Marine geotechnical investigations are subsurface exploration campaigns conducted in offshore, coastal, and estuarine environments. They provide critical data for the design of ports, harbours, bridges, pipelines, offshore wind farms, and coastal protection structures.
What Is Marine Geotechnics?
Marine geotechnics applies geotechnical engineering principles to seabed and sub-seabed conditions. Compared to onshore investigations, marine work presents unique challenges:
- Access — equipment must be deployed from vessels or platforms
- Water depth — investigations may be in 5–5,000 m of water
- Currents and tides — affect positioning and drilling stability
- Soft seabed sediments — require specialised sampling techniques
- Environmental constraints — marine fauna, turbidity, noise regulations
Investigation Methods
Geophysical Survey (Preliminary)
Before any intrusive investigation, geophysical surveys map the seabed and shallow sub-surface:
| Method | What It Maps | Typical Use |
|---|---|---|
| Multibeam echo sounder | High-resolution bathymetry | Seabed topography |
| Side-scan sonar | Seabed features | Pipelines, cables, wrecks, boulders |
| Sub-bottom profiler | Shallow stratigraphy (0–50 m) | Sediment layers, gas pockets, bedrock depth |
| Seismic reflection | Deep stratigraphy (50–500+ m) | Geological structure, fault mapping |
| Magnetometer | Magnetic anomalies | Cable/pipeline location, archaeological |
Seabed Sampling
| Method | Sample Type | Depth | Application |
|---|---|---|---|
| Grab sampler | Disturbed surficial sample | 0–0.5 m | Sediment classification, contamination |
| Gravity corer | Undisturbed core | 0–6 m | Soft sediment profile |
| Vibrocorer | Undisturbed core | 0–10 m | Sand/soft sediment profiling |
| Piston corer | Undisturbed long core | 0–30+ m | Deep soft sediment sampling |
| Box corer | Undisturbed surficial block | 0–0.5 m | Geotechnical testing of surface sediments |
In-Situ Testing
| Test | Parameter | Deployment |
|---|---|---|
| Seabed CPT | Cone resistance, sleeve friction, pore pressure | Seabed frame deployed from vessel |
| Seabed shear vane | Undrained shear strength | ROV or diver-operated |
| Ball / T-bar penetrometer | Undrained shear strength in very soft soils | Seabed frame |
| Full-flow penetrometer | Soft clay strength | Seabed frame |
Borehole Drilling (Offshore)
| Platform | Water Depth | Max. Borehole Depth | Applications |
|---|---|---|---|
| Jack-up barge | 0–30 m | 50–100 m | Near-shore, bridge piers, ports |
| Drill ship | 50–3,000 m | 200–500 m | Deep water, offshore energy |
| Dynamic positioning vessel | 50–3,000 m | 200–500 m | Deep water, no anchors |
| Permanent platform | Variable | 100–200 m | From existing offshore structures |
Downhole Testing in Offshore Boreholes
- SPT (Standard Penetration Test) — in all soil types
- Rock coring — rotary coring for bedrock samples
- Downhole geophysical logging — gamma, sonic, televiewer
- Piezometer installation — for long-term pore pressure monitoring
Laboratory Testing
Marine sediment testing includes standard geotechnical tests plus specialised marine-specific tests:
| Test | Marine-Specific Consideration |
|---|---|
| Classification | High salt content affects Atterberg limits |
| Carbonate content | Calcareous sediments behave differently to terrigenous soils |
| Organic content | Marine sediments often have high organic content |
| Salinity testing | Pore fluid salinity affects soil behaviour |
| Cyclic triaxial / simple shear | Wave loading simulation for foundation design |
| Scour testing | Erosion resistance of seabed sediments |
Typical Marine Projects
Port and Harbour Development
- Berth pocket dredge assessment
- Quay wall foundation design
- Container yard pavement design
- Channel slope stability
Bridge Foundation
- Pier and abutment foundation assessment
- Scour assessment and protection design
- Navigation channel clearance verification
Offshore Wind
- Wind turbine monopile or jacket foundation
- Cable route geotechnical assessment
- Seabed mobility and scour studies
Submarine Pipelines and Cables
- Pipeline route geotechnical assessment
- Trenchability assessment
- Pipeline stability (on-bottom stability)
- Cable burial assessment
Coastal Protection
- Seawall and revetment foundation
- Beach nourishment sediment sourcing
- Breakwater foundation design
Sampling and Testing Standards
| Standard | Title |
|---|---|
| ISO 19901-8 | Marine soil investigations |
| ISO 19901-4 | Geotechnical design of offshore structures |
| API RP 2GEO | Geotechnical design of offshore structures |
| AS 1726-2017 | Geotechnical site investigations |
| NORSOK G-001 | Marine soil investigations |
| ISSMGE TC1 | Offshore geotechnical engineering |
Key Challenges
| Challenge | Mitigation |
|---|---|
| Weather windows | Seasonal planning, weather-robust equipment |
| Positioning accuracy | DGPS, USBL acoustic positioning |
| Sample disturbance | Specialist undisturbed sampling techniques |
| Environmental compliance | Environmental Impact Assessment (EIA) before work |
| Cost | Optimise investigation to minimum required scope |
| Equipment mobilisation | Regional equipment depots, shared mobilisation |
Frequently Asked Questions
How deep are marine geotechnical investigations?
Boreholes typically reach 20–100 m below seabed for offshore structures. Very deep foundations (suspension bridges, deep water wind) may require 100–200 m boreholes.
What is the cost of marine investigation compared to onshore?
Marine investigations cost 3–10× more than equivalent onshore investigations due to vessel mobilisation, slower operations, and specialised equipment.
Can CPT be performed from the seabed?
Yes. Seabed CPT frames are deployed from vessels and pushed into the sediment using reaction weight or thrust. They can typically reach 10–30 m below seabed.
Do marine sediments need different testing?
Some tests are specific to marine conditions — especially carbonate content, salinity effects, and cyclic loading tests for wave and current loading.