Typhoon Bavi- Part 1: When extreme weather reaches the ground, threaten slopes, roads and foundations

Malaysia felt the regional effects of Typhoon Bavi when Malaysia Airlines cancelled flights between Kuala Lumpur and Taipei, joining other carriers disrupted by the powerful storm.

Although Bavi did not directly threaten Malaysia, its deadly Philippine landslides showed how extreme weather can destabilise seemingly firm ground. Beyond flight disruption, typhoons endanger slopes and infrastructure through the combined effects of rainfall, groundwater, vegetation and soil weakness.

When rain weakens the ground

A natural slope may appear firm in dry weather because partially saturated soil possesses matric suction, which provides additional strength. During prolonged rainfall, water infiltrates the ground, fills the pores and reduces this suction.

At the same time, rising pore-water pressure lowers the effective stress between soil particles and weakens the soil’s resistance to sliding. Once the downslope forces exceed the remaining shear strength, failure may occur with little warning.

Rainwater also follows cracks, root channels, weathered rock joints, construction interfaces and damaged drains, allowing it to reach deeper weak layers. Concentrated seepage may loosen soil and remove fine particles, causing shallow slips, fast-moving debris flows or larger rotational and translational landslides.

As saturation increases, the slope’s factor of safety, the engineering measure of stability, progressively declines.

Wind, trees and root-soil disturbance

Strong winds can worsen already critical ground conditions. Tree roots often reinforce shallow soil, but saturated ground provides weaker anchorage. During a typhoon, repeated tree swaying transfers dynamic forces into the root–soil system. This movement can loosen surrounding soil, enlarge cracks and disturb the slope surface.

When a tree is uprooted, its root ball may tear out a substantial volume of soil. The resulting depression exposes weaker material and forms a direct pathway for further water infiltration.

Vegetation should therefore be treated as part of the geotechnical system rather than merely as landscaping. Suitable planting may help control shallow erosion, but large trees placed too close to retaining walls, drains or steep slopes may introduce additional instability during severe winds. Poorly maintained vegetation can also block drainage channels, conceal tension cracks and restrict access for inspection.

Erosion, scour and hidden voids

Typhoon rainfall also generates rapid surface runoff. When rainfall intensity exceeds the soil’s infiltration capacity, water flows across the ground, removing topsoil and eroding exposed slopes.

At rivers, culverts and drainage outlets, fast-moving water can scour riverbanks, bridge foundations and the toes of road embankments. Once support is removed from the base of a slope, failure may progress upwards even when the upper section initially appears intact.

Rapid changes in floodwater levels create another danger. As water levels fall, groundwater trapped within a riverbank or embankment may continue to flow outward. This produces hydraulic forces capable of carrying fine particles from within the soil.

The process, known as internal erosion or piping, creates hidden channels and voids. Over time, these cavities may lead to settlement, sinkholes, embankment collapse or the failure of levees and flood-protection structures. Because the damage develops below ground, few warning signs may be visible until the surface suddenly gives way.

Roads and bridges under pressure

Roads are often among the first systems to show distress during prolonged rainfall. As saturated subgrade loses stiffness and bearing capacity, pavement weakens from below. Cracks admit more water, while poorly compacted shoulders and embankments may soften, erode or wash away.

Potholes, uneven pavement, edge cracking and shoulder collapse may appear superficial but can signal deeper foundation deterioration. Continued traffic over weakened ground accelerates rutting, settlement and loss of serviceability.

Bridge approaches are equally vulnerable. Eroded backfill behind abutments can cause differential settlement, creating a hazardous step between the road and bridge deck. Blocked or undersized culverts may also redirect concentrated flow towards embankments and accelerate washout.

Scour is less visible but potentially more serious. High-velocity flow around bridge piers and abutments can remove supporting soil, leaving a bridge standing while its foundation support is progressively lost. Detailed post-storm inspection is therefore essential, even when structural damage is not immediately visible.

Foundations on coastal and reclaimed ground

Coastal developments and reclaimed sites face added risks because the ground often contains loose, variable or poorly compacted fill. Under intense rainfall, wave action and fluctuating groundwater, these deposits may soften, settle or move laterally.

As stiffness and bearing capacity decline, foundations may settle unevenly, causing buildings to tilt, floors to crack and structural elements to distort. Lateral ground movement can also displace utilities, rupture buried pipes and damage drainage or sewer systems.

Loose saturated sand may lose strength under cyclic loading, although severe storms do not automatically trigger classical liquefaction. Ground response depends on soil type, density, drainage, groundwater level, loading and construction history.

In many reclaimed areas, the more immediate threats are differential settlement, erosion, internal washout and localised loss of support. Such sites therefore require thorough investigation, groundwater assessment and long-term settlement monitoring.

The warning matters to Malaysia

Malaysia’s mountainous roads, hillside developments, river corridors, coastal settlements and widespread residual-soil slopes make these risks especially relevant.

Many local slopes consist of deeply weathered soils that can lose strength rapidly when saturated. Highway cut slopes, hillside housing and embankments built over weak ground are particularly vulnerable during prolonged rainfall. The risk is further heightened when development alters natural drainage, steepens slopes or imposes additional loads on marginal ground.

The real warning lies beneath

Typhoon Bavi’s direct impact on Malaysia may be measured mainly through disrupted flights, but its deeper lesson lies below the surface. Extreme storms do not test buildings and transport systems from above alone; they also challenge the soil, groundwater and foundations that support them.

When the sky unleashes its force, the first failure may begin silently beneath our feet.