Glossary

10.3 Settlement of shallow foundations

5 min read•august 16, 2024

Shallow foundations transfer building loads to the ground. Understanding settlement is crucial for ensuring structural stability and longevity. This section dives into the types, causes, and calculations of foundation settlement.

We'll explore immediate, consolidation, and . We'll also examine factors influencing settlement, like soil properties and environmental conditions. Finally, we'll learn how to calculate and mitigate settlement effects on structures.

Types of Shallow Foundations

Immediate and Consolidation Settlement

Top images from around the web for Immediate and Consolidation Settlement

  • Shallow foundation - Wikipedia View original

    Is this image relevant?

  • Shallow foundation - Wikipedia View original

    Is this image relevant?

1 of 2

Top images from around the web for Immediate and Consolidation Settlement

  • Shallow foundation - Wikipedia View original

    Is this image relevant?

  • Shallow foundation - Wikipedia View original

    Is this image relevant?

1 of 2
  • occurs rapidly due to elastic deformation of soil without changes in water content, primarily in coarse-grained soils
    • Happens quickly after load application
    • Common in sandy or gravelly soils
  • Primary results from the gradual expulsion of water from fine-grained soils, leading to a reduction in soil volume over time
    • Occurs more slowly than immediate settlement
    • Typical in clay or silt soils
    • Can continue for months or years
  • settlement happens as a long-term process occurring after , caused by the creep of soil particles under constant effective stress
    • Follows primary consolidation
    • Particularly significant in organic soils (peat)
    • Can continue for decades

Differential and Total Settlement

  • Differential settlement refers to uneven settlement across a foundation, potentially leading to structural distress and damage
    • Causes tilting or warping of structures
    • Can result from varying soil conditions or uneven loading
    • Examples include cracking in walls or misaligned doors/windows
  • combines immediate, primary consolidation, and secondary compression settlements, representing the overall vertical displacement of the foundation
    • Cumulative effect of all settlement types
    • Important for overall structural performance assessment
    • Typically measured in millimeters or inches

Factors Influencing Settlement

Soil Properties and Foundation Characteristics

  • Soil type and characteristics, including grain size distribution, compressibility, and permeability, significantly affect settlement behavior
    • Sandy soils generally settle quickly but less overall
    • Clay soils often experience slower but larger settlements
  • Foundation size and shape influence the stress distribution in the underlying soil, impacting settlement magnitude and distribution
    • Larger foundations typically experience more total settlement but less differential settlement
    • Irregular shapes may lead to uneven stress distribution
  • Applied and distribution determine the magnitude of stress increase in the soil, directly affecting settlement
    • Heavier loads cause more settlement
    • Uneven load distribution can lead to differential settlement

Environmental and Construction Factors

  • Groundwater conditions, including water table depth and pore water pressure, influence effective stress and
    • High water table can increase settlement potential
    • Fluctuating water levels may cause cyclic settlement
  • Soil layering and stratification affect stress distribution and drainage paths, potentially leading to complex settlement patterns
    • Alternating layers of sand and clay can complicate settlement predictions
    • Presence of a stiff layer beneath a soft layer may reduce settlement
  • Construction methods and sequence can impact soil disturbance and stress history, influencing settlement behavior
    • Excavation and backfilling techniques affect soil properties
    • Preloading or staged construction can mitigate settlement
  • Time-dependent factors, such as soil creep and long-term loading conditions, contribute to ongoing settlement processes
    • Creep more pronounced in fine-grained soils
    • Cyclic loading (wind, traffic) may accelerate settlement

Calculating Foundation Settlement

Immediate Settlement Calculations

  • Immediate settlement calculations typically employ elastic theory, using the and of the soil
    • Based on of elasticity
    • Formula: Si=qB(1−ν2)EIpS_i = \frac{q B (1-\nu^2)}{E} I_p Where SiS_i is immediate settlement, qq is applied pressure, BB is foundation width, ν\nu is Poisson's ratio, EE is elastic modulus, and IpI_p is influence factor
  • The commonly estimates immediate settlement in granular soils, considering strain influence factors and soil compressibility
    • Accounts for depth-dependent strain distribution
    • Incorporates time effects and stress history

Consolidation Settlement Calculations

  • One-dimensional consolidation theory, based on , calculates primary consolidation settlement in fine-grained soils
    • Assumes vertical drainage only
    • Formula: Sc=HCc1+e0log⁡10(σ0′+Δσ′σ0′)S_c = H \frac{C_c}{1+e_0} \log_{10}\left(\frac{\sigma'_0 + \Delta\sigma'}{\sigma'_0}\right) Where ScS_c is consolidation settlement, HH is layer thickness, CcC_c is , e0e_0 is initial , σ0′\sigma'_0 is initial effective stress, and Δσ′\Delta\sigma' is stress increase
  • The compression index (Cc) and (Cr) serve as key parameters in consolidation settlement calculations, obtained from laboratory consolidation tests
    • Cc represents virgin compression behavior
    • Cr accounts for recompression in overconsolidated soils
  • Time-rate of consolidation estimates use the (cv) and drainage path length, allowing for settlement predictions at various time intervals
    • Enables prediction of settlement progress over time
    • Important for planning construction schedules

Advanced Settlement Calculations

  • Secondary compression settlement calculations use the secondary compression index (Cα) and are typically considered for long-term settlement predictions in highly compressible soils
    • Formula: Ss=HCα1+eplog⁡10(t2t1)S_s = H \frac{C_\alpha}{1+e_p} \log_{10}\left(\frac{t_2}{t_1}\right) Where SsS_s is secondary settlement, HH is layer thickness, CαC_\alpha is secondary compression index, epe_p is void ratio at end of primary consolidation, and t1t_1 and t2t_2 are times
  • Numerical methods, such as , can be employed for complex foundation geometries or soil profiles to estimate total and differential settlements
    • Allow for modeling of non-linear soil behavior
    • Can account for soil-structure interaction effects

Settlement Impact on Performance

Structural and Serviceability Considerations

  • Allowable settlement limits establish based on structure type, foundation design, and serviceability requirements to ensure proper performance
    • Typical limits range from 25-50 mm for most structures
    • More stringent limits for sensitive structures (nuclear plants, precision manufacturing facilities)
  • Differential settlement can cause structural distress, including cracking, tilting, and potential failure of load-bearing elements
    • Angular distortion (relative settlement divided by span length) often limited to 1/500
    • Can lead to redistribution of loads within the structure
  • Settlement-induced changes in foundation must be assessed to ensure long-term stability and safety of the structure
    • Excessive settlement may reduce effective foundation depth
    • Can alter soil properties and stress distribution
  • Serviceability issues, such as door and window misalignment, floor unevenness, and utility line disruptions, can result from excessive settlement
    • Affects occupant comfort and building functionality
    • May require costly repairs or adjustments

Long-term Performance and Mitigation

  • Time-dependent settlement behavior must be considered for long-term performance evaluation, particularly in structures sensitive to ongoing deformation
    • Important for structures with long design lives (bridges, dams)
    • May influence maintenance and repair schedules
  • Settlement monitoring and mitigation strategies, including preloading, soil improvement, or foundation adjustments, may be necessary to maintain acceptable performance
    • Preloading accelerates settlement before construction
    • Soil improvement techniques (deep mixing, grouting) can reduce settlement potential
    • Foundation adjustments (releveling, ) may address ongoing settlement issues
  • The economic impact of settlement-related issues, including repair costs and potential loss of functionality, should be considered in foundation design and risk assessment
    • Cost-benefit analysis of mitigation measures versus potential damages
    • Insurance implications for settlement-prone structures

Key Terms to Review (28)

AASHTO LRFD: AASHTO LRFD stands for the American Association of State Highway and Transportation Officials Load and Resistance Factor Design. This design methodology focuses on the reliability of structural elements by applying load factors and resistance factors to ensure safety and performance. By integrating statistical principles into the design process, AASHTO LRFD enhances the predictability of structural behavior under various loading conditions, making it crucial for evaluating both settlement in shallow foundations and the stability of retaining walls.
ASTM D1586: ASTM D1586 is a standard test method used to determine the density of soil and the resistance of soil to penetration by a standard split-barrel sampler during a boring operation. This test is crucial for site investigations as it helps in understanding soil conditions and the potential settlement behavior of shallow foundations, allowing engineers to make informed decisions during construction projects.
Bearing Capacity: Bearing capacity is the ability of soil to support the loads applied to it without experiencing failure or excessive settlement. This concept is crucial in determining the suitability of different foundation types, ensuring that structures can be built safely and sustainably, taking into account various factors like soil conditions and load distributions.
Coefficient of consolidation: The coefficient of consolidation is a parameter that measures the rate at which soil consolidates under load, specifically the time-dependent decrease in volume due to expulsion of pore water. It is critical for understanding how different types of soil behave under applied loads and directly ties into concepts such as settlement calculations, consolidation theory, and performance of foundations.
Compaction Grouting: Compaction grouting is a ground improvement technique that involves injecting a low-slump cementitious grout into the soil to increase its density and strength. This method is often used to mitigate settlement issues, particularly for shallow foundations, by reducing voids in the soil and enhancing its load-bearing capacity. The process results in minimal disturbance to the surrounding area and can effectively address settlement problems.
Compression Index: The compression index is a parameter that quantifies the compressibility of soil when subjected to an increase in effective stress during consolidation. It is crucial for understanding how much a saturated soil will compress under load, which is essential in predicting settlement behavior over time and assessing stability.
Consolidation Settlement: Consolidation settlement refers to the gradual reduction in volume that occurs in soil when it is subjected to a load over time, primarily due to the expulsion of water from the soil pores. This process is critical in understanding how structures behave after construction, as it affects the stability and performance of shallow foundations. The speed and magnitude of consolidation settlement depend on factors like soil type, drainage conditions, and the duration of the applied load.
Differential settlement: Differential settlement refers to the uneven sinking or shifting of a structure due to varying soil conditions beneath it. This phenomenon can lead to structural damage, cracks, and instability if not properly addressed. Understanding differential settlement is crucial for engineers as it influences the design and analysis of foundations, particularly when considering soil types, load distribution, and the overall integrity of civil engineering projects.
Elastic modulus: Elastic modulus is a measure of a material's ability to deform elastically when a force is applied. It reflects the relationship between stress (force per unit area) and strain (deformation) in materials, indicating how much a material will stretch or compress under load. This property is crucial in understanding the behavior of soils and foundations when subjected to loads, impacting theories of stress distribution and settlement calculations.
Finite Element Analysis: Finite Element Analysis (FEA) is a numerical method used to predict how structures behave under various physical conditions by breaking down complex shapes into smaller, manageable parts called finite elements. This method helps engineers and scientists analyze structural integrity, stress distribution, and potential failure points in designs, which is essential for optimizing performance in construction and civil engineering.
Hooke's Law: Hooke's Law states that the deformation of a solid material is directly proportional to the applied stress, provided the elastic limit is not exceeded. This principle is vital in understanding how materials behave under loads and is particularly important in assessing the settlement of shallow foundations, as it helps predict how soil will compress under weight.
Immediate settlement: Immediate settlement refers to the instantaneous change in vertical position of a foundation when a load is applied, typically occurring within a short time frame after loading. This type of settlement primarily results from the compression of soil under the applied load, particularly in saturated soils where pore water pressure may change. Understanding immediate settlement is crucial for evaluating the performance of foundations, as it directly affects structural integrity and safety.
Load Intensity: Load intensity refers to the amount of load or force applied per unit area on a surface, typically expressed in units such as pounds per square foot (psf) or kilopascals (kPa). It is a crucial concept in geotechnical engineering as it influences how structures interact with the soil beneath them, particularly in relation to settlement behavior, stress distribution, and bearing capacity of shallow foundations.
Plasticity Index: The plasticity index is a numerical value that represents the plasticity of a soil, calculated as the difference between the liquid limit and the plastic limit. It helps in understanding how a soil behaves under different moisture conditions, indicating its capacity to deform without cracking. This index is crucial in assessing soil behavior during construction, as it influences settlement characteristics, foundation performance, and the effectiveness of stabilization methods.
Poisson's Ratio: Poisson's ratio is a measure of the elastic behavior of materials, defined as the ratio of the transverse strain to the axial strain when a material is subjected to uniaxial stress. This concept is crucial in understanding how materials deform under load, affecting factors such as settlement calculations and the performance of shallow foundations. It helps predict how much a material will expand or contract in directions perpendicular to the applied load, making it important for engineers assessing structural stability and ground movement.
Primary consolidation: Primary consolidation refers to the process by which soil decreases in volume over time due to the expulsion of water from its pores when subjected to an increase in load. This process is critical in understanding how saturated soils behave under stress, as it directly impacts the settlement of structures, the stability of foundations, and overall soil mechanics.
Recompression Index: The recompression index is a measure of the compressibility of soil when it is subjected to unloading and then reloading, indicating how much the soil will compact upon being reloaded after being allowed to expand. This index is crucial for understanding how soil behaves under changes in loading conditions, particularly in predicting settlement behavior after construction. It reflects the soil's ability to regain its original volume and is a key factor in calculating settlements for structures, especially shallow foundations and during primary and secondary consolidation phases.
Schmertmann Method: The Schmertmann Method is a widely used approach for estimating the settlement of shallow foundations based on the principles of elastic theory. This method allows engineers to calculate expected vertical displacements by analyzing soil properties, foundation characteristics, and loading conditions, making it a practical tool in geotechnical engineering. By utilizing this method, practitioners can effectively assess the performance of foundations and ensure their stability under various load scenarios.
Secondary compression: Secondary compression refers to the gradual and long-term deformation of soil that occurs after primary consolidation has taken place. This process is particularly significant in fine-grained soils, such as clays, where additional settlement can continue to occur due to factors like particle rearrangement, changes in pore water pressure, and the dissipation of excess pore pressure over time. Understanding secondary compression is essential for accurate predictions of settlement in various geotechnical applications.
Settlement Plate Monitoring: Settlement plate monitoring is a geotechnical technique used to measure the vertical displacements or settlements of foundations and structures over time. This method involves placing plates on the ground surface or at specific depths in the soil, allowing for precise measurements of any shifts that may occur due to loading, soil consolidation, or other factors affecting foundation performance.
Soil Compressibility: Soil compressibility refers to the ability of soil to decrease in volume under pressure, which is crucial for understanding how soils behave when subjected to loads. This property plays a significant role in predicting settlement, especially for shallow foundations, as it determines how much a foundation will sink into the ground over time due to applied weight. Factors such as moisture content, soil structure, and the type of soil significantly influence compressibility.
Spread footing: A spread footing is a type of shallow foundation that distributes the load of a structure over a larger area to reduce the pressure on the underlying soil. This foundational approach helps in providing stability and support to buildings, especially in areas with weak soil conditions. By increasing the base area, spread footings can minimize the risk of excessive settlement and structural failure, making them crucial in the design and construction process.
Strip foundation: A strip foundation is a type of shallow foundation that consists of a continuous strip of concrete or masonry, designed to support load-bearing walls. It distributes the weight of the structure evenly across a wide area, reducing the pressure on the soil beneath. This type of foundation is particularly useful in situations where the load needs to be spread out over a long, narrow area, making it ideal for residential buildings and low-rise structures.
Terzaghi's Consolidation Equation: Terzaghi's Consolidation Equation is a fundamental formula used to calculate the settlement of saturated soil layers due to applied loads over time. It expresses the relationship between the change in effective stress, pore water pressure, and the consolidation settlement that occurs as excess pore water pressure dissipates. This equation is crucial for understanding how soil behaves under loads, particularly in the context of shallow foundations where timely and predictable settlement is essential for structural integrity.
Total settlement: Total settlement is the vertical displacement that occurs when a structure or foundation experiences a decrease in soil volume beneath it, leading to a downward movement of the entire structure. This phenomenon is crucial for understanding how foundations react to loads and the long-term behavior of structures, particularly those built on compressible soils. Total settlement can be influenced by various factors such as soil type, moisture content, and the load applied by the structure.
Underpinning: Underpinning is a construction technique used to strengthen and stabilize the foundation of a building or structure. This method is often employed when existing foundations are inadequate due to factors like settlement, changes in load, or nearby excavation work. By enhancing the foundation's support system, underpinning helps to prevent further settlement and ensures the structural integrity of the building over time.
Void Ratio: The void ratio is a fundamental soil property defined as the ratio of the volume of voids (spaces between soil particles) to the volume of solid particles in a soil sample. This term is crucial for understanding soil behavior, including how water interacts with soil, its compaction characteristics, and its strength under different conditions.
Water table fluctuations: Water table fluctuations refer to the changes in the level of groundwater in an aquifer over time, influenced by various factors such as precipitation, evaporation, and human activities. These changes can significantly affect soil moisture levels, which in turn impacts the settlement behavior of shallow foundations and can lead to structural issues if not properly managed.
© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
Back
Practice QuizGlossary
Practice QuizGlossary
Next guide