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UST rises 4"

UST rises 4"

(OP)
See attached photo.

So here is the information I have. The UST excavation was planned to be approximately 15 ft below existing grades. As you can see on the logs, grey lean clay was encountered at that depth so an additional 3 ft was excavated to the dense sand layer and replaced with compacted structural fill. The additional over-excavation was recommended by the geotechnical engineer of record. After placement of the 3 ft of compacted fill, a geofabric was placed then 2 feet of pea gravelly (gravelly sand) was placed (but not compacted). Deadmen were set, the tanks were lowered, then the tank pit was backfilled to grade with pea gravel (gravelly sand) without compaction (due to tank regulations I believe?)

Further progress was stopped due to rain. When construction resumed, the tank pit was full of water. The tanks were filled prior to the rain event; however the water level in the tanks were approximately at the same level as the water in the excavation. One of the sheet piles was vibrated to be removed, and before vertical force could be applied to remove the SSP, the end of the tank closest to the vibration rose 4 inches. All construction was stopped and the tanks and fill had to be removed because they were not in spec.

Everyone is pointing their finger on this job.

We are the testing firm for the project and were requested to perform two (2), 32 ft deep SPT borings on either side of the UST pit excavation. The owner wants our professional opinion.

Based on all the information we have, I believe liquefaction caused the tank to rise. All 3 ingredients were there, 1) groundwater, 2) very loose granular soil, and 3) vibration. I think the pore water pressures pushed up on the tank while all strength was lost in the gravelly sand and the lateral pressure increased and also helped move the tanks.

It should be noted that after the tank rose 4 inches, they let the groundwater in the tank pit drop to below the excavation depth (they may have pumped it out, Ixm not sure) and then vibrated some other sheet piles and noticed no movement of the tanks.

The owner wants to know what they may need to do different when they reinstall the tanks and my thoughts are, for one to keep the pit dry. No water = no liquefaction.

Pretty interesting happening, and someone is going to have to pay for the reinstall.

RE: UST rises 4"

I was under the impression that liquefaction would lower structures instead of raising them similar to placing your feet in wet sand and as you giggle your body the feet would sink below the sand surface. I do not know the number of deadman used in this project, however determine if the straps have not loosened and then buoyancy played a factor.

RE: UST rises 4"

Let us know of your findings when your remove the backfill. I have to say that when we install UST's for bulk storage of propane, we build reinforced concrete pads with rebar eyelets to strap the tanks.

RE: UST rises 4"

IMHO, what happened is not directly related to the boring log soil properties. It all has to do with the backfill. The only role the soil outside the excavation contributed was the clay ensured the excavation would remain flooded.

I agree with chicopee, the tank floated. Assuming the hold-down straps had some slack, the tank appears to be fiberglass and contained water so it is essentially neutrally buoyant... in ground water. However, during vibratory hammer operation the tank was submerged in "liquefied gravelly sand", not ground water. I don't know the density of liquefied sand, but it is safe to say it higher than water. Archimedes Principle applies to a submerged object floating if the object weighs less than the weight of the volume of liquid the object displaces. The weight of the volume of displaced liquid depends directly on the liquid's density.

As for who is accountable, there is plenty of "blame" to go around.

For reference, here is a photo from the web of a sewer line that "floated" due to soil liquefaction during an earthquake:




www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: UST rises 4"

(OP)
You are right slideruleera, the excavation is essentially a bathtub to perch water. I agree buoyancy contributed to the movement of the tank. I believe the pea gravel liquefying is what triggered it.

RE: UST rises 4"

(OP)
The tanks are fiberglass.

RE: UST rises 4"

Agree with Sliderule Era. I've seen migration of granular material laterally without much vibration nearby. I'd also add that the tank water may have been lower or the outside water higher. What was the thickness of the base slab and its submerged weight?

RE: UST rises 4"

So no base slab. No significance hold down resistance from "dead men" due to their weights when submerged. Vibration easily moved aside submerged granular material sitting above them to allow tank to rise because of this minimal "hold down" resistance. Take the unit weights of gravel, concrete, tank and tun the numbers with or without submergence. Big differences.

RE: UST rises 4"

Can you use well points to depress the water table and maybe a sump and pump to drain it.

Dik

RE: UST rises 4"

While I guess one could expect arguments as to exactly how much buoyant force was exerted, it sure sounds like whatever was installed to counteract same was not sufficient on one end for the flooding and so on that ensued. Perhaps the EPA reference at https://www.epa.gov/sites/production/files/2014-03..., as well as the Petroleum Equipment Institute’s (PEI) Recommended Practice 100 and American Petroleum Institute’s (API) Recommended Practice 1615 referenced therein, containing information about proper anchoring of USTs and corrective action in case same occurs may be helpful.

RE: UST rises 4"

Slide Rule Era - on a side note, in Christchurch NZ, which is highly susceptible to earthquakes, they use a Floatleess Manhole. The principle is that holes with no return valves in the side of the manhole wall allow excess pore pressures to dissipate into the manhole and prevent it floating. Thought you might be interested. The attached PDF also uses your pic above.

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