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The buoyancy issue when PVC casing is used

Geotechnical Instruments (UK) ofLeamington Spa routinely uses PVC casing, as do other UK companies, the firm's sales and marketing director TomTonkins points out after reading a draft of this article.

Almost all installations in North America use ABS casing and almost all installations in the UK use PVC casing. Elsewhere both are used, the selection often depending on which manufacturer has the most influence. Fibreglass casing is also available.

The specific gravity ofPVC is much greater than ABS, hence buoyancy forces are smaller. The two types are manufactured to different diameters. PVC tends to be more brittle than ABS, especially at low temperatures (it is believed that this is reason why ABS has become the casing ofchoice in North America).

Tonkins reports that frequent practice when installing PVC casing is to pre-grout the borehole and to lower the casing through he grout.

'When lowering through grout, quite often we do not have to fill the casing with water completely, as with the weight ofthe casing and water, this is enough to make it go down, 'he says.

Tab le 1 summarises calculated buoyancy forces for the two types ofcasing, with two different grouts. Forces and weights are per 10m ofdepth.

Note that, as indicated above, the typical density ofgrout used for installing inclinometer casing is between 1,200kg/m 3. Table 1 shows that it is necessary to use a grout with density lower than 1,170kg/m 3to overcome buoyancy when PVC casing is used.

Also note that ifa grout with a higher density that Grout 1 is used, the buoyancy force will be much larger than given in the table.

Finally, use ofPVC casing creates a 10mm annular clearance between the inclinometer probe and the casing.

On the other hand, for the smaller ofthe above two diameters of ABS casing this is 17mm. The greater clearance allows for monitoring a greater displacement. In the view ofthe authors this is a significant issue in favour ofselecting ABS casing.

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