Mekanika Tanah : Analisis Stabilitas Lereng (Teori & Studi Kasus) - Hinawan T. Santoso, ST, MT

Main ideas and lessons

• Goal of slope stability analysis

  • Determine whether a slope is stable or unstable under possible triggering actions such as:
    • Rainwater infiltration / water seepage
    • Cutting the slope to a new geometry (changing slope angle/height)
    • Adding loads (e.g., piles placed on the slope)

• Core stability concept: Safety Factor (SF)

  • The safety factor is defined as a comparison between:
    • A holding (resisting) force (or resisting shear strength)
    • A driving force (or shear stress mobilized that can cause sliding)
  • Typical interpretation:
    • SF < 1 → slope is unstable
    • SF = 1 → critical (at the point of failure)
    • SF > 1 → generally stable
  • A commonly cited threshold mentioned: SF > 1.25 for general stability.

• Shear strength components used

  • Soil shear strength is described using:
    • Cohesion (c)
    • Friction angle (φ) (implied by “internal friction angle”)
  • The discussion separates safety-factor ideas into:
    • safety factor related to cohesion
    • safety factor related to frictional shear resistance (using the tangent of the friction angle)

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Methodology / analysis approaches mentioned

The speaker lists multiple methods for slope stability analysis:

1. Practical knowledge / field experience method

   • Based on expert experience in the field
   • Usually qualitative, not ideal for strict quantitative “safe vs not safe” decisions
   • Requires high practitioner experience

2. Computer program method (numerical tools)

   • Must be used by someone with sufficient engineering skill
   • Warning: “garbage in, garbage out”
     • If incorrect soil/material inputs are used, results may not match field reality
   • Examples mentioned:
     • PLAXIS (and variants)
     • Bishop / slope programs referenced indirectly via named tools:
       • Bioslop (as stated)
       • PLAXIS 2D
       • PLAXIS Slope/WGS, SLOPE/W, LOOP, SABLE, GeoStudio / similar (names appear with some transcription inconsistencies)

3. Limit equilibrium methods

   • Use equilibrium of forces and/or moments on potential failure surfaces
   • Variants noted:
     • General limit equilibrium
     • Swedish slip circle method
     • Method of slices
       • Attributed to Fellenius and Bishop (speaker transcription: “fellenius and pisau”)
     • Stability charts/graphs
       • Named experts mentioned: Taylor (and related “Taylor methods”), plus others with unclear transcription (e.g., “koshin / paganis”)

• 2D vs 3D
  • Stability can be analyzed in two dimensions or three dimensions, depending on complexity (noted for later discussion).

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Detailed concepts and formulas presented

A) Infinite slope analysis (continuous slope plane) — without seepage

• Geometry elements

  • Height of slope: (H_a) (also written as (h_a))
  • Soil weight / unit weight: (\gamma)
  • Slope angle: (\beta)
  • Length (l) used in the derivation

• Core idea

  • Compare mobilized driving shear stress along the potential slip plane to available resisting shear strength

• Shear strength form (conceptual)

  • Resisting shear strength includes cohesion and friction contributions, conceptually like:
    • ( \text{resisting} = c + \gamma(\cdots)\tan\phi )

• Simplified SF concept

  • SF appears as a ratio where cohesion and friction contributions are treated separately.

• Special cases described

  • Coarse-grained soil: (c = 0)
  • Fine-grained soil: friction term involving (\phi) becomes 0 in the simplified intent (cohesion-only vs friction-only cases)

• Critical height

  • Solve for the maximum slope height when SF = 1.

Example #1 (infinite slope, no seepage)

• Given (as stated):
  • ( \gamma = 18.6 \,\text{kN/m}^3 ) (or a transcription mismatch mentions “18 kN/m²”)
  • ( \phi = 20^\circ )
  • ( \beta = 22^\circ )
  • ( H_a = 8 \,\text{m} )
• Claimed:
  • SF = 1.25
  • Conclusion: slope is stable
• Second part:
  • Find critical height when ( \beta = 25^\circ ) and SF = 1
  • Claimed critical height: (H_a = 11.5 \,\text{m})

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B) Infinite slope with seepage / groundwater influence

• Key modification
  • Use effective stress:
    • Total stress − pore water pressure
• Water representation
  • Water pressure is represented using a water head/height concept.
• Main change in formulas
  • Use ( \gamma_1 ) (effective/submerged-related unit weight) and ( \gamma_w ) contribution for water.
• Conclusion highlighted
  • Seepage reduces effective normal stress → reduces shear resistance → lowers SF.

Example #2 (infinite slope with seepage)

• Given (as stated):
  • ( \gamma = 20 \,\text{kN/m}^3 )
  • ( H_a = 8 \,\text{m} )
  • ( \beta = 22^\circ )
  • cohesion ( c = 18 \,\text{kN/m}^2 )
  • ( \phi = 20^\circ )
• Claimed:
  • SF = 0.78 → unstable

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C) Translational landslide on a limited slope (straight slip plane)

• Geometry

  • Slope height: (H)
  • Straight translational slip plane at angle (\theta)

• Forces

  • Driving force: based on soil weight above the slip plane (approximated via triangular area)
  • Resisting shear force: from shear strength (cohesion + friction) along the slip plane

• Safety factor

  • ( SF = \dfrac{\text{resisting shear strength-related term}}{\text{driving shear stress-related term}} )

• Critical height

  • When SF = 1:
    • ( H_{\text{critical}} \propto \dfrac{c}{\gamma \cdot \text{function of angles}} ) (speaker-stated form)

Example #3 (new embankment placed on an existing slope)

• Given:
  • Old embankment angle: ( \theta = 40^\circ )
  • New embankment angle: ( \phi = 48.5^\circ )
  • New embankment soil unit weight: 19.6 kN/m³
  • Cohesion on failure plane: 25 kN/m²
  • Internal friction angle: 17°
• Design requirement:
  • SF = 2
• Approach described:
  • Modify allowable cohesion and friction contributions using SF
• Claimed maximum allowable embankment height:
  • (H = 8.51 \,\text{m})

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D) Circular slip surface (limited slope, method of moments)

• Assumption

  • Failure surface is a circular arc with center at O

• Forces / contributions

  • (W_1): weight on the side that drives sliding
  • (W_2): weight on the side that retains (resists)
  • Cohesion along the arc contributes to resistance

• Moment equilibrium about center (O)

  • Driving moment from (W_1)
  • Resisting moment from (W_2) and cohesion along the arc

• Safety measure

  • SF expressed as ratio of resisting moments to driving moments

Example #4 (temporary excavation in clay)

• Given (with some subtitle inconsistencies):
  • Homogeneous clay:
    • ( \gamma \approx 18 \,\text{kN/m}^3 )
    • cohesion ( c = 41.5 ) (units unclear in subtitles; likely kN/m²)
    • friction angle ( \phi = 0 )
  • Excavation height: 8 m
  • Circular slip radius: 17 m
  • Slope angle: 30°
  • Arm lengths:
    • left arm ~ 16.6 m
    • right arm ~ 23.7 m
  • Geometry sector angle:
    • ( \theta = 90^\circ \Rightarrow 1.64) rad mentioned
• Claimed:
  • SF = 1.75 → slope declared stable

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E) Method of slices (general case: layered soil + groundwater)

• Motivation

  • Real slopes may be layered with different soil properties
  • Groundwater conditions may vary by layer
  • Therefore, infinite-slope or single-plane simplifications may be insufficient

• Core idea

  • Divide the failure mass into multiple vertical slices
  • For each slice, compute driving and resisting forces based on:
    • slice weight
    • interface normal stress
    • friction angle
    • cohesion along the slip surface segment
    • pore water pressure / seepage effects

• Critical-surface interaction

  • Uses a reference circle center line and transformation angles
  • Slice contribution depends on slip surface geometry and the slice midpoint

• Sign convention

  • If an angle term (noted as (\theta) or equivalent) is positive → slice tends to drive
  • If negative → slice tends to retain

• SF aggregation

  • ( SF = \dfrac{\sum(\text{resisting moment/force from slices})}{\sum(\text{driving moment/force from slices})} )

Example #5 (two soil layers, saturated/slice method)

• Given (as stated):

  • Excavation geometry:
    • cut depth ~ 14 m
    • slope ratio: 1.5H : 1V
    • additional profile depth down to 5 m below ground
  • Layered soil parameters:
    • Land 1:
      • ( \gamma_1 = 17.7 )
      • ( c_1 = 3 ) (transcription suggests “cut friction in 10”, plus unclear symbols)
      • friction angle ~ 10°
    • Land 2:
      • ( \gamma_2 = 19.1 )
      • ( c_2 = 3 )
      • friction angle ~ 24°
  • Groundwater:
    • saturated/submerged condition with free water surface affecting pore pressures

• Procedure described:

  • Divide failure width (~34.5 m) into 8 slices (~4.31 m each)
  • Additional grouping described to match layers
  • For slice #6:
    • compute slice weight
    • compute pore water pressure from water height
    • subtract pore pressure effect from total normal stress
    • compute resisting friction + cohesion from the relevant layer properties

• Claimed overall outcome:

  • SF ≈ 1.51
• Conclusion:
  • “slope tends to be stable”

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F) Taylor chart method (graphic method for clay slopes)

• Applicability

  • Homogeneous clay
  • (c) constant and ( \phi = 0 ) (implied)

• Parameters

  • Stability number concept:
    • ( N = \dfrac{c \cdot D}{\gamma \cdot H} )
  • Relationship to SF:
    • chart is used to find required geometry/angle for a given SF (as described in subtitles)

• Chart usage

  • Determine (N) and slip circle location depending on slope angle ranges
  • Slip circle placement follows Taylor rules (e.g., near toe for smaller angles, possibly outside toe for certain cases)

Example #6 (Taylor diagram)

• Given (as stated):
  • saturated clay excavation:
    • depth ( h = 10 \, m )
    • unit weight ( \gamma = 18.5 )
    • cohesion ( c = 40 )
  • hard layer at depth ( d_h = 12 \, m )
  • friction angle ( \phi = 0 )
  • target SF = 1.5
• Procedure:
  • compute a dimensionless parameter using ( d_h/h ) and the stability expression
  • use Taylor graph to find required slope angle ( \beta )
• Claimed result:
  • required ( \beta \approx 26^\circ) to (30^\circ) (subtitles suggest around 26°)

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Speakers / sources featured (as named)

• Hinawan T. Santoso, ST, MT (main presenter; shown in the video title)
• Music (background audio; no speaker)

Experts referenced

• Fellenius
• Bishop (transcribed as “pisau”)
• Swedish slip circular method (method reference)
• Taylor (Taylor chart method)
• Other chart contributors are implied but not reliably readable due to subtitle transcription issues (e.g., “Janbu / other chart contributors” suggested by garbled names)

Software/tools referenced

• PLAXIS
• SLOPE/W
• LOOP
• Plus additional tool names mentioned with some transcription errors (e.g., SABLE, GeoStudio/similar)