4: SETTLEMENT OF SHALLOW FOUNDATIONS
- Page ID
- 123406
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- 4.1: Introduction to Part 4
- This page discusses the impact of soil compressibility on shallow foundation settlement, emphasizing the need for careful design by Geotechnical Engineers to maintain structural integrity and serviceability. It outlines three settlement types: uniform, which can disrupt utilities; tilt; and non-uniform, which may lead to bending and cracking in structural elements.
- 4.2: Causes of settlement
- This page discusses settlement issues in structures, highlighting their primary cause as excessive structural loading on foundations. It also notes contributing environmental factors such as water table fluctuations, dynamic loadings, moisture-induced soil changes, and poor fill compaction. The importance of understanding these factors in relation to structural settlements is emphasized.
- 4.3: Tolerable settlement
- This page discusses the significance of limits on settlement and distortion for building stability and damage prevention. It highlights building codes like AS2870, which establish maximum allowable values for differential settlement based on the structure type. Guidelines from Poulos et al. are mentioned, providing criteria for uniform settlement across various structures.
- 4.4: Components of settlement due to structural loading
- This page discusses the components of settlement due to external loads, which include immediate settlement, primary consolidation, and secondary compression. It explains that high-permeability soils respond quickly with drained behavior, while low-permeability soils experience delayed consolidation due to excess pore pressure. Secondary compression mainly affects organic soils, with sands and stiff clays often disregarding its impact.
- 4.5: Immediate settlement of shallow foundations
- This page discusses elastic settlement of soil under flexible loadings, specifically for shallow foundations. Key factors influencing settlement include footing rigidity, embedment depth, and soil type, with variations based on drained or undrained conditions. The text explains the use of elastic theory and stresses the significance of wall friction and soil layer thickness for predicting settlement, particularly for compressible layers.
- 4.6: Primary consolidation settlement of shallow foundations
- This page covers primary consolidation settlement in fine-grained soft soils, emphasizing its significance in total settlement for structures and the influence of lateral strains on smaller foundations. It explains methods for estimating settlement, including linear consolidation theory and oedometer tests, which measure void ratio changes under controlled loading.
- 4.7: Immediate and consolidation deformations underneath shallow foundations imposing 3-D loading conditions
- This page explores elasticity theory's application in predicting soil deformation under loads, specifically for footings and embankments. It highlights the complexity of soil settlement analysis and compares analytical predictions with PLAXIS numerical simulations for soft clay. Key formulas for stress, pore pressures, and strains are presented, with findings indicating strong agreement between models.
- 4.8: Evolution of primary consolidation settlement with time
- This page discusses the relationship between pore water pressure dissipation, effective stress, and soil permeability during consolidation. It presents a closed-form solution to the one-dimensional consolidation equation under specific assumptions like uniform drainage.
- 4.9: Estimation of secondary compression settlement
- This page outlines the estimation of secondary compression or creep settlement in soil based on Hypothesis A, which suggests that soil creep starts after excess pore pressures dissipate. It presents a calculation expression for secondary compression, lists essential parameters like void ratio and soil layer thickness, and discusses determining the creep coefficient through oedometer tests, providing general ratios for clays, silts, and organic soils when specific tests are not available.
- 4.10: References
- This page discusses various scholarly contributions to geotechnical engineering and soil mechanics, highlighting Australian Standard AS2870 on residential foundations. Influential authors such as Janbu, Bjerrum, Lambe, and Poulos focus on foundation settlement and soil behavior, emphasizing the critical role of soil mechanics in ensuring the stability and design of foundations and retaining structures.
- 4.11: Additional problems
- This page addresses engineering challenges in foundation analysis, including calculations for pier settlement and angular distortion of a single-span bridge, both deemed acceptable. It determines a minimum footing radius of 3.33 m for a wind turbine and estimates consolidation settlement for a lake bed from water level fluctuations. Furthermore, it outlines a phased construction strategy for a silo on soft clay, indicating ground surface elevation at each phase.
- 4.12: Example 4.1
- This page discusses estimating immediate settlement for a square footing on saturated clay, detailing parameters such as load, embedment depth, and compressibility. It calculates the footing area and influence factors, ultimately concluding the settlement analysis. The text highlights the minimal shear resistance, suggesting a conservative wall friction factor.
- 4.13: Example 4.2
- This page details the numerical estimation of the load-settlement curve for rigid and flexible footings on sand, utilizing an elastic-perfectly plastic model based on the Mohr-Coulomb criterion. It emphasizes the significance of non-linear soil behavior through finite element modeling, leading to a maximum displacement of 30 mm and pressures of 137.5 kPa.
- 4.14: Example 4.3
- This page discusses estimating one-dimensional primary consolidation settlement in a compressible soil layer under strip pressure. With a large width-to-thickness ratio, the problem is approached using linear elastic soil behavior. It details steps for calculating settlement, focusing on the effective stress from surface pressure and its significance in the calculations.
- 4.15: Example 4.4
- This page explains how to calculate primary consolidation settlement from an oedometer test for a compressible layer subjected to strip pressure. It assumes 1-D consolidation with a pressure of 90 kPa and divides thicker layers into sublayers for more accuracy. For two 2m sublayers, the settlements are 0.14 m and 0.09 m, totaling 0.23 m. Variations in settlements are attributed to increasing preconsolidation stress with depth and the non-linear consolidation response.
- 4.16: Example 4.5
- This page discusses estimating primary consolidation settlement under a 4 m diameter circular footing, highlighting the need to consider lateral strains and model the compressible layer as an elastic half-space. It emphasizes that effective stress from footing pressure varies with depth and provides specific parameters for calculating vertical stresses and settlement, ultimately leading to a determined primary consolidation settlement value.
- 4.17: Example 4.6
- This page discusses a numerical simulation of 1-D consolidation in a compressible clay layer subjected to infinite strip pressure. The simulation follows three steps: initializing stresses, applying loading, and allowing consolidation, utilizing an undrained linear elastic model. Results show that after 100 days, consolidation is incomplete, but after 5 years, excess pore pressures are nearly zero, achieving about 100% consolidation.
- 4.18: Example 4.7
- This page presents a numerical simulation of a two-dimensional consolidation problem for an embankment on compressible clay, aiming to establish the settlement time before pavement construction to keep post-construction settlement under 10 mm. It outlines modeling steps, including pressure distribution and drainage settings, and reveals that settlement reaches the limit after about 32 days.
- 4.19: Example 4.8
- This page emphasizes the significance of boundary conditions in numerical modeling with PLAXIS for validating an analytical method for footing settlements. It reveals that thicker mesh predicts infinite settlement for strip footings, while circular footings diverge from analytical results with increased mesh thickness, attributed to numerical modeling errors.