NO OF BORE HOLE

[swelm]

hi

i have area bout 24000 square meter i want to ask about the

average number of bore hole to perpare soil report

the porpose of area mall 3 floor without basement


thanks for any comments

 

[msquared48]

I'd start off by setting up a 100 foot (about 30 meter grid) each way for the total number. Take most of the borings at the periphery of the building at the corners and probably one or two along each of four sides, and the rest at the anticipated major bearing locations at the interior.
If you run into any garbage fill areas, feel free to investigate further to better guestimate the limits of the bad strata.

Mike McCann
McCann Engineering

 

[GalileoG]

Swelm,

Perhaps you will receive more responses if you were to pose your question at the Geotechnical Engineers Area found at the link below:

http://www.eng-tips.com/threadcategory.cfm?lev2=12

 

[civilperson]

10 shallow borings,(6 meter) and 2 deep, (14 Meter).

 

[msucog]

guess ~300kip column loads. hoping for 3ksf that gives you 100sf (10x10). investigate at least 2.5B down so that's 25' borings (below planned foundation level) as a minimum target (or to something hard if encountered). it'll depend on the cut/fill depths (never stop in fill, in alluvium, or in crappy residuum). you might want to take one boring to refusal for seismic site class determination. if prelminary info is what you need, spot as many as you can afford for now understanding that you'll need to get the geotech back later. if this is more "final" and if you think it's residual soils, start out at 100' centers. if there's fill or anything oddball on the site, shrink the spacing down tighter. more imporantly, get a geotech involved and ask their opinion. help the geotech help you and provide them all the info you know (fill depths, cut depths, max column/wall loads, other heavy or area loads, settlement sensitivity of the structure, retaining wall locations, etc).

 

[swelm]

thanks for all valuable information

 

[BigH]

First off, I would "know my geology". Secondly, "know your project." That is a large area (400 m by 60 m - or 160m x 160 m). Is that the Mall's footprint or the site as a whole with parking?
civilperson gives a reasonably good idea, say, for most cases. Not knowing the general site area geology, I might go an extra borehole or two to greater depths and concentrate most borings in the mall footprint with only a few in outlyer areas for parking.
I know of one where there would have been problems with borings not going over 14 m given that the site had 40 ft of dense sand and gravel (let's not quibble that 14m is 42+ ft). But, underneath, there was some 100 ft of soft to firm clay. For footings of a 3 storey mall, that should pose no problem as the dense sand and gravel would likely support the whole of the structure without any added stresses on the underlying soft clay. However, should you say raise grade at the mall so that people can enter, say the 2nd level of the mall at one side vs the 1st level on the other side, the added fill would surely stress the underlying soft clay and lead it into consolidation settlement - and differential settlements across the width of the mall.
FYI, I was involved in one site where the designer wanted piling to rock and we went to 70 m deep and beyond to find it! Almost all of our 30 borings at this site have been in excess of 40 m. All previous work at site was piling to rock in the order of 40m or so; so the 70m+ was a major piece of site information.
In the end, there is no hard rule (many organizations have guidelines), but you need to have enough boreholes to properly model your site with reasonable accuracy. You should be prepared to drill additional holes if, as was intimated earlier, you find "garbage" or very soft clays and you want to delineate the deposit and you should be prepared to deepen borings if you find unsatisfactory conditions within the depths investigated. Drilling, the choice of hole depths and number of borings should be a flexible activity to account for varying and unknown conditions - and your client should be kept fully in the loop as you might see your site costs skyrocket!