to deal with a swamp area

[evenbetter]

I need to deal with a swamp area in a roadway project. The swamp area is about 100 feet long along the center line of the proposed road. The groundwater is near surface. A test pit performed in this area found about 7 to 8 feet of dark gray, very soft/loose alluvial soils (ML/SM) with trace roots. Below the alluvial soils, residual soils were found.

7 to 8 feet of new fill will be placed in this area to reach the final grade according to the grading plan.

My preliminary thoughts are:

1. complete removal of the alluvial soils, which is costly and goundwater issue

2. using geogrids

3. surcharging this area with drains

Do you know which option is better (considering time, cost, constuction)? Or do you have other ideas?

 

[Geocontractor]

Have you considered combination method such as a vertical concrete element installed through the soft layers and a geogrid "mattress"? Has been used on several projects for NJ highway department.

 

[inaz]

Geocontractor: I don't understand what a vertical concrete element with a geogrid mattress are. For my own education, would you please elaborate. Thanks.

 

[evenbetter]

Combination is a good idea. But you need to use two subcontractors. It may cost more.

To place geogrids on top of the swamp and add 7 to 8 feet of fill sounds simple and direct. But I don't have any experience.

 

[Geocontractor]

Evenbetter & Inaz:

The concrete elements are created using the VCC (vibro-concrete column) method. Concrete columns are formed by inserting a vibratory "probe" into the ground through the soft materials, then forming a concrete column by pumping concrete through the vibratory probe while it is withdrawn. When the columns are complete a biaxial geogrid is placed over the columns and imported granular fill is placed and compacted. Sometimes several layers of grid are needed and the "mattress" layer can be 3'-5' thick depending on the application. If you go to

http://www.haywardbaker.com

then click on "Services" then "Ground Improvement" then "Vibro-Concrete Columns" there is some additional information. If you need case histories or further information, please let me know. Depending on the location and contractor, you may need only one contractor to perform the entire scope.

Groundwater levels usually do not hinder installation and the system is ideally suited for installation through soft organic soils. Please let me know if you need any more information.

 

[Geocontractor]

One other item, check the "Projects & Literature" tab on the Hayward Baker web page. There is a case history on a VCC project in NJ. Good Luck.

 

[dirtsqueezer]

I would agree with Geocontractor and...Geocontractor.
I acted as field rep for a site with you exact profile; around 100 feet proposed road in swampland, surrounded on both sides with some very nice pond terrain. It was nice for birdwatching, but not so much for building. Hayward Baker was the sub on the project, and installed stone columns, turning the swamp into a road. I always want to recommend this method, but it seems the availability isn't entirely global. The process took on the order of weeks, and is the best method I've seen for this situation.

 

[VAD]

From the description of your soils ML/SM it appears that using a woven geotextile with a surcharge fill would be adequate. Some post settlement is to be expected. More expensive solutions would depend on type of facility etc.

Have you done any consolidation tests on the material. Can you do a partial replacement of insitu material. The vibro columns may be expensive. You need to compare costs, desired outcomes, practicality of solutions before making final choice of remedy. I see no reason to use a geogrid.

 

[evenbetter]

Thanks for all the valuable responses!

 

[BigH]

One other method you might consider since the depth of soft/very loose material is rather small is to use the rolling surcharge method. In this you work by pushing out your embankment not by dumping material over the end, but by forming a ramp up to the end and forcing small failures by running your big dozer up to the end - that would displace the soft/vloose soil. See Tschbotarioff (spelling???) who describes this method in his book (1951 - I know, it is old but many techniques in old books still have their place). I have described this system elsewhere. In Guyana, I was able to displace 40 ft of tailing slimes.