Cable trenching engineering
Dr David Cathie
Principal, Cathie Associates
Cable trenching engineering
1 April 2014, DGS Seminar, Gentofte
Cable trenching engineering
Cable trenching engineering
Types of trenching equipment – jetting, ploughing, cutting
Trenching mechanisms > Governing soil characteristics
Lessons learnt for cost-saving
Cable trenching needs engineering
Cable trenching engineering
Trenching equipment - jetting
JETTING
Propulsion ROV (free-flying) or
tracked
Performance
Clay High pressure jets essential
Trench cleaning required
Sand Good. More difficult if
dense and fine.
Coarse sand or
gravel
Satisfactory.
Cable lowering more
difficult. Jetting arm/swords
Forward jetting nozzles
Backward
jetting
nozzles
Cable trenching engineering
Jetting systems - swords
Multiple nozzles forward/downward Few (larger) nozzles rearwards
Cable trenching engineering
Jet trenching in sand
Cable trenching engineering
Jet trenching in sand
Mechanisms
Erosion/fluidisation of sand
(relative density)
Transport of sand (grain size)
Resedimentation (grain size)
Vanden Berghe et al. 2008
Cable trenching engineering
Jet trenching - cable lowering
Effect of cable stiffness and residual lay tension
Cable trenching engineering
Jet trenching/cutting in clay
Stagnation pressure causes bearing
failure – if jet pressure sufficient!
Exposure time governs depth of cut for
static jet
Traverse velocity governs depth of cut for
moving jet Slot cut by traversing jet
Machin and Allan, 2011
Cable trenching engineering
Jet trenching in clay
Processes
Jet cutting (governed by undrained shear strength, Su)
Clay lump breakage and transport (soil plasticity)
Clay lump deposition (soil unit weight)
Machin and Allan. 2010
Cable trenching engineering
Trenching equipment - ploughing
PLOUGHING
Propulsion Towed
Performance
Clay Good
Sand Good. More difficult
if dense and fine.
Coarse sand or
gravel
Good
Cable trenching engineering
Pre-cut V-trench ploughing
Plough main
beam
Spoil
heaps
Plough
share
Undisturbed
Disturbed Mould
boards
Front view of soil engagement
Cable trenching engineering
Ploughing - mechanisms
Mechanisms
Intense shearing on cutting
edge
High suction pressures in sand
(cv, M, k)
Limited undrained shear strength
rate effect in clay
Interface friction/adhesion-
sensitivity
Large
suctions
below
share
Smaller suctions
above share
due to shorter
drainage paths
Cable trenching engineering
Trenching equipment – mechanical cutting
CUTTING
Propulsion Tracked
Performance
Clay Good
Sand Possible. Not
efficient
Coarse sand or
gravel
Possible. Not
efficient
T2 General arrangement
Cable trenching engineering
Mechanical cutting - mechanisms
Mechanisms
Soils – cutting, wear and
transport out of trench
Density/strength, permeability,
hardness, plasticity, bulking
Rocks – cutting/breaking +
transport (UCS, rock quality)
Cable trenching engineering
Trenching - lessons learnt – saving cost
Carefully define the burial depth by burial risk analysis
Costs increase with D2 or D3? Excavation volume,
trencher power, harder soils, cable lowering
Carefully evaluate power and design of a trencher (not underpowered)
Sword design, nozzle pressure, flow rates
Find the peat!
Fibrous soil creates very resistant soil (ploughing or
trenching)
Avoid multi-passing in plan A
Strength, density and particle size (permeability)
within trenching zone (not just burial depth)
Cable trenching engineering
Conclusions
Resistivity survey used as basis for route characterisation on FO cable
Cable trenching is often underestimated
Qualify the systems in similar ground
conditions
Operator experience important
Trenching is a continuous site investigation –
why not collect continuous engineering data
(surface wave, refraction, resistivity) as well
as conventional data?