UST rises 4"
UST rises 4"
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.