Embedding Rock into Blinding (blinding is too thick) 2

[Ahmed_Hassabu]

Hello fellow Engineers!

I am in the process of building a large tunnel and the existing earth is extremely tough rock. We are using rock breakers to reach the required level (bottom of blinding) but I am facing a problem with accurate leveling. Rock breakers cannot be pinpoint in reaching the required levels and usually go deeper. So instead of placing a 10cm thick blinding mat as required (and paid for), I usually end up putting anywhere from 30 to 40 cm thick blinding mats. In such a large area, this is obviously a huge waste of plain concrete (30MPa as per project specs).

I was wondering if I could lay down rocks from the resulting debris and make something like a bed of screened rocks then fill all of that over with the plain concrete, this would save a huge amount of ready mix concrete. If I am to do this I need to know exactly what technical term is used for this practice and if it has been used elsewhere with any proper reference to be able to convince the supervision with this solution. The project uses British Standards but ASTM can be used for an argument.

If you have any fine tuning of the idea or if you have any other ideas that may be utilized other than this solution, I'd be grateful for your help and input. Thank you!

 

[miningman]

What is a blinding mat??? Never heard that expression. What are the tunnel dimensions?? What is the end use / purpose of the tunnel??

 

[cvg]

blinding is not usually structural concrete, but in your case it appears to be. 10 cm thick is the minimum thickness that you can generally place a thin concrete layer.
foundations are generally required to be clean, so it probably would not be allowed. and quality control would be difficult to achieve. I would not allow it.

maybe you could use a larger aggregate in your concrete mix to reduce the cement content.

 

[hokie66]

Blinding is a common term in building structures. It is also called a working mat. The purpose is to provide a hard, regular surface for placement of reinforcement and concrete. Sometimes also as a substrate for waterproofing.

Trying to occupy part of the volume with cobbles or such is too time consuming for my liking. But 30 MPa concrete is structural grade concrete, and you don't need that strength for the blinding. The rock it replaces would not be rated nearly that high.

 

[Ahmed_Hassabu]

The specifications we follow are stringent. I cannot change the 30MPa requirement nor the minimum thickness of 10cm. As hokie66 stated above this blinding surface will be used for both purposes: providing a hard and level surface for the rebar and also as a substrate for the waterproofing.

The problem is after we break out the existing rock and clean the surface the quantities consumed in this blinding tend to be huge; roughly triple the design / theoretical quantity which is claimable. This is where I'm coming from in the question, I want to reduce the amount of concrete used by filling it with rock if possible. Maybe screen the rock mechanically then pass a 10t roller above it before I cast the concrete?

 

[miningman]

If I am to help , I need more info........ basically you are taking excessive overbreak. Is your rock breaker the optimum size / power???. Perhaps you need a two pass approach where you excavate down to say 30 cm of design grade and then use a smaller hammer to achieve final grade, with less overbreak. Controlled perimeter blasting can be very effective in controlling overbreak. Are you familiar with explosives??. What is the compresive strength / RQD of the rock???? You claim you are dealing with " tough rock" That is a bit subjective........ personally I would never use rock breakers for tough rock
but I need details of the geology etc to help further.. Have you considered driving a pilot tunnel and then slashing out to full size........ another technique that can dramatically reduce overbreak.

And again , if I knew the end use of the tunnel, there are other suggestions that might be applicable

 

[Pinwards]

The term I was trying to think of to respond was "Plum Concrete". Do some research on that term and you'll find some guidance on what you propose.

#

 

[cvg]

plum concrete is a bad idea for such a thin layer and is generally used for large, mass concrete structures. however to reduce cost, fill the over break with a low strength concrete, let it set and then place the blinding over that.

 

[Ahmed_Hassabu]

Miningman (sorry don't know how to mention anyone yet in these threads) the tunnel is for heavy traffic. Roughly 30m wide by 1km long. Over the blinding we'll have waterproofing then the tunnel foundation / base which is 1.7m thick and then 4 walls above the base which carry a roof that's also 1.7m thick.

 

[miningman]

Well at those dimensions, its obviously a major project. You've certainly got adequate room to consider driving a pilot tunnel and then slashing out to full size , but I suspect that might be beyond your decision making authority. From an engineering perspective , I strongly suggest evaluating your rock breaker. If its oversized / too powerful, it may well maximise rate of advance but at the expense of excessive overbreak. Is this a conventional tunnel or cut and cover??