high water table 4

[johnhan76]

My client wants to build a residential subdivision in an area with a high groundwater table (12" depth) I am concerned with the construction and long term stability of the road. It wont have a large volume of traffic. My design will have 6" of graded aggregate base. I dont know if this rock will have any capillary action that could possibly endanger the integrity of the road. Thanks for any insight and possible solutions.

 

[SirAl]

I am interested to know where the project is located. A description of predominate native soil soil type/s would be helpful. Rural or urban setting? Will there be basements for the residential dwellings? Will there be grading associated with the subdivision and if so how will the road gradelines relate? Will the road have ditches, curb/gutter, or neither?

I certainly appreciate your concerns. A relatively clean, well graded gravel (<10% fines) would probably alleviate the capillary concern. With 6&quot; of granular on subgrade set 12&quot; above the watertable, you have a serious geotechnical issue that needs to be assessed prior to road construction. Roadbed/subgrade support and drainage is especially important with open granular roads. Even well compacted gravels will loosen and ravel with time resulting in a loss of gravel. To exacerbate the loss of structure, loosened gravels will accumulate outside main traffic lanes of the road hindering surface runoff that could result in softening of the subgrade below.

 

[jheidt2543]

A few other considerations:

1. With a high water table, particularly 12&quot; below existing grade, the cost for all underground utilities will rise dramatically.

2. The cost of any basements rises dramatically, infact with that high of a water table, I'd advise against basements.

3. What happens in the &quot;rainy&quot; season? Does the ground water table rise more?

4. Is the site in a frost zone?

 

[BigH]

An additional thought - is there any problems with US Army Corps of Engineers definition of wetland??? I seem to remember that high water tables like this, well, might be restricted in development.

SirAl is right about grading plan. If the site is flat, you might not find it possible to drain by gravity road edge drains that you would want to install to ensure drainage under the road.

After you remove topsoil, the groundwater would likely be closer - so I'd say this site is in for major imported fill considerations to provide property contouring (crowns), drainage of lots, etc. I'd also advise against basements unless you install something like those geosynthetic drainage blankets against basement walls and the footer/wall drain can be gravity drained somewhere.

 

[VAD]

The questions raised and questions raised by Sir Al,jheidt2543 and BigH are quite in order as the presence of a water table 12 inches below the ground raises a concern.

I am curious to know if there are other residential subdivisions around this location and if so what were their subsurface conditions and how are they performing. The reason for this is that it seems to me strange that a developer would want to develop this land unless the conditions that exist are not what is portrayed and what we are visualizing or what is proposed allows for the raising of the grade significantly.

Sometimes,we need to look around the neighbourhood to get some ideas if conditions are similar.

 

[johnhan76]

Thanks for all of the interest. Ill attempt to answer most of the questions so that everyone can be more informed.

1. Predominate soil type is Mascotte - description located at usda web site: (

http://ortho.ftw.nrcs.usda.gov/cgi-bin/osd/osdname.cgi?-P)

2. soil type is fine sand to a depth of 36&quot;.

3. The setting is suburban (1/3-1/2 acre lots). currently wooded

4.the roads will be graded. the client wants curb and gutter but i may suggest swales in order to raise the road out of the water. At any rate the road must be lower than the lots for the drainage to work.

5. project is located in the coastal southeast where freezing is not an issue

6. there will not be any basements

7. 12&quot; is the conservative guess for water table depth

8. wetlands have already been delineated for this site

9. property perimeter is deep ditched (>6')

10. adjacent neighborhood built on similar soils ( contractor excavated the muck, installed clean sand and ran drain tile to ditch). this may work here but i am concerned about long term.

again thanks for the help. I probably will consult a geotech but i need to make sure that i am properly informed before going to them.

 

[BigH]

You should most definitely contact a local geotechnical engineer. His guidance will be money well spent. Just as a personal matter. I probably would shy away from building my dream house so close to the groundwater level - if this is the true level, it will lead to many damp days unless you do proper contouring and grading with sufficient swales/ditches.

 

[johnhan76]

i wouldnt put my dream home here either. homeowners nearby complain of damp yards but they seem to get used to it. in your opinion how much fill needs to be placed in the lot to alleviate dampness concerns.

 

[VAD]

I think that at least 2 ft of fill would be desirable, three feet would be best. Low plasticity clay, if available, would serve the purpose as imported granular material in many cases could be cost prohibitive.

 

[johnhan76]

vad - what area of the us do you work in?

 

[VAD]

I work in Canada

 

[johnhan76]

thanks to all who replied. my current plan is to propose the following to the geotech for evaluation.

1. excavate the fine sand in the road 2-3' in depth. use this fill on the lots.

2. replace with 2-3' of free draining sand.

3. install drain tile in the free draining sand along the edge of the road and outlet to the ditch on the perimeter

if anyone sees any problems with this proposal please respond

 

[SirAl]

johnhan76

Thanks for the additional info. Good plan to consult a geotech. There are many scenarios that you can concern yourself with, but I suggest you let the geotech investigate the site and establish the subsurface characteristics. For this site, a groundwater level contour plan would be benefitial to you in your design.

Positive lot grading is paramount. The local authorities may have lot grading design guidelines/requirements. (They do in my neck of the woods). This would, in part, provide you with some parameters regarding lot fill requirements. I would certainly want to avoid final lot grading from penetrating the water table within any portions of the residential lot areas.

From a foundation perspective, I would at least consider the potential of long term subsidence associated with a falling water table. Where infrastructure (surface and subsurface) and positive lot grading is introduced to a wet area, a lowering of the watertable with time could be realized. This could trigger consolidation settlements in some situations.

Just a few thoughts for your consideration. An experienced geotechnical engineer will help you with addressing foundation issues for homes, road design & construction including subdrainage requirements (if necessary), trenching and backfilling associated with sewer and water servicing, etc. I have seen some horrible sites transformed into very nice developments.

 

[gjeppesen]

I work in southwest Florida and having the water table 12&quot; below grade is pretty typical. In seven years of working here I don't think I've ever had a job where cut anything (other than a storm pond).

Other than muck, I've never had a problem around here with this situation. Just fill, fill, fill, and make sure you are in compliance with the state Water Management Districts and local codes as far as minimum road elevations. For example, where I am the road crown must be at least 2' above wet season water table (obviously flood routing must be performed also).

If you're not familiar with Florida, I'd recommend reading a lot and asking a lot of questions. It's very different down here.

 

[johnhan76]

Gjeppesen,

Are you saying that you never cut in these situations - you always fill? So in that case I am assuming that you fill in the lots also in order that they will drain to the street? You recommended reading about sw florida. Do you have any specific recommendations. Id like to learn more about designing in this area since it is similar to my area. Thanks.

 

[lsp01]

Johnhan76,

I have to agree with Gjeppesen, sometimes it is less costly to just fill instead of excavaiting the road. You will probably run into dewatering issues considering the water table.

If you are in Florida, dewatering also requires a permit from the local Water Management District and they are very conscious of erosion and turbidity concerns.

Which leads me to another potential can of worms. Does your location require stormwater treatment and/or quantity attenuation for the development? In my area (Florida) they require both and its difficult to design a stormwater pond with a seasonal high water table 12-inches below ground without filling the site significantly! Considering that the permanent pool or normal water (NWL) of the pond is normally set a seasonal high water table, you have no storage available. Typically, we try to have about 4 feet between NWL and top of bank, this allows for 1 ft of freeboard and 3 feet for treatment and attenuation.

Even if you're not in Florida, be careful of using ditches and swales especially adjacent to wetlands. Some savy environmentalist may figure out that ditches can drawdown the water table and adversely impact wetlands. In my area they consider that a mitigatable impact, and considering the cost of wetlands ($$$$$$) it can change the whole project.

Good Luck...where are you anyway?

Lsp01

 

[johnhan76]

lsp1,

I am in the coastal area of the southeast (not florida). We also have a stormwater requirement to meet and I agree that it is difficult to meet with such a high water table. On this particulart job we are able to have 4' of storage in the pond because their is large deep ditch that we can outlet the pond to and artificially dewater the site.

on a typical site that has 12&quot; seasonal high water table how high do fill the road? fill the lots? thanks for the help

 

[dirtdoctor]

Johnhan76 -

I am a consulting geotechnical engineer in Greenville, SC, and used to work a lot in the Myrtle Beach, Charleston, and Beaufort areas. As far as undercutting the fine sand within the roadway - it can be done by digging deep ditches along the edges of the roadway - just ask Burroughs & Chapin Developers - they did this to a lot of former wetland areas (behind the Corps' eyes, of course). These deep ditches can often lower the water table temporarily so excavation of the fine sand can proceed. If you're still stuck on this idea, consult a geotech to see how much you can draw down the water table.

As far as I'm concerned, I wouldn't cut the fine sand out, but would raise grades everywhere. The one caveat to this is %fines in the clean sand - if the %fines is over 25%, then you should cut to a 'cleaner sand seam, but not more than 18&quot;. Excess fines in subgrade sands caused US 501 in Conway to be the roller coaster that it is. Clean sand fill is cheap on the coast, so try to use as much as you can. If you choose to use an open-graded stone to reduce capillary rise, be prepared to open your wallet, as stone has to be railed or trucked in. After putting 2' of clean sand fill, I would cap off with 6&quot; of clay-sand (SCDOT specs in Section 3 of blue book), then 6&quot; of Section 305A Macadam. DO NOT USE THE COQUINA/SHELL BASE COURSE.

For the residences, make sure that you remove as much clayey material as you can from the footprints and install clean sand fill, making sure that the base of footing excavations is at least 3' above the water table. Based on what I've seen down there, I would bet that the houses will be monolithic slabs or stem-wall foundations (footings w/ a poured slab interior). Differential settlements due to clay lenses above the water table can be deadly down there.

A geotech consultant for the coastal area that is very good is Soil Consultants, Inc. of Charleston and Myrtle Beach.

 

[johnhan76]

dirtdoctor and others who want to reply,

A fine post. The developer is stuck on cutting the roads b/c he feels its cheaper than raising the entire site. Raising the entire site is not a common practice here but we are seeing more of it as the lot prices rise. I am assuming Burroughs and Chapin installed edge drains in their roads after installing them? I am considering a product called Tendrain to help with the edge drains. If you are not familiar with it, it is a geotextile that provides lateral drainage from the centerline of the road to the edge drains (under drains.) I havent hired a geotech yet - will the firm that you recommended have any experience recommending underdrain design? The firms that I am familiar with do not. Thanks for your help.

 

[dirtdoctor]

Not familiar with that type of underdrain system, but have used blanket drains in coastal areas with high water tables. It's going to be your duty to tell the developer that lowering grades is going to cost a lot in the long run, especially if it's a true water table (and not a perched water condition). You could almost eliminate the underdrain system (or greatly minimize it) with properly installed side ditches and raising grades.

As far as a geotech firm, I'm sure Soil Consultants can do almost anything that you need. My first suggestion would be to do some shallow hand auger borings along the roadway alignment to see what conditions are like. Then, I'm sure they would be capable of helping you develop a site specific drainage/grading plan.