FINITE ELEMENT ANALYSIS OF SEISMIC INDUCED DEFORMATION OF BREAKWATER The CRISP Consortium Ltd/South Bank University London 15th CRISP User Group Meeting Thursday 19 th September 2002 UNIVERSITYCOLLEGE LONDON DEPARTMENT OF CIVIL & ENVIRONMENTAL ENGINEERING In association with
This FE study was carried out for Mouchel Consulting Ltd by Amir Rahim and Andrew Chan. Software used is CRISP and SWANDYNE II This FE study was carried out for Mouchel Consulting Ltd by Amir Rahim and Andrew Chan. Software used is CRISP and SWANDYNE II
A breakwater is to be constructed in an area where earthquake is likely to occur. A breakwater is to be constructed in an area where earthquake is likely to occur. The breakwater core consists of sand which is dredged from seabed The breakwater core consists of sand which is dredged from seabed The breakwater lies above a layer of sand varying from few meters up to about 20m. The in-situ sand overlays bedrock The breakwater lies above a layer of sand varying from few meters up to about 20m. The in-situ sand overlays bedrock
Criteria for liquefaction We check for liquefaction by checking whether excess pore pressure has reached vertical effective stress for a period of time after the end of the earthquake We check for liquefaction by checking whether excess pore pressure has reached vertical effective stress for a period of time after the end of the earthquake
Coast Section 7-7 Section 9-9 Port side Sea side Round head L2 U9 U12 U5 Plan of Breakwater
Earthquake Record used El-Centro earthquake (May 1940, Southern California). This was factored to a maximum acceleration or 0.3g.
Sand Breakwater (rock) Accropode 22 m 20 m Bed rock 25 m Section 9-9 at the roundhead
FE Mesh for Breakwater
Material Properties Mg is slope of critical state line and is found by using Phi Mg is slope of critical state line and is found by using Phi Mf, controls unloading modulus. We use the relationship which is based on the Relative Density. Mf=Mg x Dr Mf, controls unloading modulus. We use the relationship which is based on the Relative Density. Mf=Mg x Dr Other parameters are chosen for typical loose sand as detailed in the book by Zienkiewicz, Chan, Pastor and Shiomi Other parameters are chosen for typical loose sand as detailed in the book by Zienkiewicz, Chan, Pastor and Shiomi The armour materials were considered to be elastic with the following properties E= KPav=0.15 E= KPav=0.15 The permeability coefficient for the breakwater core material was initially taken as 5x10-5 m/s. The permeability coefficient for the breakwater core material was initially taken as 5x10-5 m/s.
Test 1 FE results with following properties: Dr=35% for breakwater sand For rock armour E= KPav=0.15 For breakwater sand k= 5x10 -5 m/s.
Excess pore pressure at various nodes in section 9-9 for duration of quake of 53 seconds and further consolidation time up to 439 seconds. Excess pore pressures at nodes 23 and 25 are zero (free drainage)
Test 2 (effect of increasing sand permeability) FE results with following properties: Dr=35% for breakwater sand For rock armour E= KPav=0.15 For breakwater sand k= 5x10 -3 m/s.
Excess pore pressure at various nodes in section 9-9 for duration of quake of 53 seconds and further consolidation time up to 439 seconds. Excess pore pressures at nodes 23 and 25 are zero (free drainage)
Test 3 (effect of stiffness of surrounding rock armour) FE results with following properties: Dr=35% for breakwater sand For rock armour E=150.0 KPav=0.15 For breakwater sand k= 5x10 -5 m/s. Here we reduced stiffness of rock armour
Deformed mesh plot of section 9-9, with smaller stiffness for rock armour
Excess pore pressure at various nodes in section 9-9 for duration of quake of 53 seconds and further consolidation time up to 439 seconds. Excess pore pressures at nodes 23 and 25 are zero (free drainage)
Test 4 (effect of compaction of breakwater material) FE results with following properties: Dr=70% for breakwater sand For rock armour E= KPav=0.15 For breakwater sand k= 5x10 -5 m/s. Here we have doubled the relative density of breakwater sand
Excess pore pressure at various nodes in section 9-9 for duration of quake of 53 seconds and further consolidation time up to 439 seconds. Excess pore pressures at nodes 23 and 25 are zero (free drainage)
Test 5 Introducing rock filter at bottom of breakwater FE results with following properties: Dr=35% for breakwater sand For rock armour E= KPav=0.15 For breakwater sand k= 5x10 -5 m/s. 3 meter layer rock filter is placed at bottom of breakwater. This has Dr=70% and k=1x10 -3 m/s
Excess pore pressure at various nodes in section 9-9 for duration of quake of 53 seconds and further consolidation time up to 439 seconds. Excess pore pressures at nodes 23 and 25 are zero (free drainage)
Conclusions This Finite Element study concentrated on four factors linked to liquefaction. These are: Effect of using more permeable material for breakwater core. The permeability was increased from 5x10-5 to 5x10-3,as detailed in test 1 and 2. Although the results show some variation in deformation and excess pore pressure, overall deformation remains the same. It can therefore be concluded that changing the permeability will only allow excess pore pressure to escape from the in-situ sand and move into the breakwater core material thus causing more liquefaction in this zone. Using more permeable material, therefore, may not reduce the effect of liquefaction. Effect of using more permeable material for breakwater core. The permeability was increased from 5x10-5 to 5x10-3,as detailed in test 1 and 2. Although the results show some variation in deformation and excess pore pressure, overall deformation remains the same. It can therefore be concluded that changing the permeability will only allow excess pore pressure to escape from the in-situ sand and move into the breakwater core material thus causing more liquefaction in this zone. Using more permeable material, therefore, may not reduce the effect of liquefaction.
Conclusions Effect of using stiffer material for rock armour. The rock armour stiffness was changed to a small value as in test 3. The results show considerable displacement and the analysis had to be limited to a short period of time (only 63 seconds) as severe deformation would cause numerical ill- conditioning. Effect of using stiffer material for rock armour. The rock armour stiffness was changed to a small value as in test 3. The results show considerable displacement and the analysis had to be limited to a short period of time (only 63 seconds) as severe deformation would cause numerical ill- conditioning.
Conclusions Effect of compaction. The relative density (Dr) specified for the breakwater core material below the water level is 35%. If this is to be increased to say twice as much, as in test 4, the settlement reduces by nearly half as much. Therefore, compaction (increase in Dr) is likely to reduce the effect of liquefaction. The most critical material parameter for the densification model is the unloading parameter Mf which is linked to the relative density of the sand (Dr). Effect of compaction. The relative density (Dr) specified for the breakwater core material below the water level is 35%. If this is to be increased to say twice as much, as in test 4, the settlement reduces by nearly half as much. Therefore, compaction (increase in Dr) is likely to reduce the effect of liquefaction. The most critical material parameter for the densification model is the unloading parameter Mf which is linked to the relative density of the sand (Dr).
Conclusions Effect of using a rock filter. Adding a 3m rock filter at the bottom of the breakwater would have two main advantages which help reduce liquefaction. These are higher density associated with such rock material, and higher permeability, allowing excess pore pressure to dissipate more quickly. Effect of using a rock filter. Adding a 3m rock filter at the bottom of the breakwater would have two main advantages which help reduce liquefaction. These are higher density associated with such rock material, and higher permeability, allowing excess pore pressure to dissipate more quickly.