Foundation subgrade problem 1

[lcruzz01]

There is project were the design foundation depth was 2 m (6.5 ft). During excavation we found 7 m (23 ft) of non-controlled fill and we suppose it has at least 15 m (50 ft). One of the options was to pour concrete with crushed rock in the first 3 ft followed by an engineered fill till we get to the original foundation depth. I suggested increasing the footing dimensions (initially 26x19 ft) to distribute the load and minimize settlements. I believe the first option can cause settlements due to the excessive depth of fill. What do you guys suggest? We can't use piles because of time delays.

 

[fattdad]

what you building? How wide is the footing? Is it a strip footing or a square/rectangular footing? What's the original design bearing pressure? What did the geotechnical engineer recommend, based on the field exploration plan?

Essentially, we don't have enough information.

Some options:

So what if it's existing fill, if it has suitable strength (and low organic content) it can support something. Just figure out the strength of the fill and design a foundation to satisfy strength and settlement.

Design an engineered subbase to support the "seat of settlement." You see, if you have a 3-ft wide strip footing, the majority of the settlement response will occur within some distance below the bottom of the footing. Now if you know how much settlement is forecast for the existing fill (i.e., some measure of soil modulus) and you have an aggregate that's much stiffer, you can calculate how thick the engineered subbase must be to limit settlement to some design criteria. It's not likely that you'll need to go all the way to 4B, however.

Grouting can work (but it's expensive).

Surcharging/preloading with a settlement monitoring program can work (it can also give you some idea of soil modulus).

Others. . .

Good luck. Maybe you can provide some more details?

f-d

¡papá gordo ain’t no madre flaca!

 

[PEinc]

So, you're in too much of a hurry to do it right with piles? Sounds unwise to me. None of your options sound any quicker than piles unless your proposed foundation scheme is so complicated that a redesign would take too long.

http://www.PeirceEngineering.com

 

[hokie66]

If this is a light structure and can tolerate differential settlement, you might be able to found it at the high level in the fill. But first, you need to know the settlement characteristics of the fill itself. What kind of material is in the fill? How long has it been there? How much more it is going to settle under its own weight?

Your original footing sizes of 26' x 19' would suggest that the structure is either heavy or there are large overturning moments. A piled solution, from the information given, would appear to be in order.

How did you get to this point before the founding condition is known? Was there no geotechnical engineering involvement?

 

[lcruzz01]

The geotechnical investigation encountered this fill and suggested drilled shafts but the structural engineer convinced the client shallow foundations will suffice (now she regrets the decision). The fill is very heterogeneous, mostly clayey sand with boulders and was placed 10 years ago. No phreatic level is present. The problem is we're behind schedule and changing to piles means a lot of bureaucratic work.

I hope the structural engineer learns from his mistake and value the inputs from geotechnical reports.

 

[dicksewerrat]

Guess you cut the delay by having the structural engr. hire a piling company or their insurance co. can do it. Time spent doing the job properly is not wasted.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Ron]

15m of fill? Not likely.

Look at ground remediation techniques (dynamic deep compaction, terra-probe, etc.).

 

[reyv]

Sounds like a very risky proposition, given that the engineering characteristics of the uncontrolled fill materials are unknown. Can you perform a plate load test to at least get some degree of confidence in the shallow foundation design?

Rey Villa, MS, PE

http://geotech-apps.com

http://engineerofrecord.com

 

[motorbiketocrank]

First, don't repeat the mistake of the structural engineer and make decisions based on convenience/schedule to the exclusion of what's technically appropriate. Obviously it will only make things worse in the long run.

Having said that, I'd agree with comments recommending you look into ground modification measures (such as deep dynamic compaction) and possibly intermediate foundation types such as aggregate piers. Frequently the firms that do this work take care of both the engineering and the construction/implementation (which speeds up the process). Often it doesn't require any structural re-design (i.e. you keep the same footings and bearing pressure and simply improve the soil so that it meets that criteria).