Liquefaction in Action Bay Fill and Earthquakes: The Hidden Dangers

Transcription

1 Liquefaction in Action Bay Fill and Earthquakes: The Hidden Dangers Overview This activity will address issues regarding wetland development and bayfill and will demonstrate the dangers of building on land prone to liquefaction. The students will create a simple and fun model of a structure built upon wetlands and observe the impact of earthquakes on bay mud and landfill. Students will use common food items to represent the bay mud, soil, and developments, and then test the impacts of an earthquake on their structure. Finally, they can eat their delicious experiment. Estimated Time Approximately one hour Objectives Students will be able to: Define liquefaction Describe the impacts of earthquakes on landfills and developed wetlands Learn about the dangers of developing wetland areas Build a model to understand natural processes in conflict with human developments Materials For entire class: sample of Bay mud, if possible For every 3-4 students: 1 regional past and present map of the San Francisco Bay: ecoatlas/habitat/maps/sfbay/ pastpresinfo1.html For each student: 1 small paper bowl, filled halfway with chocolate pudding Sharón Friedner 1 small paper plate, taped to the bottom of the bowl with duct tape half a bowl s worth of chocolate pudding 4-5 oreo cookies 3-4 graham crackers or wafer cookies 3-4 pretzel sticks 3-4 banana slices 3-4 marshmallows 1 plastic spoon 1 napkin copies of Student Pages California Science Content Standards Grades 6 Standard Set 2.d: earthquakes, volcanic eruptions, landslides, and floods change human and wildlife habitats. Grade 7 Standard Set 7.d: construct scale models, maps and appropriately labeled diagrams to communicate scientific knowledge (e.g., motion of Earth s plates and cell structure). Grades 9-12 Earth Sciences Standard Set 9.a: the resources of major economic importance in California and their relation to California s geology. Additional Resource Loma Prieta Quake Liquefaction Photos Damage1.html Save The Bay s San Francisco Bay Watershed Curriculum 77

2 Background Liquefaction is an important problem, typical of earthquakes. One possible definition of liquefaction would be: When a saturated granular soil is shaken over a period, the porewater pressure will tend to increase. When this pressure reaches the confining pressure the soil will suffer a sharp drop in strength, its behavior becoming close to that of a liquid. This results in ground deformation and settlement, tipping of buildings, landslides, failure of earth dams, or other hazards. Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. Liquefaction and related phenomena have been responsible for tremendous amounts of damage in historical earthquakes around the world. Liquefaction occurs in saturated soils, that is, soils in which the space between individual particles is completely filled with water. This water exerts a pressure on the soil particles that influences how tightly the particles themselves are pressed together. Prior to an earthquake, the water pressure is relatively low. However, earthquake shaking can cause the water pressure to increase to the point where the soil particles can readily move with respect to each other. When liquefaction occurs, the strength of the soil decreases and, the ability of a soil deposit to support foundations for buildings and bridges is reduced. Increased water pressure can also trigger landslides and cause the collapse of dams. Because liquefaction only occurs in saturated soil, its effects are most commonly observed in low-lying areas near bodies of water such as rivers, lakes, bays, and oceans. The effects of liquefaction may include major sliding of soil toward the body of water, or more modest movements that produce tension cracks. Port and wharf facilities are often located in areas susceptible to liquefaction, and many have been damaged by liquefaction in past earthquakes. Most ports and wharves have major retaining structures, or quay walls, to allow large ships to moor adjacent to flat cargo handling areas. When the soil behind and/or beneath such a wall liquefies, the pressure it exerts on the wall can increase greatly - enough to cause the wall to slide and/or tilt toward the water. Liquefaction also frequently causes damage to bridges that cross rivers and other bodies of water. Such damage can have drastic consequences, impeding emergency response and rescue operations in the short term and causing significant economic loss from business disruption in the longer term. Liquefaction-induced soil movements can push foundations out of place to the point where bridge spans lose support or are compressed to the point of buckling. Where is liquefaction likely to occur? Two conditions must exist for liquefaction to occur: (1) the soil must be susceptible to liquefaction (loose, water-saturated, sandy soil, typically between 0 and 30 feet below the ground surface) and (2) ground shaking must be strong enough to cause susceptible soils to liquefy. The most susceptible soils are generally along rivers, streams, and lake shorelines, as well as in some ancient river and lake deposits. Wetland soils are highly susceptible to liquefaction. Teacher Proc ocedur edure 1. If you have a sample of bay mud put it out in front of the class and encourage your students to observe and even touch it. Introduce the topic of wetland development and ask the students questions like: - Do you think that the Bay looks exactly the same as it did years ago? 78 Save The Bay s San Francisco Bay Watershed Curriculum

3 - What changes do you think have happened that have affected the size and shape of the Bay? - Have you ever experienced an earthquake? - What kind of effects do you think earthquakes have on land developments? - What are some things you know about wetland development? 2. Pass out the past and present maps of the San Francisco Bay and have the students look at what the Bay shoreline used to look like and what it looks like now. Have them estimate how much of our wetlands are left. 3. Explain that only 10% of our wetlands are left and discuss bayfill with them. Explain that most of San Francisco, Oakland airport, etc. are built on bayfill and point out those areas on the maps. What dangers do you foresee when we build on wetlands and bayfill? 4. Pass out materials to each student and explain that we are going to test some of the problems of building structures on wetlands and bayfill by building our own models and observing what will happen when there is an earthquake. 5. Tell the students that we are going to simulate building a structure on wetlands and bayfill. The chocolate pudding in their bowl is going to represent bay mud because it is soft and squishy. 6. Have the students put their oreo cookies on their plates and crush them up into tiny pieces and crumbs. This will represent the bayfill or dirt that we need to put on top of the mud for our development. Have them sprinkle the crumbs on top of the mud, covering the top of the mud, then place their bowls on their empty plates. 7. Next tell them the rest of their materials are their building materials and that they should build some sort of structure on their landfill. Let them be creative and have fun! 8. Now they will simulate the earthquake and see how many of their structures will stand. Have them carefully shake their plates with their bowls on them. (Make sure they shake the plate and not the bowl.) Explain that this is very similar to what happens when we really have an earthquake. Time the shaking and at 20 seconds explain that that is how long the Loma Prieta earthquake in 1989 lasted. Have them make a mental note of the damage in their bowl. After one minute, tell them that that was how long the 1906 San Francisco earthquake lasted and have them notice the damage. 9. Ask them what their bowls looked like at 20 seconds versus what they looked like at one minute. Explain that the length of the earthquake affects the amount of damage done, and that much of the Bay Area that has been filled in by bayfill would respond to earthquakes in the same way their bowls did. Clas lass s Discussion/W sion/wrap Up 1. Discuss the concept of liquefaction. Explain that it occurs mainly in soils that are saturated with water and that the movement causes the water pressure to increase and the soil particles to move freely. 2. Brainstorm ways to prevent liquefaction and discuss wetland development issues. 3. Conduct the San Francisco Airport Debate in the History of Human Impacts section of this curriculum. Save The Bay s San Francisco Bay Watershed Curriculum 79

4 Liquefaction in Action Bay Fill and Earthquakes: The Hidden Dangers INTRODUCTION Liquefaction is a common problem around the San Francisco Bay Are It occurs when soils that contain a lot of water undergo pressure or impact, such as an earthquake. You will create a simple and fun model of a structure built upon wetland soils and observe the impacts of earthquakes on bay mud and landfill. You will use common food items to represent the bay mud, soil, and developments, and then test the impacts of an earthquake on your structure. Finally, you can eat your delicious experiment! MATERIALS For every 3-4 students: 1 regional past and present map of the San Francisco Bay (from Oakland Museum or San Francisco Estuary Institute) For each student: 1 small paper bowl filled halfway with chocolate pudding Sharón Friedner 1 small paper plate taped to the bottom of the bowl with duct tape 4-5 oreo cookies 3-4 graham crackers or wafer cookies 3-4 pretzel sticks 3-4 banana slices 3-4 marshmallows 1 plastic spoon 1 napkin Student Pages PROCEDURE 1. If your teacher has a sample, observe the bay mud and touch it if you like. What does it feel like? Does it look and feel like other mud? If not, how is it different? 2. Working in groups of three or four look at the past and present maps of the Bay Are What do you notice about the Bay? How has it changed? Estimate how much of our original wetlands remain today. 3. Locate your school on the map. Is it built on wetlands? What about your house? 4. What percentage of our wetlands remain in the Bay Area today? 80 Save The Bay s San Francisco Bay Watershed Curriculum

5 5. List 3 reasons why we have lost so much of our wetlands. c. 6. Name 2 places from the maps that are built on landfill. 7. Define liquefaction. 8. After your teacher passes out all the materials, begin building your structures built on bay fill. First start by crushing up your oreo cookies on your plates. Sprinkle your fill on your wetlands or bay mud and then use the rest of your materials to build structures on your fill. 9. Now you are ready to simulate an earthquake. Carefully put your bowl on your plate and follow your teacher s instructions. Begin gently shaking your plate (not your bowl!). When your teacher calls out 20 seconds for the 1989 Loma Prieta Earthquake, make a note of the damage done to your structure, but don t stop skaking your plate! Continue shaking until your teacher calls one minute for the 1906 San Francisco Earthquake. Now look at the damage. 10. Describe the results of the Loma Prieta quake. 11. Describe the results of the 1906 quake. 12. List 2 ways to avoid liquefaction. 13. When you re finished, you can eat your liquefaction model! Then clean up your work are Save The Bay s San Francisco Bay Watershed Curriculum 81