Human impact on slopes & mass movement

Impact of human activities on the stability of slopes:

- Increasing stability (makes mass movement less likely)

- Decreases stability (makes mass movement more likely)

Some of this activity is accidental & some is deliberate (management)

Strategies to modify slopes to reduce mass movement: Pinning, Netting, Grading, Afforestation

Case studies: 

Petropolis, Brazil - (physical & human factors causing a landslide event)

Hong Kong - (management of mass movement)

Vaiont Dam, Italy

Pinning

e.g. In North Gwent, Wales 6 m long nails have been fired into slopes by high pressure guns

- Nails pin the looser topsoil to the firmer soil underneath

However (evaluation):

- Nails can bend so they are are not always successful (high pressure systems used in North Gwent have been designed to avoid this issue)

- Needs to be high level of technological and geographical understanding so so might not be available in some LICs

Rock netting is used extensively on unstable rock faces/slopes to:

- Contain & control rockfall

- Protect against the effects of erosion

- Improve stability

e.g. Along the Via Gellia Road in Derbyshire in the Peak District 235 m squared of netting has been installed to prevent rockfalls from 25m cliff next to the road

However (evaluation):

- Requires skill & experience which is less available in LICs

- Is expensive therefore less useful in LICs

- May not work if there are severe conditions (e.g. earthquakes)

e.g. Regrading at Mappleton, along the Holderness Coast

- Cliff regrading involves reducing the slope angle of the cliff to make it lower than the angle of response & planting vegetation

- This stabilises the ground preventing loose material from falling. 450m of cliff made of glacial till (a soft boulder clay) at Mappleton, along the Holderness Coast were regraded (& vegetation planted) in 1991

However (evaluation):

- Needs constant maintenance: 11 years on they have already started to show signs of slumping

- Needs expertise which is not always available in LICs

- It can fail as coastal erosion increases

- Planting trees increases interception of rainfall which reduces the water content of the soil & reducing the shear stress

- The roots of trees also hold the soil together which increases the shear strength

- In Switzerland, about 50% of the forests are managed (called 'Protection Forests')

- In Scotland (between 2018 & 2019), the government is proposing to increase the level of funding for woodland creation grants to £40 million in an attempt to reduce the frequency & severity of mass movement events

However (evaluation):

- Trees are heavy so adding this mass on the top of the slope have the potential to increase rates of mass movement

- It takes time as trees do not grow quickly so is not a quick fix

- It takes up large areas of land which may not be available or affordable or convenient

Decreasing stability (makes mass movement more likely)

There are 5 main ways that humans can make slopes less stable:

- Dumping material from mining and quarrying (spoil heaps)

- Deforestation

- Building (shanty towns) on steep slopes

- Overgrazing by animals & overfarming

- Dam construction

Landslides in Petropolis, Rio de Janerio in March 2013 resulted in 33 deaths

Key questions:

How were the landslides caused?   Physical Causes & Human Causes

How is the area now being managed?

Physical causes:

Humid climate means area receives 2200mm rainfall/year mainly between December & March = high levels of antecedent in the soil

400 mm rainfall in 24-hours on 17th March 2013 - increases shear stress/decreases shear strength

High slope angle (over 30°) - slopes are more vulnerable to to failure (if slope angle exceeds critical angle of repose, slopes will collapse) - increase in shear stress & decrease in shear strength

Shallow soil (less than 1m) - will saturate quickly making it more likely that shear stress will exceed sheer strength

Chemical weathering weakens the rock

Many boulders strewn over slopes - increases shear stress by adding mass

Deforestation - to clear area of vegetation to make room for shanty town

Shanty town built on the steep slopes - high density (40000 people/km squared) & poorly built

Lack of rubbish collection - rubbish collects & absorbs water increasing weight on slope by 20%

Like a drainage = higher water content

- Identify the Problem

- Develop Solutions

- Implement Solutions

- Evaluate Solutions

- Change, Stop or Continue

- Revegetation

- Dainage channels

- Risk assessment mapping (RAM)

ALL decrease shear stress & increase shear strength

3,000m squared / 11,000 plants were planted between July & December 2008

This lead to deep roots growing 2m in 2 years forming an extensive network of roots to bind the soil together which increases shear strength

Positives:   (Evaluation - revegetation e.g. Vetiver Grass is a sustainable solution for long term)

In the landslides of 2011 & 2013 the slopes covered by vetiver grass did NOT slide/move

The root systems basically 'fuse' the loose topsoil with the more solid lower soil

More vegetation = more interception = more evaporation = less water in the soil & also that water reaches the soil more slowly therefore less saturation

Negatives:

Takes two years to grow therefore not a short-term management solution

Requires good growing conditions for at least 2 years for the vetiver the grass to be established

Needs expertise & government support

These were installed in 2008 (after a major landslide) & 2009

1840m square of drainage channels installed

+ Drainage will decrease the amount of water in the soil - reducing shear stress & increasing shear strength

- Poor construction leads to the pipes breaking/leaking therefore more water stays in the soil meaning the soil is more saturated

- Expensive & needs skilled expertise which is not always available in MICs/LICs

- Requires constant monitoring & maintenance which requires funding & sustained interest by government

- Even with drainage, the increase in population density in the shanty town increases weight - the weight of the shanty town increases shear stress so it exceeds shear strength

RAM = Risk Assessment Mapping      Risk was calculated in 23 areas using data on rainfall & previous landslides

16 areas classified as 'high risk' & 2 areas as 'very high risk'

Risk map showed 60% of area at risk of landslides (based on a study of slope angle, water, attitude)

+ Provide predictions/warnings local government can be prepared to evacuate the local population

+ Provide data so local government can begin to stabilised this slopes with: Concrete walls, Drainage pipes, Revegetation, Prevent new housing (Need to know how do these work?)

+ Raise awareness of the danger

- Risk assessments need action as they are not a direct action so are only useful if they lead to actions such as slope stabilization methods

- Local government ignored the research leading to more landslides

If government provided alternatives to the shanty town on the slopes, then few people would live there are there therefore less buildings on the slopes (i.e. Shear stress would be reduced)

High rates of natural increase keeps increasing the population of Petropolis so therefore more shanty towns are needed therefore more weight & more shear stress which exceeds the shear strength

There were 2 Major landslides in Hong Kong:

- 1966 landslide

- 1972 Sau Mau Ping landslide

1966 landslide:

Heavy rain (3 cm in 10 minutes) increased shear stress by adding weight & reduced shear strength by lubricating rock

80 deaths due to rapid mudflows through the city

6000 homeless

1972 Sau Mau Ping landslide

14 m high cliff gave way after heavy rainfall covering Kwun Tong Resettlement Estate in 3m deep mud

Squatter settlement on top of the slope also added shear stress

71 deaths as a result of a single slide on top of the Kwun Tong Resettlement Estate

Similar landslide in 1976 triggered the founding of of the GEO (Technical Engineering Office) to manage landslides in Hong Kong

A major cause was man-made 'fill slopes' built 100 years ago that were too steep and weak to resist heavy rainfall

Sau Mau Ping today has had artificial terracing installed on the slope reducing the chance of the whole Cliff collapsing

Management of landslides in Hong Kong - Run by the Geotechnical Engineering Office

Launched the Landslip Prevention & Mitigation Program in 2010

So far, GEO has spent about $22.9 billion on landslip prevention & mitigation studies & works

5842 government man-made slopes have been upgraded

Mitigation measures for 269 natural Hillside catchments were implemented

Landslip Prevention & Mitigation Program has committed to cover all man-made slopes in vegetation to avoid a visual impact on the environment

On average about 300,000 plants are planted each year to mitigate landslides

Over 100 km of roads have been protected from rockslides using netting

Cliff drainage is compulsory for all steep slopes in Hong Kong

Overall landslide risk arising from manmade slopes has been reduced 3/4 to that in 1977

Dam constructed 100 km North of Venice in 1959

Small landslide events caused concern

Major landslide event caused a wave 250m high to overtop the dam

Villages destroyed with estimated 2500 deaths

Sliding was caused by undercutting & waterlogging of the rock above due to the water in the reservoir. This increased shear stress while reducing shear strength.

Dumping material from mining & quarrying (spoil heaps)

Spoil heap = a pile built of accumulated spoil (Reece rock removed during coal & ore mining/quarrying)