When soil gets packed in behind something like a basement wall or a retaining wall, the weight of it causes it to naturally push sideways. Lateral earth pressure at rest, or Ko, describes that weight when the soil isn’t allowed to move or shift to relieve pressure. Since the wall isn’t designed to move, it can’t tilt or lean or flex even as the soil presses down.
Getting Ko right plays a big part in building structures that last. Lateral earth pressure at rest lets engineers and contractors pick the proper reinforcement for the job, stop walls from cracking or bowing out, and dodge those costly failures from soil forces you didn’t see coming.
In this blog, we will break down what lateral earth pressure at rest (Ko) is and why it’s an important part of building or inspecting retaining walls, basement walls, and other soil-retaining structures.
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How Engineers Calculate Ko
The intensity of lateral earth pressure at rest is measured by the at-rest earth pressure coefficient, Ko, which depends primarily on the soil’s internal friction angle and its tendency to expand or contract under load. Ko is generally greater than active pressure (Ka) and less than passive pressure (Kp), meaning walls designed for Ko must withstand higher loads than those designed for movement.
How Engineers Address Ko in Design
Accounting for Ko starts with selecting soil parameters and an appropriate Ko value from lab tests or accepted correlations. Then, they are applied consistently across wall, footing, and connection checks. Engineers right-size reinforcement and wall thickness, but they also pair structure with performance waterproofing and active drainage to lower total lateral demand.
Designs must account for movement tolerance, using membranes, joints, and penetrations that can withstand soil pressure and settlement. Engineers design with constructability in mind so crews can build it right the first time.
- Structural Reinforcement Sizing: Increasing rebar size, spacing, and wall thickness to resist the higher loads
- Waterproofing System Selection: Choosing flexible membranes and joint systems that tolerate movement and pressure
- Drainage Systems: Installing footing drains, relief pipes, or weep holes to reduce hydrostatic pressure and lateral loads
- Local Building Code Compliance: Meeting NYC and NYS requirements, which mandate Ko-based design for non-yielding basement and retaining walls
By incorporating Ko early in the design phase, engineers prevent costly structural damage down the road.
Real-World Examples Where Ko Applies
- Basement and Foundation Walls: Restrained on all sides by floor slabs and designed for full at-rest pressure.
- Underground Parking Structures: Walls are held in place by slabs at multiple levels, eliminating movement and increasing lateral loads.
- Bridge Abutments: Heavily loaded structures that cannot deflect, where miscalculating Ko could cause cracking or settlement.
- Tunnel and Shaft Walls: Surrounded by soil on all sides with no chance to yield, requiring precise Ko-based design to prevent collapse.
Factors That Affect At-Rest Lateral Earth Pressure
Several site-specific factors influence Ko and resulting lateral soil pressures, including:
- Soil Type (Clay vs. Sand): Clays generally have higher cohesion and lower friction angles, while sands have higher friction angles, resulting in lower Ko values.
- Soil Unit Weight and Moisture Content: Heavier or saturated soils exert more lateral force at the same depth.
- Overburden Pressure / Depth: The deeper the soil, the greater the vertical stress and therefore the horizontal stress through Ko.
- Compaction and Surcharge Loads: Surface loads like heavy equipment, nearby buildings, or traffic increase vertical stress, which increases horizontal pressure as well.
- Excavation Sequence and Wall Stiffness: Stiffer, non-yielding walls hold more lateral stress, while flexible walls can mobilize active lateral earth pressure instead of Ko.
Ko vs. Ka vs. Kp: Understanding the Differences
Earth pressure on retaining walls is classified into three main states based on how much the wall moves relative to the soil:
- Ko (At-Rest): Occurs when the wall does not move at all. This is typical for basement walls or rigid retaining walls tied into slabs and footings.
- Ka (Active): Develops when the wall moves slightly away from the soil, allowing it to expand and relieve stress.
- Kp (Passive): Occurs when the wall pushes into the soil, mobilizing maximum resistance from the soil mass.
Because Ko > Ka and Kp > Ko, using the wrong coefficient can severely under- or overestimate wall loads. For restrained walls, engineers must design for Ko to prevent cracking or bowing.
Earth Pressure Coefficient Comparison
| State | Description | Wall Movement | Typical Value Range |
|---|---|---|---|
| Ko (At-Rest) | Soil pushing without movement | None (fully restrained) | 0.4 – 0.6 |
| Ka (Active) | Soil pushing as wall moves outward | Slight outward rotation | 0.25 – 0.35 |
| Kp (Passive) | Soil resisting wall pushing inward | Wall forced into soil | 2.0 – 4.0+ |
Consequences of Ignoring At-Rest Pressure
When a non-yielding wall is designed for active pressure (Ka) but actually sees at-rest pressure (Ko), the horizontal loads are significantly underestimated. The result is progressive deformation: hairline cracks become bowing, joints open, and water pathways form under cyclic wetting and drying.
Lateral loads also transfer into the structure, telegraphing as cracked drywall, misaligned doors, and stressed utilities. Beyond the repair costs, you risk code violations, warranty disputes, and long-term durability losses that far exceed the savings of a lighter design.
- Underdesigned walls that bow or crack due to unanticipated horizontal loads
- Waterproofing system failure as lateral stress exceeds membrane or joint tolerances
- Structural movement that damages finishes and utilities, such as cracked drywall, broken pipes, or shifted flooring
Ignoring Ko often leads to expensive remediation work that could have been prevented with proper design.
Why Choose Zavza Seal for Retaining and Basement Wall Projects
At Zavza Seal LLC, we combine geotechnical testing, structural design, and field execution to build retaining walls that last.
- 20+ years of experience handling lateral loads in Long Island soils
- Engineering-led designs developed and reviewed by licensed structural engineers
- Specialized expertise in expansive clays and high water table conditions
- Borough-ready installation crews familiar with permitting and inspection requirements
From concept to completion, we build retaining and basement wall systems that can withstand the true Ko loads your site demands.
What Is Lateral Earth Pressure At Rest: Final Thoughts
Skipping Ko calculations is a gamble with your structure’s safety. Relying on active pressure when a wall is fully restrained can lead to cracking, water intrusion, and costly failures down the line. Designing for Ko from the start protects your structure, lowers long-term maintenance costs, and ensures your wall performs as intended for decades to come.
Schedule a Structural Evaluation or Soil Test Today!
Frequently Asked Questions About Lateral Earth Pressure At Rest
What Is Lateral Earth Pressure at Rest?
Lateral earth pressure at rest is the sideways force soil exerts on a structure when the soil is not allowed to move. It occurs when walls are fully restrained, like basement or foundation walls.
What Is the Earth’s Pressure at Rest?
The earth’s pressure at rest is the natural horizontal stress within soil when there is no lateral deformation. It’s measured using the at-rest earth pressure coefficient, Ko.
What Is the Lateral Bearing Pressure?
Lateral bearing pressure is the horizontal pressure soil places on retaining or basement walls. It depends on soil type, depth, and wall movement.
What Is Lateral Earth Pressure for Non-Geotechnical Engineers?
Lateral earth pressure is how much soil pushes sideways on a wall. If the wall can’t move, this pressure builds up and must be designed for using Ko values.
What Does Ko Represent in Soil Mechanics?
Ko represents the at-rest earth pressure coefficient, showing the ratio of horizontal to vertical stress in soil when the wall does not move.
How Is Ko Calculated in Geotechnical Design?
Ko is commonly calculated using Jaky’s equation: Ko = 1 − sin(ϕ), where ϕ is the soil’s internal friction angle. Lab testing helps determine accurate values.
When Should Ko Be Used in Retaining Wall Design?
Ko should be used when walls are fully restrained and cannot move, such as basement walls, bridge abutments, and rigid retaining walls tied to slabs.
How Does Ko Differ From Ka and Kp?
Ko applies to walls that don’t move, Ka to walls moving away from soil, and Kp to walls pushing into soil. Ko creates higher loads than Ka.
What Happens If Ko Is Ignored in Design?
Ignoring Ko can cause underdesigned walls to crack, bow, or fail. It also leads to waterproofing breakdowns and costly structural repairs.
What Factors Affect Lateral Earth Pressure at Rest?
Soil type, depth, moisture content, compaction, wall stiffness, and surface loads all influence the amount of at-rest lateral earth pressure on a wall.
