Use case 4 : SPT vs Depth

[mbeaufils]

A description of the concerns about this use case.

[neilchadwick-dg]

A few observations from someone who has worked with SPT data a lot (but also of other data of course - there is life beyond SPT!)

• SPT data for an individual hole is sometimes useful when reviewing borehole logs to get a feel for the ground conditions, and possibly local variations, but it would be unusual (amongst competent designers) to base designs on the SPT profile in an individual hole. It possibly could be done in rock in some circumstances, but it is rarely appropriate in soil.

• For design a designer typically takes the totality of the SPT data for a particular area for (normally) a particular geotechnical unit. This is plotted, design lines are assessed, correlations made and other data considered alongside (e.g. undrained shear strength and/or PSD) before the relevant design parameters are assessed (which may sometimes vary with depth/elevation).

• Therefore, as I have said in other posts, the metadata (location, geological unit, maybe some other location type metadata) has a very important role to play, so that the correct data set can be filtered out.

• SPT data is more than just the N value. The detailed blowcount data (per 75mm here in UK) is sometimes useful for providing insight as to what is going on when there is an unexpected result. Also, other data such as casing depth and water depth, both recorded in AGS, can be similarly important.

• The N value reported in the factual data is not the N value seen on the plots used to inform design. It is common and often necessary for adjustments to be made, i.e. design uses a derived value. The main adjustments are for energy ratio (i.e. N60) and extrapolation where a test is terminated early, e.g. at 50 (main) blows. In coarse grained soils sometimes an adjustment for overburden pressure is made (N'). UK practice is for these adjustments to be made by the designer, i.e. they are not included in the factual data, although AGS format has recently muddied the water by adding a field for N60 (a mistake, in my opinion!).

[Didymograptus]

Use of SPT in this instance is is just a simple example, it could be any depth-related parameter. Keep it simple for now, let's not get into the interpretation yet. That's another topic.

Simple data requirements to visualise are:

Point ID, Point location (x,y,z), SPT N value, Depth of test,

The data should be able to be extracted for one or more exploratory holes.

More complicated data requirements:

Geology/geotechnical layers associated with test.