Trench Excavation Methods

Comparative study between the use of trenchers and other methods 

The main methods of the execution of trenches are: 

1. Conventional methods: excavators +/- rock breakers 

2. Drill & blast 

3. Trenching

Although the most appropriate method depends on the terrain, lithology, etc., trenching turns out to be the most efficient system in most cases:

 
DRILL & BLAST
HYDRAULIC ROCK BREAKERS AND
BACKHOES (+/- RIPPING)
CHAIN TRENCHERS
Application
 
In very strong, abrasive, massive rock.
Not suitable for rock masses composed of small, loose blocks.
Relatively easy to set up and execute.
Backhoes and dump trucks required to remove blasted material.
Particularly useful on steep slopes.
Application
 
In rock masses composed of small, loose blocks.
Not suitable in very strong, abrasive, massive rock.
Easy to set up but can be very time consuming.
Useful for tight curves.
Useful where the ground changes from soil to rock over short distances.
Aplication
 
In weak to strong rocks.
Generally not economic in very strong, abrasive, massive rock.
Unsuitable for rock masses containing loose cobbles and boulders or those containing pockets of wet clay.
Larger machines require considerable lateral working space and operate most efficiently over long distances in relatively homogeneous rocks.
Excavation effects
 
Significant over-excavation effect.
Very irregular, possibly unstable trench walls
and floor.
Sidecast spoil causes ecological damage and
increases risk of sediment release to watercourses.
Alternatively, spoil transported to temporary or permanent disposal sites.
Excavation effects
 
Moderate over-excavation.
Irregular trench walls and floor.
Sidecast spoil may cause ecological damage
locally.
Some spoil may have to be transported to
temporary or permanent disposal sites.
 
Excavation effects
 
Minimal over-excavation.
Regular trench walls and floor.
Spoil is deposited in a continuous windrow
alongside the trench.
Where space is limited, spoil must be discharged to dump trucks and transported to temporary sites for processing: no sidecast spoil.
Security Issues
 
Potential restrictions on use of explosives.
Cannot be used in buit-up areas or in
proximity to utilities, strutures and other
pipelines.
Security Issues
 
Suitable for use in built up areas or in
proximity to utilities, structures and other
pipelines
 
 
Security Issues
 
Smaller machines.
Suitable for use in built-up areas or in
proximity to utilities, structures and other
pipelines.
Some important factors to be considered in selecting appropriate trench excavation method in rock. Extracted from Bel-Ford et Pettifer, in Pipeline 
International Magazine, Sept. 2009 Issue
 
Examples of
rock type
Appropriate trenching
method
Chain trencher
tooth Wear
Potential Reuse
of spoil
Limestone
Chain trencher
Moderate
Good to fair
Dolomite
Chain trencher
Moderate
Good to fair
Sandstone
Chain trencher
Moderate - Severe
Fair
Basalt
Gneiss
Chain trencher (may need
hydraulic breaker for high spots)
Severe
Fair to minimal
Granite
Andesite
Hydraulic breaker + Backhoe
Blasting.
Note: Chain trencher economically marginal
Extreme
Fair to minimal
Chert
Ryolithe
Hydraulic breaker + Backhoe.
Blasting
Does not apply
Minimal to zero
Summary of trench excavation and backfilling methods appropriate to various rock types. Extracted from 
Bel-Ford et Pettifer, in Pipeline International Magazine, Sept. 2009 Issue