Spread Footing Adjacent to Canal + High Moisture Content Soils

[Trouser]

I am the geotechnical designer for a county bridge replacement over a canal. Spread footings are to be placed within the dense embankment fill. The subsurface profile is attached.

The canal flows seasonally and the subsurface exploration took place approximately two weeks after the canal went dry for the year. Moisture contents in the embankment fill were in the realm of 20-30%, the Tephra (30% Fines) had moisture contents of ~65%, the fine alluvium deposits moisture contents were in the 10-15%, and the flow breccia's moisture content was near 5%. The boring terminated well into a basalt bedrock unit at 70' bgs and no water table was encountered. One hole was placed within 5' of the proposed bent locations and within 10' of the canal (two holes total).

I am assuming that the embankment fill, tephra, and fine alluvial deposits are saturated during canal flow. This is based on the relative moisture contents present in the soil samples after a two week dry canal period. The drainage path is downward towards the breccia. The relative moisture conents would suggest that the Tephra has the lowest hydraulic conductivity and serves as an aquitard (the underlying alluvium has a lower moisture content). The Tephra acts as somewhat of a barrier and keeps the breccia relatively dry.

When calculating effective stresses I intend to use the buyant unit weight of soils down to the base of the fine alluvium deposit; however, below this point the soils are no longer saturated/submerged and will feel the full weight of the soils plus the water. In essence, the breccia will be unsaturated and will have vertical effective stresses calculated using the submerged unit weight of the overlying soils without subtracting the pore water pressure. Is this correct?

Are there other considerations that need to be taken into account for the construction of spread footings on soils with such high moisture contents? Are these values indicative of compressible soils?

 

[motorbiketocrank]

Curious - Is the canal lined (otherwise seems like it would have a hard time holding water)?

You mention bearing footings in the "Dense" embankment fill. I presume you mean below the loose upper 11' shown on the boring?

I think you may be analyzing the layers of soil in too much detail. Basically you have silty sands over gravel (weathered rock?) over rock. Not sure how you plan on using your various assumptions regarding water table and buoyant weights, etc. For strength related calculations you should conservatively assume a high water table with all the subsurface in a buoyant condition (will reduce bearing capacity and lateral resistance, etc). Don't think the water level issues will really factor into your settlement calculations.

Also should consider the geometry of the canal (i.e. are there slopes that will reduce your foundation bearing capacity, present global stability issues, etc). Additionally, sometimes with canals you have to worry about rapid drawdown slope stability failures, etc (which is a real problem if your foundation fails with the slope).

I'd really be strongly considering deep foundations. And ensuring that they're far enough away from the canal to not be impacted by possible slope stability issues.

 

[Trouser]

Thanks for the input geobdg. The canal is not lined. I would say that there is a significant amount of water loss based on the sections of cattails strewn about. I think this serves as "irrigation" for many of the fields adjacent to the canal.

I was thinking through my scenario last night and came to the same conclusion as you recommended. Just assume that the canal's water level is the water table. As for the footings, we plan to excavate the loose embankment fill and place footings on the denser fill. The footings are bearing +- 2 feet from the toe of slope and are located ~5' beyond the top of slope. There are no stability issues. My pimary concern is serviceability, which is why I still have the question regarding the compressibility of soils with such high water contents (Tephra @ w = 65%).

This breccia, as our geologists speak of it, is typically found above basalt flows and is angular rock fragments in a fine grained matrix. These can be thought of as the debris that roll along above the flow as the flow progresses.

 

[cvg]

you have 22 feet of tephra with variable densities which should be assumed to be collapsible. adding water and a significant load could cause a collapse.

 

[Riggly]

Based on your boring log, this profile looks pretty good for a shallow foundation system. However,it would depend on the type of bridge and loading How big of a bridge are we talking, what type - what span length is being supported? Have that canal ever overflowed it's bank, or will the foodings be outside the flood plain? If not scour during severe storm could be a concern with the introduction of the footings in the possible flow path.

I would think that practically all settlement will occur during construction. And if further settment occurs I think it would be minimal. If this was over a roadway I would be more concerned about settlement. But typically the problem I notice with a lot of bridge is that the approach slab typically settles significantly more than the bridge itself and that poses a maintenance and ridership problem. Those bridges are generally on deep foundation, and I wonder if this differential settlement would be made smaller with bridges on shallow foundations.

If you are conerned about that sandy soils in your bearing zone how about considering some deep soild treatment rather than drilled shaft?