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Sunday, November 16, 2008
Thursday, September 25, 2008
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Posted via Pixelpipe.
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Wednesday, September 24, 2008
Thursday, January 17, 2008
Rockfall - Yosemite National Park Example
Upper Yosemite Rockfall
This was how Upper Yosemite looked like during winter.
This was how Upper Yosemite looked like during winter.
In July 10th, 1996, a spectacular rock fall took place when a huge slab of granite about 150m wide, about 10m long, and 6m in thickness plunged near 500m off the valley wall to the valley floor below.
The rock mass, which weighted some 162kilotons, accelerated to about 160mph, before crashing onto the valley floor, where it generated an air blast, leveling 2000 trees, and killing 1 person who was crushed by a tree blown down by the air blast in front of the fallen rock mass.
Here are some pictures of some falls trails.
The rock mass, which weighted some 162kilotons, accelerated to about 160mph, before crashing onto the valley floor, where it generated an air blast, leveling 2000 trees, and killing 1 person who was crushed by a tree blown down by the air blast in front of the fallen rock mass.
Here are some pictures of some falls trails.
Rockfall - Understanding
After learning about landslides, we shall move on to learn more about rockfalls!
And here is a detailed summary of what we have to know abour rockfalls. (:
Rockfalls
Rockfalls occur on slopes of at least 50-90 degrees. It occurs when the angle of friction is greatly exceeded, overcoming the internal strength of the rock, thus causing the rock to fall. The particle or rock fragment would first become detached then it starts falling, rolling and bouncing until it reaches a point where it can come to rest ( where the slope angle is low enough). This movement causes rockfalls and rock toppling. Most falls involve individual fragments which disintergrate on impact, but occasional major slope collapses take place.
Rockfalls result in :
(a)recession of steep rock walls
(b)resultant slope form is mainly free face and rectilinear or constant gradient slope
(c)scree slope can be concave
(d)talus slope have a high angle of repose,35-40 degree
Causes of rockfalls
1) Geological weakness
-presence of joints, faults and fissures offer lines of weakness along which detachment of blocks is facilitated.
-allow for weathering processes like freeze-thaw or freeze wedging to take place which will subsequently result in rock falls of felsenmeers.
-lines of weaknesses cause the shear strength to decrease
2)Weathering processes
-frost action
-thermal disintegration
-water pressure in pores and joints
-climate related processes thus seasonal pattern of rock fall is present
-maximum in spring and autumn
-gravity plus structural weakness causes the rocks to fall after acted on weathering actions
3) Earthquakes
-quakes and vibrations resulted in increase in shear strength stress and cause imbalance
-hence triggering major rock falls
4) Severe Storms and Heavy Rainfall
-pore water pressure
-role of water in freeze-thaw action
And here is a detailed summary of what we have to know abour rockfalls. (:
Rockfalls
Rockfalls occur on slopes of at least 50-90 degrees. It occurs when the angle of friction is greatly exceeded, overcoming the internal strength of the rock, thus causing the rock to fall. The particle or rock fragment would first become detached then it starts falling, rolling and bouncing until it reaches a point where it can come to rest ( where the slope angle is low enough). This movement causes rockfalls and rock toppling. Most falls involve individual fragments which disintergrate on impact, but occasional major slope collapses take place.
Rockfalls result in :
(a)recession of steep rock walls
(b)resultant slope form is mainly free face and rectilinear or constant gradient slope
(c)scree slope can be concave
(d)talus slope have a high angle of repose,35-40 degree
Causes of rockfalls
1) Geological weakness
-presence of joints, faults and fissures offer lines of weakness along which detachment of blocks is facilitated.
-allow for weathering processes like freeze-thaw or freeze wedging to take place which will subsequently result in rock falls of felsenmeers.
-lines of weaknesses cause the shear strength to decrease
2)Weathering processes
-frost action
-thermal disintegration
-water pressure in pores and joints
-climate related processes thus seasonal pattern of rock fall is present
-maximum in spring and autumn
-gravity plus structural weakness causes the rocks to fall after acted on weathering actions
3) Earthquakes
-quakes and vibrations resulted in increase in shear strength stress and cause imbalance
-hence triggering major rock falls
4) Severe Storms and Heavy Rainfall
-pore water pressure
-role of water in freeze-thaw action
Landslide - Jalan Dermawan Example
“Heavy rain causes landslide at Jalan Dermawan”
Due to heavy rains on 18 December 2006, a landslide occurred on 19 December at Jalan Dermawan where residents complained of hearing a cracking sound at the back of their homes at 10 am in the morning. Residents were evacuated from their homes for safety precautions. Fallen trees from the landslide covered the road which became impassable to traffic and mud and trees had also fallen into the garage areas of some of the houses. Thankfully, no one was hurt during the landslide.
This example of landslide in Singapore is caused my heavy rainfall which is one of the common causes for such movement as water plays an important role in de-stabilizing the slope. The most important effect of the water is that it increases pore pressures. This reduces the frictional forces between the soil particles and destabilizes the slope. The presence of water results in increased in weight, lubricating effect between rock layers and reduced friction between soil particles thus causing the landslide at Jalan Dermawan.
Mitigation Measures
1) Slope Drainage
Slope drainage is the cliff-face method of control which involves reducing pore pressures using drainage lines within cliff faces, field drains, gravel trenches and by intercepting overland flows. Shallow surface drains are used to intercept the overland flow while vertical drainage is emplyed to remove the water from the cliff face as well as the body of the cliff.
This method is especially useful in clay cliffs which are susceptible to slumping triggered by high water content in the clay. This method deals with the main factor of cliff face failures which is the action of water and therefore it is one of the most effective way of strengthening the cliff. However, changes in cliff hydrology can have impacts on the ecology and land uses of the cliff top. Successful drainage schemes may also result in subsidence of cliff-top land as the cliff dries out. Implementation of a comprehensive drainage network sometimes requires a considerable level of technical knowledge and expertise.
2) Cliff/Slope modification
The aim of this modification is to reduce the height of the cliff. If the height and slope angle of the cliff are decreased then there is a reduced chance of mass movements occurring as the greater slope angle and height results in an increased shear stress. Cliff modification involves blasting the upper cliff to reduce its height and very often the blast materials are used to stabilize the cliff foot as toe support.
If the blasting is done with care, the mass movements can be minimized as the shear stress caused by height is reduced. However, this method of modification can backfire and result in increased mass movements as the blasting can cause instability in the rock structure.
Due to heavy rains on 18 December 2006, a landslide occurred on 19 December at Jalan Dermawan where residents complained of hearing a cracking sound at the back of their homes at 10 am in the morning. Residents were evacuated from their homes for safety precautions. Fallen trees from the landslide covered the road which became impassable to traffic and mud and trees had also fallen into the garage areas of some of the houses. Thankfully, no one was hurt during the landslide.
This example of landslide in Singapore is caused my heavy rainfall which is one of the common causes for such movement as water plays an important role in de-stabilizing the slope. The most important effect of the water is that it increases pore pressures. This reduces the frictional forces between the soil particles and destabilizes the slope. The presence of water results in increased in weight, lubricating effect between rock layers and reduced friction between soil particles thus causing the landslide at Jalan Dermawan.
Mitigation Measures
1) Slope Drainage
Slope drainage is the cliff-face method of control which involves reducing pore pressures using drainage lines within cliff faces, field drains, gravel trenches and by intercepting overland flows. Shallow surface drains are used to intercept the overland flow while vertical drainage is emplyed to remove the water from the cliff face as well as the body of the cliff.
This method is especially useful in clay cliffs which are susceptible to slumping triggered by high water content in the clay. This method deals with the main factor of cliff face failures which is the action of water and therefore it is one of the most effective way of strengthening the cliff. However, changes in cliff hydrology can have impacts on the ecology and land uses of the cliff top. Successful drainage schemes may also result in subsidence of cliff-top land as the cliff dries out. Implementation of a comprehensive drainage network sometimes requires a considerable level of technical knowledge and expertise.
2) Cliff/Slope modification
The aim of this modification is to reduce the height of the cliff. If the height and slope angle of the cliff are decreased then there is a reduced chance of mass movements occurring as the greater slope angle and height results in an increased shear stress. Cliff modification involves blasting the upper cliff to reduce its height and very often the blast materials are used to stabilize the cliff foot as toe support.
If the blasting is done with care, the mass movements can be minimized as the shear stress caused by height is reduced. However, this method of modification can backfire and result in increased mass movements as the blasting can cause instability in the rock structure.
Landslide - Mitigation
In order prevent landslides from happening, mitagation efforts should be done. And there are ways to which landslides could be prevented.
Vulnerability to landslide hazards is a function of location, type of human activity, use, and frequency of landslide events. The effects of landslides on people and structures can be lessened by total avoidance of landslide hazard areas or by restricting, prohibiting, or imposing conditions on hazard-zone activity. Local governments can reduce landslide effects through land-use policies and regulations. Individuals can reduce their exposure to hazards by educating themselves on the past hazard history of a site and by making inquiries to planning and engineering departments of local governments. They can also obtain the professional services of an engineering geologist, a geotechnical engineer, or a civil engineer, who can properly evaluate the hazard potential of a site, built or unbuilt.
Floods and landslides triggered by monsoon rains close to 100 people dead or missing on the main Indonesian island of Java on December 27 Landslides hit villages in densely populated Central Java's Karanganyar and Wonogiri districts early Wednesday after heavy downpours, with floods also swelling in several areas, leaving 42 dead and 42 missing.
This video shows a landslide that happened at Little Guilin.
Vulnerability to landslide hazards is a function of location, type of human activity, use, and frequency of landslide events. The effects of landslides on people and structures can be lessened by total avoidance of landslide hazard areas or by restricting, prohibiting, or imposing conditions on hazard-zone activity. Local governments can reduce landslide effects through land-use policies and regulations. Individuals can reduce their exposure to hazards by educating themselves on the past hazard history of a site and by making inquiries to planning and engineering departments of local governments. They can also obtain the professional services of an engineering geologist, a geotechnical engineer, or a civil engineer, who can properly evaluate the hazard potential of a site, built or unbuilt.
Floods and landslides triggered by monsoon rains close to 100 people dead or missing on the main Indonesian island of Java on December 27 Landslides hit villages in densely populated Central Java's Karanganyar and Wonogiri districts early Wednesday after heavy downpours, with floods also swelling in several areas, leaving 42 dead and 42 missing.
This video shows a landslide that happened at Little Guilin.
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