Rockfall - Yosemite National Park Example

Upper Yosemite Rockfall

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.

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

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.

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.

Landslides - Understanding

Let us first understand the movements that happen during landslides ((:
There are two nature of movements of landslides.
- Rotational Slide
- Slumping


Rotational Slide

Rotational slides move downward and outward on top of curved slip surfaces. Movement is more or less rotational about an axis parallel to the slope. This type of slide has an arcuate slide plane, along which sliding occurs and they produce slumps by their backward rotational movement. Rotational slides leave a series of concave slope forms. The rotational slide is characterized by sliding of land as a coherent large slump block.

Rotational slides are common on steep slopes and usually occur on weaker rocks. They are also commonly associated with permeable caprock overlying an impermeable substratum and characterized by over-steepened slopes such as marine cliffs or actively retreating escarpments. But sometime, rotational slides can take place on slope made up of one type of rock/soil.

Coherent blocks of material slip down across one or more converging slide planes so that back tilted slopes are found at the surface and may trap sediment or standing water. Rotational slides move only short distances, and their accurate movements tend to restore equilibrium soon because the driving mass decreases and resting mass increases.



Slumping

Slumps have rotational movements along a curved slide plane. The coherence of the mass is then lost. This movement also takes place as a flow, near the toe. It results in (a) an arcuate scar at the head where the material evacuated, (b) a linear tongue of mobile material and (c) a bulging toe dominated by flowage. Slumping results in sliding of masses of rock but not as coherent as a whole. The coherence of the mass is lost and often flow is observed at the toe of slumping. Usually, the resultant slope form is concave and sometimes the undulating nature of toe of the slope will depict a miniature series of concave and convex at the base of the slope.

There are several causes to this movement.
1) Heavy rainfall
Water increase pore pressures. This reduces the frictional forces between the soil particles which in turn results in destabilizing the slope. The presence of water also results in increase in weight, lubricating effect between rock layers and reduced friction between soil particles and between rock layers.

2) Geographical Conditions
The nature of rock structure (jointing planes and bedding planes) is an important factor. If permeable rocks such as Chalk, Sand or Cherts lies above the impermeable rocks, rotational landslides are very likely to happen. Water is able to percolate through the porous rocks but it cannot pass through the impermeable layer. As a result, water accumulated at the base of the impermeable rock layer, which leads to the increase in weight, lubricating effect between rock layers and reduced friction between soil particles and between rock layers.

3) Earthquakes
Earthquakes causes vibration and thus trigger slope instability. The vigorous shaking if an already-unstable slope by seismic waves may cause slope failure. The higher the magnitude of an earthquake, the more vigorous the shaking will be and thus the more mass wasting will occur.

4) Human Activities
Construction of roads and houses causes oversteepening of slopes as such activities results in the removal of toes which effectively reduces the resisting mass, leading to slope instability and in turn, slope failure. Building of houses on slopes will also add weight to slope, adding on to the fact that human activities introduces a lot of ground water beneath the homes, lubricating effect and weakening of the soil/rock layer beneath occurs.

This picture shows a slumping failure.

Introduction to Mass Movement

HAHA HELLO EVERYONE! :D

In this nice little island SINGAPORE, natural processes are constantly taking place around us.
Mass movements such as landslides happen in Singapore too.
But first of all, let us look at the some definations which will help with our understanding of Mass Movements, with some examples that happened in Singapore, along with her mitigation efforts.


Mass Movement:
- refers to the downward (downslope) movement of material (rock,regolith and soil) on slopes
- under the influence of gravity without erosive agents like wind,ice or running water.
- mechanisms of mass movements are water,gravity,shear strength and shear stress
- 3 types of movement: slide,flow and heave
- examples : Landslides,mudflows,rockfalls and avalanches.

Slide:
- involves movement if coherent blocks of material along a well-defines surface
- examples: rock slide, avalanche or slump

Flow:
- involves continuous movement of material as a viscous fluid
- no sharply defined failure surface
- shear is distributed throughout moving mass
- differential movement (mass surface moving faster than the base)
- examples: mudflow,debris flow, earthflow

Heave:
- expansion of material near surface at right angles to the slope
- gravity causes material to move slightly
- caused by repeated freezing/thawing or wetting/drying cycles
- may result in creep

Alrights, after learning these definations, we will be looking further into 2 types of mass movements -- Landslides and Rockfall. Secondly, we will also be looking into real examples of such mass movements that happened in Singapore. Lastly, we will be looking at the mitagation efforts taken to deal with these mass movements. ((: