Quiz 4 Coastal

Types of hazards

• Episodic = short time

• Hazardous conditions = equilibrium conditions that are design challenges

Common episodic merine geohazards

• Earthquakes

• Submarine landslides

• Methane expulsions

• Turbidity flows

• Hurricanes/cyclones

Common hazardous conditions marine geohazards

• Soil creep or lateral spreading

• Problematic soils

• Mud and salt geoforms

• Scour

• Ice scour

Uncommon hazards

• Animals

• Rogue waves

• Tsunamis

Earthquales

• Can occur offshore and in coastal areas

• Ground motions need to be included in design

  • Liquefaction

Submarine landslides

• High frequency on geological time scale

• Previous slides visible on seafloor in holocene soils

• Slopes with angles as low as 0.5 deg

• Can be large

• Triggering mechanisms:

  • increase load - increases shear stress in slope

  • decrease resistance - increases pore pressure → decreases eff stress and lowers strength

• Natural triggering processes

  • Underconsolidation - reduction in strength / rapid sedimentation increasing pore pressure faster than dissipate

  • Erosion and deposition - increases load / erosion @ toe shorten slope / deposition @ crown increase load

  • Increase in methane volume - reduce strength / methane expands → pore volume increase

  • Earthquakes - increase load / acceleration increase slope weight

• Human triggering processes

  • Drilling blowout - decrease strength / mvmt of fluid → higher pore pressure

  • Heat from drills, pipes, elec cables - dec strength / inc temp = inc pore pressure and pore volume

  • Subsidence - depete reservoir / can change slope angle increasing load in slope

  • Installation of foundations at slope crown - increase load / vertical stress inc

  • Installation of anchors - inc load / horizontal force/stress

Methan expulsion

• Release methane into water

• Methane = breakdown of biological matter or petroleum deposits

• Methane hydrate = dense water ice on seabed w/ methane in it

Turbidity flows / currents

• Flows of water near seabed w/ significant suspended soil solids (underwater sandstorm)

• Move large amts of soil and put loads on structures

• Causes:

  • Submarine landslides

  • Strong currents

  • Earthquakes

Hurricanes

Cyclones, typhoons, tropical storms, etc

• Large, rapidly rotating storms centered on low pressure depression

• Wind 65+ knots

• Large load on structs

• Erode seabed

• Storm surge

  • Mvmt of water onshore by hurricane, like tsunami

Soil creep / lateral spreading

• Creep = slow mvmt and settlement of ground

  • Secondary compression

• Lateral spreading = creep downhill

Problematic soils

Carbonates, glauconites, micas, etc

Mud and salt tectonics

• Geological units of mud and salts, low density, move relative to other geological units

• Similar to slow continuous earthquakes

• Salt diapir = when salt deposit rises out of seabed (wisdom teeth)

Scour

Erosion around structure

• Undermining

  • Reducing embedment depth of pile

  • Removing soil from under a foundation

• Remedies

  • Embed pile more

  • Add skirts to shallow foundations

  • Scour protection

    • Piles of stone around structure

    • Concrete mats, stones (“jacks”), aprons

  • Reduce undermining

    • Skirts

    • Increase embedment below scour depth

Ice scour

Depression in ground from iceberg dragging on seafloor

Mitigate w/ burying pipes and cables

How to access geohazards

3-phase process

What are the 3 phases?

1) Desktop study

2) Geohazard impact assessment

3) Final reporting

What is Phase 1?

Desktop study

Task:

• Analyze exploration geophysical data

• Compile and review published work and unpublished internal documents

• Develop a GIS model

• Identify critical engineering issues

Product:

• Report and provide data packages

• Regional scale GIS map w/ potential hazards identified

What is Phase 2?

Geohazard impact assessment

Task:

• Analyze high-res geophysical and geotech data

• High-res GIS hazard map

• Identify geohazards in project zone

• Create geohazard susceptibility map w/ other enge teams

• Identify information required to assess geohazard

• Recommend special studies to address challenges

Product:

• Report and data packages

• Workshop w/ all science and engineering teams

What is Phase 3?

Final reporting

Task:

• Analyze data from special studies and review new info

• Complete detailed geohazard assessment

Product:

• Report used to predict

  • Final site location

  • Foundation design

  • Methods for assessing potential economic loss

Risk

Economic, life, environmental loss due to an event over a given time

3 factors of risk

Event, loss, exposure

Bayesian analysis

Statistical method for calculating risk

P(Damage event) = P(event) * P(Impact | Event) * P(Damage | Impact)

How to deal with risk?

• Can more easily assess for more frequent events

• Damage given event hard to predict

• Easiest way to reduce is to avoid an event

• If can’t avoid, engineer a solution to reduce damage

• Geotech engineers assess risk, not establish acceptable risk

  • Client, gov, insurers do that