When Slopes Fail: What Actually Causes Landslides on Infrastructure Projects

Landslides along roadways, rail corridors, and developed slopes are often described as sudden or unexpected. In reality, most slope failures are the result of long-term conditions quietly worsening over time until one triggering event finally pushes the slope past its limit.
 

Understanding why slopes fail is the first step toward preventing costly damage, service disruptions, and safety hazards. While every site is unique, most infrastructure-related landslides share a common set of underlying causes.
 

Water: The Most Common Culprit

Water is involved in the vast majority of slope failures.  Poor drainage, blocked culverts, leaking utilities, and uncontrolled surface runoff all increase pore water pressure within soil and rock. As water pressure rises, the soil’s ability to resist movement decreases.
 

Key water-related contributors include:

• Inadequate surface drainage

• Subsurface seepage paths not accounted for during original construction

• Aging or failed drainage systems

• Extreme rainfall events acting on already marginal slopes

Importantly, slopes often tolerate these conditions for years before finally reaching a tipping point.

Toe Erosion and Loss of Support

Many infrastructure slopes rely on the material at their base—the “toe”—for stability. When streams migrate, ditches deepen, or erosion goes unchecked, that toe support can be removed.
 

Common causes of toe erosion include:

• Channel downcutting

• Scour during flood events

• Poorly protected embankment toes

• Maintenance activities that unintentionally steepen slopes

Once the toe is compromised, even a previously stable slope may begin to move.
 

Legacy Construction and Hidden Conditions

Much of today’s infrastructure was built decades ago, often with limited subsurface investigation by modern standards. Older fills, undocumented repairs, and variable construction practices can leave behind weak zones that are invisible at the surface.
 

Examples include:

• End-dumped fills with poor compaction

• Buried organic layers

• Unreinforced retaining structures

• Changes in alignment or grade over time

These legacy conditions frequently explain why failures occur long after construction is complete.
 

Geometry and Over-Steepened Slopes

Slope geometry matters. As slopes are widened, cut steeper, or loaded with new infrastructure, factors of safety can be reduced without obvious warning signs.
 

This is especially common where:

• Roads have been widened multiple times

• Retaining structures were designed for earlier loading conditions

• Slopes were cut steep to minimize right-of-way impacts

Small geometric changes can have outsized effects on stability.
 

Triggering Events: The Final Push

While long-term conditions set the stage, landslides are often triggered by a specific event:

• Prolonged or intense rainfall

• Rapid drawdown after flooding

• Freeze-thaw cycles

• Construction vibrations or excavation

These events don’t usually cause the problem, they simply expose it.

Why Early Identification Matters

Most slope failures give warning signs:

• Tension cracks

• Leaning guardrail or trees

• Pavement distress

• Bulging at the slope face

• Persistent wet areas

Early investigation and targeted stabilization can often prevent emergency conditions, reduce costs, and improve long-term performance.
 

A Smarter Approach to Slope Stability

Effective slope remediation requires more than a single solution. Successful projects consider:

• Site history and performance

• Drainage improvements

• Constructability constraints

• Long-term durability

• Monitoring and verification

At GeoSpecialties, we believe that understanding why a slope is failing is just as important as deciding how to fix it. Thoughtful evaluation leads to safer, more reliable infrastructure, and fewer surprises down the road.