Pertemuan 08 GEOTEKNIK

1. Pertemuan 08GEOTEKNIK Matakuliah : S0182/Studi Kasus Dalam Teknik Sipil Tahun : Juli 2005 Versi : 01/01

2. Learning Outcomes Mahasiswa dapat membandingkan kasus-kasus yang terjadi dengan berbagai alternatif yang dipilih  C4

3. Outline Materi • Analisa pemecahan masalah • Beberapa alternatif pemecahan masalah • Kasus kegagalan konstruksi yang mungkin terjadi

4. Slope Failures • On September 12 of 1717 an avalanche cascaded down the Troilet, Italy glacier, gaining speed on a cushion of air, reaching a falling velocity of 320km/hr over a 3600 m fall. Two towns were destroyed, with 7 people killed and 120 cows lost. The slosh of the avalanche ran up the far side of the valley at a speed of 125 km/hr. • Snow avalanches, rock avalanches, debris flows, mud flows, and rock falls are failures of the surface under the action of gravity. The basic physics controlling the stability or instability of landforms is relatively simple and well understood, but the hazards are not always recognized, even when geological deposits indicate past slope failures in the region. In many cases, surface instabilities of this type are compound events, associated with earthquake or volcanic processes, which enhance their catastrophic potential. From the surface geology perspective, landslides and debris flows are important landscape modifying agents, and play as large of a role in eroding topography and depositing debris as is played by other mechanisms such as rainfall and runoff.

5. Slope Failures

6. Slope Failures There are two basic classes of surface failures: 1. Single rock failures • These often are blockfalls of individual rocks falling from an eroding surface, which is usually very steep, or "over steepened" beyond the stable angle of repose for the surface materials. The blocks pile up in an apron of debris, called a talus slope or talus cone. 2. Flows of the surface, involving large volumes of rocks and soil. • Slow flows- (creep, solifluction, earthflows) • Fast flows- (snow avalanches, landslides, mudflows, debris flows) • Fast flows leave jumbled deposits, often piles upon piles, that can be recognized in the geological record by their shape and the poorly sorted (many rock sizes intermixed) nature of the deposit. The fast flows tend to be fluidized, either by a mixture of rock and air or rock and water. The relative components of the mixture determines how the failure will be named: a mudflow is water with lots of clay and silt material, which is a runny mud, while a debris flow is a jumble of rock fragments with some water. The fluidized nature of the flow accounts for the last flow velocities attained, and the associated hazard posed.

7. Slope Failures

9. Landslides • Landslides occur when a portion of a hillslope becomes too weak to support its own weight. This weakness is generally initiated when rainfall or some other source of water increases the water content of the slope, reducing the strength of the materials. Other causes of landslides include earthquakes and loud sounds. Many types of landslides move seasonally or periodically and may lie dormant for years. • Landslides are generally classified into slides, falls and flows. Slides move as large bodies by slipping along one or more failure surfaces. Falls of rock or soil originate on cliffs or steep slopes. Large rockfalls can be catastrophic events. An earthquake off the coast of Peru in 1970 started a rockfall which accelerated to more than 170 miles per hour and buried more than 18,000 people. Flows are landslides that behave like fluids. Mudflows involve wet mud and debris. Earthflows involve wet, claylike material. Solifluction is the downslope flow of soil that occurs on arctic and alpine hillsides when thawed ice or snow saturates the soil cover.

10. Landslides • Landslides constitute a major geologic hazard because they occur in all 50 states, causing $1-2 billion in damages and more than 25 deaths each year. Landslides commonly occur with other major natural disasters, such as earthquakes and floods. The 1980 eruption of Mount St. Helens in Washington was preceded by the development of a large landslide on the north side of the volcano. The Northridge earthquake in 1994 in the San Fernando Valley triggered thousands of landslides in the Santa Susanna Mountains north of the epicenter. Landslides

11. Landslides • Even though landslides are generally not as spectacular or costly as earthquakes, major floods, hurricanes and some other natural catastrophes, they are more widespread and may cause more property loss than any other geologic hazard. Much of the damage attributed to earthquakes and intense storms is really due to landslides. • The most expensive landslide in U.S. history occurred in Thistle, Utah, in the spring of 1983. It reached 1/2 mile from top to bottom and ranged in width from 1,000 feet to about 1 mile. Total costs attributable to this landslide exceeded $500 million. In May of 1970, an earthquake in Peru cost about 70,000 lives. In Alberta, Canada, in 1903, a mass of about 30.6 million cubic meters slipped from the top of Turtle Mountain and fell to the floor of the adjacent valley. The mass of earth and rock spread across a 2 mile wide valley, annihilating the town of Frank and killing 70 people.