GeoTech II

What is soil liquefaction?

Soil liquefaction is when wet soil (usually sand) suddenly loses its strength and starts behaving like a liquid instead of a solid

What is effective stress?

Effective stress is the actual stress carried by the soil grains—this is what gives soil its strength and stability. (σ′=σ−u)

What is pore water pressure?

Pore water pressure is the pressure of the water inside the voids (pores) between soil particles.

coarse grained soils

Gravel, sands, water drains easier, low pore pressure buildup

Fine grained soils

Silts and clays, high pore water pressure

What is the continental margin?

It is continental slope and continental rise, important for oil and gas industry, and is about 20% of ocean floor

What are the two types of ocean sediments?

Terrigenous (land-derived) and pelagic (ocean-derived)

Explain terrigenous sediments

sands, gravel, silt clay; on continental slopes

Is transported by wind, currents, iceberg rafting

What is turbidity currents?

underwater current due to the flow of sediments over steep slopes

What are slide deposits?

The slide deposits are the product of submarine landslides and are found as slumps on the slope bottom.

What are glacier marine soils

Coarse-grained sediment produced by glacial scouring (carving/erosion of underlying ground surface by the top ice layer).

What is abyssal clay?

high plasticity, less than 30% silt, tiny particles that settle slowly

What is authigenic deposits?

Deposit generated where it is found, not transported. Mineral precipitated and crystallized in seawater. (forming directly on seafloor)

What is biogenous Oozes?

Dead marine organisms and plants can be calcareous ooze (calcium carbonate) or siliceous ooze (diatoms/ radiolarians).

What is undrained shear strength?

is the maximum shear stress a saturated soil can handle without letting water escape

what are the survey types?

Regional (data collected from a large area, limited sampling and needs geophysical methods) and Site specific (smaller area with lots of detailed info and in-situ testing)

What are the two properties important for design and analysis of geosystems?

Shear strength and buoyant unit weight properties

What is CCD?

Carbonate Compensation depth at which carbonate materials will dissolve due to lower temps, higher pressure and CO2, and low pH (below 4500m)

What is subbottom profiling?

A method that uses sound waves (acoustic signals) to image layers beneath the seafloor.

• Sound waves travel down and reflect off different sediment layers

• The time it takes to return is used to estimate depth

• Shows layering and structure, not detailed soil properties

What is limited soil sampling?

A method of collecting small, shallow sediment samples to identify soil type.

• Provides basic properties and sediment type

• Does not give full geotechnical data

What is side-scan sonar?

A method that uses acoustic waves to create a 2D image of the seafloor surface.

• Produces a plan view (top-down image)

• Detects objects like:

  • Pipelines

  • Shipwrecks

  • Debris

• Used to identify hazards and avoid bad sites

What is visual observation in seabed surveys?

Direct viewing of the seafloor using:

• Divers or

• Cameras/ROVs

What are the 2 types of site-specific sampling?

Shallow-water soil sampling and deep-water soil sampling

What is the difference between shallow and deep soil sampling?

• Shallow sampling (0–20 ft) → gravity cores, vibrocores

• Deep sampling (>10–12 ft) → drilling rigs, wireline sampling

How are shallow-water and deep-water sampling performed?

• Shallow water → jackup barges or fixed platforms

• Deep water → floating vessels

What is wave breaking?

When the wave crest becomes too steep and collapses

what is coulomb damping?

a mechanism where energy is dissipated through friction between two surfaces in contact

What determines if water is shallow, intermediate, or deep?

The ratio h/L (depth vs wavelength)

What is a progressive wave?

A wave that moves forward and carries energy

How do water particles move under waves?

Water particles beneath waves move in orbital paths, which are circular in deep water and become elliptical in shallow water due to interaction with the seabed

What is effective stress?

Effective stress is the portion of total stress carried by the soil skeleton and is equal to total stress minus pore water pressure

What happens when pore pressure increases?

Effective stress decreases → soil weakens

When does liquefaction occur?

When pore pressure = effective stress

What is ru= u / σ0′​?

Ratio of pore pressure to initial effective stress

What does ru=1 mean?

Full liquefaction

Which soil causes the most wave damping?

clay

What happens when saturation increases?

Pore pressure increases

what happens to soil strength when saturation increases?

it decreases

What is shear modulus?

soil stiffness, waves have less effect with more stiff

What increases failure risk?

when applied stresses increase or when soil shear strength decreases, particularly due to elevated pore water pressures, weak soil properties, or external loading.

Why do waves lose energy over seabeds?

Waves lose energy over seabeds due to bottom friction, sediment transport, and turbulence in the boundary layer, which dissipate wave energy as heat and motion

What is instantaneous liquefaction?

When wave pressure quickly reduces soil strength and makes it behave like a liquid

Why does lower saturation increase liquefaction?

Air in pores → more compressibility → larger pressure changes

What combination gives the MOST liquefaction?

• Low Sr​ (less saturated)

• Long wave period

• Shallow water

What controls whether wave-induced liquefaction occurs?

The balance between wave loading (H/L) and soil strength

What happens if friction angle (ϕ′) increases?

Soil becomes stronger → less likely to liquefy

How does slope affect seabed response?

Steeper slopes → larger stresses and higher instability risk

What happens when wave height increases (H₀)?

Liquefaction zone becomes larger and deeper

What is δ?

The friction angle between the wall and the soil

Which is more effective: vertical or inclined drain?

It intercepts water along the failure plane, not just vertically

What is surcharge?

Extra load on the ground (cars, buildings, etc.)

What is one major difference between nearshore renewable devices and offshore platforms?

Nearshore devices must handle breaking waves and fluctuating loads, while offshore platforms deal with longer-period waves and less breaking.

How do calcareous oozes behave?

More compressible and weaker while silicous oozes are stronger and more stable

How does slope change from shore to deep ocean?

• Continental shelf → gentle

• Continental slope → steep (steepest)

• Continental rise → flattens

• Abyssal plain → very flat

What is sub-bottom profiling?

Uses sound waves to image layers below the seabed

What is sidescan sonar?

A 2D acoustic imaging system for seabed surface and it shows seabed texture and

What does higher cone resistance (qc) indicate?

Higher soil strength and friction angle

How does increasing wave period affect seabed pressure?

Longer wave period → longer wavelength → pressure penetrates deeper → higher seabed pressure

How does decreasing water depth affect seabed pressure?

Shallower water → stronger interaction with seabed → higher pressure

What is instantaneous liquefaction?

Occurs within one wave cycle when effective stress drops to zero due to wave pressure

What is residual liquefaction?

Occurs over many wave cycles as pore pressure gradually builds up

Key difference between instantaneous and residual liquefaction?

• Instantaneous → short-term, cyclic

• Residual → long-term, cumulative

Which is more likely in shallow water?

Residual liquefaction

Why do standing waves promote liquefaction?

They create strong pressure gradients → increase pore pressure → reduce effective stress

How can structures suppress residual liquefaction?

• Dissipate wave energy

• Redistribute stresses

• Reduce pore pressure buildup

Why does sediment near structures help?

It can drain or redistribute pore pressures, reducing liquefaction risk

Why is soft normally consolidated clay prone to failure?

• Low undrained shear strength (su)

• Cannot resist wave-induced stresses

What controls how deep wave pressure reaches?

Wavelength (longer waves = deeper penetration)

Which item is the best-supported set of advantages of seabed-mode CPT over down-hole CPT in this paper?

Easier sensor zeroing at fixed depth near seabed and often cost-effective/high quality results in many cases.

why does Coulomb Theory Overestimates Kp When (friction angle b/w wall and soil) is Large

Because actual passive failure surface is not linear and it is much easier to fail along logspiral than along planar surface