Foundation Engineering Basics

Foundation Engineering

• Foundation: Part of an engineered system that transmits loads from the superstructure to the underlying soil or rock.
• Superstructure: The engineered part of the system that applies loads to the foundation.
• Foundation Engineer: A person with the training and experience necessary to design foundations. This involves understanding soil mechanics, geology, and foundation engineering, as well as structural aspects like reinforced concrete and steel design.

Steps in Foundation Design

• Site Location and Load: Locate the site and the position of the loads to be supported.
• Site Inspection: Physically inspect the site for geological or other potential problems, and supplement with existing data if available.
• Field Exploration: Develop a program to collect necessary soil data through field and laboratory testing.
• Soil Design Parameters: Establish the required soil design parameters using test data, scientific principles, and engineering judgment.
• Foundation Design: Design the foundation using the soil parameters, considering cost effectiveness, construction personnel, and practical tolerances.
• Collaboration and Oversight: Maintain close communication with the client, other engineers, architects, and contractors to prevent overdesign and ensure risk is managed appropriately.

Foundation Classification

• Shallow Foundations: Include bases, footings, spread footings, or mats. Generally, their depth is approximately less than the building width or breadth (D/B < 1).
  • Spread Footings: Support a single column
  • Mat Foundations: Support multiple columns, potentially the entire building structure.
  • Bases: Support machinery.
• Deep Foundations: Include piles, drilled piers, or drilled caissons. Their depth is greater than the building width or breadth (D/B > 4).

Retaining Structures

• Retaining Structures: Structures that hold back soil or material in a shape different from that given by gravity. Constructed from materials like wood, metal sheeting, concrete, and earth.

Soil Exploration/Sampling

• Soil exploration is an essential step for economical substructure design and is needed to determine foundation type, load capacity, settlemet predictions, and groundwater table depth.
• Economic evaluations of site exploration should be made in the early stages of a project to assess costs.
• The main objective of site investigation is to ascertain adequate information to help in foundation type determination, calculate allowable load capacity, make accurate predictions of settlement, and locate groundwater.

Soil Boring Methods

• Hand Tools: Backhoes and hand tools can be used to excavate test pits for obtaining undisturbed soil samples or obtaining samples for testing in non-vertical orientation.
• Mounted Power Drills: Used for numerous borings to deeper depths. Includes methods like wash boring and rotary drilling. Continuous-flight augers, especially common in North America and Europe are now the most often used method for soil exploration.

Additional Considerations

• Foundation Depth: Sufficient depth is required to avoid seasonal ground shifts (freezing/thawing), lateral squeezing beneath the foundation, and possible disturbances to adjacent properties.
• Expansive Soils: Conditions in soil can cause swelling during moisture changes.
• Corrosion: Potential for materials within the foundation to be affected by corrosive components in the soil.
• Future Changes: The foundation should accommodate possible future changes in superstructure design or loading.
• Environmental Concerns: The foundation design must meet environmental standards.

Bearing Capacity of Foundations

• The soil must have the capacity to support loads from the structure without failure. This often involves considering shear failure and settlement.
• Settlements are time-dependent and often cause structural problems, in which case intervention must occur
• Bearing capacity of rocks may be much higher than that of soils, but the structure or material properties of the rock must be considered.
• Water tables can significantly impact bearing capacity calculations, utilizing the effective unit weight of the soil.

Soil Improvement Techniques

• Consolidation: Preloading (surcharge fill) or the installation of vertical drains can be used as methods to increase consolidation of soil.
• Compaction: Methods such as excavation and recompaction, vibrocompaction, deep dynamic compaction, and vibro-replacement.
• Adhesion: Chemical stabilization, slurry grouting, jet grouting, and thermal stabilization.
• Reinforcement: Soil and rock anchors, mini-piles, soil nailing and soil mixing
• Physical alteration: Electro-osmosis, lime columns, and vitrification.
• Biological transformation: Soil mixing.

Additional Considerations

• Grouting methods involve injecting grout into the soil to control displacement, increase strength, and prevent settling.
• Stabilization using admixtures involves mixing material like Portland cement or lime to improve soil properties
• Soil inclusion refers to methods designed to prevent subsidence and to give structure support on compressible soils, including excavation with or without displacement, driving, vibrating the ground, and using various fillings for the creation of superficial foundations
• The choice and application of soil improvement methods depends on many factors; environmental characteristics, soil type and condition, economic constraints, technical feasibility, required bearing capacities, and time scales are some important factors to consider.