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MENGENAL TANAH LONGSOR DAN
MITIGASINYAMITIGASINYA
Dr. Ir. H. Erizal, MAgr.Dr. Ir. H. Erizal, MAgr.
APA ITU TANAH LONGSOR?APA ITU TANAH LONGSOR?
Tanah longsor boleh disebut juga dengan Tanah longsor boleh disebut juga dengan gerakan tanah. Didefinisikan sebagai masa t h t t i l l tanah atau material campuran lempung, kerikil, pasir, dan kerakal serta bongkah dan , p , glumpur yang bergerak sepanjang lereng atau keluar lereng karena faktor gravitasi bumikeluar lereng karena faktor gravitasi bumi.
PR S S RJAD NYA N S RPROSES TERJADINYA LONGSOR
air yang meresap ke dalam tanah akan g pmenambah bobot tanah. Jika air tersebut menembus sampai tanah tersebut menembus sampai tanah kedap air yang berperan sebagai bidang gelincir maka tanah menjadi licin dan gelincir, maka tanah menjadi licin dan tanah pelapukan di atasnya akan bergerak mengikuti lereng dan keluar bergerak mengikuti lereng dan keluar lereng.
TIPE LONGSORANTIPE LONGSORAN
• longsoran rotasi (rotational slip)• longsoran translasi (translational slip)• pergerakan blok• runtuhan batu • rayapan tanahrayapan tanah• aliran bahan rombakan
longsoran rotasilongsoran rotasi
longsoran translasilongsoran translasi
pergerakan blokpergerakan blok
runtuhan baturuntuhan batu
rayapan tanahrayapan tanah
aliran bahan rombakanaliran bahan rombakan
TIPOLOGI LERENG RENTAN TIPOLOGI LERENG RENTAN LONGSOR
• Lereng timbunan tanah residual yang dialasi oleh batuan kompak
• Lereng batuan yang berlapis searah lereng topografi• Lereng yang tersusun oleh blok‐blok batuan.
PENYEBAB TANAH LONGSOR
• Faktor alam
• Faktor manusia• Faktor manusia
MITIGASIMITIGASI
• Tahap awal (preventif)Tahap awal (preventif)• Tahap bencanap
• Tahap pasca bencana
Bagan alir sistem manajemen bencana longsor (Karnawati, 2002)
SLOPE STABILITY
Dr Ir H Erizal MAgrDr. Ir. H. Erizal, MAgr.
Types of landslideyp• Rock failure
f il l d t i d– failure plane pre-determined
• Soil failure– failure plane along line of max stress
Types of landslideTypes of landslide
• Rock failure– failure along pre-determined planes of g p p
weakness• Soil failure• Soil failure
– failure along lines of max. stress• frictional, cohesive = rotational• frictional, incohesive = planar
Rotational landslip analysisRotational landslip analysis
• For undrained frictionless failure– total stress analysisy
• For cohesive and frictional failureth d f li– method of slices
– Bishop’s conventional method (can take into account pore water pressure)
Rotational slipRotational slip
• total stress analysis or φu = 0
• strength parameters are those of undrained soil
where
Cr θ2F = restraining moment
disturbing moment
WeCrF θ
= C = cohesive strength (Pa)r = slip circle radius (m)θ= slip sector in radiansW = weight of sliding sector (N)e = eccentricity of sliding sector (m)
Method of slicesMethod of slices
• Swedish circleSwedish circle method
• For use with cohesive• For use with cohesive and frictional soils
∑+n
nNCrF 1
tanφθ
∑∑= n
n
n
TF
1
1
∑ n1
Effect of a tension crackEffect of a tension crack
• Reduces the angle of the sliding sector
Height of tension crack:
F f i ti l il
Ch 2=
For frictionless soil
ρhc =
Cohesive and frictional soilC = cohesive strength (Pa)ρ= unit weight of soil (N m-3)φ = friction angle
⎟⎞
⎜⎛ += 45tan2 φCh
Cohesive and frictional soil
⎟⎠
⎜⎝
+2
45tanρ
hc
Location of slip circle centreLocation of slip circle centre
• No simple way – trialNo simple way trial and error
• F more sensitive to• F more sensitive to horizontal movements than verticalthan vertical movements
Effective stress analysisEffective stress analysis
θ
GWL r
hP = h x f wρ
L
∑∑ −+
= n
nnfn
T
LPNCrF 1
tan)( φθ
∑ nT1
Other methods of analysisOther methods of analysis
• Taylor’s stability analysis– used for frictional and cohesive soils– uses a dimensionless number to iterate
towards a solutiontowards a solution• Bishop’s method
– effect of forces on each side of slice considered
– iterative method
I R iI. Review
Method of slices
Fellenius Method
Simplified Bishop Method
p pMethod
II Spencer / Morganstern Price MethodsII. Spencer / Morganstern-Price Methods
Spencer and Morganstern-Price Methods
III J b M th dIII. Janbu Method
Janbu Method
Correction Factor
Example Problem – Janbu Method
IV. Sliding Block Analysis
V. Stabilization TechniquesV. Stabilization Techniques
Monitoring Techniques
Stabilization Methods
Selection Criteria:Selection Criteria:
• Subsurface conditions & potential modes of failure• The present & required topography• Physical constraints – property lines or buildings• Consequences of failure• Availability of materials, equipment, & expertise• Local performance history• Aesthetics• Time required for construction• Cost
Slopes Flattened or Benched
Berm Provided at ToeWeight Increases the Resistance to SlidingWeight Increases the Resistance to Sliding
Protection Against Erosion Provided at Toe
Lowering of GWT to Reduce the Pore Pressures
Drainage of Slopesg p
Drains are added to stabilize slopesDrains are added to stabilize slopes
Use of Driven or Cast-in-Place Piles
Retaining Wall OR Sheetpiling OR Cylinder Piles Provided to Increase Resistance to SlidingProvided to Increase Resistance to Sliding
Plan for Building Design to aid Slope Stability
Landslip monitoringLandslip monitoring
FlowslidesFlowslides
• Soil, clay, rock debris may behave like liquid; water content is > liquid limitq ; q– flowslide
• Flowslides are extremely mobile– e.g. Yungay, Peru, 1970
Mt. Huascaran, Peru, 1970
• earthquake triggered flowslide
• hit towns of Yungay and Ranrahirca, 18 km away, at around 150 km/hr
• Yungay completely buried, 66,000 deadburied, 66,000 dead
Flowslide Slumgullion ColoradoFlowslide, Slumgullion, Colorado
• National natural landslide laboratory
• Major slip ~3500 years ago, present y gslip ~1000 years ago
The Mam Tor head scar – looking west
Mam Tor landslideMam Tor landslide
O d d t l i ll t d• Occurred due to glacially oversteepened slopes
• Age ~3600 years, from radiocarbon dating of tree remains recovered from boreholes
• ~300 m wide and ~1000 m longUpper part• Upper part– multiple rotation landslide
• Lower part– debris flow
Cross-section through the Mam Tor landslip
Geological map and movements at each station1996 t 2002- 1996 to 2002
CorrelationCorrelation of movementsmovements with rainfall
An analogue for sedimentation i h lf bin half-graben.
DerbyshireDerbyshire County Council is the transportation agent!
Mam Tor referencesMam Tor references
• Skempton, A.W. et al., 1989, The Mam Tor landslide, North Derbyshire, Phil. , y ,Trans. Royal Soc. Lond. 329, 503-547
• Rutter, E.H. et al., 2003, Strain displacements in the Mam Tor landslip, Derbyshire, England, J. Geol. Soc. Lond.Derbyshire, England, J. Geol. Soc. Lond.160, 735-744.
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