Foundation on Compacted Select Fill

[MattR]

We are in the process of designing a crane footing (10ft. square)on controlled fill (20 ft of fill with footing embedment of 5 to 10ft). My primary concern is settlement since the applied loads are in the range of 2 tsf and bearing should not be an issue. I ran an analysis assuming a sandy soil compacted to both 90% and 95% modified proctor yielding a relative density of 50% and 75% respectively and an assumed modulus of 365 and 626 tsf respectively as well. Using Schmetmann I calculated acceptable settlments of about 0.5in to 1.29in. and was going to recommend that the fill (up to 20 ft thick) be plaaced in 8in. lifts to 95% (about 0.5in).

Is this reasonable, are the construction recommendations reasonable, any comments would be greatly appreciated.

 

[SlideRuleEra]

From a constructability view, 8 inch lifts with 95% modified proctor are quite reasonable for good quality fill material.

 

[DRC1]

What type of crane is giong to be in a 10 ft square area? Most crawlers require atleast 12 by 15.If you are placing a fill you will need tighter compaction because you do not want any settlement. Crane loads do not distribute uniformly. Bearing pressure is highly dependent upon the geometry of the pick. Most cranes require differential settlemnt to be with in on degree of level. Also any settelement will occur during the lift and a one inch drop can cause a lot of problems. Usually 12x12 timber mats are placed unde the crane to spread the load. Usually the manufacterer of the crane will supply excelent technical support. A Really good book on the subject is "Cranes and Derricks" by Howard Shapiro, published by McGraw Hill.

 

[MattR]

The crane is actually set to sit on a group of footings on top of a frame (permanent structure), the frame will be shimed level before the crane is used. Most of the predicted settlement will occur before the crane is used (about 80% of it), the actual service loads that the crane will hoist are small in comparison with the load created by the footing, frame and overburden.

 

[Focht3]

"Most of the predicted settlement will occur before the crane is used (about 80% of it), the actual service loads that the crane will hoist are small in comparison with the load created by the footing, frame and overburden."

Immediate settlements are of greater importance for sands and silts, so the maximum crane loads are of greater importance for these soils. What are you using for fill? "Sandy soil" isn't very descriptive -

By the way, your D_R values look too low to me - I would expect values of 65-70% and 80-90%. I didn't check the modulus values, though.

Have you thought about using a crane "load test" to force the immediate settlements to occur in a controlled manner? This can be done while the crane is very low to the ground. Be sure and measure the settlements; pay for the surveyor yourself if necessary. It's a great opportunity to learn! I'll bet that your present estimates are too high by a factor of at least 3...



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[MattR]

Thank you for your response, a test lift is a very good idea and can be built into the construction contract. upon a safe lift of maximum service load the crane can be checked for level and shimmed if needed.

The fill materials are known as "select fill", the exact gradation can vary although materials passing the #200 sieve must be less than 12%, passing the #4 30-100% and no particles greater than 4".

About 80% of the load will be placed on the foundations before the crane is in operation so in reality I'm only concerned with the immediate settlements that the lifts will induce (the crane will be shimmed level on its frame after it is installed). Based on this my estimates are very conservative and the functioning crane will not induce much of the predicted settlment during its service life.

The relative density estimates were taken from a statement in Holtz Kovacs "An Intro to Geotech Eng" that stated a 1% change in relative compaction equals a 5% change in relative density (pg 142). Can you recommend another reference or method to correlate compaction to density?

The relative density was then correlated to a description (medium to dense for 50% and dense to very dense for 75%) as per Bowles. Finally a modulus was estimated based on these descriptions as per Bowles.

 

[Focht3]

First, let me tell you that I'm not a big fan of Bowles' book - and your problem is a good example of why. It's a good reference for "ballpark" numbers and estimates, but you can get into trouble if you use them for design. (I wish Bowles had used ranges, not specific values.) Also be careful with the Holtz & Kovacs citation - the 1% / 5% correlation is based on some pretty specific circumstances, and represents a general trend, not a "hard and fast" rule.

How are you getting from stress increase (Schmertmann) to change in relative compaction?

Be sure and plug the surveying into the crane test procedure. Include a shot at each corner of the foundation. If your client wants to know why this is necessary, tell him that foundation tilt is a crucial performance issue, and the only way to effectively measure it is to shoot the four corners. He'll bite. You will need shots before, at 25%, 50%, 75% and 100% of maximum load, and post-loading.

And please post your measurement results in this thread. We would also appreciate statistics on the compaction of the fill as well as the gradation and description of the material actually used for this project. That way, we can all learn a bit from your project, myself included. Think of it as a tangible "thank you" -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[cap4000]

Keep in mind the load pressure distribution from the footing is safely 2V on 1H using stone backfill along the 4 sides. Remember this pressure can push the sides of the fill-soil outward creating a huge settlement problem. I would over size the footing accordingly and use cruhed stone 3/4 gravel or 2 inch stone for the entire depth(20ft?)down to virgin ground. By using stone you will eliminate the compaction nightmare in this deep hole! Use a large excavator to tamp the stone and leave the people and equipment out of it.