Every rainy season we hear news of landslides blocking mountain roads, riverbanks collapsing, or cut slopes behind houses sliding. All of these share the same engineering principle: "slope stability". This article explains the mechanisms, causes and prevention approaches from a geotechnical engineering perspective.

What is a landslide, and how does it differ from a debris flow?

A landslide (or slope failure) is the downslope movement of a mass of soil or rock that occurs when the shear stress acting on the soil mass exceeds the soil's own shear strength, so the mass slides along a failure surface.

In everyday Thai usage, "din slide" usually refers to a soil mass sliding down a slope, while "din thalom" (literally "collapsing earth") is often used for violent events in which soil, rock and water rush down rapidly, such as mudslides in the mountains. In engineering terms both belong to the same group, mass movement; they differ in speed, water content and mode of movement, but the root of the problem is the same: the slope's stability drops until it can no longer resist the forces acting on it.

Landslide mechanism on a hillside: a slope stable under normal conditions compared with the same slope failing after heavy rain, when infiltrating water reduces the resisting force The left panel is a cross-section of a hillside under normal conditions, with a house on the crest. The surface soil on the slope rests on a layer of weathered rock or dense soil, and the interface between the two layers is a weak plane that could become a slip surface. The groundwater level is low, and the resisting force from the soil's shear strength still exceeds the driving force from the weight of the soil mass along the slope. The right panel shows the same slope after prolonged heavy rain: rainwater infiltrates the soil, the groundwater level rises, and the increased pore water pressure on the weak plane reduces the resisting force, while the saturated soil becomes heavier and increases the driving force. FS falls below 1 and the soil mass slides down along the plane as a block, producing a tension crack behind the crest, a tilted and settled house, posts leaning the same way, and bulging soil with turbid seepage at the toe, which are warning signs. The strip at the bottom summarizes the three groups of prevention principles: manage water, reduce the acting forces and increase the resisting forces. Landslides: how a stable slope fails in heavy rain Not to scale Normal: slope stable Resisting > Driving (FS > 1) Driving Resisting Weathered rock / dense soil Surface soil Low GWL Weak plane (potential slip surface) ! After heavy rain: failure Resisting < Driving (FS < 1) 1 Crack behind crest 2 House / trees tilt 3 Bulging + seepage at the toe GWL rises Pore pressure u ↑ On plane → less resistance Saturated → more driving 1 Manage water Surface drains · slope drains 2 Reduce loads Flatten slope · add berms 3 Add resistance Retaining wall · soil nails
A slope fails when the shear stress acting on it (from the weight of the soil mass and the slope angle) exceeds the soil's shear strength — engineers assess this with the Factor of Safety (FS). Infiltrating rainwater raises pore water pressure and reduces resistance, which makes it the number-one trigger.

What causes landslides? 4 main causes

Although each event differs in detail, the factors that destabilize a slope usually fall into 4 main groups.

1. Water (the number-one trigger)

Rainwater infiltrating the soil mass raises the pore water pressure, which lowers the soil's shear strength while also adding to the weight of the soil mass. This is why landslides occur most often during prolonged heavy rain or shortly after a downpour.

2. Slope angle

The steeper the slope, the larger the component of weight pulling the soil mass downslope, and the higher the shear stress acting on it. Slopes cut steeper than that soil type can withstand are at particular risk of failure.

3. Soil type and condition

Each soil type has a different shear strength. Soft clay, silt, or layers containing cracks and weak planes are riskier than dense soil or hard rock. Soil layers that dip parallel to the slope also form planes along which the soil mass can slide easily.

4. Excavation and human activity

Cutting into slopes for roads, building houses on hillsides or excavating the toe of a slope removes the material that supports the soil mass below, making the slope higher and steeper and upsetting its balance. Placing fill that adds weight at the crest, or discharging wastewater onto a slope without a drainage system, can also accelerate failure.

⚠️ A note on disaster statistics

Most landslides in Thailand are linked to heavy rain in the mountainous areas of the North and South. If you need to cite statistics or risk areas, use data from official agencies such as the Department of Mineral Resources (DMR) and the Department of Disaster Prevention and Mitigation (DDPM), which have produced subdistrict-level landslide risk maps.

Which areas are at risk of landslides?

Site features that engineers watch closely include:

  • Foothills and steep slopes in high-rainfall areas, especially where a thick soil layer covers the underlying rock
  • Slopes cut for roads, buildings or site grading without retaining structures or drainage
  • River and canal banks whose base is eroded by water, so the bank gradually steepens until it fails
  • Areas with traces of past movement, such as ground cracks, leaning utility poles or curved tree trunks
  • The outlets of watercourses, ravines, or foothill plains that lie on the path of soil and mud flows

What are the warning signs of a landslide? 5 signs to watch for

Many landslides give warning signs days to weeks in advance, but landslides triggered by heavy rain sometimes happen very quickly with almost no warning. If your home or land is on a slope or near a riverbank, keep an eye out for these 5 signs, especially during prolonged rain.

  1. Ground cracks parallel to the slope edge — long, curved cracks at the crest or behind the house (tension cracks) are the first sign that the soil mass is starting to move. The faster the cracks widen, the higher the risk.
  2. Posts, fences or trees leaning the same way — utility poles, fence posts or tree trunks that were once straight gradually tilting downslope show that the ground beneath them is moving.
  3. Bulging or heaving soil at the toe — ground, roads or walls at the foot of the slope bulge up or are pushed outward as the soil mass above pushes down and accumulates at the toe.
  4. Seepage, muddy water or new springs where there were none — groundwater changes its flow path when the soil starts to crack. If nearby ponds or streams turn unusually muddy after rain, be especially cautious.
  5. Building cracks, sticking doors and windows, and floor settlement — diagonal cracking in structures on the slope, floors sinking into depressions, or water pipes repeatedly breaking for no apparent reason are usually caused by foundations moving with the soil.
⚠️ If you see 2 or more signs

Move people out of the area during heavy rain and have a geotechnical engineer inspect the site as soon as possible. Waiting for the rain to stop before inspecting is often too late, because landslides usually occur during or shortly after prolonged heavy rain. The Department of Mineral Resources uses measured rainfall above 100 millimeters in 24 hours as the alert threshold for landslide and flash-flood watch (Khaosod, 3 Jul 2026)

How can landslides be prevented by engineering?

Effective prevention is rarely a single method on its own; it combines three groups of principles: managing water, reducing the acting forces and increasing the resisting strength.

← Scroll to see the full table →

ApproachPrincipleBest for
Drainage systemReduce infiltration and pore water pressure (surface drains, horizontal slope drains)Almost every slope at risk from water
Slope regradingFlatten the slope or add berms to reduce shear stressSlopes with enough room to regrade
Retaining structuresRetaining walls, sheet piles or soil nailing for supportHigh, steep slopes that cannot be regraded
Slope surface protectionGrass planting and erosion-control materials to reduce runoff scourShallow slopes, riverbanks, general reinforcement

For high slopes or riverbanks that need a permanent retaining structure, engineers select the wall type to suit the height, soil type and water level. Read more in the article 7 types of retaining walls and how to choose

💡 Water is the key

In practice, water management is often the most cost-effective and effective measure, because water is the main trigger of landslides. Many slopes that look stable in the dry season fail in the rainy season because they lack a proper drainage system.

The 3D view below brings together in one picture what engineers look at on a real hillside: from surface signs, the slip surface and groundwater level in the cross-section, and movement-monitoring instruments, through to each group of prevention measures.

3D model of a landslide-prone hillside behind a house, with monitoring and engineering prevention measures Conceptual 3D view of a block of ground shaped as a hillside, with a flat top on which a house stands, sloping down to the right to the flat ground at the toe. The front cross-section shows surface soil over weathered rock. A red curved slip surface runs from behind the crest down to daylight at the toe, and the soil mass above it is shaded light red. The blue dashed line is the groundwater level that rises after heavy rain. A tension crack appears on the ground surface behind the crest. An inclinometer casing is drilled through the slip surface and anchored in the rock to measure subsurface movement. On the slope are horizontal drains and soil nails long enough to pass through the slip surface; at the toe is a retaining wall, and at the back a drainage ditch intercepts surface water. The right-hand box gives the Department of Mineral Resources watch threshold of rainfall above 100 millimeters in 24 hours, and the three groups of measures — manage water, reduce driving forces and increase resisting forces — together with monitoring instruments. 3D model: landslide-prone hillside, monitoring and protection Not to scale 1 Crack behind crest First sign of soil movement 4 Inclinometer Measures subsurface movement 3 GWL rises after rain Pore pressure ↑, resistance ↓ 2 Slip surface Potential sliding mass 5 Soil nails + drains Extend past slip plane 6 Toe wall Adds resistance at the toe Monitor + protect Rain > 100 mm in 24 hrs = landslide watch level (Dept. of Mineral Resources) 1 Manage water Interceptor ditch · slope drains 2 Reduce driving Flatten slope · add berms 3 Add resistance Soil nails · toe wall · piles 4 Monitoring Inclinometer · Piezometer Design FS to meet criteria in both dry and wet states Conceptual view, not to scale
3D model — ① A tension crack behind the crest is the first sign that the soil mass is starting to move ② The curved slip surface and the potential sliding mass ③ The groundwater level rising after rain increases pore water pressure and reduces resistance ④ An inclinometer drilled through the slip surface and anchored in rock measures subsurface movement ⑤ Soil nails extending through the slip surface, combined with horizontal slope drains ⑥ A wall at the toe adds resistance. Right-hand box: the Department of Mineral Resources watch threshold of rainfall above 100 mm in 24 hrs, and the three groups of measures with monitoring. Conceptual illustration (image: SPN Soil Engineering)

Where does risk assessment start?

Before designing any protection, you must first understand the soil and water at the actual site. The steps a geotechnical engineer follows start with:

  • Site reconnaissance — look for traces of past movement, water flow directions and slope angles
  • Soil investigation — collect samples to determine the soil layers, the depth to firm strata and the groundwater level
  • Soil strength testing — determine the shear strength parameters for use in stability analysis
  • Slope stability analysis — calculate the factor of safety (FS) for both dry and wet conditions (see the criteria in Slope stability factor of safety)
  • Design of protection measures — choose methods that bring FS within safe criteria for the type of project
  • Monitoring with instruments — an inclinometer measures subsurface movement and a piezometer measures pore water pressure, confirming that the measures are working

Complex slope work, such as high slopes, riverbanks or areas that have failed before, should be assessed and designed by the responsible engineer, because safe slope angles and protection methods depend on site-specific soil and water conditions; there is no fixed formula that works everywhere. See real project examples in the hillside cut slope case study and riverbank erosion protection design

SPN Soil Engineering provides soil investigation and soil strength testing and geotechnical engineering design for slopes, riverbanks and retaining structures, with reports certified by licensed civil engineers (Council of Engineers Thailand). Contact us for free advice and a quotation within 24 hrs.