Consolidation Settlement (Sc)
Estimated Settlement
What Is Geotechnical Consolidation Settlement?
Consolidation settlement is the gradual compression of saturated clay soil under sustained load, caused by:
- Increase in effective stress in the soil skeleton
- Slow drainage of pore water from the voids
- Reduction in volume of the clay layer
- Result: Downward movement (settlement) of the ground and the structure
In other words:
When you put a long-term load on saturated clay, the water trapped between soil particles slowly squeezes out. As water escapes, clay particles rearrange themselves and move closer together. That volume change appears as surface settlement.
This settlement is time-dependent and is described by classical Terzaghi one-dimensional consolidation theory.
Your Consolidation Settlement (Sc) calculator is built exactly around this behaviour:
it helps you estimate how much settlement occurs and under what stress conditions, especially for:
- Normally Consolidated (NC) soils
- Overconsolidated (OC) soils
Why Consolidation Settlement Is So Important
In geotechnical design, consolidation settlement matters because it can:
- Damage structures over time, even if bearing capacity is safe
- Create differential settlement, leading to cracks and distortion
- Affect serviceability much before ultimate failure
- Cause long-term issues in:
- Buildings
- Tanks and silos
- Embankments and approach ramps
- Retaining structures
- Pavement subgrades over soft clay
Ultimate strength is about “Will it stand?”
Serviceability and settlement are about “Will it stay usable and comfortable?”
Consolidation settlement directly affects serviceability, and your calculator helps you quantify this.
The Role of Your Consolidation Settlement (Sc) Calculator
Your calculator is focused on primary consolidation settlement in one dimension.
It lets the user:
- Select Soil Consolidation State:
- Normally Consolidated (NC)
- Overconsolidated (OC)
- Input key soil and loading parameters:
- Clay layer thickness (H)
- Initial void ratio (e₀)
- Initial effective stress (P₀)
- Stress increment due to applied load (ΔP)
- Compression index (Cc)
- For OC soil:
- Recompression index (Cr)
- Pre-consolidation pressure (Pc)
- Get a clear output:
- Total settlement (Sc) in the same unit as H
- A short description of which formula was used:
- NC formula
- OC Case 1 (Stress < Pc)
- OC Case 2 (Stress > Pc)
- See user-friendly messages:
- Estimated settlement clearly displayed
- Error messages if inputs are missing or invalid
In simple terms, the tool translates soil behaviour into one clear settlement number, with context.
Key Inputs Explained in Plain Language
Let’s walk through the key input parameters one by one and connect them to real soil behaviour.
Clay Layer Thickness (H)
H is the thickness of the compressible clay layer considered in your analysis.
- If H is small → less volume to compress → less settlement
- If H is large → more compressible volume → more settlement
In your calculator, H also defines the unit of settlement Sc.
So if H is entered in metres, Sc will come out in metres; if in feet, Sc is in feet.
Initial Void Ratio (e₀)
The void ratio e is:
Ratio of volume of voids to volume of solids in the soil
The initial void ratio e₀ represents how “loose” or “open” the soil structure is before loading.
- High e₀ → clay has many voids → more room to compress → higher settlement potential
- Lower e₀ → denser structure → lower compressibility
Your calculator uses e₀ in the denominator (1 + e₀), meaning:
For the same stress change and Clay thickness, soils with higher void ratio compress more.
Initial Effective Stress (P₀)
P₀ is the existing vertical effective stress in the clay before you apply any new loading.
It depends on:
- Overburden soil thickness
- Unit weight of soil
- Groundwater level
It’s the “starting stress condition” of the soil.
Stress Increment (ΔP)
ΔP is the additional effective stress imposed on the clay due to:
- Footing or raft
- Embankment
- Added fill
- Structural loads
The final effective stress becomes:
Pᶠ = P₀ + ΔP
The bigger the ΔP, the greater the driving force for compression, and therefore, higher settlement.
Compression Index (Cc)
Compression Index (Cc) describes the compressibility of soil in the virgin compression range.
This is usually used when:
- Soil is normally consolidated (NC), or
- For overconsolidated soils, when the final stress exceeds preconsolidation pressure Pc
Properties of Cc:
- Higher Cc → more compression per log cycle of stress → more settlement
- Soft clays often have high Cc
- Stiff clays have lower Cc
In your calculator, Cc is needed for both NC and OC modes.
Recompression Index (Cr) – Only for OC Mode
Recompression Index (Cr) applies when the soil is:
- Overconsolidated, and
- Stressed within its past stress range (i.e., stresses up to Pc)
Cr is smaller than Cc and represents:
“How much the soil compresses when reloaded in the already experienced stress range.”
In the OC case:
- For stresses below Pc, settlement is controlled by Cr.
- For stresses beyond Pc, the soil follows Cr up to Pc, and then Cc beyond that.
Your calculator activates Cr input only when “Overconsolidated (OC)” is selected.
Pre-Consolidation Pressure (Pc) – Only for OC Mode
Pre-consolidation pressure (Pc) is the maximum effective stress the soil has experienced in the past.
It defines whether:
- The current stress (or new final stress) is still in the recompression zone, or
- Has entered virgin compression.
In the calculator:
- You must enter Pc in OC mode.
- Pc is compared with P₀ and Pᶠ = P₀ + ΔP to decide which settlement formula to use.
Normally Consolidated vs Overconsolidated Mode in the Calculator
Your calculator is cleverly designed to handle two different soil states:
- Normally Consolidated (NC)
- Overconsolidated (OC)
Normally Consolidated (NC) Mode
When the soil is normally consolidated, it means:
- The current effective stress is close to the maximum stress it has ever experienced.
In the NC option:
- Pc and Cr are not needed.
- Settlement is computed directly with Cc over the entire stress range from P₀ to Pᶠ.
Conceptually:
Sc (NC) ∝ (Cc / (1 + e₀)) × H × log₁₀(Pᶠ / P₀)
Your calculator runs this logic in the background when Soil Consolidation State = NC.
This mode is ideal for:
- Young marine clay deposits
- Recently formed alluvial clays
- Soft, normally consolidated clay layers
Overconsolidated (OC) Mode
Overconsolidated soils have:
- A preconsolidation pressure Pc greater than the current initial stress P₀
- Been loaded more heavily in the past and then unloaded
- A stiffer response at low stress ranges
In OC mode, the calculator distinguishes between two sub-cases based on the final stress Pᶠ = P₀ + ΔP.
OC Case 1: Final Stress ≤ Pc (Stress stays below or at Pc)
Here:
- The soil is only recompressed, never pushed into virgin compression.
- Settlement is calculated only using Cr: recompression index.
Conceptually:
Sc (OC Case 1) ∝ (Cr / (1 + e₀)) × H × log₁₀(Pᶠ / P₀)
The calculator labels this as:
“Formula: OC Case 1 (Stress < Pc)”
This often results in smaller settlement compared to an NC soil.
OC Case 2: Final Stress > Pc (Stress goes beyond Pc)
Here:
- The soil first experiences recompression up to Pc.
- Then it enters virgin compression beyond Pc.
So the total settlement Sc becomes the sum of two parts:
- Recompression from P₀ to Pc (using Cr)
- Virgin compression from Pc to Pᶠ (using Cc)
Your calculator automatically performs this split and displays:
“Formula: OC Case 2 (Stress > Pc)”
This is a very realistic situation in geotechnical design, especially where:
- New loads are large
- Past overburden has been removed
- Deep clays have complex stress histories
How the Calculator Improves Understanding and Design
Beyond just giving a number, your Consolidation Settlement (Sc) calculator supports:
Educational Clarity
- Shows the formula context used (NC vs OC cases).
- Encourages users to think about Pc, Cr, Cc, P₀ and ΔP relationships rather than blindly plugging numbers.
Practical Design Insight
Engineers can:
- Compare settlements under different load increments ΔP.
- Study the effect of clay thickness H or void ratio e₀.
- Understand how treating a soil as NC vs OC changes the settlement prediction.
- Use Sc to check against allowable settlement limits for different structures.
Error Reduction
The tool:
- Validates inputs to ensure values are positive and present.
- Shows clear messages like “Input Error” when required parameters are missing, especially in OC mode where Pc and Cr are essential.
This reduces the risk of incorrect settlement results due to incomplete data.
Engineering Use and Limitations
While your calculator gives an accurate primary, one-dimensional consolidation settlement estimate, it is important to remember:
- It assumes one-dimensional compression (vertical strain only).
- It does not include secondary consolidation (creep).
- It assumes consistent unit systems (all stresses in same unit, H and Sc in same length unit).
- It is based on classical consolidation theory, which is an idealization.
Therefore:
- Use it as a design support and educational tool, not as a blind replacement for engineering judgement.
- For critical projects, combine the calculator output with:
- Detailed site investigation
- Oedometer lab tests
- Layered soil modelling
- Numerical analysis, if needed
