Pavement Design

[jsullivan52]

I am a young E.I. looking for some advice on things to look out for in a pavement design. The firm I work for has a soil lab, but mainly for construction material testing and quality control. Not really set up yet for design related testing. I have several questions.

1) The first question I have is there anything wrong with using estimated values of CBR obtained from soil properties?

2) What are the depths of bores needed for road construction? I have heard 3' below top of sub-grade and five' below top of sub-grade. Then run estimated CBR on the worst material.

3) What should be the distance between bores? Again I have heard 200' and 500'.

4) Is Hydrometer analysis needed?

I work in northern mississippi in mostly mississippi delta land. I would greatly and help and/or direction in learning the most I can about this subject. Presently the firm mainly just follows the State Aid Procedure and assumes a CBR value of 3.

Thanks for any and all help.

 

[fattdad]

For work on VDOT (Virginia DOT) projects a CBR is required every 500 ft. When I know of a project that will go to VDOT, I do borings at 500 ft spacing that extend at least 5 ft below the design subgrade. This is somewhat problematic in fill areas as you may drill a hole that's 5 ft deep but the subgrade for the road will actually be coming from somewhere else. If it's from an on-site cut, well, that may be o.k.

I do pavement desing based on CBR data. I rarely rely on assumed CBR values. Heck a CBR with Proctor in our lab is a $300.00 discussion - that's a couple hours of a senior engineer - well worth it in my opinion. The problem is that a CBR test takes about 10 days to complete (four days for the proctor and four days for the soak).

I use the AASHTO desing method for industrial pavement desgin. I use the VDOT "Vaswani" design method for secondary roads. In both instances I use 2/3rds of the average laboratory CBR value for my design.

When I do have CBR data for an industrial/commercial project I do use correlation from CBR to subgrade modulus, which allows a more meaningful soil property for the structural engineer that's designing the slab on grade.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[jgailla]

jsullivan,
I am using subgrade to mean the natural undisturbed ground, subbase to mean the imported select material or disked/mixed material to acheive a certain CBR, and base for crushed rock (limerock in my area). Our local regs dictate a minimum 1.5" asphalt, 6" base, and 12" subbase (LBR 40~=CBR 32, usually mixed into the top part of the subgrade), which is OK for residential and low truck use roads.
1. Estimated CBR is usually used. Once you get your subbase down, the underlying CBR doesn't really matter unless it's really crap, in which case it should be removed. CBR of 3 should be a good conservative number.
2. Five feet below top of subbase should be good. You won't save any money doing 3' augers instead of 5'. You need the depth to groundwater and general soil characterization.
3. Distance between borings is dictated by local regulations. 300' spacing is a good number if you don't know the local regs.
4. You really shouldn't have to characterize the subgrade very much. Unless it's crap, you can build on it, and you don't need a hydrometer to check.

I have worked in north Florida and south Georgia. There are a host of other concerns in other parts of the country which may invalidate my responses. Depending on where you are, you may need to worry about frost heave, swelling clays, etc., probably no frost heave in MS

 

[jsullivan52]

Thanks for your help. That clears up some confusion on the testing side of things. I hope to get the lab out soon. The project we are working on is 3 miles of rehab. The existing structure is "believed" to be 8" of black base and 3-4" of asphalt. The current condition of the road is poor, with noticable rutting as well as swelling on about two thirds of the 3 miles, and the rest is in a little bit better condition. Any Suggestions?

 

[jsullivan52]

Oh by the way, does anyone have a good soil property/CBR correlation table? We have one from 1983 that state aid handed out.


Thank you

 

[msucog]

we estimate sometimes but seldom for a bona fide design. when estimating, we use local numbers. i suggest that you perform the lab tests to see what you've really got. we usually check out to 3-5' below subgrade but run tests no deeper than about the upper 2'. we also perform tests on haul in material too. as far as frequency, that's job/client specific.

and being from "missippi" myself, watch out for the gumbos...regardless of whether it's supposed to be there or not. check with someone more experienced in dealing with these soils if you think you've run across them.

i'll try to remember and check my old notebooks from school when i'm back in the office later in the week since i think we had local/state info...it was probably the hand out you mention though.

 

[lovethecold]

As far as depth goes, as others have alluded to, depth of boring should be determined by the amount of fill present. I would recommend at least one sample into natural soil, or sample through peat or other very soft soil/highly organic soil.

I can't comment on the soils in Mississippi, but would recommend using the low CBR value for the given soil type, without a CBR to say otherwise. Also, lean on others more experienced with your soils in the area. I myself am pretty new to the geotech field (a recent grad, coming up on my first year). Geotechnical engineering has a lot to do with actual experience and knowing the soils in the various areas that you serve. I agree with fattdad, $300.00 is cheap.

It sounds like you work for a similar company that I do. We try very much to not do pavement designs. For the most part my bosses consider it too much liability for the amount we get paid. But, when we do do them, it tends to be for parking lots and coutry backroads. It is surprising how little a civil engineer at a company that does pavement designs doesn't even know about the basic traffic data for the road in question. Mostly we find that when someone who wants a pavement design, for something other than a parking lot or backcountry road, just wants the money for doing a design without taking the risk; and usually they want a desing without doing proper testing, or giving the necessary traffic assumptions, etc. So, be careful about doing a road design when it is something you tend not to do. IMHO, designing pavement is as much practical experience as being a soils engineer. At present, I am finding some of this out the hard way.

Welcome to the profession, and good luck.

 

[marblaze]

please visit link for some useful info regarding this matter.

http://malilipotbuilders.blogspot.com/2008/04/cbr.html