5: BEARING CAPACITY OF SHALLOW FOUNDATIONS
- Page ID
- 123529
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- 5.1: Introduction to Part 5
- This page discusses the application of shallow foundation bearing capacity theory, emphasizing footings with shallow embedment depths. It provides a theoretical derivation of bearing capacity formulas and highlights design aspects such as Ultimate Limit State and Load and Resistance Factor Design per Australian Standards.
- 5.2: Some fundamentals- Load and Resistance Factor Design (LRFD) and Shear strength of soils
- This page explores geostructure design principles to endure collapse loads, highlighting the use of conventional and Load and Resistance Factor Design (LRFD). It details the Mohr-Coulomb model's application to soil behavior, emphasizing shear strength parameters and limitations. The discussion includes the significance of drained vs. undrained conditions, especially for fine-grained soils, where undrained shear strength varies based on stress history.
- 5.3: Some fundamentals of the bearing capacity of shallow foundations
- This page discusses bearing capacity of foundations, focusing on maximum safe pressure, failure definitions for different soil types, and the importance of collapse load for geotechnical strength. It outlines calculation methods using limit theorems, failure mechanisms, and conditions for accurate assessments. The analysis covers strip and shallow footings, emphasizing undrained and drained conditions, along with key theoretical mechanisms and relevant equations.
- 5.4: Common bearing capacity equations and practical considerations
- This page covers the theoretical and practical aspects of calculating the bearing capacity of footings on homogeneous soil, addressing conditions under total and effective stress analyses. It emphasizes necessary corrections for factors like soil weight and footing shape, and acknowledges limitations in failure mechanisms. Additionally, it discusses the bearing capacity for loose-to-medium sands and soft clays, recommending conservative shear strength adjustments based on local failures.
- 5.5: Bearing capacity of footings on layered soils
- This page examines foundational engineering principles, emphasizing the critical thickness of the soil layer and the need to assess failure surfaces in non-uniform profiles. It covers strategies for estimating bearing capacity in various scenarios, such as soft clay over stiff layers, and encourages the excavation of weak soils or alternative filling for stability. Additionally, it references Salimi Eshkevari et al.
- 5.6: Numerical modelling of footings resting on the surface of drained soil
- This page discusses using equivalent strength parameters with an associative flow rule to achieve stable estimates of foundation bearing capacity, avoiding numerical issues related to friction and dilation angles.
- 5.7: Bearing capacity from Standard Penetration Test (SPT) results
- This page discusses challenges in obtaining undisturbed samples from coarse-grained soils for shear strength analysis, leading to reliance on in situ tests like the Standard Penetration Test (SPT) and empirical correlations for assessing footing bearing capacity. The AASHTO methodology suggests using average SPT values at specific depths, adjusted for overburden stress and groundwater depth, for footings under vertical loads without eccentricity.
- 5.8: Bearing capacity from Cone Penetration Test (CPT) results
- This page summarizes empirical methods for estimating the bearing capacity of shallow foundations through cone penetration tests, highlighting formulas from Eslaamizaad and Robertson (1996) for coarse- and fine-grained soils. Recommended conversion factors are Kφ (0.16 to 0.30) for coarse and KSu (0.30 to 0.60) for fine soils. To manage uncertainties, it suggests using the lower bounds of these factors in calculations.
- 5.9: References
- This page discusses standards and publications for bridge and foundation design, emphasizing geotechnical principles. It references AASHTO LRFD Bridge Design Specifications, Australian bridge design standards, and studies on soil mechanics and bearing capacity. Key authors like Budhu, Craig, Salgado, and Wu delve into foundational theories and practical assessments of soil strength, load testing, and soil behavior in engineering contexts.
- 5.10: Additional problems
- This page addresses the required thickness of an improvement layer and the estimation of vertical loads on a silo foundation on saturated clay, identifying a minimum thickness of 0.60 m. The short-term vertical load is calculated at 2785 kN and the long-term at 3649 kN. It also questions how a rise in the groundwater table affects the bearing capacities without considering reduction factors for shear strength parameters.
- 5.11: Example 5.1
- This page discusses designing a circular footing for a wind turbine on a saturated clay layer according to AS5100.3 standards. It details analytical methods to ascertain the minimum footing radius while examining bearing capacity under undrained loading. Key factors include undrained shear strength from triaxial tests, footing rigidity, and roughness. The ultimate geotechnical strength is calculated, with a reduction factor applied to ensure compliance with safety design parameters.
- 5.12: Example 5.2
- This page details the numerical estimation of short-term undrained collapse load for footings using PLAXIS software. It describes methods to validate the analytical collapse load and examines footings at a specific depth through total stress analysis with the Undrained Mohr-Coulomb model. The findings reveal a strong correlation between numerical and analytical bearing capacities, confirming the numerical approach's effectiveness in undrained conditions.
- 5.13: Example 5.3
- This page discusses the estimation of the design capacity of a rectangular footing in saturated clay according to AS5100.3 standards, addressing both short-term (undrained) and long-term (drained) conditions. Design capacities are calculated using specific equations with adjustments for shape and embedment, emphasizing the importance of the lower capacity, primarily influenced by undrained conditions in normally consolidated clay.
- 5.14: Example 5.4
- This page discusses the design capacity of a strip footing on a two-layer formation of soft saturated clay and a compacted coarse-grained layer, using AS 5100.3. The undrained shear strength of the clay is 30 kPa, and a punching-type failure is considered. Two calculation methods were used: a simplified method and one by Salimi Eshkevari et al., yielding similar results, although the simplified method overestimated capacity due to footing width variations.
- 5.15: Example 5.5
- This page details the design of a mat foundation for a storage facility on medium-stiff saturated clay, highlighting that the initial short-term bearing capacity of 180 kPa falls short of the required 200 kPa. To address this, a 1 m thick layer of well-compacted coarse material is proposed, validated through a numerical trial-and-error method and comparison with analytical results, confirming its effectiveness in enhancing bearing capacity and minimizing excessive settlement.
- 5.16: Example 5.6
- This page outlines the process for estimating footing design capacity in sandy soil based on Standard Penetration Test (SPT) measurements, as per AS 5100.3. It includes calculating corrected SPT values, determining average corrected N′ values, and applying equations to assess bearing capacity while considering groundwater influences. A conservative geotechnical strength reduction factor is also integrated to conclude the design capacity.
- 5.17: Example 5.7
- This page discusses estimating the design capacity of a strip footing using Cone Penetration Test (CPT) data from silty sand subsoil. The average cone resistance is 4.4 MPa, indicating medium-dense sand. A conservative geotechnical strength reduction factor of 0.40 is applied to calculate the design capacity in stress terms, adhering to AS 5100.3-2 guidelines.