Blasting techniques used in Tunnels

 The engineering concept behind Tunneling is always a matter of interest for Civil Engineering  individuals. One of the most fascinating thought in general is the procedure of Blasting used for Tunneling. For some, it's just a regular practice however, in reality, it is as complex job in it's own. 

Today we will learn about Blasting techniques commonly followed during Tunneling works as well as in Mining activities depending upon various Engineering parameters.

Blasting is an engineering practices used in mining and civil works for production of ores, surface, semi-surface, subsurface, and underground excavations, and demolishing of structures. It comes in several forms of blasting engineering as follows:

 

-          ResiBlast: Residential blasting with PPV = 2 - 9 mm/sec

-          CommBlast: Commercial blasting with PPV = 10 - 24 mm/sec

-          IndBlast: Industrial blasting with PPV = 25 - 59 mm/sec

-          InfraBlast: Infrastructure blasting with PPV = 60 - 119 mm/sec

-          CtldBlast: Controlled blasting with PPV = 120 - 449 mm/sec

-          MineBlast: Mining blasting with PPV = 450 - 499 mm/sec

-          ProdBlast: Production blasting with PPV = 500 - 599 mm/sec

-          UnCtldBlast: Uncontrolled blasting with PPV ≥ 600 mm/sec

 

Any type of breaking with PPV < 2 mm/sec would not be considered as blasting and instead it would be considered as ME (Mechanized Excavation), Non ExBreak (Non-Explosive Breaking), and/or ManDigg (Manual Digging).

 

As mentioned above there are 8 classes of blasting in which all categories are based on Peak particle velocity (PPV). Therefore, in an engineered blasting the main parameter to control is PPV. To obtain and serve the purpose following technique are utilized:

 

-          Increase in delay numbers

-          Decrease in charge per delay

 

To record the PPV sound vibration measuring machine/equipment to be utilized. An engineered blasting must have the following passes:

 

• -          Blasting patter design
• -          Charge calculation
• -          Pattern projection at face
• -          Drilling
• -          Charging and detonating
• -          Stemming and tamping
• -          Circuit closure and firing
• -          Vibration measurement
• -          Post blasting analysis

 

Output of an engineered blasting is as follows:

 

1. -          Uniform fragmentation of mucks
2. -          Achievement of required pull length (PL)
3. -          Enhanced safety
4. -          Proper profiling
5. -          Damage prevention to surrounding ground
6. -          Controlled overbreak
7. -          Controlled ground vibration
8. -          Controlled noise
9. -          Minimised ore wastage

 

Following technique are used in Engineered blasting:

 

-          Cushioning

-          Line-drilling

-          Pre-splitting

-          Smooth blasting

-          Muffle blasting

 

In an engineered blasting pattern principal parts are as follows, which requires a detailed design and revision based on observational assessment for high efficiency blasting results:

 

-          Burn Cut

-          Production Holes

-          Protection Holes

 

Burn cuts are a series of holes that carefully designed to generate sufficient free space for expansion of the rock mass prior to the ignition of the production holes. Burn cuts in blasting patterns are classified as following types:

 

-          P-Cut (Parallel Cut): A series of parallel holes to the production and protection holes

-          V-Cut (V-shape Cut): A series of V-shaped holes that intersect at ends and angled to the production and protection holes

-          A-Cut (Angled Cut): A series of angled holes that do not intersect at ends and angled to the production and protection holes

 

Production holes are utilized to break the rock mass and produce a uniform fragmented muck. These holes are fully charged, tamped, and stemmed.

 

Protection holes’ function is to protect the periphery by prevention of transmitting the wave to the surrounding ground. These holes are drilled in utilized in two forms; charged, and uncharged. Charged ones are not tamped and stemmed; they act as pre-splitting or smooth blasting holes, which make a film of air to reverse the direction of passing waves from the surrounding ground toward the production mass. Uncharged protection holes are in the form of close spacing empty holes to make a row of air to ban the transmission of the wave to the surrounding ground.

The Tunneling/Mining in itself is a very interesting topic. More about the Tunneling Engineering work will be released soon. Hope the reader will find it useful.