Grounwater encountered during trenching 2

[LarryTx]

Significant flow of groundwater is entering the trench from the side, along a long segment of the storm sewer pipes. The pipes then turn and run in the direction of flow. Geotech and contractor suggest placing rock in the trench with filter fabric above the rock. This seems like a common, reasonable solution.

The developer has asked for assurance, in writing, that this is a viable "long-term" fix and that streets and building foundations won't be affected in the future. As a general civil engineer, I am in no position to offer such a "guarantee", particularly when the extent of the seepage (between two rock seams) is unkown. My "guaranteed" solution would be to construct an interceptor trench and direct it around the site, leaving it open, but this would result in the loss of several housing units and significant re-design.

I'd love to hear any thoughts or advice as to handle this situation.

Thanks

 

[cvg]

perhaps the developer should be asking for "insurance" (from his broker) instead of "assurance" from his engineer

 

[LarryTx]

Agreed! Our insurance wouldn't cover our assurance.

 

[fattdad]

In very permeable soils, you can dig the dirt from the trench faster than the water can come in but it still wants to. It wants to flow into the trench and restore the original ground water level. If you did nothing, the trench would fill and then the flow will stop.

If you replace the dirt with open-graded gravel, compaction moisture content is no longer an issue and the pipe backfill will be proper for strength (with minimal tamping to support the haunches).

When the backfill is complete and surface fill is installed, you will have a stormwater pipe surrounded by saturated gravel. The water table may leak into the pipe, but soil fines are likely to migrate with this flow. Filter fabric atop the gravel is good practice, but not likely to affect long-term performance. Filter fabric on the subgrade may actually be better, to separate the gravel from the wet subgrade.

Just some evening ramblings. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[beej67]

Well here's my two Spanish Doubloons:

I would be very careful not to "guarantee" ANY solution, when it comes to groundwater or stormwater. Groundwater comes from stormwater, and stormwater comes from the weather, and the weather comes from God Himself, or agents thereof. Who are you to guarantee what God will do? (or "the weather dice," if you're atheist, whatever)

There are no guarantees in this corner of the business, just best practices. In asking you to guarantee an engineering solution will work, he is asking you to know with certainty what the weather is going to be like in the future. You need to make him understand that you cannot do that. The best you can do as his engineer is give him the best available solution within his budget.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[LarryTx]

Papa Gordo es el mejor! Thanks much - I think the developer has visions of pressurized water shooting out of the pavement. I'll have to school him on the laws of gravity.

beej67, nice summary. I suppose I could ask if his roofers must guarantee that their roofs will never sustain hail damage.

I really appreciate everyone's thoughts and musings.

 

[oldestguy]

While i won't get into guarantees, you might think about what flowing water in bedding for pipes can do.

This usually is a "during construction thing", but ordinary bedding stone is not a filter and unless you have a filter between that soil, beside, under, and above that bedding, you can have erosion take place.

Given that flow for a significant time, you can lose some support for the pipes. This usually means, during construction, since once closed off, with backfill, flow usually stops.

However, that loosening of support may not show itself right away.

The city of Rhineland, WI had this happen for a sanitary sewer project, with the settlement resulting in pipe breakage continuing for some time after the job was done.

The best practice is to keep the water from flowing, by catching it or diverting it before it gets there. Well points, deep well, etc. That responsibility is the contractors, IF you have written the spec to say so.

 

[LarryTx]

oldestguy, thanks for weighing in - I was hoping you would!

I failed to mention that the excavation is in rock. There is a 1 to 2 foot clay layer at the surface, with a hard limestone layer beneath, and weathered limestone through which the groundwater appears to be flowing at a depth of 4-5 feet. The geotech recommends placing gravel pipe bedding, and larger gravel (1" to 3") above the bedding across the area where water is flowing. On top of the larger gravel is filter fabric, then backfill, then geogrid, then road base and asphalt. Thus, any groundwater will have a flow path through the gravel. Although flow will be impeded by the pipe and gravel, water can also move through the seam (bisected by the trench) as it always has.

With a rock cut, we don't expect a significant loss in pipe support.