Strippings Depth (Waste Soils) 4

[Rookie2]

Hi,
I was hoping for some advice concerning a project that I am working on that is currently under construction. I realize that earthwork calculations are somewhat of a best guess, and that strippings depths vary greatly from site to site / region to region.

The problem that I am having on this project (heavily wooded site) is that we do not have enough dirt to do the fill proposed by the plans. My calculations were based on the assumption that there would be 9" of strippings required; The grading contractor has stripped the site to a depth of 1.5' (300 acre subdivision), which of course creates a big problem. We have more strippings to waste than anticipated; and we have more fill, and less cut to do it with. The client did not allow us to get soil borings due to time constraints.

My questions are:

Does 1.5' of strippings sound reasonable? We have a similar site in the same area where the soil borings reported that the top soil depth was 4".

What effect does the fact that the site is heavily wooded have on the strippings depths?

Are there seasonal factors involved that I should be aware of? The majority of the earthwork was done in the Winter which tends to be wet around here; however the the site slopes at 10% or more, so it is well drained.

How can I prevent this problem from happening in the future?

Thanks alot for any input.

 

[BigH]

1.5 ft of stripping does not sound 'normal' - but then again it may be a fact of life at your site as well as others. In southern Ontario it used to be about 4 to 5 inches as a general rule. I doubt that the time of year makes much difference as you are stripping in order to remove the topsoil. In a wooded area, you are likely to have much more of a root system - but then, most of your area, at one time, was heavily wooded - just that the trees were cut down in past years. A caveat is that if the area is quite wet, perhaps your depth of stripping was influenced by rutting of the contractor's equipment - and had he used, say tracked equipment, it might have resulted in less stripping.

But, there is some serious questions to ask. You have indicated that the client did not permit soil borings due to time constraints and yet he was risking a 300 acre development?

1. Most good local geotechs could have given you preliminary information within a day or two of the finish of the borings or test pits. This would have been reasonable AND better than no information at all.
2. Where were your firm (or the client) when the contractor was stripping? Anyone on site? . . . or did the contractor just go on 'is merry way. See my last comment above on the type of contractor's equipment. If he caused extra stripping to occur, then this should be on his account - both for the stripping and for the filling required. If you weren't there (or no compentent person looking after the owner's interests were there), then, well, how can you tell if the "large" stripping was a result of the ground conditions or contractor's methodology/equipment.

Some advice from an old man (besides being Big) - as to how to prevent this in the future: Stripping is ALWAYS a contentious issue. In development work, you need to be quite careful (one reason I have never liked it). So, borings or no borings, you should go dig a few hand dug test pits to determine the thickness of topsoil and or unsuitable material so you don't get burned. If you have borings, it was our company policy not to use the "spoon" representation of the thickness of the topsoil but to actually dig three shallow holes near by and measure in situ - directly. I think this is some good advice. On 300 acres, assuming a per inch basis for cost and, say $5/yd3 for unit rate (for example), even one inch would be a volume of 40k yd3 or, say, a cost of $200k. Wow!! . . . and spending a couple of grand on a quick and dirty geo investigation . . . So, your 9 additional inches of excavation over and above your 9 original inches means nearly $1.8 million! and this doesn't include the imported fill to raise the grade back up 9 inches.
and sorry for the length.

 

[jheidt2543]

BigH's comments are right on the money! But I'll add a few of my own:

1. I wouldn't call earthwork calcs a "best guess", but they are more art than science and rely a good deal on local knowledge. Also, once you get those big machines on site, they seem to keep moving! Look at the difference 1" of thickness makes on a large site in BigH's comments. Topsoil isn't consistant over an entire site and neither is the first cut of a scrapper exactly 4".

2. Most of the good dirt contractors I've worked with have their own drill rigs to check a site or at least dig a few pits to check not only topsoil, but the thickness of various layers. This is important, particularly on a big site, even if there are soil borings by a geotech. They need to figure out where stock piles of different materials are going to be placed. (Really! The good contractors actually have a plan!)

3. It seems some owner's set up a job for problems. How can anyone go into a 300 acre job without soil borings? Yes, we all want the job, but sometimes you have to tell the owner he is full of XXXX, that's what he is paying an engineer for.

 

[Rookie2]

Thank you very much for the great advice. To answer your questions, we recommended to the client that he get soil borings; we were primarily concerned with rock. He had some one dig test pits and they didn't hit rock so he chose to move on. The client hired an indepedent project manager to do the construction inspection, We do site visits only to inspect the sewer and water construction (per the clients request). Luckily we were only grading the roads so the problem isn't over the entire 300 acre site.

I will keep all of these things in mind in the future; thanks again.

 

[cvg]

It also seems that your independant project manager didn't do anybody any favors. Why wasn't he on the phone to you and to the client first thing on day one when the contractor began to overexcavate? Depth of topsoil could have also been confirmed at that point before allowing the contractor to continue. The Contractor should have also been concerned with a changed condition as it usually requires a change order to get paid for it... Sounds like a 3-ring circus out there.

 

[civilperson]

The proceedure in most Department of Transportation for the states is to pay clearing and stripping by AREA and then the inspector is at war with the contractor to keep excavating soil with roots, organics and deleterious materials. After stripping, a preliminary survey is completed for measurement of excavated quantities.
Thickness of organic laden "topsoil" varies from zero in the deserts of Nevada and Arizona to 7-8 feet in valley bottoms of Missouri and Iowa.

 

[LCruiser]

As mitigation you may want to revisit your ground surface elevations - either some localized areas, or lowering the entire site by 9" - to avoid the big borrow extra. That would need to be done quickly though, as I imagine the contractor is bulding pads now?

 

[Dozerman56]

When I was in New Hampshire I typically assumed 1' of strippings unless I had specific information otherwise. Here in central NC we typically figure 6" unless proven different. But stripping a heavily wooded site in very wet conditions could substantially increase your strippings - maybe 50% or more. If your site is sloped 10% there must be some substantial fills/fill slopes. Perhaps some of your strippings could be incorporated here. Or pick an area that won't be built on, mine your borrow, and replace with the strippings. Some, but not all developers subscribe to the 'time is money' philosophy and understand that accelerating a project will increase their construction costs but get their product to market faster. Hopefully this is part of your owner's plan.

 

[BigH]

Dozerman56 - but why "assume" the depth of strippings. It is an easy case to dig a few pits - even by hand - to get a good handle on the depths to be removed. When costs of even an inch or two might be worth 100k or so, it seems a small price to pay. Also, you can have a good handle on whether the overstripping is contractor caused or not.

p.s. You know what assume means, I presume? ass u me .

 

[Rookie2]

Thanks for all the input. We met with the contractor today. He says the excess strippings were caused by the clearing crews. Their equipment created ruts and wet areas that they had to cut out; Exactly as BigH said. We are lowering the roads to generate the dirt that we need, but of course the contractor wants an extra. Thanks again for the advice.

 

[TDAA]

If the contarctor hired the clearing crew, he should have been in control of them. The contractor should not be paid for extras that they cause.

"Well, Rookie2, we undercut the site, then overfilled it, but don't wory, we got it right the third try. We just need $45 million for all the extra work we did."

 

[LCruiser]

TDAA is right. Was the clearing crew a sub of the contractor? If so, it should be on him because he didn't use appropriate equipment for the conditions.

 

[Rookie2]

Just an added note, This particular developer just gave us a new job (200+/- lot subdivision). It is going for rezoning June 6th. He has requested that we have plans ready to submit June 7th, as he expects the rezoning to be approved. At this point he has a boundary survey and gis 2' contour interval topo. No wetlands delineation, soil borings, etc, etc. He needs to have the plans APPROVED by the middle of July so that he can start moving dirt. Things just don't happen that quickly around here (45day +/- review time frames; soil borings 3 weeks min.; Wetlands delineations...don't ask.) Sorry, if this is an inappropriate post for this topic; but I think it is relevant based on some of the posts.

I should also mention that the developer is a semi-retired civil engineer whom I have alot of respect for. I'm sure there's some wisdom in there somewhere.

 

[Dozerman56]

BigH:
You're right, "assume" was a poor choice of words. But 6" of strippings is the local standard benchmark. It's rarely more than 9" or less than 3", and since it's usually fairly easy to incorporate it into fills it really doesn't matter unless we're on a flat site with minimal cuts and fills - no place to put it and bridge over.

 

[BigH]

Dozerman56 - I agree that local benchmarks may be such - and for planning, it might be a good idea - but if you are going to develope 300 acres, you should be prepared to send out a lab tech with a shovel and have him dig a few pits for half day or so to see if there are some surprises. I've seen where the topsoil stripping caused major headaches to the consultant before and this is one headache that can be done without. (I've been caught on ass-u-me a few times! myself).

The most important point is that the depth of stripping wasn't accurately known and the stripping guy used inappropriate equipment/methodology causing an overrun as a result of his action - how can you then 'ding' him and for how much? There is little to no basis for the extra costs deduction.

 

[Rookie2]

Dozerman and BigH, Thanks so much for all of your input; I don't mean to beat a dead horse, but I still have questions.

Dozerman you mentioned that you worked in Central NC; I work in the Charlotte area. We DO assume strippings (usually 6"-8"depths), even when we have soils reports (usually 4"-6"). The reason is, we have no idea what the guy driving the equipment is going to decide is a reasonable strippings depth. I have no doubt that he has never seen the soils report; Usually I wonder If he has even seen the plans.

Even if he has, very likely he is going to err on the conservative side so he doesn't have trouble with proof rolls, density tests etc.

I work for a small site development firm (9 people). We don't have the resources to be on site daily to inspect construction. When we have offered this, the developers are not willing to pay for our time. Sometimes they pay us to be involved with the contract preparation, sometimes not.

When I asked how do I prevent this problem (strippings) from happening in the future; I should have asked how do I prevent It from being my fault in the future.

I realize I will probably get some negative responses; but I'm just trying to learn.

 

[cvg]

I think it has to do with the clientele you work for as much as anything. if you are expected to do it right, with no data and insufficient time and then are not paid to supervise, but yet are to take the blame for any failures - seems like a win-lose situation (for the developer). These are the kind of clients that we generally prefer not to work for.

 

[TDAA]

cvg is right. Although you may have respect for your client's knowledge, their idea of how to get the job done right is a bit skewed.

I used to work in the Geotech dept of a multi-disp. firm in Richmond VA. I also did a lot of construction admin for them. As you said, the reports will typically tell you 4-6", we usually said 6-9". We had the same situation develop in heavily wooded areas, where the final depth is 18", so when I read your initial post; I knew exactly what was going on.

From my experience in the area: Your spec likely says to remove / strip the organic material form the site. With the trees you have there in the heavily wooded areas, the roots tend to be wide spread, and much of them are in the 12-16” range. When they cleared, they likely just went through and pushed them over, leaving holes some of which can be 2 feet deep (even if they cut them for the lumber, they still normally pull the stumps). They will lightly grade the site as they go to avoid having all those holes, and thus mix the topsoil a bit. This will increase the total to be stripped, but needs to be done to some degree. Even if the roots were left in the ground, as they come through with scrapers, they will expose so much wood that no inspector would tell them to leave it.

So, what can you do?
1. Make it clear, in writing, to your client that you cannot accurately tell the depth of topsoil without geotech information of some sort.
2. Make it clear, in writing, to your client that you cannot be responsible to verify quantities for unit rate bids if you are not there, and that cannot be responsible for contractor overages unless you have the ability to see the differing site conditions and be able to act in a timely fashion.
3. Make sure your spec is clear as to what you want the contractor to clear/strip. Method is up to them, but have a definable goal that could be followed by a field inspector. Indicate you should be contacted if conditions differ, the contractor does not get paid for over excavation, and they are responsible for achieving the design grades.
4. Be sure all know geotech data is disclosed to bidders, but that they know it is informational only, and not contractual. They should have the right to inspect / dig on the site as well.
5. Revise your bids to be lump sum (if your spec is clear what they are to do). This way, the general will watch the clearing / stripping guys to avoid having to do more work. The inspector needs to be sure the spec is followed, and the work is done properly.

My original post was in reference to things you had said about how your current situation came about; however, your contractor will burn you every time (when they need / want to) if your specifications leave them room to that.

So, after all the details, the summary: Define your scope. Define your limitations. Define contractor’s scope. Define how they get paid.

 

[Rookie2]

cvg and TDAA, thanks alot. This information is invaluable to me.