Large Underground Circular Water Reservoir

[Nonlinearer]

I am designing an underground circular tank for water retention. Its a big storm water tank. It has 25 meter diameter and 30 meter deep. The outer strata is rock (qu=20000 to 25000 KN/Sqm), its lime stone. Its dry density is almost as of hardened concrete, i.e 2.46 MN/m3. Water table is also not an issue its just 4 to 5 meter from bottom of tank. The top is covered with a roof and some backfill on top. Some internal columns are also provided to support roof. There are two scenarios in design, I am considering;

1- I have talked to a soil expert who has experience in deep shafts and mining. According to him, there is not any structural circular wall is required. Only shotcrete on the internal face of rocks (after excavation) will be enough with some rebar mesh.

2- If I think structurally, considering some gap between structural wall and outside rock, the internal water pressure is governing and I am ending up having very high reinforcement for hoop tension. Probably pre-stressing would be required.

Can anybody shed light on designing of such a Tank?



IR

 

[MiketheEngineer]

I am not an expert on this.

Properly installed shot-crete will use the rock wall as support.

Many a swimming pool is made just this way.

 

[hokie66]

Sounds like this is a good application for a shotcrete lined tank. What is the end useage of the water, or where does it go? Is the limestone fractured?

 

[Nonlinearer]

I have to check if the limestone is fractured. The end usage, I am not fully sure, probably will be pumped out somewhere. As the shotcrete looks like an obvious solution. How can we check the design or adhesion of shotcrete to the rock? the thickness of the shotcrete? As in a normal circular wall, whether to check shotcrete for this hoop tension from water? If adhesion is proved to be OK, whether this hoop tension will be transferred to rock face and how it should be checked. I am thinking of doing shotcrete in circular patterns of let's say 2 meter high to form continuous rings of shotcrete on rock face?

IR

 

[MiketheEngineer]

Talk to a shot-crete guy or maybe there is an association

 

[bimr]

Before the design of the tank can be initiated, you have to know more of the design conditions.

Does this tank have to be water tight? Will the tank normally be empty? What will the stored water be used for. etc.

 

[miningman]

This type of thing is bread and butter for any competant underground mining contractor. However the advice you have been given misses the fundamental issues. I'm guessing that your tank is essentially being excavated from surface. If so I will guarantee that there are sufficient fractures and planes of geological weakness that without some sort of lining,water will escape(or possibly flow into) your excavation / tank. Additionally 25 meters diameter is a big hole and local areas of geological instability will be present. Shotcrete at about 3 inches thickness would provide adequate strength and prevent water flow, but I would anticate that unless you are looking for only a 5-7 year life expectancy, a poured concrete wall will be necessary.

I have worked in a number of deep shafts and get nervous with more that 40 feet of unsupported ground above me. I suspect you'll end up bolting and screening the walls on the way down, possibly with localised shotcrete, and then form up to pour your concrte liner, mininum 2 feet thick , probably 3 feet.

 

[Nonlinearer]

miningman,
Exactly what I was thinking, shotcrete itself might be ok, though high quality control would be required in implementation of shotcrete, but the rock face can not be guaranteed for the life span of the tank which is minimum 50 years. Specially it lot depends upon the excavation/ rock removal techniques which may not be fully controlled and micro fracturing in the rock face may become a big crack thus affecting the strength and water-tightness.

IR

 

[miningman]

OK, start assuming that excavation will be by drill and blast techniques. Overbreak can be controlled by perimeter blasting techniques but dont even think for a minute that this will be a nice smooth wall like you might hope for. Short term rock support will require split sets and wire mesh, and depending on the geotech's reading of ground conditions, probably 8 foot resin grouted rebars.

I'll almost gurantee that the 30 feet of rock closest to surface will be what I would call " rotten rock" Nothing that experienced mining perssonnel cant deal with, but it might appear horrendous to engineering personnel who atre unfamiliar with blasting and poor ground conditions.


I can think of a few ways I might excavate this, none will be cheap. In fact I predict the owner wil be horrified when he sees the first estimate prepared by someone who knows what he is talking about. Just the removal of the broken rock from the bottom of the hole as it gets deeper presents some interesting challenges.

 

[Nonlinearer]

30 feet seems to me too much. Is it experience based? or relevant to the rock properties which I mentioned in my first post on this thread?

IR

 

[miningman]

Experience based. It might only be 12 - 18 feet, but it will be more that 6-8 feet. Thick about the concept of geological erosion and weathering over say 10-20,000 years. Freeze / Thaw cycle, possible glaciation, etc etc. Theres probably a layer of organic matter, clay, grass etc on surface right now but that masks what you'll be dealing with once the surface deposits are stripped off.