Are there any generally accepted rules of thumb for determining an appropriate number of borings for a building structure?

I understand that local knowledge and other factors will affect the decision. I was just thinking about a possible situation where you take several borings and find nothing out of the ordinary. The project goes to construction and an adverse sub-surface anomaly is found that was not detected by the borings, thus leading to significantly increased costs as well as delays.

What if the owner tries to pin the increased costs on the design professionals, arguing that an improper number of borings were taken? You have the burden of proof to demonstrate that you followed the standard of care.

abusementpark - see Eng-Tips thread: http://www.eng-tips.com/viewthread.cfm?qid=217899

It can get you thinking in the right direction.

nothing further than about 200 ft and no extrapolation (interpolation is o.k.).

f-d

¡papá gordo ain’t no madre flaca!

Thanks for the link BigH!

In the wrong location, 200' can be disastrous! Always look for local knowledge. I have sen a report showing boreholes 50' apart hitting bedrock at 3' and nothing at 100' because of a buried escarpment. Fortunately the strata are well known to those of us in the area.

. . . and in the right location, 400 may be appropriate. Here we sit in cyberland trying to field a question that is without context. As a geologist-turned-geotechnical engineer there is no doubt the relevance of local geologic knowledge. That said, in the most uniform conditions if somebody went with 500 ft spacing, they'd set them self up for some measure of dispute during construction. It's just too easy for contractors to claim changed condition and when the boring spacing is too broad it's an open door.

I'm very familiar with the need for closer-spaced borings. Heck, we drill the location of each pier on our bridges!

f-d

¡papá gordo ain’t no madre flaca!