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