Frequency of laboratory Testing along Boreholes

[ONENGINEER]

Hi, I have three 250ft boreholes with only SPTs every 5ft for a heavy foundation of 1000 m[sup]2[/sup]. The soils are predominantly silt, some clayey silt and (silty) sand in a till context. The silts to me err on the sand type rather than clay. I will have some Atterberg limit tests to verify.

Grain size distribution (GSD), PI and moisture content (mc) tests have come in my mind. My challenge is how many of GSD tests they should carry out to avoid ending up with an excessive number of lab testing, assuming the overall soil types are not much different.

I understand the importance of moisture content tests for silt/clay when compared to PL. However, for sand type soils (all below water table) I am not sure how the moisture test results are utilized in design if i decide not do do these tests. Similar boreholes by others got 50-100 moisture content tests and no indications in their geotechnical report if the mc results were actually used.

In other words is there a minimum GSD and mc test numbers below which the investigation could look insufficient. Thank you.

 

[geotechguy1]

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.

 

[dik]

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

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-Dik

 

[MTNClimber]

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.

 

[ONENGINEER]

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.

 

[EireChch]

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.

 

[ONENGINEER]

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.

 

[geotechguy1]

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

 

[EireChch]

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.