Steel Tank Partially on Fill
Steel Tank Partially on Fill
(OP)
We have a large steel tank (>100 ft. diameter) that's planned for the side of a hill. The hydraulics and property line issues dictate that the best location is partially on excavated hillside and the rest on filled hillside. Some of the fill is twenty feet deep. The tank will have a ringwall. My question is:
Does anyone have a concern about this, in particular due to settlement? We'll put in flexible pipe connections. In my opinion, steel tanks are not as susceptible to differential settlement as other types of tanks or, for that matter, other types of structures. Other senior people in our company are worried. We'll ask for very controlled fill compaction, but contractors never can be trusted with earthwork. So I'm not under any illusions that we can eliminate settlement. But I think that the tank is flexible and as long as the settlement is reasonable, it should be OK.
Does anyone have a concern about this, in particular due to settlement? We'll put in flexible pipe connections. In my opinion, steel tanks are not as susceptible to differential settlement as other types of tanks or, for that matter, other types of structures. Other senior people in our company are worried. We'll ask for very controlled fill compaction, but contractors never can be trusted with earthwork. So I'm not under any illusions that we can eliminate settlement. But I think that the tank is flexible and as long as the settlement is reasonable, it should be OK.





RE: Steel Tank Partially on Fill
I believe that you might have some worries about edge failure if it is very near a slope crest heading downhill. 20 ft of fill is a lot - and while it might be compacted well, you might want to consider building the fill as a reinforced fill (see some past experiences with steep reinforced slopes for railway embankments - one that comes to mind is near Hamilton Ontario). Also how are you going to blend the new fill with the existing ground onto which it is to be placed (see Ohio's DOT tech memo on sidehill fills).
While not exactly the same, the paper by Bjerum in the 1957 London ISSMFE conference would be worth a look - in it they had an edge failure of a tank on clay.
RE: Steel Tank Partially on Fill
You may consider installing settlement plates on the fill side and see when the settlement levels off. This tells us when the fill settlement under its own weight has completed, however, we will still have more settlement under the 20 ft fill than under the cut section. In this case, try to take consolidation tests, say 15 ft below the tank floor on the cut and on the Fill side.
From the consolidation test results, your geotechnical firm will be able to tell you if differential settlement is a concern, if shallow ring foundations are still feasible, or if you need deep foundation support below the ring wall foundation, such as drilled piers. If you decide to support the Fill side with piers, you will need to check differential settlement between the pier supported and the shallow foundation on the cut side.
Then there is daylight distance requirement to safe guard against the edge failure, BigH mentioned. I would agree with the concerns of the senior people.
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RE: Steel Tank Partially on Fill
RE: Steel Tank Partially on Fill
RE: Steel Tank Partially on Fill
RE: Steel Tank Partially on Fill
Would the problem - the threat to the tank and its foundation by movement of the fill - be reduced by any kind of retaining wall around the fill, instead of a simple fill and natural slope of the fill earth? Obviously, any retaining wall is both more expensive to build than an open un-restrained slope, but is the advantage of "no slump" greater than the higher pressure of the wall's weight over a smaller area of fill? Could that ringwall be incorporated into the retaining wall design?
Also, is a threat of the higher part of the hill coming down onto the tank over time? Seems like that happens frequently on highway cuts when the rains return.
RE: Steel Tank Partially on Fill
RE: Steel Tank Partially on Fill
RE: Steel Tank Partially on Fill
I you are worried about differential settlement you can use rigid inclusions for the filled area in order to reduce the total settlement in that area and therefore eliminate differential settlement . You will have to consider a 1 to 1.5 m thick compacted distribution layer over the whole area to transfer the loads to the rigid inclusions network. From a construction point of view, rigid inclusions are piles ( steel driven or ACIP, or any kind of piles ) and from a design point of view, they are designed to settle with controlled settlement values. Design is generally carried out using FEM.
RE: Steel Tank Partially on Fill
RE: Steel Tank Partially on Fill
RE: Steel Tank Partially on Fill
RE: Steel Tank Partially on Fill
RE: Steel Tank Partially on Fill
You are talking about 3,000 yards or so additional grading, that is not very much money to fix your problem.
RE: Steel Tank Partially on Fill
I highly recommend a raft foundation system as effective solution to the problem.
RE: Steel Tank Partially on Fill
RE: Steel Tank Partially on Fill
RE: Steel Tank Partially on Fill
RE: Steel Tank Partially on Fill
This is a steel tank, a cement treated aggregate base will exhibit a compressive strength of more than 5 MPa at 7 days (at 4 to 5% cement - and well graded crushed stone) and it will likely increase to greater than 10 MPa at 28d - clearly this is pretty close to the rigidity of the rock and should not, for a steel tank, be of a concern - a few mm, maybe 10mm, of settlement? As for the ring wall, many a large steel tank have been built without them to "support the tank wall and roof. I've been involved with several - one of a 150ft diameter and >50 ft high. No problems