What is Sliding in Concrete Foundation Design

Foundations fail in a number of ways.

When we say a foundation fails in sliding, we are not talking about cracking concrete or soil punching.

We are talking about something more fundamental:

The entire foundation block tries to move sideways as a rigid body.

Sliding vs Other Failures

Bearing failure → soil beneath fails in compression (local or general shear).

Overturning → foundation rotates about an edge due to moment.

Sliding → horizontal forces overcome lateral resistance at the base.

These are independent failure modes.

A footing can be:

Safe in bearing ✔

Safe in overturning ✔

But, Unsafe in sliding ✖

Why Sliding Is Often Ignored

1. Engineers focus heavily on vertical loads and soil bearing capacities.

2. Sliding does not “look dramatic” in drawings.

3. Friction is assumed to be “automatically sufficient.”

There isn't heavy settlement if sliding failure occurs and you always have good frictional force that resists sliding which itself increases with vertical loads. Most of the time sliding failure occurs and the effect is practically invisible. 

In real projects, sliding governs more often than people admit, especially if:

1. Lateral Loads are large but vertical weight is limited.

2. Equipment is producing horizontal and cyclic forces.

3. Presence of Water table is reducing effective weight.

Structures Where Sliding Is Critical

1. Pump & compressor foundations

2. Equipment pedestals with lateral loads

3. Retaining walls

4. Tank ring walls

5. Foundations with heavy seismic loadings

I have seen multiple pump foundations pass bearing and overturning checks comfortably yet develop service issues purely due to overlooked sliding.

Forces That Cause Sliding

Sliding is not caused by horizontal load alone. It occurs when lateral forces find insufficient resistance at the soil–foundation interface.

Lateral Forces

1. Wind loads → superstructure transfers base shear

2. Seismic loads → inertia forces proportional to mass

3. Earth pressure → lateral soil thrust

4. Dynamic forces → unbalanced machine loads

5. Hydrostatic forces → water pressure on substructures

Inclined & Eccentric Loads

1. Inclined column loads have a horizontal component.

2. Eccentric vertical loads create moments → which:

        - Reduce contact area (Refer Loss of Contact in Concrete Foundation)

        - Reduce effective normal force

        - Indirectly reduce friction

Free-Body Diagram

Soil does not pre-resist sliding. It reacts only after the foundation tries to move. If a foundation has no lateral forces, it won't have any lateral reaction due to soil as well.

This distinction is critical.

What Resists Sliding?

1. Base Friction ()

It is defined as the product of Normal force acting on the body and friction coefficient 

$\mathrm{where}$

μ = interface friction coefficient (soil & concrete)

N = effective vertical load

If there is no vertical load, there won't be any friction. Uplift forces reduce sliding resistance instantly.

Typical values of friction coefficient for:

• Concrete on soil: 0.35–0.5

• Concrete on rock: 0.6–0.7

Always refer to geotechnical report for any technical values related to soil-concrete interface, soil-soil interface and so on.

Weight of footing and overburden forces also increase "N" that subsequently leads to increase in frictional resistance. 

Overburden forces are conservatively ignored but can be considered only if:

1. Soil is permanent

2. Soil cannot be eroded or be removed

2. Passive Soil Resistance ()

Passive soil resistance is defined as the pressure exerted by the soil on the footing when lateral forces cause a significant lateral movement. 

As the foundation attempts to slide, it pushes against the soil in front of it. The compressed soil responds with an equal and opposite reaction which opposes the sliding motion. 

The passive resistance for a unit width of foundation is given by,

where

 = Coefficient of Passive Earth Pressure

 = Unit Weight of Soil

 = Depth of Bottom of Foundation from FGL

 = Depth of Top of Foundation from FGL

The top 1 ft or 300 mm of soil is usually not considered to account for soil mass not mobilizing for resistance.

Thus, the formula modifies to,

It is highly dependent on Confinement and Excavation condition. Often passive resistance is reduced heavily (Around 50%) or ignored in conservative designs.

Engineers use passive resistance only when:

1. Soil in front of the foundation thickness is permanent and protected.

2. Excavation will not remove the soil in adjacent areas, and undermining will ever occur.

3. Codal provisions that specifically allows or denies partial mobilization.

3. Shear Keys ()

These are vertical projection below footing used to resist the lateral forces on the foundation. 

Major advantage of shear keys is that they convert sliding against soil into bearing against soil for the same lateral force. Soil is stronger in bearing than sliding because it is easier to slide on the soil surface than to compress it. The maximum capacity against sliding for any soil will be bearing capacity times the friction coefficient (which is always less than one).

The sliding resistance offered by a shear key is equal to the passive earth resistance mobilized against the vertical face of the shear key.

Where

​ = passive earth pressure coefficient

 ​= unit weight of soil

 = depth of shear key below base

 = width of shear key (perpendicular to sliding)

Shear keys are an effective solution but:

1. They increase excavation work

2. They add detailing complexity

3. They introduce stress concentration

Always remember, Shear keys are not default solutions, they are engineering interventions in cases where typical design won't suffice.

4. Cohesion and Adhesion

Adhesion is defined as the force of attraction that causes unlike particles to stick together. 

Adhesion exists only because:

1. Fresh concrete roughness interlocks with soil

2. Some chemical attraction may exist initially

3. Suction may temporarily develop in unsaturated soils

This sounds helpful but none of it is dependable and thus adhesion is assumed to be negligible between soil and concrete particles.

Cohesion is defined as the force of attraction that causes like particles to stick together. 

Factors affecting Cohesion are:

1. Moisture:  This force in soil is dependent on the amount of moisture present. For a low moisture soil, the soil particles sticking together will be higher resulting in higher cohesion.

2. Cyclic loading: Under cyclic and dynamic loadings, cohesion forces change. Soil bonding breaks while friction is still available.

All this leads to an unsafe design, if considered, for long-term reliability. A foundation designed using cohesion may be safe today and unsafe 10 years later.

A conservative engineer assumes:

Sliding resistance comes from friction and geometry, not soil “stickiness.”

Sliding Check Calculations

Sliding is a force balance problem.

Driving Force

H =  All Horizontal Forces (in worst direction)

Resisting Force

 ​

Safety Condition

​

Why Friction Is Not “Just μ × W”?

(where W = Total Vertical Load)

Because along with Total Vertical Load "W", moments and lateral forces also act on the footing that can result in reduction in contact.

Normal Force "N" is a contact force whose direction is perpendicular to the actual area of contact.

Uplift due to buoyant forces can also reduce effective weight resulting in reduction in Normal Force "N"

Effective vertical load must be considered keeping in mind:

1. Uplift

2. Load eccentricity

3. Load combination effects

Code Philosophy & Safety Factors

Why Sliding Is Checked at Service Level?

Sliding is not checked to prevent collapse but to prevent movement.
And movement is a serviceability concern, not a strength one. 

Sliding depends on interface behavior, not material strength. Interface behavior is uncertain and displacement sensitive. 

Even a few millimeters of sliding can cause:

1. Misalignment of machinery,
2. Overstressing of connected pipes,
3. Cracking of superstructure elements,
4. Loss of operational tolerance.

The structure may still be standing, but it is no longer functional.

Sliding is also a sudden movement of a rigid-body which itself is geometry-controlled. When the moment resisting forces equal driving forces, the foundation starts moving as a block.

There is no “reserve ductility” after that.

So, checking sliding at ultimate loads would mean accepting that the foundation is allowed to move.

If sliding starts, the foundation has already failed long before collapse.

Typical Factor of Safety

Static loading

FOS ≥ 1.5

Seismic loading

FOS ≈ 1.1–1.2

May vary as per Codes.

Practical Design Mistakes

Some of the common mistakes I have encountered are:

1. Using total weight instead of effective vertical load as per appropriate load combination.

2. Ignoring worst load direction. (use direction with more lateral forces) 

3. Counting full passive pressure blindly (use 50% or lesser)

4. Forgetting uplift in seismic cases (DL + EL is one of the most governing LC)

5. Adding shear key when increasing size was also a solution. (It's always preferred to increase size then introduce shear key)

Good engineers simplify smartly, not blindly.

Step-by-Step Sliding Check Workflow

1. Identify governing load combinations

2. Resolve all horizontal forces

3. Determine effective vertical load $N$

4. Select realistic friction coefficient

5. Add allowable passive/shear key resistance (if any)

6. Compute FOS

7. If FOS < Required FOS:

   1. Increase footing size

   2. Increase footing thickness

   3. Add or modify shear key embedment

   4. Improve soil

Final Thought

Sliding is not about formulas. 

It is about how the foundation wants to move. Soil does not “hold” the foundation but only resists when movement is attempted.

Good foundation design is understanding motion before resisting it.

If you understand sliding at this level, you are no longer “checking” foundations,

you are designing them.

This is the fourth blog in the series.

Next, we will discuss:

1. Overturning in Concrete Foundations
2. Reinforcement Design for Concrete Foundations

Stay Tuned.

Image by brgfx on Freepik

This article is intended for educational understanding of foundation behavior. Designers must refer to relevant codes, standards, and professional judgment for actual design.