Public road on peat & muck

[MSU95]

I am working for a private developer that in it's negotiations with the local road commission have agreed to improve 1/2 mile of an existing gravel road. The agreement was not read over by an engineer and they have agreed to remove all organics and unsuiltable soils. At the time of the signing of the agreement there had not been a geotechnical investgation to determine the subsurface materials and their suitability of the existing subgrade.

Recently, we had a local geotechnical firm drill 1 - 10' deep borehole every 200' at the outer edge of the existing 18' wide road. The results were not what the developer had hoped for to say the least. It turns out that the road bed has an average depth of 5' of peat and muck, some areas 3.5' and a few areas as deep as 10'. The road commission is standing firm that the full depth of peat and muck must be removed and replaced with suitable engineered fill. The existing roadbed has been built up over the years to consist of roughly 3-feet of road gravel. The road gravel is of varying quality and it does not appear that it would be able to be re-used on the road reconstruction.

My question is, assuming we remove the 3' of unacceptable gravel, how deep does the removal of the peat and muck need to be before we can expect to achieve a suitable road bed. This also assumes that the design would include the installation of some form of geo-grid or styro-foam block.

There is also a county drain located along the full length of the road that is wet year-round.

Thanks in advance for your time and advice.

 

[VAD]

A few questions:

1. What is the performance of the existing roadway

2. Is this road to be paved later

3. What is the nature of material below the peat and muck. Is it stiff, soft etc

4. What is the ground water disposition re water levels from driling

5. What is the topography of the roadway site and surrounds

6. How long has the road been in existence

7. Is it possible to raise the roadway grade or is this restricted by site and other conditions.


I would venture to say remove at least 1m of the peat and muck. However, I would be careful about the characteristics of the underlying soils. If material below 5 ft is stiff then I would remove to that depth. If it is very soft, then I would stay at the 3 ft level. The use of a geotextile may be required at the excavation depth before placing new material. Generally, depending on site conditions some may want to excavate the 10 ft deep deposits. However, this may be costly and would also depend on whether the road will be paved or not. It appears that it will be paved hence the reason for the City requiring removal of all unsuitable material.

With the drain being wet all year round I would be inclined to think that there is a spring source. Depending on topography, this may be providing water within the roadbed and causing problems as well. This shoud be addressed.

Boreholes are good but for the situation presented, a few testpits with a backhoe can be very revealing. I would recommend that you undertake such an investigation as well, it could help in the decision making.

Regards

 

[Ron]

A few questions come to mind...

Has the existing roadway settled? Is there evidence of undulation of the subgrade due to the variable peat/muck depths?

If not, the 3 feet or so of gravel is providing a high modulus bridging layer that should be left in place. If no significant settlement and no additional fill needed to achieve desired grade, consider adding base and surface courses onto existing material.

Check proposed traffic to make sure no significant changes.

 

[jimbo]

If the agreement says "...to remove all organics and unsuiltable soils," Does your client intend to honor that statement?

The client may be stuck, as it appears. There is no evident option to not remove all organics and unsuiltable soils.

jimbo

Buy a dictionary, keep it nearby and USE it. Webster's New World Dictionary of American English is recommended, and Webster's Collegiate Dictionary.

 

[dmoler]

Ouch, jimbo, harsh ending. Though I agree that that was idiotic to agree to something without first checking it out.

As far as reusing the gravel, why not? In Florida, the limit is 5% organics for subgrade. However, there are depth control lines for removal. Go to

http://www.dot.state.fl.us/rddesign/rd/RTDS/Standard_Index.htm

scroll down to index no's 500, 501, and 505 and download for some good guidelines on removal beneath roadways of organics, and limits on embankment utilization of fill.

I am not sure why you would remove muck and replace with granular fill and also use a geogrid. I guess you have some kind of combination in mind with some removal and then geogrid?

I have used two layers of geogrid (BX1400 and then BX1200) before for similar circumstances. Remove gravel, put bottom of geogrid on muck, fill, put another beneath base layer. Actually works pretty well.

 

[DRC1]

I have used geogrd for unpaved haul roads. Generally needed 3-5 feet good material over the grid. Check out

http://www.tensar.com.

They can put you in touch with the local engineering rep who can sell his stuff not just to you but the road commission as well. Granted agreeing to remove all organics prior to knowing what all was was not the best move, and I'm sure wont happen again, but in your sides defense, nobody knew and everbody knew nobody knew, so the expectation goes to what was resonable, and if you can explain what you thought was reasonable in a rational manor, that should become the benchmark for your obligation.
Good Luck and let us know what happens.

 

[rockchip]

Definitions and geogrid systems aside, the essentials of the site are similar to what occurs in Western Washington and Oregon: Thick deposits of saturated orgainc soil right where we want to put roads and buildings. Based on the described problem, I do not believe full removal of all the organic soil is warranted or practical. One of the most critical elements of design on organic soil is the compressibility of the soil, and for this we run consolidation tests on the organic soil and take care to obtain the time rates of consolidation for several load increments in the range of anticipated real-world stress. On important thing to note: the presence of organics means the consolidation of the soil will continue long after the primary compression has been achieved (this is time dependent consolidation as opposed to load-dependent consolidation). This implies the existing gravel road in question, even if nothing is done, will probably continue to settle because the original 3 feet of organic soil was replaced with 3 feet of even heavier gravelly soil. Thus, care should be taken to avoid adding more fill which would cause even more settlement. To cut to the chase, we have found preloading to be useful for these types of soils. Preloading requires up to 10 feet of temporary fill to be placed on the subgrade and left for up to 6 months or more. Monitoring of settlement is necessary to square the predicted with the real settlement at the site. Be aware of bearing failures on soft ground below the preload fill. Creative solutions used by local jurisdictions also involve lightweight fills (sawdust or sawdust/fill mixes, or styrofoam) and combinations using geogrid. Beware! Using styrofoam in high groundwater will result in a floating roadway!

 

[dmoler]

Rockchip,

I have not used the stryofoam you are talking about, but have much experience in organics (unfortunately). What would be the best, say, definitive, works and studys on the styrofoam approach? I have heard of this but dont know much about it. Reply greatly appreciated.

 

[jheidt2543]

Just an addition few thoughts from a different perspective to consider:

First, as a few have already noted, to make this agreement prior to doing the borings was foolish. But, now that you have seen the borings, you MIGHT make a case for "changed conditions". If you can show that the subgrade is so different from what is normally expected in the area, then fulfilling the contract would be an "unjust hardship", the phrase I think the attorneys use.

Another tactic might be to show that the local road commission knew of the problem and didn't share that information with the developer which would be "unjust enrichment". This won't work if it is a local developer.

However, I'm not an attorney, so perhaps instead of fighting it out in court, the threat may produce a more reasonable attitude. I think the geogrid ideas noted above would be the least expensive alternative all around. Spend the money on the problem, not on the attorneys.

One caution to keep in mind, never threaten something you are not prepard to carry out. Good luck!