Storage Tanks - Borehole Spacing

[Trin.Eng.]

The textbook entitled Foundation Analysis and Design states that there is no binding rules for the number or depth of soil borings.

However, PIP VECTA001 recommends for large tanks a minimum of 3 borings. The PIP STE03020 defines a large tank as 50 ft. diameter or greater.

Please advise whether there are any other available guidance with respect to the number, depth and location of borings for the design and construction of storage tanks?

 

[Ron]

I like the 3 hole approach as well. It follows my approach for any testing process....a minimum of 3 tests. My reasoning....one test can be very misleading. Two tests with different results can be confusing. Three tests will help determine the minimum correlation between tests (it will usually validate one of tests, one or two)

 

[EireChch]

Also remember, at least one of those boreholes should extend to approx 2 times the tank diameter, so 100ft.

We have done 120m deep boreholes for a 60m tank last month.

 

[GeoEnvGuy]

In my opinion an investigation is required to provide the design engineer with a reasonably accurate conceptual model of the subsurface conditions at the site. Because of the complexity of natural deposits no one method of exploration is best for all situations. The field and laboratory investigation program must be adopted to the soil conditions and size of the job, so that the design engineer can provide a safe and economical design as well as inform the construction engineer about the materials and conditions he will encounter.

 

[1503-44]

120m deep for a tank? Seems way OTT to me. I'd much rather have 6 to 20m. In any case more is definitely better.

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[HTURKAK]

The following is excerpt from PIP VECTA001
Tank Selection Guide ;

(....Generally the norm for large tanks is a minimum of three borings, equally spaced at 120 degrees on a circumscribing circle 5 feet outside the foundation..).

This criteria could be norm and representative of soil conditions for a single large tank.. However, in the case of a tank farm with 10 -20 tanks, the no. of borings should be decided acc. to size of the tank farm and geology . The number could change acc. to outcomes of drilled boreholes..and if geotechnical group feels necessary, they could increase the number..

 

[1503-44]

Check it out on Google Earth
36°53'45.66"N
35°55'17.06"E

[highlight #CE5C00]100m Diameter Tanks[/highlight][highlight #EF2929][/highlight]

6.2.1 Geo-technical conditions
As part of the soil investigation studies that have been previously performed, a total of 6 borings have been made on site and the samples revealed that the top layer of the soil consists of sandy / gravelly clay with a layer thickness up to approximately 6 m.

Under this layer are located layers of vesicular basalt and basaltic breccia.

Tests made on the samples have shown that the vesicular basalt and the basaltic breccia layers consist of highly to moderately weathered material.

Uniaxial compressive tests on the samples have shown various degree of strength, ranging from 48 MPa to 278 MPa for the vesicular basalt and from 10 MPa to 38 MPa for the basaltic breccia.

The previously performed TCR and RQD test results, have shown the vesicular basalt and the basaltic breccia as highly to moderately weathered.
Also the previously performed pressure meter test results have shown a modulus of deformation of minimum 16 MPa for basaltic breccia and minimum 56 MPa for vesicular basalt. These values can be used for estimating the settlement of the tanks.

The soil pressure under the tank, derived from the height of the liquid column and weight of the tank is 0.20 – 0.22 MPa maximum.

These values are far less than the uniaxial compressive strength values of the rocky sub grade and therefore the bearing capacity of the vesicular basalt and the basaltic breccias layers of the sub grade is sufficient.

Soil characteristics of the top layer of sandy / gravelly clay are not determined. These values shall be obtained by tests during detail design.

In order to obtain a sufficient bearing capacity for this top layer, compaction to 90% of the Proctor value is required.

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The borings shall be drilled with rotary type machines equipped with a hydraulic feed or hollow stem auger type drilling equipment and capable of securing satisfactory samples of the required diameters to a[highlight #EF2929] depth of maximum 60 meters[/highlight]. Casing shall be used where necessary to keep the hole open and enable the boring to advance. When bedrock is encountered during the drilling process, rock shall be cored to a depth of 3 m to confirm soundness of the rock. If rock is sound no further drilling is required.
==============================================================================================

Im happy with the boring depth given there. I'd think a boring depth to 1/2 the tank's diameter would be sufficient in most cases.

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[EireChch]

"Its OTT" - thats the main issue in geotechnical engineering. Projects with tanks of up to 100mm id dia are probably turning over millions-billions of (insert currency) a year. The budget cant take another 20k to drill a deep borehole??

If youre happy to take the risk and go to half D then go for it but I would not be happy to do that. A 100m tank has a zone of influence of approx 1.5-2B so 150 to 200m. A settlement calc needs parameters to at least 150m-200m. You are making assumptions below 60m at your own peril.

Ive been there before and reluctantly made those assumptions but not after I have first highlighted that we want a borehole to 2B.

Edit 1 - I agree with a shallower borehole if you are confident your in continuous rock.
Edit 2 - I noticed in my earlier post that I said 120m for a 50ft tank. That was an error, should have said 2B so 100ft!

 

[1503-44]

EireChch, Thanks for addressing my concern. I was thinking about smaller tanks as well (100ft), when I mentioned 20m, so it is true that I might be pushed to accept a 1D boring depth, if there was sufficient reason to justify deeper borings. I also point out that the 60m depth there was not conditional with finding rock. As those are about the largest tanks, both in diameter and height, you will ever find and the area is subject to severe earthquakes, there was probably considerable risk, yet they did not recommend deeper bores. The tank's liquid load alone when full is also around 4000 psf, with additional 20m shell load on the rings, so tank loads just don't get much higher than that. The tanks have been there for 15yrs, so fortunately they seem to be performing well.

I have always been told that differential settlement is the greatest risk for tanks. We know that there will always be settlement, so that is accounted for by flexible pipe configurations at connection points. The 3 borings are made to determine uniformity sufficient to preclude differential settlements. Would not the most risk for differential settlement occur in the shallower zones? Is it not unusual to encounter soils sensitive to settlements below 60m? That's at least about 1500psi soil weight at that depth.

Justifiable costs are not argued about too much, but then again, if I doubled the cost of everything on the project, I wouldn't be around long.

So, do you think they (I) accepted too much risk with the 60m bores?




Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[EireChch]

Yes I 100% believe that 60m deep borehole is totally inadequate for a 100m dia tank. One borehole should be taken to at least 1.5B.

Just because you "got away it" doesnt mean its justifiable. If you are in granular soils then the risk is greatly reduced. However when you get into CLAY, even if its OC CLAY, the settlements add up. Secondary compression in OC CLAY, although quite minimal will still add up if you have sufficient thickness of it.

Any soil, not matter how deep can and will experience settlements if there is an increase in load.

I agree that differential is the main concern and also that this is more prevalent in the shallower layers. Hence why I would go for 3 boreholes with at least 3 CPTs per tank.

 

[1503-44]

No problem with 3.

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[BigH]

The depth of boreholes . . . of 1.5 to 2 x diameter. For 50 m storage thanks that would be up to 100 m. Fine with a few caveats - IF the stratigraphy is soil throughout the full depth. Would one go that deep if they hit bedrock at, say, 15 m? - and prove bedrock by at least 5 m and review of geological setting? Doubtful.

 

[1503-44]

No. Typically bedrock drilling stops once the integrity of the rock is proven, even at 15m depth.