STAGNANT WATER IN FILL LAYER OVER CLAY 2

[CISGeotechnical]

I am a Geotech and I'm involved in a residential development project in which a granular fill (silty clayey limestone fragments, about 20-30% fines) is being placed over the existing clayey soil. The fill will be about 2m max. height. Stagnant water is evident within the fill layer just above the existing clayey soil when excavations for the storm sewer system manholes are being performed.

The main concerns are:
1. The proper excavation without water.
2. Possibility of settlement problems in the long term due water in the fill layer.

I was thinking of recommending a french drain system in a low area to connect with the storm sewer system to collect the stagnant water from the fill layer. This may be possible when looking at the original grading of the existing clayey soil. Also I thought that stagnant water may be pumped from sump pits in the excavations for the manholes.

Could any one suggest me another option to deal with this problem based on experience?

Thanks in advance.

 

[georam]

You did not mention whether there has been some settlement at the site. In addition, the consistency and moisture content of the in-situ clay was not reported. Is it a soft clay ? Is the moisture content is relatively high or closer to its liquid limit ? Is the water table high within the clay layer.

My guess is that the water can come from two sources :
- squeezing or consolidation of the underlying clay by the fill, resulting in drainage path at the clay-fill interface. Stagnant water can be cause by development of water pockets in the area, where they could not drain properly. Perhaps if you placed a good layer of granular fill or some geofabrics before placing the fill, and with a good grade, the water would have drained horizontally.
- water may come from the surface (rainfall, surface run-off, etc), water sprinkles, leaks in toilets, etc. It is trapped above the non permeable clay layer.

French drain seems to be a good idea. Wrap the granular fill or the gravel with a geotextile fabric to prevent quick clogging of the drain. Recently, I hear about some horizontal drain PVC pipe which can be inserted/drilled using horizontal drilling technique. This can be done if you can access the end of the fill at fill/clay contact. Horizontal drilling and insertion of drainage PVC pipe can be useful if you do not want to "wreck" the fill by digging trenches for french drains. The perforated PVC pipes can be wrapped in PVC sock before inserting into the holes (as drilled casings were pulled out).

There are also drainage pipes which can be placed in trenches , and can pick up water from different depths within the fill (like multiple plastic/drain pipe stacked against each other, forming a "wall&quot.

Hope that this helps.

 

[KAM_imported]

You didn't say where the groundwater table was located prior to filling. It is possible that you have encountered perched groundwater in the fill from surface water that cannot get through the clay. Compressibility of the limestone fill itself should not be a major issue as long as it is properly compacted. Settlement of the underlying natural clays may be an issue; however, due to the weight of the fill, and should be evaluated. Depending on the thickness of the clay, its water content, and its pre-consolidation pressure, significant time-dependent settlements may occur. The amount of water that gets squeezed out of the clay will be directly related to the amount of primary settlement that occurs. You may want to consider monitoring the fill-induced settlements, and not initiating construction until they have stopped. It would have been useful to grade the original clay surface to drain prior to placement of the overlying fill. Some type of underdrain system may be of value at this time.

 

[CISGeotechnical]

Thanks georam and KAM.
I am involved in this project doing the earthwork monitoring. The geotechnical investigation was performed by another geotechnical consultant. At first I thought the problem might be stagnant water because when I entered the project, due the clayey soil of the site and a quick view thru the boring logs, no groundwater problems were anticipated. But right now, giving a thorough review, I should perform some borings on my own since the groundwater has scatered variations even in the clayey soil which has some "sand traces".
I would call the client to give me time to make a thorough study and evaluate the possible implications since the water may be either stagnant or coming up from the weight of the fill consolidating the clayey soil, which was at first thought by the geotech consultant that it wasn't a problem at all.
I'll let you know.
Thanks again.