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?