Presentation on theme: "Liquefaction, Kobe Earthquake Matt Greaves, Tom Baker."— Presentation transcript:
Liquefaction, Kobe Earthquake Matt Greaves, Tom Baker
Kobe Earthquake Kobe, Japan. 5.46am Tuesday 17 th January 1995. 20 seconds in duration. 7.2 Richter scale, 6.8Mw. 6,434 people died and 26,000 injured. 100,000 buildings completely destroyed. One fifth of Kobe population left homeless. $200 billion damage. The costliest natural disaster to befall any one country. Guinness World Record.
Effects Initial effects caused the collapse of buildings, bridges and roads. Subsequent effects were fires, congestion, closure of businesses and people made homeless.
Geological Context Convergent plate boundary. Triple junction with 3 subduction zones. –Philippine plate was subducted beneath the Eurasian plate. –Pacific plate was subducted beneath the Philippine and Eurasian plates. Fault Rupture length: 30 – 50km. Epicentre 20km from Kobe on the Awaji Island. Focus 16km beneath.
Liquefaction What is Liquefaction? –Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. Where does it occur? –Liquefaction occurs in loose saturated cohesionless soils - soils in which the space between individual particles is completely filled with water –It’s effects are most commonly observed in low-lying areas near bodies of water such as rivers, lakes, bays, and oceans
Liquefaction - How does it occur? Before an earthquake –Individual particles in the soil are randomly assembled –Each particle is in contact with a number of neighbouring ones –Contacts produce contact forces between particles and thus give the soil it’s strength When an earthquake occurs –Rapidly applied loading –Particles attempt to move into denser configuration (consolidation) –No time for water to be squeezed out, and remains trapped in the soil –Large increase in water pressure, reducing contact forces and thus effective stress –Soil becomes weaker and loses strength –Extreme case, particles lose contact and strength becomes so low –Soil acts as a liquid
Liquefaction What Happened at Kobe? –Some of the soil in Kobe experienced liquefaction –Support provided by foundations for buildings and bridges reduced –The Port and waterfront facilities suffered extensive damage –Built on reclaimed land consisting of loose to medium dense cohesionless fills, which are prone to liquefaction –Quay wall displaced, causing the shore line to move 2-3m outwards and settle up to 1m
Engineering Solutions Three possibilities to reduce liquefaction hazards: –1. Avoid Liquefaction Susceptible Soils –2. Construct Liquefaction Resistant Structures –3. Improve the Soil 1. Avoid Liquefaction Susceptible Soils –Carrying out site specific investigation to establish susceptibility. –There are various criteria to establish the liquefaction susceptibility. Historical Criteria e.g. Has liquefaction taken place before. Geological Criteria e.g. Saturated soil deposits created by sedimentation. Compositional Criteria e.g. Single size soils are more susceptible. State Criteria e.g. Density & effective stress at time subjected to loading.
Engineering Solutions 2. Construct Liquefaction Resistant Structures –Design the foundation elements to resist the effect of liquefaction. –Shallow Foundations All tied together Settle/move uniformly Buried utilities should have ductile connections. –Deep foundations Piles must be able to resist vertical and horizontal loads and bending moments induced by lateral movements. Ductile connections.
Engineering Solutions 3. Improve the Soil –This can be achieved by densification of the soil and/or improvement of its drainage capacity. –Dynamic Compaction Dropping a heavy weight of steel/concrete from a height. Invasive process. –Compaction Grouting Slow flowing water/sand/cement mix is injected. Good for strengthening foundations of existing buildings. –Drainage Techniques Drains of gravel, sand or synthetic material. Often used in conjunction with other soil improvement techniques.