Over-water site investigation from jack-ups, barges and vessels

How bridges get built · Site investigation and testing · Over-water site investigation from jack-ups, barges and vessels

Also called: marine site investigation, offshore geotechnical investigation, nearshore drilling, seabed CPT

How it is done

Plan from the desk study: bathymetry, tides, currents, waves, navigation and permits; run the hydrographic and geophysical survey first (multibeam, side-scan sonar, sub-bottom profiler, magnetometer) to set and clear the borehole positions.
Select the platform by water depth, waves, current and seabed: spud barge or pontoon in shallow sheltered water, jack-up in exposed or tidal water, a drilling vessel in deep water or for long campaigns.
Position by RTK-GNSS; preload jack-up legs beyond the expected storm and drilling loads and check for punch-through where a stiff layer overlies soft clay; moor barges on anchors or spuds.
Run a conductor casing from the deck to below the seabed, measure water depth and tide at every stage, and reduce every level to the project datum.
Drill and sample with wireline tools, alternating push samples, downhole CPT strokes, SPTs and rock coring; use heave compensation on floating units, and seabed CPT frames for continuous profiles between boreholes.
Log weather and downtime, record a quality class for each sample, keep samples from drying and vibration, and run simple strength index tests on deck as a check.
Seal the holes where required, demobilise, combine the geophysics and boreholes into the marine ground model, and drill a second phase at the final pier positions.

Key numbers

Typical working water depthspud barge or pontoon up to about 10–15 m; anchored barge to about 30–40 m; geotechnical jack-up about 40–60 m; drilling vessels in deeper water.
Floating units usually stop drilling at significant wave heights of about 1–1.5 m; an elevated jack-up keeps working in much rougher seas.
Positioning by RTK-GNSS to about ±0.1–0.5 m; every level reduced to datum with tide correction.
Seabed CPT frames100–200 kN thrust, 20–50 m penetration in soft to firm soils; downhole CPT in strokes of about 1.5–3 m.
Cost per metrecommonly three to six times a land borehole.

Where it fits

Every pier in water: rivers, estuaries, lagoons, straits and sea crossings.
Long marine alignments, where geophysics and seabed CPTs can cover the gaps between a limited number of boreholes.

Where it does not

Floating platforms in exposed water when high-quality samples are needed: heave disturbs samples and stops work.
Fixing pier positions before the hydrographic and geophysical survey: a borehole may land on a buried channel, wreck or cable.

Choosing it

Choose a jack-up over a floating barge in exposed, tidal or wavy water and wherever sample quality and continuous working matter; accept the higher mobilisation cost and the punch-through check.
Choose a spud barge or pontoon in sheltered shallow water, and a temporary trestle only where it will also serve construction.
Prefer a geotechnical vessel with seabed CPTs for long alignments in deeper water, then return with a jack-up for boreholes at the final pier positions.
Do the geophysics before the drilling, not after: it is what lets a few expensive marine holes represent a whole alignment.

Plant, pace and money

PlantJack-up platform, spud barge or geotechnical drilling vessel; rotary rig with wireline system and heave compensation; seabed CPT frame; survey vessel with multibeam, side-scan sonar, sub-bottom profiler and magnetometer; tug, crew boat and RTK-GNSS.

ProductivityOne 50–80 m borehole every 2–5 days including moves; several seabed CPTs per day; weather downtime is often a large share of time in exposed water.

CostIndicative only: about USD 600–1,500 per metre of borehole including the platform, three to six times a land hole; jack-up mobilisation and weather standby are priced separately and can rival the drilling cost.

Risks and controls

What goes wrong

Jack-up punch-through or scour around the legs during preloading or storms.
Errors in tide and datum reduction that put rock head or founding levels metres out.
Sample disturbance from heave on floating units; holes lost in strong currents.
Collision with shipping, and striking unexploded ordnance, cables or pipelines on the seabed.

Quality assurance

Daily positioning checks against fixed control, tide gauge records tied to the datum, a quality class for every sample, weather and downtime logs, and a site-specific assessment for each jack-up location.

Origins

Grew from offshore oil and gas practice of the 1960s and 1970s (wireline sampling, seabed and downhole CPT, heave-compensated drilling) and was carried into bridge work on the large sea crossings of northern Europe, Greece and East Asia from the 1980s onward.

Examples

A typical sea-crossing campaign starts with a hydrographic and sub-bottom survey of the whole corridor, then a borehole at every main pier with CPTs between them, from a jack-up; on large rivers in South Asia, spud pontoons work in the dry season.

Case studies

Rion–Antirion BridgeGreece · 2004Investigations in up to 65 m of water reached 100 m without finding rock; the weak, variable upper 20 m led to 2 m diameter steel pipe inclusions 25–30 m long and 90 m diameter caissons on a 3 m gravel layer.

Padma Multipurpose BridgeBangladesh · 2022Over-water borings at the design stage covered 13 of 42 piers; drilling at every pier during construction found clay layers that forced the redesign of 22 pier foundations.

Fehmarnbelt Fixed LinkDenmark / Germany · late 2000s (investigation)Marine campaigns of boreholes, CPTs and geophysics along the 18 km crossing defined glacial deposits and highly plastic Palaeogene clay for both the bridge and tunnel options before the immersed tunnel was chosen in 2011.