mengenal tanah longsor dan mitigasinyaweb.ipb.ac.id/~erizal/mektan/slope stability-rev.pdf · apa...

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.