MASS WASTING

What is mass wasting ?

Mass wasting is the downslope movement ofEarth material, primarily under the influenceof gravity.

The word landslide is a general term for masswasting and for the landforms created bysuch movements.

Image: University of Minnesota Gravity

Weight of

Material

Friction

Upward

force of

supporting

materials

Driving Forces and Resisting Forces

Driving forces (tekanan

geser) menyebabkan material

bergeser

Resisting forces (kekuatan

geser) mempertahankan

lereng agar tidak longsor.

Stabilitas lereng tergantung pada hubungan antara tekanan geser dan kekuatan geser material pada lereng. Apabila kekuatan geser > Tekanan geser maka lereng akan stabil.

STABILITAS LERENG

BATUAN

TANAH

HASIL

PELAPUKANFaktor pembentuk tanah

iklim

batuan induk

topografi

organisme (flora dan fauna)

waktu

manusia

Fase 1Proses pelapukan dan pembentukan tanah yang

dipengaruhi banyak faktor

1

(sumber: Sutikno)

BATUAN

Fase 2Proses berlanjut dan tanah hasil pelapukan akan menebal, batu-batubesar masih utuh (tidak lapuk)

2

(sumber: Sutikno)

Fase 3 Penebalan tanah berlanjut dan lapisan atas akan semakin melemah/ melunak

akibat mengecilnya butir tanah dan akibat adanya pelindian unsurpengikat butir

tanah. Air tanah mulai mengumpul membentuk lapisan akifer

BATUAN

Air tanaah

Air tanah

3

(sumber: Sutikno)

HUJAN

BATUAN

Fase 4 Tanah hasil pelapukan lunak semakin tebal sehingga melebihi akar vegetasi. Initerjadi karena vegetasi hanya salah satu faktor pembentuk tanah, faktor-faktor lain tetapberjalan. Permukiman mulai merambah kepinggir lereng dan umumnya tidak tahu kalaurawan

4

(Sumber: Sutikno)

BATUAN

Fase 5 Tanah berukuran halus dan lunak sudah tidak mampu lagimenempel pada lapisan di bawahnya akibat sudut kemiringan lerengtajam. Retakan-retakan mulai bermunculan. Tanah lunak siap longsor

5Retakan-retakan

(Sumber: Sutikno)

BATUAN

Fase 6 Tanah mulai longsor bila dipicu oleh faktor dan akan mengalir sebagaialiran massa yang sangat berbahaya serta akan menerjang apasaja yang dilewatinya serta membawa apa saja yang ada di atasnya.

6

(Sumber: Sutikno)

BATUAN

Fase 7: Longsor terjadi akibat beban lereng

sangat berat akibat penjenuhan tanah

7

(sumber: Sutikno)

BATUAN

Longsor berhenti atau terus berjalan

Masuk ke lembah sungai menjadi banjir bandang

Mata air akibat longsor

8

(sumber: Sutikno)

Landslides body

Jatuhan Longsoran Aliran

Pembagian tipe Gerakan Masa

1 2 3

a. Terjadi pada bidangtegak (vertikal)

b. Terjadi pada bidangdatar (horisontal)

Merupakan gerak jatuhbebas material tanpa bidanggelincir.

Gerakan material menurunilereng melalui bidang gelincirtertentu, dapat lambatataupun cepat.

a. Longsoran lambat- Tipe rayapan (creep)

b. Longsoran cepat- Translasional- Rotasional

Gerakan materialmengalir akibatkeberadaan air (flows).Terjadi perubahanstruktur internal yangsignifikan.

a. Aliran debris (debrisflows) dan lumpur(mudflows)

b. Debris avalanche

Falls are abrupt, downward movements of rock or earth, or both,

that detach from steep slopes or cliffs. The falling material

usually strikes the lower slope at angles less than the angle of

fall, causing bouncing. The falling mass may break on impact,

may begin rolling on steeper slopes, and may continue until the

terrain flattens.

Rockfall

A topple is recognized as the forward rotation out of a slope of a massof soil or rock around a point or axis below the center of gravity of thedisplaced mass.

Toppling is sometimes driven by gravity exerted by the weight ofmaterial upslope from the displaced mass. Sometimes toppling is dueto water or ice in cracks in the mass. Topples can consist of rock, debris(coarse material), or earth materials (fine-grained material). Topplescan be complex and composite.

Topple

Topple

A slide is a downslope movement of a soil or rock

mass occurring on surfaces of rupture or on

relatively thin zones of intense shear strain.

Movement does not initially occur simultaneously

over the whole of what eventually becomes the

surface of rupture; the volume of displacing

material enlarges from an area of local failure.

Slides

Rotational Landslide A landslide on which the surface of rupture is curved

upward (spoon-shaped) and the slide movement is more

or less rotational about an axis that is parallel to the

contour of the slope.

The displaced mass may, under certain circumstances,move as a relatively coherent mass along the rupture

surface with little internal deformation.

The head of the displaced material may move almostvertically downward, and the upper surface of the

displaced material may tilt backwards toward the scarp. If

the slide is rotational and has several parallel curved

planes of movement, it is called a slump.

Rotational Landslide

The mass in a translational landslide moves out, or down andoutward, along a relatively planar surface with little rotationalmovement or backward tilting.

This type of slide may progress over considerable distances if thesurface of rupture is sufficiently inclined, in contrast to rotationalslides, which tend to restore the slide equilibrium.

The material in the slide may range from loose, unconsolidatedsoils to extensive slabs of rock, or both.

Translational slides commonly fail along geologic discontinuitiessuch as faults, joints, bedding surfaces, or the contact betweenrock and soil.

Translational Landslide

An extension of a cohesive soil or rock mass combined with

the general subsidence of the fractured mass of cohesive

material into softer underlying material.

Spreads may result from liquefaction or flow (and extrusion)

of the softer underlying material. Types of spreads include

block spreads, liquefaction spreads, and lateral spreads.

Spreads

A form of rapid mass movement in which loose soil, rock andsometimes organic matter combine with water to form a slurry thatflows downslope.They have been informally and inappropriately called mudslidesdue to the large quantity of fine material that may be present in theflow.Occasionally, as a rotational or translational slide gains velocity andthe internal mass loses cohesion or gains water, it may evolve into adebris flow.Dry flows can sometimes occur in cohesionless sand (sand flows).Debris flows can be deadly as they can be extremely rapid and mayoccur without any warning.

Debris Flows

The word lahar is an Indonesian term. Lahars are also known asvolcanic mudflows. These are flows that originate on the slopes ofvolcanoes and are a type of debris flow.A lahar mobilizes the loose accumulations of tephra (the airbornesolids erupted from the volcano) and related debris.

Lahars (Volcanic Debris Flows)

Debris avalanches are essentially large, extremely rapid, often open-slope flows formed when an unstable slope collapses and theresulting fragmented debris is rapidly transported away from theslope.In some cases, snow and ice will contribute to the movement ifsufficient water is present, and the flow may become a debris flowand (or) a lahar.

Debris Avalanche

Earthflow

FACTORS THAT CONTROL MASS WASTING

III. Faktor-faktor pengontrol dan pemicu longsorlahan

FAKTOR PEMICU

FAKTOR PENGONTROL: PERMUKAAN FAKTOR PENGONTROL: BAWAH PERMUKAAN

1. Morfologi lereng2. Kemiringan lereng3. Arah hadap lereng4. Kerapatan aliran5. Keberadaan lembah/ cekungan

Geologi:1. Tipe batuan2. Struktur geologi3. Dip/ kemiringan perlapisan batuan4. Pelapukan batuan

1. Curah hujan2. Gempabumi3. Faktor manusia (penggunaan

lahan, pemotongan lereng,jalan)

4. Keberadaan vegetasi

Tanah:1. Ketebalan tanah2. Sifat fisik dan geoteknik tanah3. Kadar air

Hidrologi:1. Kelengasan2. Keberadaan air tanah3. Kerapatan aliran4. Adanya tubuh air

(a) Sedimentary rock layers dip parallel to this slope. (b) If a road cut undermines theslope, the dipping rock provides a good sliding surface, and the slope may fail. (c)Sedimentary rock layers dip at an angle to this slope. (d) The slope may remainstable even if it is undermined