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Is this unreasonable?

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SCGeo

Geotechnical
Jun 9, 2005
20
I have performed a geotechnical exploration for a proposed parking lot. The area planned for construction is currently a field and will require 3-4 feet of fill from present ground surface in order to meet required grades. The site is approximately 150' by 350' and was explored with test pits in 6 locations.

In one of the test pit locations, buried topsoil and organics (branches up to 3 inches in diameter) were encountered from 3.5 to 5 feet below existing surface elevation. In another test pit location, buried topsoil was encountered at a depth of 5 feet and was approximately 1 foot thick.

At the remaining locations, mixed soil and rock fill (boulders up to 2 feet in diameter) with some asphalt debris was encountered to depths of up to 8 feet below existing grade. The soil within the mixed soil and rock was near optimum moisture based on natural moisture testing and experience with similar materials.

My report stipulated that provided the owner was willing to assume some risk associated with undocumented fill, the existing mixed soil and rock fill across the majority (90%)of the site could remain in place if it was shown to be stable through proof-rolling.

I made it clear that at the two locations where it was encountered, the buried topsoil and organic debris should be over-excavated and "chased" laterally in order to completely remove it from the area planned for construction and backfilled with engineered fill. In my limited experience, if you find buried trash at one location, you can bet that it extends laterally from the point across the site.

The developer says that he needs more options for repair. He believes my recommendations are too conservative and wants options to stabilize the lot through bridging or with geogrid.

If it were simply weak soils, I would agree. But 1.5 feet of topsoil and branches as big as my wrist are another matter altogether. I can't think of any constructed geotechnical project where it is acceptable to fill on top of buried topsoil or wood.

Am I wrong?


 
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VDOT for example lets stumps, vegetation, etc. remain iin place if they are 5 feet or more below the proposed surface.

Chec your local DOT spec on when clearing and grubbing is required. Inclusion of a grid or non-woven fabric, pinned often, may also help, but weigh the costs.
 
In this neck of the woods parking lots are often constructed over peat (100% or so organics) if bridged with geotextile and 3' min. gravel. A surcharge period is used to reduce future settlement. I agree owner should be allowed to take risk that some settlement will lead to minor repair/shorter pavement life which may be much more economic than excavating buried organics.
 
You could potentially give him recommendations for using a geogrid or woven fabric. However, you may need multiple layers with aggregate in between.

I have successfully constructed road and parking areas over much more compressible soils.

There are still significant risks of unsuitable performance, especially if the underlying soils are as variable as you indicate.

Maybe you can contact your local geogrid supplier and they can give you some guidance. Just take what they say with a grain of salt. They are trying to sell a product after all.
 
SCGeo,

If the developer were buidling any type of above grade structure, I would agree with you completly. However, given that they are building a parking lot, and that they are willing to accept the risk fo future settlement. I would strip and proof roll the site. If the proof rolling is generally good then I would place all but about 18-inches of the fill with soil compacted to a little higher than normal for a parking lot (say 95% modified). I would then place 9-inches of parking lot base rock then a layer of biaxial geogrid such as Tensar BX1100 (1200 if the loads are heavy), then the remainder of the rock. Make sure the rock is well compacted then pave it.

I like the Tensar geogrid because it is stiff and in my experience it developes its load at a lower strain and therefore works better. I like putting it in the middle of the rock, again because it seems to develope its strength at a lower strain, the rock locks into the holes better.
 
I like the Tensar BX products the best myself. Remember, you need at least 10 inches of material (including asphalt) on top of the BX1100 geogrid to develop its strength. This thickness is less for the BX1200 and stiffer grids.
 
Thanks to everyone for the guidance. I'm still a relative geotech rookie and I guess I have heard so many horror stories in this regard that I'm still a bit overly conservative.
 
You might want to be descriptive in the title of the thread. I think you would get more responses. People just kind of gloss over things like "Is this unreasonable."
 
Hey every body: Which one of you that recommends all this extra work (fabric, excavation, special fill, strippping, grubbing, etc.) has ever seen a site like this fail when nothing special was done. I'll bet that number is zero.

First off, what is the parking lot for? If there is a settlement in some area, what will be the problems resulting, such as where slopes are "tight". Will there be water puddles that people may slip in and sue the owner? Can grades be made steeper?

If some pavement tends to fail early, what will that cost and will it be a problem.

I have seen very many parking lots on buried topsoil, corn stalks, municipal land filling, junk, logs, house demolition stuff, etc. that function fine for their intended purpose with no special work done. Even ashes work if you can stand some gradual settlement. In this case the added fill planned is more than many of those I have seen and makes it even less likely for problems later.

Lacking better desctiption for the use, it would be my opinion that it has to be a very fancy use that would require any of the special work recommended or in the report.

I do assime that the imported fill will be reasonably compacted in lifts and the usual local parking lot pavement design will be used.

Isn't one of the functions of an engineer to design the job at least cost, considering the risks? Why load it up with work that may not be needed?

So, who has seen this or similar type of job fail because of no special preparation please?
 
I had the experience of developing a 20 acre parking lot for a docking facility for Japanese cars coming into North America years ago. The general area was some organics over soft clay over sands. The site was two level as excavation fill had been placed over 1/2 of the site from downtown excavations. We simply graded the site moving excess fill from the high area to the low area, put on some 300mm of river sand and topped with 4 inches of 1.5 inches of crushed stone base course. It worked out quite well. In the 20 acres, we had some 15 square metres where we had some soft clay pump up through the fill - we fixed this - but not a bad track record (0.01% problem). No geotextile was used.
 
I forgot to ask another question.

What do you do with the stuff you want to excavate and dispose of? Does it go to a low site where some other owner will want a parking lot some day?

I can't reall how many sites I have worked on where we removed crap from the building site and then placed it outside the building at the parking lot. No sweat if you are careful.
 
It really depends on the owner's expectations for performance. If they want very little maintenance and have the parking lot last for 20 years, then you will need to do more work. It also depends on the vehicle loading. If this is a distribution center for a trucking facility, then you will need to be more careful than for a private, business lot with no trucks.

You may be able to get by with doing any remedial work in the main traffic aisles. If there is a refuse truck path with a dumpster, then concentrate on that path.

However, with the conditions that you describe, the fill is extremely variable with soft spots in some areas and hard spots in others. If the owner doesn't want to spend money up front, then they will pay more on maintenance. That maintenance may include tearing up portions of the lot and repaving.

Also, if this is a commercial establishment, they may be real touchy about how their lot looks over time.

The essence is that you need to sit down with the owner and go over the relative costs for several options. Then, find out their expectations for performance. Let them know that, generally, the more careful they are up-front, the less they will have to spend long-term.
 
oldestguy said:
So, who has seen this or similar type of job fail because of no special preparation please?
I have, many of them, both those with & without special preparation. Go to Charleston, SC, or many other Southeastern US coastal locations for a first hand look.

[idea]
 
OK, we got one good answer so far. I was hoping to hear from those above with the "extra work" recommendations as to why the recommendations.

I should have defined what "failure is" I suppose.

I don't consider some uneven surfaces as failure as long as the lot functions to park cars, etc. without considerable re-paving, patching early, etc.

Don't forget, the case here has considerable fill over the poor stuff and pavement structure has to be designed for the loading.

Many of the parking lots I have been involved with have piping and electric cables buried beneath. Nice to get them in early, but will the site ever get added trenching? Ask a back-hoe operator what he thinks of grids or fabric under or in the pavement he has to dig through. Or, in case of a gravel lot, a grader operator that blades the shoulders.

For Sliderule Era, do they have to do early overlays or big undercuts to do the fix? Or no fix? Maybe some discussion of what is tolerable or not so to help in future reports by SCGeo. I find people seem to be pretty tolerant of uneven parking lots as long as the mud is gone, water drained, no pot holes, etc.
 
You have to be very careful in assuming what "failure" for your client. It is much more preferable to discuss with them what they can expect for performance for several alternatives and let them decide. After all, it is their money and their pavement.

They could sue your butt off for the most minor variations, settlement, or cracking if they expect perfection.

On the other hand, I have had clients that only expect the pavement to last 3 to 5 years and just good enough so cars don't sink in the mud.

Don't be afraid to discuss expectations with your client and the rest of the project team. If you can let them make the decisions, then your liability goes way down.
 
I think we all have to be careful when using the word "failure" - I try to keep away from it and use "distress" even if part of the damn slope has slid down the hill! But you are right - one needs to clearly identify and cater to expectations. Many times expectations are unrealistic and, you will need to decide if you want the job or not (?).
 
oldestguy - Removing 2' to 5' of unsuitable material and replacing with properly compacted suitable soils is the typical solution, but some times even this does not work.

The worst case that I was involved with was as a Contractor on construction of a coastal boat landing / parking area. Per specs, we excavated 4' of organic material and replaced it with 4' of acceptable compacted sand/clay. Asphalt pavement was placed over this. Within 6 months, everything (4' of compacted fill, parking lot, sidewalks, signs, etc. - but not the pile supported reinforced concrete boat ramp) had settled 6". I will say that the compacted fill did do one thing nicely - no differential settlement - it all went down uniformly. Of course the Owner held us (Contractor) accountable. An independent soils lab was called in... to make a long story short, we were vindicated (and paid). The Engineer of Record (a respected national firm) was held accountable.

Other times (as an Owner's representative) we have used undercutting (often more than once for the same spot), asphaltic concrete overlays, or just replacement paving (on occasions, several inches thick)

I like the comments by eric1037 & BigH about Owner involvement and the definition of "failure". Often, when the large retail chains (Wal-Mart, etc.) open a new store here they just clear, grub, remove/replace a minimum of bad material (maybe a foot), and pave. They know that the parking lot will be "trash" in a year or so. However they make enough profit by keeping to a tight construction schedule and opening the store quickly to pay for the extensive repairs/replacement that is inevitable.

A few years ago construction of a new chain restaurant store had fallen behind schedule. Wet weather had prevented parking lot construction. During a break in the weather, the Contractor was authorized (and did) hire a helicopter to hover over the work site for an entire day to try to dry the sub base to meet the minimum paving requirements. The next day they paved. Parking lot lasted about a year, but the restaurant opened on schedule.

[idea]
 
SlideRuleEra hinted at something else of interest to me. I came onto a job after some organic material had been removed and replaced with engineered fill - for a roadway embankment widening. Settlement happened at the interface of the new and old. The site did have some 4 to 5 ft. of organic/peat but then a lot of soft normally consolidated clay. So, to get rid of the peat, they ended up loading additional weight as the replacement (say 5 x 120 pcf) which, of course, caused settlements of the normally consolidated clay. Sometimes there is a trade off of one vs the other.
 
Hello once more:
All I can say, I must have a good guardian angel.
 
More on related subject: Building pavements directly on topsoil in Wisconsin and Illinois.

In the '70's many a K-Mart was done on a lot where we had to "use what was there". No hauling in of fill, if possible.

Pretty much standard specs were "strip only the sod and use the topsoil". Topsoil generally about 5 percent loss on ignition. So most subgrades were topsoil, either natural or filled, to make the grades required.

One cardinal rule may have saved the day. "No rutting over 2 inches to fully loaded tandem-dual truck traffic prior to adding base course."

I will say that some "adjusting" of base course thickness had been experienced from time to time, regardless of subgrade type, to be able to build on it without major undercutting. This was done by field trial to see what thickness would carry that fully loaded truck

To my knowledge no special repairs were required in the 25-30 year life experienced for these jobs and similar clients. I know of one such pavement that now is over 40 years without full overlays, but with some patches.

It was my experience that topsoil worked as well as non-organic soil, except it took on water and softened quicker than non-organic soil. Therefore, once the base was on a firm subgrade, this worked as if soil was non-organic.

So, I ask, why routinely strip topsoil?

PS: On the earlier postings, you did not see me recommending leaving in peat and building over it unless some special work was done.
 
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