Depth of Boring

[NBRY1]

How deep should a boring extend when areal fill is placed on a site of deep sand or clay?
For square and strip footings, depths of 2B and 6B would extend to isobar of 0.1q; however, for areal fill, what is prudent depth?

 

[EireChch]

I dont think there is a guideline on this topic. Settlement calculations beneath fill areas are also a little objective, since as you said there is no guidance on 0.1q.

I have posted about it before but dont think anyone gave a comprehensive answer.

I think it depends on the situation, how much is the thickness of fill?

If its 2m, I would probably limit boreholes to 30m depth.

If its 5m, I would limit to 50m.

Interpolate in between. Where did i get these numbers? Just a complete gut feel.

I would cut settlement calcs off at these depths.

You could do a PLAXIS 3D and model it to get a feel for depth of influence. Might be a large model and take some time to run but it is probably the most accurate way to determine it.

 

[1503-44]

It would also depend on the type of structure or equipment it is supporting and how sensitive to dynamic movement and long term settlements that would be and what kind of loads are being applied, static, downward or uplift, or type and frequency of dynamic load. A shopping center load and sensitivity could be quite different than loads from equipment in an industrial plant.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[EireChch]

1503-44 lets assume that it is just something simple. 3-5m of fill with residential structures on top.

Interested to know what BH depth you would go for.

Also interested in others opinions too. Gun to head kinda question

 

[1503-44]

Personally I'd be more concerned with the quality and placement of the fill than borehole depth. 5m of fill is likely to be more than your structure load in many cases.
Gun to head, I'd do 15-20m, just because you can probably do that for about the same price as 10. (Below original grade). But the geotech guys always say I need 2x the depth that I think. Here's your chance to do it again. Oh, I see you already did that in your post above. I don't get too excited about 1-2 story residential structures. If it was industrial, I could agree with your 30m above. 50m seems pretty excessive to me, at least when not having a specific reason to go that deep. I'd hit oil in a lot of places I worked.

My perspective is... With all the residential structures in the world that have been built in the last 2000 years, most without any highly technical geotech study. most do seem to drop from the top down (earthquakes or erosion/termites/no maintenance), rather than from bottom up (soil bearing failure). Just saying. But then again I don't know how advanced the Mayans, or Incas, Greeks etc. were with their geotech studies. It could be interesting to know more about that. The Romans didn't even know that Vesuvius had any volcanic potential.


Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[msucog]

Been a decade since I've been here but glad to be back. 0.1q is completely appropriate or minimum 20' deeper than deep foundation or to top of rock...many variation exist. For area fill, you can reference simply stress distribution plots to determine expect 0.1q based on area of loading which means a lot deeper than the footing stress. Then plan another 20% if you're worried about it. 30%q is reality of where it is critical but must understand deeper. The question I pose regularly is "You're trying to shave 20' off borings? Why? To save a couple thousand bucks? What is the risk cost? What have you accomplished? What is the added cost once a contractor sees the borings are too shallow and they are on the hook?". Speaking from experience as consultant and contractor and other, the costs are exponentially higher if/when the geotech tries to cheapen their work. I would vote to calc the 0.1q depth then add 50% at least for enough boring coverage that value engineered options can be considered, then remain engaged with the owner through the final design and construction...help them see the value of great geotechnical engineering. Dont allow yourself to become a glorified crappy drilling company. There are indeed references out there and more specific ones giving minimum guidance coming.

 

[MTNClimber]

You need to talk to a local geotech for an answer. How deep I would drill in Denver would be different than how deep I would drill in Boston.