LECTURE 1

LECTURE 1

Bearing capacity

• Introduction- Foundation types and loading types

• Modes of shear failure, definitions, parameters and tests

INTRODUCTION

Typical problem:

• Foundation: - part of the structure in direct contact with the ground

• - transmits the load of the structure to the ground

INTRODUCTION

• Bearing capacity: there is a need to consider the foundation arrangements, the parameters that describe the performance criteria of the superstructure.

The design process must include:

– Limiting settlement or movement

– Safety against ultimate shear failure

– Functional serviceability and material durability

– Economy- construction and maintenance phases

INTRODUCTION

• THREE main criteria to be considered in the design of shallow foundations:

– Adequate depth

– Limiting settlement

– Factor of safety against shear failure- refer to the modes of shear failure

INTRODUCTION

• AIMS

• evaluate the ultimate bearing capacity of the soil beneath a

foundation (Limit state design)

• limit the settlement to a tolerable amount (Serviceability limit state)

• Durability to resist attack by substances in the ground or

environment

• Designed in accordance with BS 8004 and BS 8013 and Eurocode

7- Geotechnical Category 1

Solutions for:

• A. Limit states-collapse-factors of safety applied to applied load

and to resistance of ground (material factors)

• B. Service states -settlements/deformations(ensure that

deformations or deflections do not damage the appearance or

reduce the useful life of a structure or cause damage to installation in a structure)

Types of loading:

A. Compression

B. Tension

C. Shearing

FOUNDATION TYPES

• Pad

• Strip

• Raft

• Pile

Types of loading:

A. Compression

B. Tension

C. Shearing

Modes of shear failure

Modes of shear failure

Modes of shear failure

Types of shear strength

Strength envelopes

Failure criteria

• Need to evaluate the parameters which can be used in design.

• Apply a failure criterion to a set of shear test resul

• Two applicable failure criteria:

– The Tresca criterion- Stated in terms of total stress

– The Mohr Coulomb – Stated on terms of effective stresses

Shear strength tests and parameters

• Three main stages and two main types of drainage conditions exist

• Stages: saturation, consolidation and shearing/axial loading

• Drainage conditions: – drained test (no increase in pore water pressure after

consolidation- COMPLETE dissipation of pore water pressure)

– Undrained test- constant limiting shear stress is displayed at all values of normal stress- NO dissipation of pore water pressure

Shear strength tests and parameters

Types of shear strength test

• Laboratory – Shear Box

– Simple shear

– Ring shear

– Triaxial compression test

– Stress path

• In-situ – In- situ shear box

– Standard Penetration and cone penetration

– Shear vane

Shear box test

Shear box test • Features include:

– A known normal force is applied- vertically- consolidation occurs

– The shear displacement (∆l) is applied and the corresponding shear force T is measured.

– The vertical displacement (∆h) ismeasured.

Stress strain relationships – Types of stress

Questions

• How will peak and critical strengths vary with soils of different densities (drained tests)?

• Elaborate on the following two applicable failure criteria:

– The Tresca criterion- Stated in terms of total stress

– The Mohr Coulomb – Stated on terms of effective stresses

Nest week

• Shear box and triaxial tests

• Ultimate, safe and allowable bearing capacity

• Shallow foundations:

Bearing capacity of foundations on cohesionless soils.