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# Geotechnics

### Ground stress, pore pressure, permeability and shear strength

Question | Answer |
---|---|

Stresses in the ground are expressed as..... | Total stress, effective stress or pore water pressure |

The stress developed between two solid particles in contact acts in..... | One direction only (normal to the plane of contact between them) |

The stress developed in the pore water.... | Is a hydraulic pressure and acts equally in all directs |

Define total stress | On a plane within a soil mass, being the force per unit area transmitted in a normal direction across the plane |

Define effective stress | On a plane, representing the stress transmitted through the soil skeleton only |

Define pore pressure | Being the pressure of the water filling the void space between solid particles |

What is the relationship between them? | Total stress = effective stress + pore pressure |

Describe effective stress and explain its importance | On a plane, representing the stress transmitted through the soil skeleton only. It is important because a change in effective stress can cause changes in strength and volume/shape, of the soil |

Settlement occurs..... | as the volume of the soil layers change |

What is the coefficient of lateral earth pressure? | The ratio of the horizontal effective stress to the vertical principal effective stress |

Vertical effective stress will vary..... | Linearly with depth |

Horizontal effective stress will vary.... | depending on the degree of consolidation of the ground |

The "at rest" condition implies what? | That no deformation occurs |

How do you calculate the lateral total stress? | add the pore water pressure |

At any depth, the pore pressures.... | in all directions are equal |

In Darcy's experiment, what three things did he find? | That flow was proportional to: the head difference the cross-sectional area inverse of the soil sample length |

The pores in soils are so small that.... | The flow through most soils is steady and laminar |

Define laminar flow | Occurs when fluid flows in parallel layers, with no disruption to the layers |

When would ground water flow occur? | When there is a difference in the total head between two points in a soil mass |

What did Darcy conclude from his experiment? | Q = K x A x [change in head/ change in length} |

i = change in head/change in length What is i? | The hydraulic gradient |

V = Q/A What is V? | The Darcy/superficial velocity |

Q = K x A x [change in head/ change in length} What is K? | Coefficient of permeability |

Define the shear strength of a soil | The maximum internal resistance to applied shearing forces |

Define the internal friction angle | It is a measure of the shear strength of a soil |

Define cohesion | A measure of the forces that cement particles of soil |

Define un-drained shear strength | The shear strength of a soil when sheared at a constant volume |

Define the critical or ultimate state | Stresses reached when continuous shearing occurs at constant shear stress and constant volume |

Define dilation | Is a measure of the change of volume of a soil when it is distorted by shear |

What are the three main uses of shear strength of soil | Slope stability Retaining Wall Bearing capacity/Foundations |

The internal friction angle and cohesion are known as what? | The shear strength parameters |

Created by:
Civilman