Creep settlement is the time-dependent deformation that occurs in soils and other geomaterials when subjected to a constant effective stress over an extended period of time. It is a major consideration in the design and construction of foundations and earth structures. Creep settlement occurs in addition to immediate settlement and consolidation settlement.
What Causes Creep Settlement?
Creep settlement is caused by the rearrangement of soil particles and deformation of the soil structure over time under a constant effective stress. The main factors that contribute to creep settlement include:
- Stress level – Higher effective stresses produce more creep deformation
- Soil composition – Clays, especially plastic clays, are more susceptible to creep than sands
- Clay mineralogy – Montmorillonitic clays are more creep susceptible than kaolinitic clays
- Soil structure – Dispersed, flocculated, and oriented clays exhibit more creep than aggregated masses
- Permeability – Less permeable soils consolidate and creep slower than pervious soils
- Temperature – Higher temperatures accelerate creep deformation
- Cyclic loading – Creep is enhanced by cyclic stresses such as wave, wind or traffic loads
The viscous nature of clays makes them particularly prone to creep effects. The clay platelets can slowly slip and reorient over time under constant effective stress, causing continuing deformation.
Stages of Creep Settlement
Creep settlement occurs in three distinct stages over time:
- Primary Creep – This initial stage exhibits a decreasing creep rate that gradually tapers off. It involves a breakdown of the original soil structure.
- Secondary Creep – The second stage is characterized by a nearly constant creep rate and contributes the majority of the creep settlement. This constant rate reflects deformation of the reconstituted structure.
- Tertiary Creep – In the final stage, the creep rate exponentially accelerates until failure occurs. The acceleration is caused by the development of critical microstructural flaws.
The duration of each stage depends on the soil properties and stress conditions. Stiffer soils and lower stress levels lengthen the primary and secondary stages.
Estimating Creep Settlement
There are several methods used by geotechnical engineers to estimate creep settlement:
- Empirical methods – Using empirical correlations based on field observations of creep in similar soils.
- Laboratory testing – Measuring creep settlement in 1-D consolidation or triaxial tests on undisturbed soil samples and extrapolating to field conditions.
- Numerical modeling – Advanced constitutive soil models can simulate the complex stress-strain-time behaviors leading to creep.
Laboratory testing provides the most direct measurement but is time consuming and expensive. Empirical methods provide a quick estimate but contain high uncertainty. Numerical modeling provides a compromised approach but requires experienced resources.
Key Parameters for Estimating Creep Settlement
Some key parameters used to estimate creep settlement include:
- Cc – Compression index
- Cr – Recompression index
- Cαe – Secondary compression index
- σ’p – Preconsolidation stress
- OCR – Overconsolidation ratio
- Cc/(1+e0) – Modified compression index
- H – Thickness of compressible layer
The secondary compression index Cαe, which measures the strain per log cycle of time in the secondary creep stage, is a particularly important parameter. Higher Cαe values indicate more creep susceptibility.
Effects of Creep Settlement
Some key effects of creep settlement on soils and structures include:
- Additional settlement over time leading to excessive distortions or deformations.
- Downward drag on deep foundations such as piles and drilled shafts.
- Downdrag forces on subsurface utilities and pipelines.
- Loss of effective stresses and shear strength on potential failure surfaces.
- Cracking and deterioration of structural elements.
These effects can occur gradually over a period of years. As such, creep settlement requires careful consideration in design to avoid long-term impacts.
Mitigation of Creep Settlement
There are several design approaches to mitigate the effects of creep settlement in soils:
- Allow for anticipated creep settlements in design calculations and provide articulation.
- Use end-bearing piles that gain capacity with time as negative skin friction develops.
- Preload areas early in construction to induce creep prior to building.
- Replace on-site soils with properly compacted fill or deep foundations to stronger layers.
- Use reinforced earth structures and geosynthetics to provide confinement.
- Implement drainage measures to accelerate consolidation.
The preferred mitigation method depends on the specific project conditions and constraints. Creep settlement effects should be assessed early in the design process.
Examples of Creep Settlement
Some examples of creep settlement issues include:
- The Transcona Elevator in Winnipeg, Canada experienced significant creep settlement over 8 years after construction, requiring jacks to re-level.
- The San Jacinto Monument in Texas tilted and cracked decades after construction due to creep, requiring foundation repairs.
- The main building at Dulles Airport outside Washington D.C. experienced long-term creep settlement requiring jack lifts to stabilize.
- A 20-story hotel in Bangkok saw 10 inches of creep settlement over 5 years after construction.
These examples illustrate the need to properly account for creep in the design of foundations on clay soils.
Creep Settlement Rates in Different Soils
Creep settlement rates vary widely depending on the soil type. Typical secondary compression index values include:
| Soil Type | Secondary Compression Index Cαe |
|---|---|
| Clean sands | 0.001-0.005 |
| Silty sands | 0.002-0.015 |
| Plastic clays | 0.01-0.05 |
| Organic clays | 0.03-0.1 |
As shown, clays exhibit significantly more creep settlement than granular soils. Plastic clays and organic clays are especially prone to creep issues.
Creep Settlement Rate Estimation Methods
Common methods to estimate creep settlement rates include:
Empirical Correlations
- Mesri 1973 – Cαe = 0.05(IL)-0.5 for Louisiana clays
- Mesri 2007 – Cαe = 0.004(IL) for soft solids
- Cαe = 0.01 to 0.04 for Bangkok clays
Where IL = Liquidity index
Laboratory Testing
- 1-D consolidation test
- Triaxial creep test
- Rowe cell consolidation test
Laboratory testing on undisturbed samples provides the most accurate creep rate determination.
Numerical Modeling
- CAM clay model
- Modified Cam clay
- Soft soil creep model
Numerical models allow simulation of complex soil behavior leading to creep settlement.
Creep Settlement of Shallow Foundations
The creep settlement of shallow foundations on clay soils can be estimated as:
- ΔSc = Cαe H log(t2/t1)
Where:
- ΔSc = Creep settlement
- Cαe = Secondary compression index
- H = Thickness of clay layer
- t1, t2 = Elapsed times
Creep settlement accumulates logarithmically with time. Settlements are typically estimated over the design life span of a structure, often taken as 50 years.
Example Creep Settlement Analysis
Given:
- 3m thick layer of soft clay
- Cαe = 0.02
- Estimated creep settlement over 20 years
Creep settlement ΔSc = 0.02 x 3 x log(20/1) = 0.072m = 7.2cm
This demonstrates a typical application of the creep settlement estimation method.
Creep Settlement of Piles
Piles driven through creeping clay soils experience downdrag forces and settlements over time. The creep settlement of piles can be estimated as:
- ΔSp = Cαe H log (t2/t1) fs
Where:
- fs = Shaft adhesion
- Other variables as defined previously
The dragload on the pile is calculated as:
- Qc = ΔSp P
Where:
- P = Perimeter of the pile
These creep effects must be accounted for in the geotechnical design of the deep foundation system. Frictional resistance and toe capacity may be reduced over time.
Example Creep Settlement Analysis for Piles
Given:
- 0.5m diameter piles
- 20m through soft clay
- Cαe = 0.04
- fs = 8kPa
- Estimate creep settlement and dragload over 10 years
Creep settlement ΔSp = 0.04 x 20 x log(10/1) x 8 = 3.2cm
Dragload Qc = 3.2 x π x 0.5 = 5kN
The results show potentially significant creep effects that must be considered in pile design.
Conclusion
In summary, creep settlement is time-dependent deformation of soils under constant effective stress. It occurs in clay soils and contributes to vertical settlements over time. Proper estimation and mitigation of creep effects are crucial in foundation design and construction on clayey sites. Careful evaluation of soil properties, creep rate estimation, and design approaches are needed to manage potential creep issues and avoid problems.