Landslides

These pictures below were taken at Labrador Park.

Taken from: http://www.flickr.com/photos/wildsingapore/1170382716/

This picture shows a landslide at Labrador Beach. The nature of movement of landslide is rotational slide and slumping. Rotational slides are common on steep slopes. When coherent blocks of material slip down, back tilted slopes are formed at the surface. Secondly, slumps have rotational movements along a curved slide plane. They are characterized by over-steepened slopes such as marine cliffs or actively retreating escarpments. They may also take place on the slope made up of a single type of rock or soil.

Taken from: http://www.flickr.com/photos/wildsingapore/489508936/

This second picture shows mitigation to prevent further mass movement. The canvas is placed over the cliff to ensure that rain would not reduce the cliff's shear strength. This is because water can add weight to the cliff and dissolves some of the materials that bind the rocks together. The canvas ensures that water on it would flow downwards, thereby not affecting the cliff.

Mitigation

i.Modifying the event

Slope stability can be increased by a variety of engineered techniques which, although expensive, enable successful control of hazardous mass movements like landslides.

1. Protective Barriers

As the slide moves downslope and larger pieces collide with each other, the block from rockfalls usually breaks up into smaller pieces. These pieces can travel a long distance due to the force of the falling rock. When the base of the mountain is removed for the roadbed, road cuts are susceptible to rockfalls and slides. Therefore, to make the roads safer for motorists, some protective barriers have been designed to catch the falling rocks. Steep slopes can be covered or sprayed with concrete to prevent rock falls.

2. Slope/Cliff Pinning

Function: The cliff is pinned through the likely shear planes in the cliff. Piles, made up either of metal, plastic or wood, are used to pin the potential mass movements. In order to provide toe support, toe revetments are often built at the foot of the cliff. This can serve as an alternative to the slope managers who are especially dealing with the unstable cliff subject to frequent mass movements. It is relatively inexpensive and if the potential shear plane is effectively identified, mass movements can be minimised. The best results can be obtained if the slope manager combine the cliff pinning with the cliff-foot strategies. However, pinning can further disturb the cliff materials and at times mass movements are accelerated as the materials become less compacted with the intrusion of foreign materials. Identification of potential shear planes sometimes proves to be a difficult task.

3. Cliff/Slope modification

Function: The aim of cliff modification is to reduce the height of the cliff. This is because when 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 stablise the cliff foot as toe support. If the blasting is done with care, such as controlled blasting that does not weaken the rock structure internally, mass movements can be minimised with the reduced height. The blast materials come in handy as materials for toe support.
However, cliff modification can backfire and result in increased mass movements.
Blasting can cause instablity in the rock structure of the remaining cliff and this can accelerate slope failure.

4. Terracing (benches) and drainage

Steep slopes can be made more stable through regrading the slope to lower gradient via terracing or benches. Drainage can increase shear strength of the materials by reducing the pore-water pressure. Regrading the slope can produce more stable angles.

Next, by loading the toe and retaining walls, materials deposited at the slope foot reduces the shear stress. Retaining walls can also stabilise the upper slope. Stablisation can be further enhanced through rock anchors on the the upper slope and shotcrete used to reduce infiltration into the hillside.

Lastly, for toe stablisation, the toe can be stablised by gabion and earth fill. A hazard-resistant design can also be applied.

5. Slope drainage

Function: Slope drainage is the cliff-face method of control which is most widely used for cliffs with a high clay content. Pore water pressures can be reduced by drainage lines within the cliff face, field drains, gravel trenches and by intercepting overland flow. Shallow surface drains are also used to remove water from the cliff face as well as the body of the cliff.

• This method is particularly useful in clay cliffs which are susceptible to slumping and rotational slides which are triggered by high water content in the clay.
• It also deals with the main factor of cliff-face failures-the action of water- and therefore it is the most effective way of strengthening the cliff.
• However, this method might alter the cliff hydrology if successful and impact 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.

Mudflow

Picture taken in an old Asian Village in Japan. The picture shows a devastating mudflow which had taken place many years back and had destroyed nearly the entire village.

Mudflows are very quick flows of earth materials which behave more like a liquid rather than a solid. They involve finer sediments and are less viscous than earthflows. Thus, they travel at a high speed too. Mudflows occur on lower slopes than earthflows and are common in areas with very sparse vegetation. Torrential downpours also contribute to the occurrance of mudflows as they add a huge amount of water content to the earth materials.

Because of the high velocity and long distance travel, mudflows can be hazardous to people. Mudflows that result when a volcanic eruption causes ice to melt or water to be drained from the crater lakes. These mudflows are termed as lahars. Sometimes, lahars can be extremely hot when they are created as a result of eruptions of hot tephra.

Causes of Mudflows :

• Role of water and heavy rainfall
• Volcanic eruptions

Other Examples of Mudflows :

• 1980 eruption of Mount St. Helens which triggered off mudflows and debris avalanche
• 1985 eruption of Nevado del Ruiz volcano in Canada which triggered off devestating mudflows, killing 23,000 people

Rotational Slide and Slumping

About

Slumping is similar to rotational slides. Slumps have rotational movements along a curved slide plane. The coherence of the mass is lost. the movement may take place as a flow near the toe of the slide. Theses processes result in an arcuate scar at the head where the material had evacuated, followed by a linear tongue of mobile material, and ending with a bulging toe dominated by flowage.

Causes

Heavy Rainfall :

• Water increases pore pressures ->reduces frictional forces between the soil particles & destabilises the slope
• Water adds to the weight of the soil and has a lubricating effect between layers of rock.

Geological Conditions :

• Rotational landslides are very likely if a permeable rock layer lies above an impermeable rock layer
• Water percolates through the porous rock and settles between the two rock layers
• Lubricates them & adds to their weight.

Earthquakes :

• Earthquakes cause vibration & trigger slope instability
• The vigorous shaking of an already unstable slope may cause it to fail

Human Activities :

• Over-steepening of slopes caused by construction of houses & roads results in removal of toes
• Effectively reduces the resisting mass, leading to slope instability & failure
• Building houses on the slope can add weight to it
• Human acitvities introduces a lot of ground water beneath the homes, lubricating and weakening the soil or rock layer beneath.
  
  Process
  
  
• Rotational slide will result in a series of rocks along a curved fracture with much of the materials moving as a coherent large slump block. A back tilted slope is a characteristic feature.
• Slumping also results in sliding of masses of rock not as a coherent whole. The coherence of the mass is lost and flow is observed at the toe. An arcuate scar at the head may be formed, with a bulging toe at the end of the slump.
• In both rotational slide and slumping, there are prominent crown, major scarp, minor scarps and a toe.
• Rotational slides will leave a series of concave slope forms
• Slumping results in a concave slope form and sometimes the undulating nature of the toe will depict a miniature series of concave and convex at the base of the slope.

Animation depicting the cross-section of a mudflow

Rock Fall

Credits: members.tripod.com/~Jenn2000/fall.html

Rockfalls occur when a particle or rock fragment breaks off from the parent rock. The movements involved in rockfalls are falling, rolling and bouncing. The result of these movements are individual fragments that disintegrate on impact. Occasional major slopes collapses may take place.

Causes Of Rockfall:

• geological weakness
• weathering processes
• earthquakes
• severe storms
• heavy rainfall

The resultant slope form is mainly free face and rectilinear/ constant gradient slope. The surface from which rocks were detached forms the steep cliff while the rock debris accumulates at the base and forms the talus slope.

newspaper articles on landslides in s'pore

Here are two newspaper articles from The Straits Times that shows the damage caused by mass movements in Singapore.

Hillview Estate Hit By Another Mudslide

Section: Home
By: TRACY SUA, TEH JOO LIN
Publication: The Straits Times 05/02/2007
Page: H1
No. of words: 556

BCA declares three homes out of bounds after latest incident.

ANOTHER mudslide, probably triggered by wet weather, hit semi-detached houses in Upper Bukit Timah's Hillview estate.
This "avalanche" in Jalan Dermawan happened recently, along the same slope and about 100m away from the first one.
The first time it happened on Dec 19, three houses were swamped by mud and debris. The affected families have since been given the all-clear and have moved back home.

The Building & Construction Authority (BCA) has now declared three houses – Nos. 100, 102 and 104 – out of bounds.

The families in 100 and 102 were spared the shock of seeing mud and debris crashing into their homes because they managed to evacuate three hours before the retaining wall in their backyards gave way; the family in 104 made it out of there just in time.

The BCA issued the order to the families to evacuate after it had been alerted to soil movements by the Defence Ministry, which had installed movement sensors there in late December.

On the evening of the second mudslide on Jan 12, Daniel Lim, a 16-year-old student living in No. 106, was playing computer games in his room at about 9pm when he heard a crash.
He recalled: "It was not that loud, more like a table crashing...Then I heard one of the neighbours shout 'Get out!' and all of us rushed out."

A mess in the backyards of Nos. 100, 102 and 104 greeted the residents: Soil and vegetation, till then held back by the wall, was all over the backyards.
The most affected house, No. 102, had debris about one-storey high covering the maid's room and kitchen; No. 100 had a new wall of mud kissing its back wall. A mini basketball court that used to put 10m between the house and the slope was gone; No. 104's backyard was piled with broken pieces of the wall and vegetation.

The next day, the land owner, Far East Organization, laid canvas sheets to protect the slope. It teamed up with Mindef's professional engineers to remove soil from the hilltop to reduce the load on the slope and to stabilise it. Movement detectors have also been installed to check for shifts in the soil, which has seemed stable since.
One of the three households has accepted Far East Organization's offer of accommodation.

Hong Kah GRC MP Ang Mong Seng, who recently met the estate's residents, BCA, Far East Organization and Mindef about the situation, said Mindef and Far East Organization were doing "a good job" with slope works and clearing the debris.

At Jalan Girang in Braddell estate and Westlake Avenue off Thomson Road, slope-shoring works are also going on, following landslides in late December.

A Land Transport Authority spokesman said the debris from a Jan 12 landslide at Jalan Anak Bukit near Beauty World has been cleared. The slope has been covered with canvas and the slip road to the Pan-Island Expressway has been reopened.

Meanwhile, as the rains abate and the Chinese New Year approaches, nurseries along Thomson Road are going all out to recoup the millions in losses they incurred in the December floods. Ms Sharon Goh, who runs Candy Greenhouse & Flowers, said: "We have to earn back to cover our losses."

Homes Hit By Landslides: Who Pays Bill?

Section: Home
By: TRACY SUA
Publication: The Straits Times 10/01/2007
Page: H6
No. of words: 634

AFTERMATH OF DECEMBER FLOODS: Home owners still unsure if they will get help for repairs

JALAN Dermawan resident James Tan's home is in shambles.
Its broken metal grille gate is lying inside the compound, a 7m-long perimeter wall is gone and there is mud everywhere.
"I was shocked. I had no idea it was this bad," said Mr Tan, 60, after visiting his two-storey semi-detached house last week.

On Dec 19, the Hillview Gardens estate unit was hit by tonnes of soil and vegetation, which slid down a 50m-high nearby slope as Singapore was battered by its heaviest rainfall in 137 years in the month of December.

More than 100 people were affected as landslides and floods hit several parts of the country.

Apart from Mr Tan and his wife, two other families living near the bottom of the slope have been asked to stay away until the land is stabilised and declared safe.

Jalan Dermawan is one of three areas where people were asked to evacuate their homes after being hit by landslides and landslips over the last two weeks of last month.

In Jalan Girang in Braddell estate and Westlake Avenue off Thomson Road, six houses were evacuated.

Most of those affected, including Mr Tan's family, have not been able to return home as repair works are still under way.
Mr Tan, an entrepreneur, was not aware of the full extent of the damage to his house because he left for Shanghai shortly after the landslide, returning to visit his house only on Jan 2.

Since the landslide, more than 1,600 truckloads of debris and soil have been removed from the area.

The Ministry of Defence, which owns the slope near Mr Tan's house, has been working to stabilise the ground and has put Mr Tan up in a chalet at The Chevrons, a club for warrant officers and specialists.
Mr Tan does not know how much the damage to his house will set him back, but a family car which was flipped over will cost about $42,000 to repair.
It is still unclear who will pay for the repairs – whether the owners of the affected houses will pay in full, or whether they will get help.

Mindef public affairs director Colonel Benedict Lim said: "To render the area safe for residents, the contractors are working to stabilise the lower sections of the slope, before proceeding to remove soil from the overhanging upper sections."
Work is expected to be completed by the end of the month.

The Building and Construction Authority (BCA) has written to 20 government departments and agencies with slopes or earth-retaining walls to remind them to inspect and maintain them regularly.
It estimates that advisories will be sent to more than 100 private land and home owners as a precautionary measure.

Landslides and landslips are rare in Singapore, with the BCA saying that, besides last month's incidents, there has been only one recorded case in the past three years.

The incessant rainfall last month also caused havoc in some areas – particularly among nurseries in Thomson Road.

Hawaii Landscape executive director Lilian Koh, 55, said: "Everytime there's a heavy downpour, I get scared" because a flood is likely to follow in the low-lying area.
The landscape company, which was flooded on Dec 19, estimates it has suffered damage of $500,000.

At nearby landscape firm Nature Company, three truckloads of debris have been carted away.
Nature Company assistant general manager Ivan Tan, 49, said the company raised the level of the compound by 300mm to 400mm three years ago and by about 500mm just two months ago.
"Raising it further will be very costly as we would have to elevate the whole building," he said.
He estimates the company has lost more than $200,000 in products damaged by water.

Credit: http://newslink.asiaone.com/