--- title: "Landslide causes and slip surface techniques" subject: "Engineering Geology I" chapter: 6 key: "semester_3/engineering_geology_i:landslide-causes-and-slip-surface" source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/landslide-causes-and-slip-surface site: BCT Engineering --- # Landslide causes and slip surface techniques Engineering Geology I · Chapter 6: Physical Geology - Asked in: 2061 Baisakh (old course), 1+3+4 marks - Video: [LANDSLIDE-WHAT ARE THE CAUSES?HOW TO MITIGATE?||ENRICH GEOGRAPHY](https://www.youtube.com/watch?v=HT9SBe7NZgI) — ENRICH GEOGRAPHY - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/landslide-causes-and-slip-surface - Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-6 ## Question What is landslide? Describe its causes. Explain the techniques to determine the slip surface of landslides. ## Short answer A landslide is a gravity-driven downslope movement along a slip surface when driving stress exceeds strength. Causes include steep slopes, weak rocks, heavy rain, earthquakes, toe cutting, and loading. Slip surfaces are located by surface mapping, boreholes, inclinometers (most reliable), geophysics, and back analysis. ## Answer A **landslide** is the downward and outward movement of rock, soil or debris down a slope along a slip surface, under gravity. It occurs when the driving shear stress exceeds the shear strength of the slope material. #### Causes **Natural causes** - Steep slope, weak or weathered rock, and adverse orientation of joints and bedding. - Heavy rainfall and rise of the groundwater table (increase of pore-water pressure). - Earthquake shaking and volcanic activity. - Undercutting of the toe by river or wave erosion. - Loss of vegetation due to forest fire. **Man-made causes** - Cutting of the slope or toe for roads (the main cause in Nepal). - Loading the top by buildings, dumps and reservoirs. - Deforestation and poor drainage, leaking pipes and irrigation canals. - Blasting and vibration, excessive mining or quarrying. #### Techniques to determine the slip surface The slip (failure) surface is the base of the moving mass; its depth and shape are needed to design stabilisation. 1. **Surface mapping:** location of the main scarp, cracks, bulges at the toe, and tilted trees or poles. It gives the approximate outline and shape of the slide. 2. **Test pits and trenches:** excavation exposes shear surfaces directly, shown by polished, slickensided or remoulded clay layers. Suitable for shallow slides. 3. **Boreholes and core logging:** drilling through the slide shows the change from disturbed to intact material; soft and sheared zones, loss of core, and different moisture identify the slip zone. 4. **Inclinometer:** a casing in a borehole is read at intervals; the depth at which the casing bends or shifts gives the slip surface and rate of movement. It is the most reliable method. 5. **Slip indicators and shear strips:** a flexible tube, a wire or a column of rings or beads in a borehole is sheared at the slip depth. 6. **Time domain reflectometry (TDR):** a coaxial cable in a borehole gives a signal change when it is sheared. 7. **Geophysical surveys:** seismic refraction, electrical resistivity imaging and ground penetrating radar show the contrast between the slide mass and the stable ground beneath. 8. **Piezometers:** water pressure readings help locate the water-bearing zone along the slip surface. 9. **Back analysis:** a trial slip surface is found by stability analysis for $FS = 1$ using the known strength and geometry. Using more than one method gives better confidence. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A landslide occurs when: A. The slope is dry and gentle B. Shear strength exceeds shear stress C. The driving shear stress exceeds the shear strength of the slope material D. Pore-water pressure falls to zero Answer: C. Slope material fails when the driving force exceeds resistance. (Key point: Failure condition) 2. What is the main man-made cause of landslides in Nepal? A. Planting trees on slopes B. Building check dams C. Reducing irrigation D. Cutting of slopes or toes for roads Answer: D. Road cutting removes support of the slope toe. (Key point: Main man-made cause) 3. How does heavy rainfall trigger landslides? A. It raises the groundwater table and pore-water pressure, reducing strength B. It increases friction between particles C. It cements the soil D. It reduces the weight of the slope Answer: A. Higher pore pressure lowers effective stress and shear strength. (Key point: Water effect) 4. Why is the inclinometer considered the most reliable method to locate a slip surface? A. It measures surface cracks only B. The depth where the casing bends or shifts shows the slip surface and rate of movement C. It reads rainfall at the site D. It gives the colour of rock cores Answer: B. Casing in a borehole deforms at the slip depth. (Key point: Inclinometer) 5. Back analysis finds a trial slip surface by: A. Assuming FS = 5 B. Drilling boreholes only C. Stability analysis for FS = 1 using known strength and geometry D. Counting tilted trees Answer: C. A failed slope has FS about 1 at failure. 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