Modeling of shotcrete and rock 1

[Nonlinearer]

I am designing an underground circular water tank which will be built in rocky strata. The tank is almost 30 meters deep and 25meter in diameter. Due to high strength rock (Limestone). Water table is not an issue, its just 2 to 5 meter from bottom of tank but seasonal variation may exist. I have talked to Geotech/Mining expert who is suggesting to use only shotcrete (with mesh) on rock as a lining to form the tank. As a structural Engineer, if I design this tank, assuming some gap between outer rock and my structural tank or if the displacement occurs from the tank due to internal water pressure and mobilize the hoop tension in the tanks circular wall, I end up having very thick circular wall and probably prestressing would be required. I wanted to go with Geoteck/Mining expert's opinion. I wanted to check the shotcrete adhesion to the rock and the strength of rock to take the water pressure of this 30 meters water head. Rock is obviously very strong but want to run my numbers. My questions are:

1) Which approach is recommended, shotcrete on rock or separate circular structural wall inside?

2)If shotcrete on rock then how to check the shotcrete adhesion on rock and adequacy of rock just behind the shotcrete. I want to model it in some software as a small segment of the tank's cross section with two materials shotcrete and rock as a solid model where I could apply the water pressure on shotcrete and could see the atresses inside?

IR

 

[chanskat]

Nonlinearer,

Interesting project. Regarding:

1) Shotcrete is a natural for either rock face stabilization or creating a circular wall. If building a structural wall, shotcrete can be placed with reinforcement identically to the cast concrete option, but without the need for any formwork, casting and vibration of fresh concrete. Shotcrete is routinely used to stabilize tunnels and deep caissons.

2) You need to get a geotechnical engineer or geologist to tell you if the rock is competent to withstand the hoop tensile forces generated by the water pressure. They should also be able to give you an idea of the elastic modulus of the rock. You can run a simple analysis using the modulus of the rock and the shotcrete (same as concrete) with tensile forces generated by the water pressure. If the rock is competent and fairly stiff then a relatively thin, reinforced shotcrete "liner" should be fine. The concept would be to make the liner as thin and flexible as possible. This would be similar to a slab on grade where the majority of the load is transferred directly to the supported soil beneath. If the rock isn't too good you can consider a structural wall though it will take a lot of reinforcing to accommodate the water pressure with a 30m depth.

Regarding the shotcrete adhesion, it really depends on the rock. You could do a pull-off test to verify the adhesion, but I strongly suspect, the failure would be in the limestone, not the shotcrete.

Regards,

Charles

 

[Nonlinearer]

During my discussion with some of very experienced Engineers, I have been advised, do not depend upon shotcrete to form water retaining structures. Underwater shotcrete may not perform well during the service life of the tank. They suggest shotcrete is good for shafts, tunnels or for lining purposes where it will not be exposed to water and will not be loaded by water pressure. Shotcrete may also fall apart if the rock face get cracked/fractured.

Contrary to above, if a proper reinforced/ post tensioned tank is proposed inside the rock face, then this will be a more conservative, safe and professional solution. This will definitely be more costly than the shotcrete one but I believe is more recommended.

IR

 

[chanskat]

Hi again,

Well that is interesting because at least in the USA, shotcrete has been used on many hundreds (if not thousands) of above-grade tanks with capacities up to 25,000,000 gallons that have been proven watertight for decades of service. A quality concrete mix design shot using a professional, ACI-certified shotcrete nozzleman, with appropriately sized and maintained equipment will be the equal or superior of cast concrete in strength, permeability and durability. Also, you should note many, if not most tunnels have external water pressure from ground water. Shotcrete has also been used extensively on large building basement walls where water pressure is present on the outside face of the wall. Shotcrete is also used extensively in soil nailing wall applications where ground water and soil pressures may be substantial.

A properly designed, reinforced shotcrete section will act identically to a cast concrete section with the same reinforcement. The statement that shotcrete will "fall apart if the rock face get cracked/fractured", simply isn't true for properly designed and placed shotcrete.

Of course a well-designed PT concrete tank will perform well too. However, you will need to add the cost of over-excavation in competent rock to get room for construction of the outer form, allowing crew access for setting/stripping the forms, and PT operations after the form is stripped. Additionally, a 30m high form system designed for casting full height would be quite substantial, and expensive. Full height casting is required by ACI 373 Design and Construction of Circular Prestressed Concrete Structures with Circumferential Tendons where in Section 4.1.3.3 "Concrete should be placed in each vertical segment of the wall in a single continuous operation without cold joints or horizontal construction joints."

A PT tank option is indeed much more conservative, and expensive. It is not any safer or more "professional" than a properly designed and executed shotcrete design.

Regards,

Charles

 

[hokie66]

Agree with Charles completely. In this application, a reinforced shotcrete liner would be the method of choice, provided the rock will stand without any stabilization. Because of the method of placement which compacts the concrete, properly performed shotcrete is always more water-resistant than cast in place concrete.