Saturated Soil, Settlement Problem? 1

[aapl2k7]

We're building a shrimp hatchery in Nicaragua (Light Gage Stainless Steel Structure, Reinforced Concrete Tanks), the site is about 350 meters from the beach, today we ran into this problem, over the last week or so, we cut the area of work to remove the top layer of the soil (black mud, highly expansive soil, about 0.85 meters deep), then we started to backfill with stone, throughout the week it rained a lot (we can expect more rain since it's that time of the season), today we found some water so we decided to explore some more and found out that below the first layer backfill was very humid, so we decided to dig a trench, and a couple of hours later the trench filled with water.

My major concern is because of the concrete tanks and the drainage pvc pipes, a little of settlement could be a disaster, i walked through the trench but the soil below felt stable, hard; since it rained a lot i'm sure the lower layers of soil are saturated, but i'm not sure if it's safe to keep filling and compacting. I need help with this one.

 

[aapl2k7]

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[aapl2k7]

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[aapl2k7]

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[aapl2k7]

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[iandig]

Just a quick observation, has the water seeped out from the granular backfill. Its unclear from the images where the water is entering the excavation. If as you describe the underlying soils appear compentant, then it may be the backfill has acted as a french drain and collected the recent rainfall. The water would be 'stored' within the granular backfill until such time as it has attenuated away through the underlying soils, or evaporated etc..

 

[moe333]

It does appear that the the recent rains are collecting in the rock fill you have placed. You want to make sure you haven't constructed a bathtub where rain and surface runoff from adjacent areas will collect in your rockfill. A little collected water won't hurt but if the perched water comes close to the foundations, you could get some softening, and maybe induce some settlement. However, the rockfill may not be affected by the water significantly, particularly if it is basically clast supported as it appears to be. It would be a good idea to drain the base of the fill to a lower area so that the perched water drains before getting too high. You could do this with a series of french drains more permeable than your rockfill.

Perhaps the bigger issue is have you performed a geotechnical investigation with borings? You say settlement would be disastorous. There may well be compressible layers below the firm surface layer you describe. This may be a an upper dessicated layer with softer soils below. The loading from the rockfill and structures may induce settlement if there are compressible layers below.

 

[civilperson]

For construction purposes, pump the excavation down below the rock fill and continue with the fill/compaction. Upon completion of the fill, install drain tile piping below the base of the tank flowing to either daylight or to a sump with a pump.

 

[escrowe]

This condition is difficult to evaluate without knowing depth to groundwater,
the site elevation, and the nature and condition of the
native material underlying the new fill.

If the water entering the trenches is a result of precipitation,
then the impact on a well-compacted fill should be minimal.

However, if the water entering the trenches is a result of
consolidation/drainage of the underlying native soils,
then future settlement is definitey an issue.

Hopefully a geotechnical exploration was completed during the design stage of this project.
As Moe suggests, you need good geotechnical data, including water table measurements,
and Standard Penetration Test (SPT) borings, Cone Penetration Test (CPT) borings or similar.

In the absence of other data, you might consider digging a deeper test pit to observe the underlying soils.
Such a test pit should be located outside of the footprint of your proposed construction.