Soil Mechanics Lecture 9 Flashcards
What defines a small mass movement?
- mass movement is within a small, contained area
- retaining structures can be designed/ built to ‘retain’ the collapsed soil
- Additional stabilisation methods are usually not required
What defines a large scale mass movement
- Movements over a large area - slop stability analysis required
- retaining structures may be too expensive to build
- alternative stabilisation/ monitoring systems may be needed
What is are examples of large scale mass failure?
Falls - detatched from a steep slope; descends mostly throught the air by free fall, leaping rolling. very to extremely rapid movements
Topples - forward rotation about a pivot point under the action of gravity
Slide -
Lateral spread - shear failure or liquefaction along nearly horizontal soil layers
Flow - flow slide in sand
What different types of slides are there?
- Shear failure causing slump
- Debris slide in shear become a flow slide
- Translational movement of a major part of the slip surface
What are the purposes of retaining structures?
- Create temporary or permanent space (contruction sites, underground parking, subway stations)
- Retain soil
- stabilise slopes or excavations
What is the design goal of retaining structures?
To keep withing the servicibility limit states and to prevent damage to existing infrastructure
What are different kinds ultimate limit states?
- Bearing capacity failure
- Sliding
- Overturning
- General instability
What are the most checked ultiate limite states?
Sliding and Overturning
What are some examples of externally stable retaining walls?
- Gravity
- Cantilever
- Tieback
- Braced
What are some examples of an internally stable retaining wall?
- Reinforced soil (soil nails (natural soil), geotextiles/grids (fill), metallic strips(fill))
Where does the resistance come from in a gravity wall?
The resistance comes from self weight - bear in mind the soil has to support the weight of the wall - also bear in mind that it is retaining soil, so it also needs to counteract the load from the pressure of the soil
The reistance comes from the weight and the friction
Where does the resistance come from in a cantilever wall?
the volume below the ground surface
Where does the resistance come from in a tieback wall?
The volume of the wall below the ground surface and the friction from the tieback - the lenghth and angle of the tieback has to be good enough to make sure it goes to other zones that are not going to collapse
Where does the resistance come from in a braced wall?
the braces
How do metallic strips work?
They pull the surface under tension and keep the soil behind stable
