are so well developed and understood in engineering that it\u2019s motherhood.\u00a0 The ground may be complex but finding this out and doing something about it is fairly straightforward.\u00a0 Not building on the ground is one solution.<\/p>\n Simple, preliminary investigation<\/em><\/span><\/p>\n There\u2019s a list of geotechnical techniques in the Appendix.\u00a0 They are roughly in the order they might be carried out.\u00a0 You can repeat some depending on what you\u2019re finding.<\/p>\n One of the least expensive at the beginning of an investigation is a good walk-over survey of the ground.\u00a0 This would be accompanied by a study \u2013 before, during or after the walk-over, or all three \u2013 of published topographic and geologic maps of the area and published aerial photographs.\u00a0 New and old sink holes like those reported would be seen in a good walk-over.\u00a0\u00a0An engineer experienced in terrain analysis could also pick out big sink holes on aerial photographs even those taken from 1,000s of feet high.<\/p>\n Such a walk-over and study are standard procedures in geotechnical investigation.\u00a0 They’re cast in stone.<\/p>\n Video and stills taken from drones 10s and 100s of feet high have been available for a few years now.\u00a0 They have been invaluable in my work.\u00a0 Sink holes and unusual features on the ground show up well in aerial video.<\/p>\n Before drone photography I hired a small plane and had the pilot fly low over a site as I took photographs of the ground.<\/p>\n These preliminary techniques would be standard at the beginning of a geotechnical investigation of a site like the one evacuated near Vancouver.\u00a0 The results would indicate if more expensive investigative work was justified like that mentioned above and listed in the Appendix and how to plan and do it.<\/p>\n Investigation and\/or use of findings questioned<\/em><\/span><\/p>\n Of course, good and thorough geotechnical investigation must be followed up with good use and implementation of the findings.\u00a0 The evacuation would seem to call into question the investigation and\/or the use of the findings.\u00a0 What’s wrong here is that something went wrong and shouldn’t have.<\/p>\n Appendix\u00a0<\/strong><\/p>\n There are many techniques that could be employed during a geotechnical investigation.\u00a0 I\u2019ve used all of them in my consulting engineering practice at different times over the years:<\/p>\n <\/p>\n<\/div>\n","protected":false},"excerpt":{"rendered":" I was surprised at the news last week about the evacuation of 14 homes in Sechelt, on the Sunshine Coast near Vancouver because of sink holes and unstable ground. What\u2019s wrong with the picture is why such unstable ground was … Continue reading \n