Frequency of laboratory Testing along Boreholes

There's not really a one-size-fits all rule. It's really better if you start at the end and think about what questions you need to answer in your reporting or design / design support, and then work backwards to understand what testing you need.


When last I worked in North America it was normal to do water content tests every 0.3m / 1 foot. They are very cheap, should only really run $5 a test at most (you put some dirt in a cup. Weigh the cup. Put cup in oven. Wait. Weigh cup again). For soils with some plasticity where the water content is relative to the liquid limit and plastic limit gives you some crude information potentially about strength, stiffness, stress history. We used to just do them regardless of soil type - I guess you could optimize; you'll probably spend more time on your hourly rate deciding which ones to test and which ones not to test than just testing all of them. Normally a smaller number of Atterbergs are done. In this school of geotech you can use the atterbergs to derive strength parameters. PSD's - hard to say. Used to do sieves and hydrometers again with a lower frequency since I think they cost $70-80 a test or something at the time.

In addition, I'd be more concerned about changes in material.

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I'll echo geotechguy and say it depends on your area, the type of construction/site improvements proposed, and sometimes depends on the soil types. There is no written rule of a minimum. I would recommend talking to your supervisor. They should know the standard of care in your area and give you guidance.

I understand the difficulties to answer a general question of this nature but thanks all and in particular geotechguy1 for your useful hints. While the moisture content is a useful data for fine grained soils, I am still interested to know how to utilize the moisture content test results in design considerations of sand soils below the ground water level.

MC every 1 ft! that seems excessive. OP moisture content can be used to determine void ratio. Not a parameter that is used that much when assessing sand, maybe for seismic work.

I think a good ball park, for a standard report that we would do, which would give a description of ground conditions and parameters, and some outline foundation/engineering recommendations and bearing pressures, would be a MC, PSD and atterberg every 2-3m.

Though we often get 20m boreholes with 3 MCs, PSD and Atterberg's. Not ideal but we deal with it and move on.

Also - PSDs can be used to help determine friction angle for granular soils using coefficient of uniformity, as per BS8004 and others.

Thanks EireChch to point out the friction angle vs coefficient of uniformity, I could not readily find BS8004 but saw the attached figure and wonder how widely it is used in practice without consideration of the soil compactness.

Are the labels on the vertical axes the right way round?

The BS8004 correlation is used as a supplementary check. Ideally you would have SPTs which are actually useful in sands/gravels. I would never use uniformity coefficient on its own to determine phi.