Excavating near the toe of a dyke 1

[georam]

 A 25 ft high dyke sit on top of compact fine sand. The water table at the downstream toe of the dyke is close to the tail water level, ie. the fine sand is almost completely saturated. Removing the sand material beyond the downstream toe of the dyke will cause instability of the dyke. What techniques or methods which can be used to ensure the removal of the sand can be carried out
without jeopardizing the stability of the dyke (want to remove the sand as close as possible from the D/S toe of the dyke). The excavated sandy material may be replaced with rockfill or other fills in the future, if required for long term stability. Excavation may extend to about 20 ft-25 ft deep, which will be in fine sand below water table. Are there techniques where sand can be removed and replaced simultaneously (other than techniques similar to the installation of slurry trench, or stone column placement) ?
PS. Experience in excavation of the sand below water table in the area has been difficult. The sand turned into soupy, flowing consistency. Removing this deposit nearby the dyke, without any shoring or ground improvement techniques may result in catastrophic failure of the dyke.

Would like to hear any ideas from the forum. Thanks.

 

[KAM_imported]

You might want to consider reinforcing elements such as stone columns, lime-cement columns, or vibro-concrete columns in the area to be excavated. Contact a reliable ground improvement contractor such as Nicholson Construction Co. or Hayward-Baker, Inc. for more info and pricing. They will do preliminary design for free usually if you give them field and lab test data.

 

No bentonite slurry is allow. Tough! Are you ready for some big extra from the contractor? How about ground freezing?

 

[emmgjld]

georam

I think we need a little more information. It would be helpful to know where this project is or at least what 'level' of technique might be availible. Are the restrictions due to lack of equipment or lack of access.

 

[bmike]

You might be able to excavate and backfill in narrow trenches excavated at right angles to the line of the dyke. You may not end up with a nice and clean backfill zone but this may give you enough end effect to stabilze your excavation, for a while. It will also limit the dyke area that will fail if your excavation collapses. We have had some success excavatung and backfilling in tidal marshes this way, but the material was somewhat cohesive and had some stand up time. You will want to be careful in sand.

You might also try a steel sheet pile along the dyke toe which you could extract as you backfill.

 

[dirtman]

There is a technique for putting in a drain in saturated sands. Go to a search engine such as Google.com and input for the search string, "bio-polymer slurry trench". Also, go to inquip.com and geocon.net for more information.

 

[drc]

Wow! This is a very tricky problem. You probably need to sheet the excavation. The fine sand has little shear strength, and with a 25 foot head and satuated condtions, could easily blow. The sheeting would probably need to be tied back, and the depth of the toe of the sheets would be controlled by the pumping requirements. The fine sand can make a considerable quantity of water, and without the cut-off, would be difficult to dewater. The soupy condition you refer to is a condition known as running sand, and can be extremely hazardous in which to work without proper shoring. The concern is not only for the excavation, but also for the base stability of the embankment. Even if the embankment does not fail, defomation could damage the core or filters, leading to a piping problem, which in time could destroy the embankment. Also, if you plan to fill the excavation, it should be filled with a graded material to prevent piping. Rock fill would allow the fine sand to move into the voids, weaking the base of the embankment. This is definately a design problem for an engineer with considerable sheet pile design or dewatering design experience.
You should also get an experienced and reputable sheetpiling contractor. This is not a project where the first concern should be the bid price.