- Title
- Bearing capacity of surface strip footings on layered soils
- Creator
- Salimi Eshkevari, Seyednima
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Estimating the bearing capacity of shallow foundations on layered soils is frequently required both in off-shore and onshore geotechnical engineering applications. The conventional bearing capacity theory, developed for homogeneous soils is not valid for such layered systems. Granular working platforms for tracked plant is a good example of footings on layered soils with relatively thin top layer. This problem is currently treated in practice using a simplified method, presented in the Building Research Establishment publication BRE470-2004. Available bearing capacity solutions are mainly based on empirical models, interpreted from experimental test results that cover limited range of material parameters. While it is generally accepted that such models may be applicable to soil properties and footing geometries outside the range tested experimentally, they offer limited insights on how the assumed failure mechanism affects their range of application. This research considers the problem of estimating the ultimate bearing capacity of rigid surface strip footings on layered soil profiles, comprising a dense sand layer of finite thickness, overlaying a deep layer of different potentially problematic soils i.e. very soft to stiff clay and very loose to medium dense sand. Rigid strip footings resting on the surface of the top layer are studied in the thesis, as they resemble tracked plant. The ultimate bearing capacity of the footings were estimated from the average of rigorous upper and lower limit collapse loads of the footing, calculated using the Finite Element Limit Analysis (FELA) method, developed in University of Newcastle. Adaptive remeshing was employed to obtain upper and lower bounds that closely bracket the true collapse loads. The geometry of the collapse mechanisms was interpreted from upper bound FELA analyses to gain insight into the contribution of layered soils parameters on the shape of the mechanism. Detailed investigations indicated that the assumed punching shear failure mechanism of Meyerhof (1974) cannot accurately describe the geometry of the shear planes for all studied material parameters. For layered soils with cohesive subgrades, it was found that the inclination of failure planes beneath the footing drifts from the assumed vertical in Meyerhof’s model, as strength of the bottom layer increases relative to the top layer. As such, a more detailed investigation was undertaken that resulted in a new bearing capacity model that considers variable geometry of the failure mechanism as a function of the key parameters of the problem. The model provides results that are in close agreement with published experimental studies, and allows treating simple problems, such as the design of working platforms, without having to resort to numerical simulations. The model is extended to clays with increasing shear strength with depth, using expressions derived for bearing capacity factor as a simple function of the dimensionless rate of increase of soil strength. The same methodology was followed to develop a new bearing capacity model for strip footings on strong sand overlying relatively weak sand. The inspiration behind this model was the observation that the mechanism, assumed in the classical Hanna (1981) solution, only provides acceptable estimation of the collapse loads for limited range of material parameters, where the top sand is significantly stronger than the bottom layer. A new form of failure mechanism (transitional failure mechanism) was identified via FELA analyses when the relative strength of the top layer decreases, resulting in Hanna’s bearing capacity model (1981) overestimating the ultimate bearing capacity of the footing. A method was developed to predict the dominant mode of failure based on the key parameters of the problem, and a new bearing capacity factor was proposed for predicting the ultimate bearing capacity of the footing, where transitional failure mechanisms apply. The new method was benchmarked against published experimental data and was shown to provide accurate estimates of the bearing capacity for a wide range of the problem parameters. The use of FELA results in this research led to the refinement of the existing bearing capacity models for footings on two layered soils. The main novelty of these models is the consideration of the failure mechanisms of variable geometry, depending on the parameters of the problem, which allows capturing the failure mode for a wide range of problem parameters. The proposed models comprise simple formulas and design charts, hence can be efficiently used in practice for the quick design of working platforms for tracked plant without having to resort to numerical analyses.
- Subject
- bearing capacity; strip footings; layered soils; Finite Element Limit Analysis; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1401100
- Identifier
- uon:34870
- Rights
- Copyright 2019 Seyednima Salimi Eshkevari
- Language
- eng
- Full Text
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | ATTACHMENT01 | Thesis | 5 MB | Adobe Acrobat PDF | View Details Download | ||
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